Science Tech Talk: Fusion Steam Engine, Torus Radiator, etc.

[Eagle1Division]

There's a number of engineering concepts I'm going over for some science fiction and want to discuss a little.

First is the Fusion Steam Engine. Essentially, it's Magnetically Confined Fusion. (I think I'll run with D-T fusion, since mission times are in days/weeks) It's for the engines on a an orbital shuttle, so to give the fusion engine enough thrust to liftoff (preferably T/W > 1 ), water is injected as a fine spray into the "throat", just before the nozzle. The water instantly vaporizes/explodes, and lowers the exhaust velocity to get a much higher thrust by introducing extra reaction mass. The rate of water injection can be slowly lowered through the duration of the flight, so that the SSTO can have the advantages of a high thrust first stage and high impulse second stage without any staging.

(i.e., higher thrust/lower impulse at liftoff and for altitude climbing, and lower thrust/higher impulse for the second part of the ascent.)



Then there's the Torus radiator. The Torus radiator is for Fusion and Antimatter engines that produce gargantuan amounts of waste heat. It's a torus that contains plasma, where a large amount of the container is transparent on the IR frequency. The Torus shape is to create more surface area. The plasma is heated by a heat exchanging system similar to what air conditioners use; another purpose of the Torus shape is to circulate the plasma so it maintains an even temperature and doesn't heat up near the heat exchanger.
Aside from that, a fission nuclear reactor is used to power the heat exchangers and the magnets on the Torus.
A large amount of conventional radiators are needed to handle the waste heat from the nuclear reactor and the heat exchanger.

 

[Wishbone]

http://www.projectrho.com/rocket/

 

[Eagle1Division]

http://www.projectrho.com/rocket/

Yeah... I've gone through that site a number of times. Don't think it ever mentioned water injection into the throat of a fusion engine, or a Torus Radiator / plasma radiator of any kind...

 

[T.Neo]

I don't think propellant injection is a new concept, the Discovery II heated propellant with a kind of compact tokamak reactor thing, and my feeble mind is desperately trying to wrap itself around an ICF engine that does pretty much what you're describing (injecting propellant to increase mass flow; increasing thrust at the expense of exhaust velocity).

However, I would regard water as a poor choice of propellant; LH2 would be better. This is because the ionisation energy and soforth is higher (AFAIK), leading to lower efficiency, and because the exhaust of a water-rocket will have more massive particles and thus a lower exhaust velocity (most of the mass of a water molecule is in the oxygen atom).

If you want to compromise exhaust velocity a bit for a bit of a gain in propellant density, perhaps methane would be an attractive propellant. It'd probably be a better option than water.

As far as I know, inertial confinement fusion (similar to that used in the Daedalus concept, for example) is the only fusion propulsion design that can achieve the high power/mass ratios that can allow high acceleration propulsion. I suppose that for launch you can drastically cut your exhaust velocity, but even then you could require hundreds of gigawatts (the Saturn V is a good example).

The problem with D-T is although it's an easy reaction to 'ignite', it carries away most of the energy in the form of neutrons, and tritium decays as it is radioactive. You might be able to intercept a lot of the neutrons in the propellant if you're using a hydrogen rich propellant (like... hydrogen), but you'll always get some neutrons spilling out- neutrons that could fry the crew and the ship. D-D is better in that it requires only mundane deuterium, but still releases around a third of its energy as neutrons, has a lower energy density, and a higher lawson criterion.

He3-He3 is entirely aneutronic, but is apparently more difficult to ignite and requires rare Helium-3. He3-D is a good compromise; only requiring "some" He3, having a high energy density (higher than that of D-T fusion), and having a lawson criterion roughly half that of D-D fusion. It does, however, still lead to some neutron radiation via D-D side reactions.

You will also face problems with the bulk of the drive system, the mass of the drive system, the aerodynamics of the drive system, how the drive system copes with being filled with air (as opposed to being in a vacuum), high pressure operation inside the drive system, and vastly differing dynamics as exhaust velocity is shifted through an order of magnitude.

The problem with your torus radiator that I can see, is if it works on the principle of an airconditioner, why can't airconditions run merely on the heat within the rooms they cool? My knowledge of thermodynamics isn't that good, but my instinct tells me that it violates something. Airconditioners use a good deal of electricity, and your "airconditioner radiator" would probably require more power to operate, then it would shed from the vehicle (hence defeating its own purpose).

I don't think the torus shape is particularly good either. A torus with a major diameter of 40 meters and a minor diameter of 4 meters would have roughly the same area as a (smaller) 28x28 plate. In addition a torus will me more difficult to build than a flat plate, have a more intricate coolant pipe/path system, and have problems radiating into its own structure as well as that of the rest of the ship (and potentially even into its own structure, though I don't know how this would affect things).

 

[Eagle1Division]

I don't think propellant injection is a new concept, the Discovery II heated propellant with a kind of compact tokamak reactor thing, and my feeble mind is desperately trying to wrap itself around an ICF engine that does pretty much what you're describing (injecting propellant to increase mass flow; increasing thrust at the expense of exhaust velocity).

However, I would regard water as a poor choice of propellant; LH2 would be better. This is because the ionisation energy and soforth is higher (AFAIK), leading to lower efficiency, and because the exhaust of a water-rocket will have more massive particles and thus a lower exhaust velocity (most of the mass of a water molecule is in the oxygen atom).

If you want to compromise exhaust velocity a bit for a bit of a gain in propellant density, perhaps methane would be an attractive propellant. It'd probably be a better option than water.

Water's extremely hard to turn to steam, but water also expands a LOT and carries a LOT of force when it turns to steam. AFAIK this is why it's used in nuclear powerplants to turn turbines, despite the fact it takes a tremendous amount of energy to vaporize, apparently the extra amount of force it carries is well worth it.
You could say they do that because water is cheaper, but you don't necessarily have to lose the fluid you work with, it could merely be condensed, cooled, and re-used if using a different liquid is more efficient.

This is also for a colony world with limited infrastructure, so using something easily available is a huge bonus. Not to mention water is an extremely dense propellant, which also helps greatly since the craft needs an aerodynamic shape.

Also the fact it's a very heavy particle is one of the main reasons I find it attractive; if you want more exhaust velocity and less thrust you can simply lower the injection rate, but because it's so heavy when the injection rate is full the thrust is tremendous. It'll cause a major pain on the Delta-Vee, but it's only an orbital shuttle, and it's using a fusion reactor, so my guess is it'll still have enough Delta-Vee to make orbit even with a drastically hurt exhaust velocity; if there's any significant mass Ratio, anyways.

As far as I know, inertial confinement fusion (similar to that used in the Daedalus concept, for example) is the only fusion propulsion design that can achieve the high power/mass ratios that can allow high acceleration propulsion. I suppose that for launch you can drastically cut your exhaust velocity, but even then you could require hundreds of gigawatts (the Saturn V is a good example).

The problem with D-T is although it's an easy reaction to 'ignite', it carries away most of the energy in the form of neutrons, and tritium decays as it is radioactive. You might be able to intercept a lot of the neutrons in the propellant if you're using a hydrogen rich propellant (like... hydrogen), but you'll always get some neutrons spilling out- neutrons that could fry the crew and the ship. D-D is better in that it requires only mundane deuterium, but still releases around a third of its energy as neutrons, has a lower energy density, and a higher lawson criterion.

He3-He3 is entirely aneutronic, but is apparently more difficult to ignite and requires rare Helium-3. He3-D is a good compromise; only requiring "some" He3, having a high energy density (higher than that of D-T fusion), and having a lawson criterion roughly half that of D-D fusion. It does, however, still lead to some neutron radiation via D-D side reactions.

I'm working with a vehicle more along the lines of 75-150 tons, not 3,000 >.< . Thanks for the review on the nuclear fuels, I've read the short section on Project Rho, what I'd like to know is the kind of radiation shielding necessary as a result of using different types. I think D-D may be the best option, since these flights will be as routine as possible; and although the engine only runs for a few minutes for takeoff; doing this dozens of times in your life may have some bad side effects if the reaction produces neutrons, even with some shielding.

You will also face problems with the bulk of the drive system, the mass of the drive system, the aerodynamics of the drive system, how the drive system copes with being filled with air (as opposed to being in a vacuum), high pressure operation inside the drive system, and vastly differing dynamics as exhaust velocity is shifted through an order of magnitude.

The vast majority of the engine would be something a mix of MPD and Magnetic Confinement fusion, the confining magnetic fields both initiate the fusion reaction and propel it out the back; so in that way it's like the MPD and magnetic confinement.

Water injection only comes in at the very back, sort of at the "throat", behind the rest of the engine system and directly in front of the nozzle. And the nozzle will certainly need to actuate...
I'm thinking something like the "Turkey Feather" designs that jet fighters use; mentioned at the bottom of this article, in order to dilate and change the expansion area for different pressures during ascent.

The problem with your torus radiator that I can see, is if it works on the principle of an airconditioner, why can't airconditions run merely on the heat within the rooms they cool? My knowledge of thermodynamics isn't that good, but my instinct tells me that it violates something. Airconditioners use a good deal of electricity, and your "airconditioner radiator" would probably require more power to operate, then it would shed from the vehicle (hence defeating its own purpose).

I don't think the torus shape is particularly good either. A torus with a major diameter of 40 meters and a minor diameter of 4 meters would have roughly the same area as a (smaller) 28x28 plate. In addition a torus will me more difficult to build than a flat plate, have a more intricate coolant pipe/path system, and have problems radiating into its own structure as well as that of the rest of the ship (and potentially even into its own structure, though I don't know how this would affect things).

The main point I'm working on here, is the big, juicy, Rt^4 in
∂Q/∂t = Re * (5.67x10e-8) * Ra * Rt^4
That's just begging to be used, and a solid-state radiator can get so hot; so the logical thing to do is to make it hotter. And heavens knows the heat "compaction", so to speak (dumping heat from something colder into something hotter a.k.a. air conditioning), would not be so great when you're using an antimatter engine that produces gargantuan amounts of waste heat anyways.

It doesn't need to operate at an extremely high temperature; it only needs to be ionized/electrically charged, and work far hotter than anything solid could... Which is an extremely high temperature... :shifty:

And although that's said; the whole purpose is to work hotter than a solid could take; so it needs to be magnetically confined to avoid melting the walls of the container, thus the Torus shape for a Tokamak container.

And yes, the subsystems of compressing the working fluid, flowing, magnetic fields, and etc. would produce a lot of extra heat, I do realize that, but once again I stress the Rt^4, to the fourth, part of the equation.

273 K, room temperature radiator, 273K^4
= 5,554,571,841
1600 K, a liquid droplet radiator, 1600K^4
= 6,553,600,000,000
But 3,000 K, a little less than twice that, 3,000K^4
= 81,000,000,000,000
Making a radiator ten times as efficient per area:
(12.36x liquid droplet radiator)

And since it's using magnetic confinement, you can make the radiator plasma even hotter without melting the container; Say at, 4,000K^4
= 256,000,000,000,000
(39x liquid droplet radiator)
Or 5,000K^4
= 625,000,000,000,000
(95x liquid droplet radiator)

And if the Tokamak/Doughnut-shaped container isn't necessary, then you could work it into a more convenient shape for more radiating area.

 

[T.Neo]

From what my feeble mind can tell, thrust isn't related to the mass of individual particles, but rather to the overall mass flow.

To my knowledge it has to do with how much kinetic energy you can put into each particle. If you can put a set amount of kinetic energy into a particle, you'll get a higher exhaust velocity if you use lighter particles than if you use heavy ones, and the propulsive efficiency of a drive depends on the exhaust velocity.

As a comparison, a kilogram with 1 kJ of kinetic energy would have a velocity of around 45 m/s. A 1000 kilogram object with the same amount of kinetic energy would have a velocity of only around 1.5 m/s.

A nuclear powerplant and a rocket engine are two different things. For one, I'm pretty sure nuclear powerplants don't ionise their coolant. Your vehicle might not do that in low-ISP mode, but it will if you throttle it up enough. And even then, it'll probably be operating at a temperature higher than a (healthy) nuclear powerplant.

Orbital shuttles have to fit into a lot of annoying parameters that a strictly orbit-orbit craft can escape completely. Propellant density can be one of those things. That's why I suggested methane; liquid methane has a density of 416 kg/m^3, vs ~67.8 kg/m^3 for LH2. It is lower than water, but methane will definitely offer a higher exhaust velocity.

Water is 2.4 times denser than liquid methane. Liquid methane is over six times denser than liquid hydrogen. The density combined with the exhaust velocity would make it, IMO, the best option physics-wise.

Resource-wise is another issue, but it shouldn't be too much trouble to synthesise. On the other hand having a huge ship with a huge amount of required propellant (water-propelled) might actually be worse in terms of supporting infrastructure.

I'm working with a vehicle more along the lines of 75-150 tons, not 3,000 >.< . Thanks for the review on the nuclear fuels, I've read the short section on Project Rho, what I'd like to know is the kind of radiation shielding necessary as a result of using different types. I think D-D may be the best option, since these flights will be as routine as possible; and although the engine only runs for a few minutes for takeoff; doing this dozens of times in your life may have some bad side effects if the reaction produces neutrons, even with some shielding.

If you don't want a 3000 ton engine, scale it down to fit the requirement. Most of the time it should be about relationships and ratios and stuff, not about the total mass. I think. I may be horribly wrong.

The reaction produces neutrons. The advantage is that this is only roughly 33% of the total energy released by the reactions, as opposed to most of the energy, as with D-T fusion. Fusion reactors will also emit X-ray radiation, produced by [ame="http://en.wikipedia.org/wiki/Bremsstrahlung"]Bremsstrahlung[/ame] (I have no idea why or how, but it happens, and X-rays are scary, so one better not mess with them ).

The only detailed figures I could find for fusion drive efficiencies is of dubious applicability and is located here, a most of the stuff on Orion's Arm is pretty silly but they do have some interesting concepts.


For shielding against X-rays you want something dense, like lead or tungsten. For shielding against neutrons you want something that has a lot of hydrogen atoms in it- like your propellant!

You will (unfortunately) have to lug around some heavy shielding, but you can at least partially use your propellant for shielding purposes, at least for a portion of the launch.

There's an equation on Project Rho that describes how to calculate a needed thickness of shielding. Attenuation depth figures can be found all over the internet, if you can find them... :shifty:

An advantage of D-D is that deuterium is fairly common, so it'd probably be easier to get a hold of than Tritium or He3.

The vast majority of the engine would be something a mix of MPD and Magnetic Confinement fusion, the confining magnetic fields both initiate the fusion reaction and propel it out the back; so in that way it's like the MPD and magnetic confinement.

Water injection only comes in at the very back, sort of at the "throat", behind the rest of the engine system and directly in front of the nozzle. And the nozzle will certainly need to actuate...
I'm thinking something like the "Turkey Feather" designs that jet fighters use; mentioned at the bottom of this article, in order to dilate and change the expansion area for different pressures during ascent.

A mix of MPD and Magnetic Confinement fusion? Have you done the calculations for it?

Those magnets could be painfully, painfully heavy. :dry:

Well, for starters you want to inject at least some propellant in the reaction chamber, or at least use it to cool off the reaction chamber, to prevent everything from melting.

If you crank up the exhaust velocity- 100 000 m/s, 200 000 m/s, 500 000 m/s... then it's likely that the exhaust would erode the interior of your engine badly, especially the 'turkey feather' nozzle. And it might be poorly directed, too. You need a magnetic nozzle for that kind of exhaust velocity.

That's just begging to be used, and a solid-state radiator can get so hot; so the logical thing to do is to make it hotter. And heavens knows the heat "compaction", so to speak (dumping heat from something colder into something hotter a.k.a. air conditioning), would not be so great when you're using an antimatter engine that produces gargantuan amounts of waste heat anyways.

It doesn't need to operate at an extremely high temperature; it only needs to be ionized/electrically charged, and work far hotter than anything solid could... Which is an extremely high temperature...

And although that's said; the whole purpose is to work hotter than a solid could take; so it needs to be magnetically confined to avoid melting the walls of the container, thus the Torus shape for a Tokamak container.

And yes, the subsystems of compressing the working fluid, flowing, magnetic fields, and etc. would produce a lot of extra heat, I do realize that, but once again I stress the Rt^4, to the fourth, part of the equation.

It's about thermodynamics. You can't get a free lunch from thermodynamics. If you could run an airconditioner off the heat within a room, I'd believe you. But it's certain that all the associated equipment for your "aircon radiator" would be far, far, far larger and far heavier and far more power-hungry than any conventional setup.

And while your radiating plasma would not physically be in contact with any solid component, the whole process could impart a lot of heat to various components. For example, even the mundane transparent housing for the plasma would not be totally transparent, and thus it would have to have its own cooling system, going off to other radiators.

The key isn't to deal with your waste heat and cycle it through the system. That only ends sadly. The key is to seperate the energy from the ship as much as possible. You want to make your drive chamber extremely reflective, for example, or make it out of light materials such as beryllium, aluminium and carbon, so that ionising radiation such as gamma and X-rays would mostly pass through, reducing heating.

The whole key is to isolate the waste heat from the ship to the best extent possible.

 

[Eagle1Division]

From what my feeble mind can tell, thrust isn't related to the mass of individual particles, but rather to the overall mass flow.

To my knowledge it has to do with how much kinetic energy you can put into each particle. If you can put a set amount of kinetic energy into a particle, you'll get a higher exhaust velocity if you use lighter particles than if you use heavy ones, and the propulsive efficiency of a drive depends on the exhaust velocity.

As a comparison, a kilogram with 1 kJ of kinetic energy would have a velocity of around 45 m/s. A 1000 kilogram object with the same amount of kinetic energy would have a velocity of only around 1.5 m/s.

A nuclear powerplant and a rocket engine are two different things. For one, I'm pretty sure nuclear powerplants don't ionise their coolant. Your vehicle might not do that in low-ISP mode, but it will if you throttle it up enough. And even then, it'll probably be operating at a temperature higher than a (healthy) nuclear powerplant.

Orbital shuttles have to fit into a lot of annoying parameters that a strictly orbit-orbit craft can escape completely. Propellant density can be one of those things. That's why I suggested methane; liquid methane has a density of 416 kg/m^3, vs ~67.8 kg/m^3 for LH2. It is lower than water, but methane will definitely offer a higher exhaust velocity.

And that's what I meant; methane will offer a higher exhaust velocity at the same volume injected per second, but less thrust. But you could use just a smaller amount of water volume injected per second and get the same effect.
However, you can get a lot more thrust out of water using the same injection rate. But if you want the exhaust velocity the methane would offer, you just inject less water volume per second.

This applies if it's the overall mass flow that matters, as you said earlier. Water is much easier to obtain and is denser, working well for the heavily limited volume of the aerodynamic craft.

Do you mean gas-state methane? Water could easily be used as steam by running it over the "combustion chamber" first, and vaporizing it.

Anyways, I'm not drastically redesigning the fusion engine (sort of). Everything that's normally there for a MCF engine is still there, except in addition to that there's a tiny region directly behind the engine, where water can be injected at a very high pressure in a fine mist, to instantly vaporize in the fusion engine's exhaust stream. There's no reason this portion needs to be in the path of the fusion exhaust, the water is sprayed into the fusion exhaust but the injector can be safely out of the way. The entire part will be out of the way entirely during normal non-water-injecting operation. It will be the Injection Area.

Behind this portion is a turkey-feather nozzle, while water injection is off, it'll open up all the way until it's almost like a disk, completely clearing the path of any significant fusion exhaust. When water injection is turned on, however, it's used to create an expansion area for the expanding steam.

The injection rate of water can be throttled, so you can exchange thrust for impulse in a smooth way.

One issue I see is the exhaust stream from the fusion engine impacting the water, deflecting, and then impacting the walls of the Injection Area. To counter this, the inside walls of the injection area can be covered with strong plates, perhaps even replacable plates if the exhaust products from the fusion reaction, even after deflecting off the water, are harmful enough that a permanent plate would shorten the lifespan of the engine.

This section could be regeneratively cooled, as well, by the inflowing water. Since the amount of heat imparted on it will be directly proportional to deflected fusion exhaust, and deflected fusion exhaust will be directly proportional to water injection amount.

Resource-wise is another issue, but it shouldn't be too much trouble to synthesise. On the other hand having a huge ship with a huge amount of required propellant (water-propelled) might actually be worse in terms of supporting infrastructure.

I still don't understand why Methane would be different than water; if you want the higher exhaust velocity of Methane, just inject less water per second and you'll get the same effect.

If you don't want a 3000 ton engine, scale it down to fit the requirement. Most of the time it should be about relationships and ratios and stuff, not about the total mass. I think. I may be horribly wrong.

The reaction produces neutrons. The advantage is that this is only roughly 33% of the total energy released by the reactions, as opposed to most of the energy, as with D-T fusion. Fusion reactors will also emit X-ray radiation, produced by Bremsstrahlung (I have no idea why or how, but it happens, and X-rays are scary, so one better not mess with them ).

The only detailed figures I could find for fusion drive efficiencies is of dubious applicability and is located here, a most of the stuff on Orion's Arm is pretty silly but they do have some interesting concepts.


For shielding against X-rays you want something dense, like lead or tungsten. For shielding against neutrons you want something that has a lot of hydrogen atoms in it- like your propellant!

You will (unfortunately) have to lug around some heavy shielding, but you can at least partially use your propellant for shielding purposes, at least for a portion of the launch.

There's an equation on Project Rho that describes how to calculate a needed thickness of shielding. Attenuation depth figures can be found all over the internet, if you can find them... :shifty:

An advantage of D-D is that deuterium is fairly common, so it'd probably be easier to get a hold of than Tritium or He3.



A mix of MPD and Magnetic Confinement fusion? Have you done the calculations for it?

Those magnets could be painfully, painfully heavy. :dry:

Well, for starters you want to inject at least some propellant in the reaction chamber, or at least use it to cool off the reaction chamber, to prevent everything from melting.

If you crank up the exhaust velocity- 100 000 m/s, 200 000 m/s, 500 000 m/s... then it's likely that the exhaust would erode the interior of your engine badly, especially the 'turkey feather' nozzle. And it might be poorly directed, too. You need a magnetic nozzle for that kind of exhaust velocity.

Like I said, when the water injection is off, it operates just like a regular MCF engine, the water Injection Area is built to be out of the way (it doesn't need any significant length), and the Turkey Nozzle is out of the way when opened all the way. Perhaps some small trace amount of exhaust will still hit these components, but not any amount significant enough to do significant damage.

12,741,179 m/s exhaust velocity for D-D fusion...

Maybe the best part of water injection is that you can use it to get rid of the unholy amounts of waste heat a powerful fusion engine produces, by using open-cycle cooling with water, or whatever fluid is chosen. Just like a chemical rocket engine, run it around the engine to cool it before using it as reaction mass.

So, am I correct to assume that doubling the amount of reaction mass, also doubles thrust and halves specific impulse?
I don't think so...

I'm assuming the extra performance comes because a fusion engine, compared to a chemical rocket, is much more energetic per kg of fuel. The exhaust velocity is directly proportional to the energy generated by the "combustion", chemical or nuclear or even just heat, divided by the mass. Larger mass/lower energy = lower exhaust velocity. Lower mass/Higher energy = higher exhaust velocity.
However, exhaust velocity comes from Kinetic Energy, which is
KE = 1/2mv^2
Meanwhile, the actual movement of the vehicle comes from momentum, which is
p = mv
So decreasing the mass to increase the amount of energy per unit of mass would work on a square root. Multiply the energy per mass by 4, and you've multiplied the momentum imparted by 2.
Multiply the energy per mass by 1/2, and you've multiplied the total momentum imparted by Sqrt(1/2).

So if you double the reaction mass without increasing the energy (i.e, by spraying water), your total Delta-Vee will be Sqrt(1/2) it's original, and your thrust will be double.

I know I'm right. I just tested it... Now I just need to bash the equations around until they prove it :lol: .

88.065 TJ/Kg on average for D-D fusion.
Where can I get thrust for a D-D engine? There's equations for exhaust velocity, but I can't find thrust anywhere on the engine list...

---------- Post added at 03:15 AM ---------- Previous post was at 03:06 AM ----------

Ah, here it is:
Finding total Delta-Vee when reaction mass is changed. Energy is constant, created by fusion reactor at full power. However, KE per particle is what determines exhaust velocity (since the exhaust is made of particles ), so doubling the reaction mass essentially halves the KE.

1/Ti = KE
Ti, or Total injection, includes the normal exhaust from the fusion reactor, so no water injection would be 1.

Finding Delta-Vee (momentum) change for different propellant injection rate.
KE = 1/2 mv^2
p = mv

KE / v = 1/2 p
p = 2 * KE / v
p = 2 * (1/Ti) / v

Finding propellant injection rate for different Delta-Vee (momentum).
p = 2 * (1/Ti) / v
v * p / 2 = (1/Ti)
Ti = 1 / ( v * p / 2 )

multiply p by the craft's Delta-Vee without injection to get Delta-Vee with water injection.

Only problem... v is on both sides of the equation Oooooh my.
Arrg, I'm too tired for this... :facepalm: (Thank goodness I'm not on the forums' time zone. 3am!)

I think I've seen some of this before...
I have a feeling this has already been done on Atomic Rocket and I just didn't notice it. :shifty:


The Square-Root trick still works, though. I've tested it. Here's my test runs:

KE = 100
m = 10

KE/m = 1/2 V^2
2 * (KE/m) = V^2

v = 4.47

p = mv
p = 44.7
So the momentum is 44.7.

KE = 100
m = 5
(I've halved the mass)

2 * (KE/m) = V^2

V = 6.32

p = mv
p = 31.62

44.7 was the momentum for 10 mass. 44.7^2 = ~ 2,000
31.62 was the momentum for 5 mass. 31.62^2 = ~1,000

Doubling the mass doubled the square of the total momentum, so I'm working on the hypothesis that this always applies and that to half the momentum takes the square root of 2x the mass.

But anyways, it's still workable. I'll assume a mass ratio of 2.5 for a fuel-laden aerodynamic craft. (747's have mass ratio of 2).
Dv = Ve * ln[R]
R = 2.5
Ve = 12,741,179
Dv = 11,674,624
Dv needed for LEO insertion = 11,000 m/s (Figure I calculated from the STS Delta-Vee a long time ago, It's a lot bigger than 7,800 so if there's error then it's probably on the side of caution.)
Dv is 1,061x greater than that needed for LEO.
So my V can be 1 / 1,061 that of the engine without any propellant injection.

Sqrt(1,061) = 32.57.

I can inject so that I have 32.57x as much reaction mass, and 32.57x the thrust of a regular D-D MCF engine. Whatever the regular thrust is...

So with a Mass ratio of 2.5, I can inject 32.57x as much reaction mass to get 32.57x as much thrust and still have a Delta-Vee of 11,000 m/s.

---------- Post added at 03:55 AM ---------- Previous post was at 03:15 AM ----------

Okay, I actually didn't know how it worked for a bit, because the test equations seem to be wrong at first sight; they seem to say that a higher exhaust engine is less efficient. Actually, that's right, an engine with 2x the exhaust velocity requires 4x as much energy, so it's less efficient per amount of energy.

Aha! So that's why higher exhaust velocity engines produce much more waste heat.

Anyways, those equations are for a single instant of engine power.

The less fuel-efficient engine produced 44.7 momentum, and the more efficient produced 31.62 momentum, using only half as much mass. For the same amount of mass, it produced 1.4148x the momentum, but over twice the time, which means less thrust.

I think I just reinvented the wheel. But it's exciting, from a new viewpoint where it all makes sense to my own understanding.

 

[T.Neo]

And that's what I meant; methane will offer a higher exhaust velocity at the same volume injected per second, but less thrust. But you could use just a smaller amount of water volume injected per second and get the same effect.

Except we're not talking volume injected into the engine, we're talking mass injected into the engine- the mass flow.

Volume is generally important only when you want to make pipes and containers.

However, you can get a lot more thrust out of water using the same injection rate. But if you want the exhaust velocity the methane would offer, you just inject less water volume per second.

Yes, at lower thrust. Remember that it's a relationship between mass flow, thrust, and thrust power (which is effectively the kinetic energy of the mass flow... if that makes sense). The key with the lighter particles is that each particle has a higher velocity for the energy delivered to it.

This applies if it's the overall mass flow that matters, as you said earlier. Water is much easier to obtain and is denser, working well for the heavily limited volume of the aerodynamic craft.

Aerodynamic craft are also heavily limited in terms of mass. Hence why you want a compromise between best volume attributes and best mass (mass ratio determined by ISP) attributes. Methane fills the role perfectly.

Do you mean gas-state methane? Water could easily be used as steam by running it over the "combustion chamber" first, and vaporizing it.

I don't think I intended to refer to methane in a gaseous state. Methane would be stored as a cryogenic liquid, piped to the engines, potentially used for regenerative cooling, and then fed into the engine to be heated and expelled out the nozzle.

Behind this portion is a turkey-feather nozzle, while water injection is off, it'll open up all the way until it's almost like a disk, completely clearing the path of any significant fusion exhaust. When water injection is turned on, however, it's used to create an expansion area for the expanding steam.

What directs the exhaust in high specific impulse mode? What's the top range specific impulse?

I still don't understand why Methane would be different than water; if you want the higher exhaust velocity of Methane, just inject less water per second and you'll get the same effect.

At lower thrust. If particle mass wasn't important, you'd see all NTR concepts using water due to its far superior density and availability traits. But you see most of them using hydrogen, which is cryogenic, far less dense than water, and requires far more effort to obtain...

Like I said, when the water injection is off, it operates just like a regular MCF engine, the water Injection Area is built to be out of the way (it doesn't need any significant length), and the Turkey Nozzle is out of the way when opened all the way. Perhaps some small trace amount of exhaust will still hit these components, but not any amount significant enough to do significant damage.

That's fine, but how do you redirect the exhaust, how do you prevent the fusion exhaust from hitting the walls of the reactor and ablating them away, for example?

Maybe the best part of water injection is that you can use it to get rid of the unholy amounts of waste heat a powerful fusion engine produces, by using open-cycle cooling with water, or whatever fluid is chosen. Just like a chemical rocket engine, run it around the engine to cool it before using it as reaction mass.

The problem with using open-cycle cooling for such a powerful engine is that the amount of coolant needed ends up being a very large amount, with mass flows approaching that of an engine with a far lower exhaust velocity (thus negating the reason for having a drive with a high exhaust velocity).

So, am I correct to assume that doubling the amount of reaction mass, also doubles thrust and halves specific impulse?
I don't think so...

I'm assuming the extra performance comes because a fusion engine, compared to a chemical rocket, is much more energetic per kg of fuel. The exhaust velocity is directly proportional to the energy generated by the "combustion", chemical or nuclear or even just heat, divided by the mass. Larger mass/lower energy = lower exhaust velocity. Lower mass/Higher energy = higher exhaust velocity.
However, exhaust velocity comes from Kinetic Energy, which is
KE = 1/2mv^2
Meanwhile, the actual movement of the vehicle comes from momentum, which is
p = mv
So decreasing the mass to increase the amount of energy per unit of mass would work on a square root. Multiply the energy per mass by 4, and you've multiplied the momentum imparted by 2.
Multiply the energy per mass by 1/2, and you've multiplied the total momentum imparted by Sqrt(1/2).

So if you double the reaction mass without increasing the energy (i.e, by spraying water), your total Delta-Vee will be Sqrt(1/2) it's original, and your thrust will be double.

Wait, did I say anything about increasing energy while decreasing mass flow?

A variable mass flow with a set amount of delivered energy (a set thrust power) will have a variable specific impulse. To scale thrust and keep exhaust velocity the same you have to scale the amount of energy delivered to the propellant.

Of course in reality there can be a whole lot of things occuring in the engine that could make things more complex.

Where can I get thrust for a D-D engine? There's equations for exhaust velocity, but I can't find thrust anywhere on the engine list...

That's because thrust depends on engine design. I don't think there are any D-D engines on the engine list, but a D-D engine might be sufficiently similar to another type of engine to use similar to one of the engines there.

I think I just reinvented the wheel. But it's exciting, from a new viewpoint where it all makes sense to my own understanding.

I've afraid you've completely lost me. I make heavy use of online calculators and information provided elsewhere on the internet, so your heavy use of maths may have fried my brain. :facepalm:

 

[Eagle1Division]

Except we're not talking volume injected into the engine, we're talking mass injected into the engine- the mass flow.

Volume is generally important only when you want to make pipes and containers.

My point was that water is denser; but that makes no effect on the amount of mass required.

Yes, at lower thrust. Remember that it's a relationship between mass flow, thrust, and thrust power (which is effectively the kinetic energy of the mass flow... if that makes sense). The key with the lighter particles is that each particle has a higher velocity for the energy delivered to it.

I covered this in my math section....

Aerodynamic craft are also heavily limited in terms of mass. Hence why you want a compromise between best volume attributes and best mass (mass ratio determined by ISP) attributes. Methane fills the role perfectly.

Except you'd just need more Methane, you'd need the same amount because it's the mass that matters. Mass flow, doesn't matter what type of mass it is, it's mass flow.

What directs the exhaust in high specific impulse mode? What's the top range specific impulse?

Like I said, regular MCF fusion engine with an extra piece on the back; when water injection is off it's just a regular MCF fusion engine. Magnets direct the exhaust. Top specific impulse is 1,300,120 s, when water injection is off; the specific impulse of a D-D fusion engine.
(12,741,179 m/s)

At lower thrust. If particle mass wasn't important, you'd see all NTR concepts using water due to its far superior density and availability traits. But you see most of them using hydrogen, which is cryogenic, far less dense than water, and requires far more effort to obtain...

I still don't quiet get it though. How does this work with the math? What matters in the math is the relationship in-between the energy and the mass. And I thought hydrogen was used because it expands greatly when heated, making it a great substance to use since the NTR heats the propellant to drive it.

But this fusion concept I'm talking about does't use heat, it uses a direct transfer of kinetic energy (impact), a direct transfer of energy to propel the exhaust, so it's heat expansion isn't important, only the mass. Heat expansion comes as a bonus on top of that, which water is good at.

Also, water has an extremely high heat capacity, so heating it using the NTR would probably be very inefficient, whereas hydrogen would heat up instantly.

That's fine, but how do you redirect the exhaust, how do you prevent the fusion exhaust from hitting the walls of the reactor and ablating them away, for example?

It's a regular MCF engine with those extra bits at the end; I'm not concerned with the design of an MCF engine, I'm talking about the design of a water (or fluid) injection system to increase the thrust.

Magnetic fields, btw, are used to keep fusion exhaust from ablating the reactor walls.

The problem with using open-cycle cooling for such a powerful engine is that the amount of coolant needed ends up being a very large amount, with mass flows approaching that of an engine with a far lower exhaust velocity (thus negating the reason for having a drive with a high exhaust velocity).

I don't need the exhaust velocity of a fusion engine; so that's acceptable. A mass ratio of 2 (as is used in the B747 airliners) is very much acceptable. I use water to trade impulse for thrust.

Wait, did I say anything about increasing energy while decreasing mass flow?

A variable mass flow with a set amount of delivered energy (a set thrust power) will have a variable specific impulse. To scale thrust and keep exhaust velocity the same you have to scale the amount of energy delivered to the propellant.

Of course in reality there can be a whole lot of things occuring in the engine that could make things more complex.

No, I said changing the ratio of energy to mass flow. The whole idea is this:
A fusion engine doesn't provide the T/W ratio needed for an orbital shuttle, but has a far higher exhaust velocity than you need for an orbital shuttle. So by injecting water into the exhaust stream, you increase the mass flow, but the energy from the fusion engine stays the same.

You get a much lower exhaust velocity, but a much higher thrust. You lose specific impulse but you don't need the specific impulse of a fusion engine.

By injecting water, you lose impulse, and gain thrust. Fusion engine has excessive impulse, but not enough thrust; so you inject water to get the thrust you need, and although you lose specific impulse, you don't need it all anyways.

That's because thrust depends on engine design. I don't think there are any D-D engines on the engine list, but a D-D engine might be sufficiently similar to another type of engine to use similar to one of the engines there.

I think I'll use the D-T engine and slice the thrust and ISP to 1/3, since D-D fusion has 1/3 the energy, IIRC. I'll check up on that...

I've afraid you've completely lost me. I make heavy use of online calculators and information provided elsewhere on the internet, so your heavy use of maths may have fried my brain. :facepalm:

Don't worry, my own head was fried quiet badly after that. :lol:
It's odd. I sort of knew it but didn't know how I knew it. I should do that more often. It was neat

 

[T.Neo]

I covered this in my math section....

I wrote that before I read your math section. :shifty:

Except you'd just need more Methane, you'd need the same amount because it's the mass that matters. Mass flow, doesn't matter what type of mass it is, it's mass flow.

No, you'd need less mass in terms of methane than you would need in terms of water, because the specific impulse would be higher.

I still don't quiet get it though. How does this work with the math? What matters in the math is the relationship in-between the energy and the mass. And I thought hydrogen was used because it expands greatly when heated, making it a great substance to use since the NTR heats the propellant to drive it.

But this fusion concept I'm talking about does't use heat, it uses a direct transfer of kinetic energy (impact), a direct transfer of energy to propel the exhaust, so it's heat expansion isn't important, only the mass. Heat expansion comes as a bonus on top of that, which water is good at.

Also, water has an extremely high heat capacity, so heating it using the NTR would probably be very inefficient, whereas hydrogen would heat up instantly.

I don't know why it works, but does. I think it has something to do with the whole particle mass, kinetic energy relationship.

As I can understand it, the temperature of a material depends on the kinetic energy of its particles, averaged out. The [ame="http://en.wikipedia.org/wiki/Temperature"]Wikipedia article[/ame] gives a better explanation.

You're doing the same thing with a high or low performance drive; transferring heat from the fusion products to the propellant. Except in a high performance drive, you're transferring far more heat...

High heat capacity will reduce efficiency.

It's a regular MCF engine with those extra bits at the end; I'm not concerned with the design of an MCF engine, I'm talking about the design of a water (or fluid) injection system to increase the thrust.

Magnetic fields, btw, are used to keep fusion exhaust from ablating the reactor walls.

Have you seen a design for an MCF engine with this sort of power/weight?

I'm yet to find an engine that meets my wacko requirements beyond (maybe) ICF. Even if you have a thrust power of 200 gigawatts or thereabouts, you could end up with an engine massing thousands of tons, and that just wouldn't be workable.

I don't need the exhaust velocity of a fusion engine; so that's acceptable. A mass ratio of 2 (as is used in the B747 airliners) is very much acceptable. I use water to trade impulse for thrust.

You do need a somewhat high exhaust velocity. Assuming that this planet has the same surface-low orbit dV that Earth does (maybe 9500 m/s) and your engine had an exhaust velocity of 10 000 m/s, you would need a mass ratio of 2.6. For a mass ratio of 2, you'll need an exhaust velocity of 13 700 m/s.

For a 100 ton ship at a mass ratio of 2, you'd need about 240 m^3 of volume to accomodate that amount of methane, and about 1475 m^3 to accomodate liquid hydrogen (if my calculations are correct). The 747-400 ER can apparently carry roughly 240 m^3 of fuel, so that's pretty close.

I think to get the same exhaust velocity with heavier particles, you need a higher temperature. I think. I'm not sure; my knowledge of the physics involved there is quite poor.

No, I said changing the ratio of energy to mass flow. The whole idea is this:
A fusion engine doesn't provide the T/W ratio needed for an orbital shuttle, but has a far higher exhaust velocity than you need for an orbital shuttle. So by injecting water into the exhaust stream, you increase the mass flow, but the energy from the fusion engine stays the same.

It starts to become less a problem of thrust to weight and more of a problem of power to weight. If your fusion reactor has a specific power of 10 kW/kg, you end up havin an engine massing 20 000 tons!

And that can, for example, be a ~30 kN engine operating at full specific impulse, or a ~30 MN newton engine operating at 13 700 m/s. In either case, it is a pretty bad number.

By injecting water, you lose impulse, and gain thrust. Fusion engine has excessive impulse, but not enough thrust; so you inject water to get the thrust you need, and although you lose specific impulse, you don't need it all anyways.

But if you inject methane, you'll have a higher specific impulse for a certain amount of mass (?) injected than you would with water. I think, I dunno.

Just because you have 'overkill energy' doesn't mean that mass ratios and stuff like that don't factor in.

I think I'll use the D-T engine and slice the thrust and ISP to 1/3, since D-D fusion has 1/3 the energy, IIRC. I'll check up on that...

One should note that the conversion efficiency of the fusion products of D-D fusion will be higher since less energy is in the form of neutrons. Though there could be other factors affecting engine mass; for example, the lawson criterion of D-D fusion is higher than that for D-T fusion...

 

[Eagle1Division]

I wrote that before I read your math section. :shifty:



No, you'd need less mass in terms of methane than you would need in terms of water, because the specific impulse would be higher.


I don't know why it works, but does. I think it has something to do with the whole particle mass, kinetic energy relationship.

As I can understand it, the temperature of a material depends on the kinetic energy of its particles, averaged out. The Wikipedia article gives a better explanation.

You're doing the same thing with a high or low performance drive; transferring heat from the fusion products to the propellant. Except in a high performance drive, you're transferring far more heat...

High heat capacity will reduce efficiency.

If the transfer of energy was done by heating the water. But it's not: It's primarily done through direct kinetic energy transfer: I'm spraying the water into the exhaust stream, the particles directly come in contact and transfer kinetic energy. The expansion from heating is an added bonus.

Have you seen a design for an MCF engine with this sort of power/weight?

I'm yet to find an engine that meets my wacko requirements beyond (maybe) ICF. Even if you have a thrust power of 200 gigawatts or thereabouts, you could end up with an engine massing thousands of tons, and that just wouldn't be workable.



You do need a somewhat high exhaust velocity. Assuming that this planet has the same surface-low orbit dV that Earth does (maybe 9500 m/s) and your engine had an exhaust velocity of 10 000 m/s, you would need a mass ratio of 2.6. For a mass ratio of 2, you'll need an exhaust velocity of 13 700 m/s.

For a 100 ton ship at a mass ratio of 2, you'd need about 240 m^3 of volume to accomodate that amount of methane, and about 1475 m^3 to accomodate liquid hydrogen (if my calculations are correct). The 747-400 ER can apparently carry roughly 240 m^3 of fuel, so that's pretty close.

I think to get the same exhaust velocity with heavier particles, you need a higher temperature. I think. I'm not sure; my knowledge of the physics involved there is quite poor.

http://www.projectrho.com/rocket/enginelist.php#dtfusion

Thrust Power:
1.2
Exhaust Velocity:
22,000 m/s
Thrust:
108,000 N
Mass:
10,000 kg
T/W ratio:
10.8 m/s^2 (for the engine).

I think the requirements will force me to use a D-T engine, if a D-D engine would have lower performance than that... :shifty:

Dv = Ve * ln[R]
Ve = Dv / ln [R]
Dv = 9,800 m/s
R = 2.5
so
Ve(required) = 10,695
Ve(D-T) = 22,000
22,000 / 10,695 = 2.057
Sqrt (2.057) = 1.434

So I can inject a measly 43.4% extra propellant, so the engine will have the characteristics of:

Thrust Power:
0.8368
Exhaust Velocity:
10,695 m/s
Thrust:
154,872 N
Mass:
10,000 kg + 1,600 kg?
11,600 kg
T/W ratio:
13.35 m/s^2 (for the engine).

Here's the best part, though. Since the water injection is throttle-able, You can change your engine characteristics through the flight. Let's say I start with the highest thrust, and end with the highest impulse to get the required 9,800 m/s of Delta-Vee.
If I run with no water injection, I have 2.057x more Delta-Vee than I need. So, if the thrust throttle-down through the flight is linear, then I can start off the flight with 1/2.057x the impulse I need (and since Delta-Vee and Impulse are proportional, 1/2.057x the Delta-Vee I need). Engine characteristics at liftoff:

1/2.057 = 0.486145

Dv = Ve * ln[R]
Ve = Dv / ln [R]
Dv = 9,800 m/s * 0.486145 = 4,764 m/s
R = 2.5
so
Ve(required) = 5,199
Ve(D-T) = 22,000
22,000 / 5,199 = 4.2316
Sqrt (4.2316) = 2.057
(is this just an amazing coincidence or what?)

So during liftoff;

Thrust Power:
0.58
Exhaust Velocity:
4,764 m/s
Thrust:
222,156 N
Mass:
11,600 kg (doesn't change)
T/W ratio:
19 m/s^2 (for the engine)

And performance just before MECO:

Thrust Power:
1.2
Exhaust Velocity:
22,000 m/s
Thrust:
108,000 N
Mass:
11,600 kg
T/W ratio:
9.31 m/s^2 (for the engine).


Now, I could raise the thrust even higher during liftoff if I used the lower thrust for only a very short amount of time. However, since I have a significant mass ratio (2.5), it should be noted that the acceleration will probably change very little throughout the flight, since the T/W ratio provided is just for the engine and not the entire vessel including fuel.

Oh, and one more note, since with this setup vertical takeoff is impossible: this only accounts for transfer of kinetic energy. Expansion of the fluid caused by rapid heating also produces significant thrust (since that's how NTR works...), and that's not accounted for at all in these numbers.

Since that's the only way that the NTR produces thrust, which it produces a lot of, my gut and my head both tell me the actual thrust will be far, far, far greater than the numbers given above. (Now if only I could find an equation for thrust given by vaporizing fluid...)

But if you inject methane, you'll have a higher specific impulse for a certain amount of mass (?) injected than you would with water. I think, I dunno.

Just because you have 'overkill energy' doesn't mean that mass ratios and stuff like that don't factor in.

If I'm using the performance of the given D-T engine, then I will probably need methane for it to make it to orbit. Though to be really honest, I still think water would still work, because the thrust power is 0.58, which is rediculously high compared to any chemical engine, and I absolutely don't doubt that the water will either be completely vaporized upon contact with the fusion stream or even turned to plasma, that is, if it hasn't already been vaporized via open-cycle cooling.

One should note that the conversion efficiency of the fusion products of D-D fusion will be higher since less energy is in the form of neutrons. Though there could be other factors affecting engine mass; for example, the lawson criterion of D-D fusion is higher than that for D-T fusion...

I think the lower performance of D-D is fatal at this point. The design barely works with D-T fusion, I wouldn't want to use a reaction that gives less performance...

---------- Post added at 02:18 AM ---------- Previous post was at 02:00 AM ----------

Oh, actually I think water would work better than Methane. Since the temperatures here are very much high enough to instantly vaporize the water (thrust power 1.2 meets fine spray of water for resulting average thrust power of 0.58) I really doubt heat capacity plays a major role. Rather, the expansion of the fluid upon reaching boiling temperature, it's a guess, but something tells me it's related to the speed of sound in the fluid. And in this case, a higher speed of sound means it "explodes" faster when it's vaporized, and has a tremendous amount of expansion.

Hydrogen or methane would work better when heat transfer rate is a major issue, like in a NTR, since the NTR can't operate at a temperature above ~2,000 C (unless it's melting).

But the exhaust stream of the fusion engine is thousands of times hotter than that, so heat transfer is not so much an issue. However, force created by vaporizing is always an issue, so you'd aim for a fluid that creates more force when it's vaporized, and for that, water would work fine.

 

[T.Neo]

If the transfer of energy was done by heating the water. But it's not: It's primarily done through direct kinetic energy transfer: I'm spraying the water into the exhaust stream, the particles directly come in contact and transfer kinetic energy. The expansion from heating is an added bonus.

What I was trying to say is that temperature is a kind of kinetic energy. By transfering the heat of the fusion products to the propellant, you're doing just that- transferring kinetic energy.

To me it sounds like you want to accelerate the fusion products first and then basically collide them into the propellant. I don't know, but I think every propellant-injected design I've seen injects the propellant into a sort of "combustion chamber" for maximum heating before expelling it all out the nozzle.

I think. :shifty:

I think the requirements will force me to use a D-T engine, if a D-D engine would have lower performance than that...

Don't use D-T. The problem with D-T is that tritium has to be manufactured and it decays. If you really have to, use D-He3- at least the He3 occurs naturally somewhere, and the D-He3 fusion reaction is aneutronic, unlike the D-T reaction which takes away most of the energy in the form of neutrons (though a D-He3 reactor will still produce some neutrons).

Your thrust-weight is likely far too low to be practical, and I'm having a hard time understanding your mass figures... they seem too 'frankenstein', and not enough 'We know this will work this way because of this, this and this".

And what figures is the thrust power in? Gigawatts? Megawatts? Petawatts?

If I'm using the performance of the given D-T engine, then I will probably need methane for it to make it to orbit. Though to be really honest, I still think water would still work, because the thrust power is 0.58, which is rediculously high compared to any chemical engine, and I absolutely don't doubt that the water will either be completely vaporized upon contact with the fusion stream or even turned to plasma, that is, if it hasn't already been vaporized via open-cycle cooling.

Have you calculated the density of the 'fusion stream', the mass flow through the reactor, and the amount of energy the fusion reaction releases?

Have you done even a little research into how the fluid interacts with the 'fusion stream'? Have you calculated the amount of mass needed for the open-cycle cooling?

I think the lower performance of D-D is fatal at this point. The design barely works with D-T fusion, I wouldn't want to use a reaction that gives less performance...

The engine you're using was never meant for this purpose at all. It is meant as an ultra-low thrust continuous acceleration engine for interplanetary missions. It doesn't have any of the attributes that a high-thrust, high specific power engine has.

It's far too late now, but tomorrow I'll try and post how I'd try to get at the goal you're getting at.

Not that my ersatz methods of research, calculation and determination are much good.

---------- Post added 05-29-11 at 00:17 ---------- Previous post was 05-28-11 at 05:34 ----------

Ok, let's assume that;

- Body that the vehicle is launching from has an dV to orbit of 9800 m/s.
- Vehicle masses 100 tons.
- Payload is 30 tons.
- Mass ratio is 3 or under.
- Atmospheric and gravitational properties are sufficiently similar to those of Earth.

It's best to work backwards. Let's say the engine has two 'stages' a high thrust, low ISP stage for low in the atmosphere, and a low thrust, high ISP stage for later on in the flight.

The low ISP mode exhaust velocity will be 13 700 m/s. The high ISP mode exhaust velocity will be ten times that; 137 000 m/s.

Upon takeoff the vehicle will have a mass of 390 tons. Let's say the vehicle has a L/D ratio of 6. Let's give the vehicle a thrust of 1000 kN in first gear; that is a thrust power of 6.85 gigawatts, if my math is correct.

In terms of mass ratios and variable ISP ships, it's best to work backwards; Let's say that the 'second gear' stage is responsible for 4000 m/s of the overall dV. This requires a mass ratio of 1.03, or 3.9 tons of propellant, if my math is correct.

Let's say that in second gear, the spacecraft needs to accelerate at 6 m/s^2. For a 135 ton ship, that is 810 kilonewtons. At the second-gear ISP, that is a thrust power of ~55.5 gigawatts. Oops. Maybe that can work in your favour, however- it could allow you to have a slightly higher thrust power in first-gear mode, or the engine could be designed as such that it copes better with higher power loads for example, in second-gear.

If you want to keep power production steady at 6.85 GW, you would have a thrust of 100 kN at 137 000 m/s. Which is still pretty good, but only enough to accelerate a 135 ton ship at about 0.74 m/s^2. I don't know if this is enough to maintain a proper trajectory while thrusting to orbit, though it'd probably be desirable to go for the lowest acceleration that can still sufficiently cope with gravity losses, for example.

Anyway, you've still got 5800 m/s to deal with using the first-gear mode. You'll need a mass ratio of about 1.53 for this; that would entail 71.5 tons of propellant (for 130 ton ship + 5 tons of propellant used in the "second stage"). Mass flow should be around 73 kilograms/second.

A little test I did with the Deltaglider and varied thrust levels led to a time of 5-7 minutes to reach mach 5.5 and 27km altitude. Let's say it takes this vehicle 15 minutes to accelerate to 5800 m/s; 900 seconds. At 73 kg/s, that is roughly 65 tons, which equates quite closely with the previous figure of 71.5 tons. In order to recuperate for losses over the atmospheric phase of the flight, an overall propellant mass of 80-90 tons might be suitable, and still definitely within the overall mass limit.

For the first-gear burn you would need to burn roughly 0.0786 grams of D-D fuel per second (or about 70 grams over a 15 minute burn). For the second-gear 55.5 GW burn you would need to burn roughly 0.6370 g/s (or about 430 grams over the entire burn); note that this is not the amount of fuel that has to be fed into the engine, which would be quite a bit more (due to reactor and energy conversion efficiency issues).

From what I understand you basically want an engine that does stuff that no other engine design does (probably). Fusion engines- fusion reactors- fusion anything, is extremely finicky, tricky stuff, and I haven't a clue about how they work beyond saying that the way you're doing it probably won't work- the specific power is too low, the magnets will be too heavy, the engine would be too bulky, it could flood with air or something and not work, all sorts of stuff like that.

It's also doubtful that the ship will mass only 100 tons; you might get all the peripheral stuff under that mass, but the engine and radiation shielding will be the most massive components, and the engine specifically will almost certainly be very heavy. But that's what overkill is important for; managing extra growth. You'll even see it in NASA studies; it is not a good thing to be caught with a ship that can't carry enough mass.

I'm not saying that you should use my figures -my figures are very likely wrong- but the key here is to understand the concept of the ship, to visualise it. Figure out the requirements and the limitations of this ship. And then the key is to understand the relationship between power, and mass, and the rocket equation. If they do not fit, change the requirements- or change the relationship.

You might want to experiment with VTOVL for example. Or a lower second-gear exhaust velocity. Or some sort of airbreathing propulsion, such as a scramjet. The possibilities are endless...

 

[Eagle1Division]

What I was trying to say is that temperature is a kind of kinetic energy. By transfering the heat of the fusion products to the propellant, you're doing just that- transferring kinetic energy.

To me it sounds like you want to accelerate the fusion products first and then basically collide them into the propellant. I don't know, but I think every propellant-injected design I've seen injects the propellant into a sort of "combustion chamber" for maximum heating before expelling it all out the nozzle.

I think. :shifty:

Not really. It's true on an atomic level, but in this sort of application it's more accurate to say "thermal energy" as different from "kinetic energy".

For instance, if I hit a baseball with a bat, I'm imparting kinetic energy. It's extremely different from heating the baseball until it explodes, and directing it's vapor backwards. It's not anything nearly the same.

In this sort of way the design is "hitting" the water with the fusion exhaust, and thus directly transferring kinetic energy by impact rather than heating.

The purpose of putting it in a fine spray is to make the thermal transfer as fast as possible by maximizing the surface-area-to-mass ratio of the water.

And since it's nothing but a low-mass, high velocity stream of air hitting the high-mass water, I would NOT want to confine it to a chamber. Reason simply being that the fusion products cannot be allowed to impact the walls of the fusion chamber. If the water injection occured in a chamber with a "throat", then the slowing of the flow would backtrack and the fusion products would impact the walls of the fusion chamber.

Best example: Compare the pressure in a watering hose when it's on, as opposed to when you've got your thumb over it. The pressure is much higher with your thumb over it because something is obstructing the flow; the same would happen if the injection happened in a chamber, and the increase of pressure would mean the fusion products would impact the walls of the fusion chamber.

Either that, or you'd need much more powerful, heavy magnets to keep the fusion products from hitting the walls of the chamber.

Don't use D-T. The problem with D-T is that tritium has to be manufactured and it decays. If you really have to, use D-He3- at least the He3 occurs naturally somewhere, and the D-He3 fusion reaction is aneutronic, unlike the D-T reaction which takes away most of the energy in the form of neutrons (though a D-He3 reactor will still produce some neutrons).

Yes, but Deuterium has to be manufactured, too... :shifty:

Sure tritium has a short half-life... measured in years. I don't need it to last years any more than the space shuttle needs to hold LH-2 in the ET for years.
And it can be bread in a chamber lined with a certain isotope of Lithium, though lithium itself is hard to get, I still think that'd be easier than mining He3 from elsewhere in the solar system and shipping it to the habitable planet.

Your thrust-weight is likely far too low to be practical, and I'm having a hard time understanding your mass figures... they seem too 'frankenstein', and not enough 'We know this will work this way because of this, this and this".

Once again, I'd like to calculate how the rapid thermal expansion effects the thrust and impulse. Doubtless it adds to both, but since I don't know where to get equations that describe thrust provided by vaporizing water, and it's such a specific question I really doubt google will work...

Here's the science behind Frankenstein: The new systems include a turkey feather nozzle, a short portion of metal for the injection area, and some pipes to transport the water, a pump for the water, with a negligable mass of spray injectors. Really, I doubt that all would weigh 1.8 tons, but I'd prefer to err on the side of caution...

And what figures is the thrust power in? Gigawatts? Megawatts? Petawatts?

Same unit the rest of the Engine list uses: Gigawatts.

Have you calculated the density of the 'fusion stream', the mass flow through the reactor, and the amount of energy the fusion reaction releases?

Have you done even a little research into how the fluid interacts with the 'fusion stream'? Have you calculated the amount of mass needed for the open-cycle cooling?

Frankly I don't see the point. I'm not designing an engine system as a professional. I'm playing with an idea. I don't pretend to be a professional who can actually design every last detail on something like this.

When it comes to measuring the performance of the vehicle, it's really more of the ratios that matter; more or less I've mathematically solved those.

Even "a little research" in fluid dynamics is quiet out of my league :blink: - what I can say though, is you spray a very fine mist of water over an exhaust stream of hypersonic superheated plasma-state hydrogen, you'll likely get a little bit of an explosive result

As for the mass for open-cycle cooling, are those on Atomic Rocket? Those would be interesting to find, I somehow doubt I'll have enough, though there's no reason the water can't help, and let radiators take what heat is left. (The Space Shuttle uses Flash Evaporators until it's payload doors can open in orbit; doubtless I can use a similar system.)

The engine you're using was never meant for this purpose at all. It is meant as an ultra-low thrust continuous acceleration engine for interplanetary missions. It doesn't have any of the attributes that a high-thrust, high specific power engine has.

It's far too late now, but tomorrow I'll try and post how I'd try to get at the goal you're getting at.

Not that my ersatz methods of research, calculation and determination are much good.

I know it wasn't made for this; that's why I came up with a way it can be used for this .

I won't argue against myself; but any discussion is better than none.


For instance, now I realize I'll need caps over the engine nozzle before ignition. It goes like this: Vehicle is taxiid to the runway the same way an airplane is pulled out from a jetway (hitched on a small utility, truck-like vehicle. I know they have a name...).

Before passengers board, one of the pre-flight ops is to place a small, thin seal over the fusion combustion chamber (which will sort of be like a nozzle behind the injection area and inside the Turkey Feather Nozzle.)
Once it's sealed off, a small pump turns the combustion chamber into a vacuum, allowing ignition to occur. As ignition occurs, water injection is enabled, and the massive force of the fusion products blows the tiny plastic or paper seal off like it's not even there. And that's how you start a fusion engine in atmosphere .

---------- Post added 05-29-11 at 00:17 ---------- Previous post was 05-28-11 at 05:34 ----------

Ok, let's assume that;

- Body that the vehicle is launching from has an dV to orbit of 9800 m/s.
- Vehicle masses 100 tons.
- Payload is 30 tons.
- Mass ratio is 3 or under.
- Atmospheric and gravitational properties are sufficiently similar to those of Earth.

It's best to work backwards. Let's say the engine has two 'stages' a high thrust, low ISP stage for low in the atmosphere, and a low thrust, high ISP stage for later on in the flight.

The low ISP mode exhaust velocity will be 13 700 m/s. The high ISP mode exhaust velocity will be ten times that; 137 000 m/s.

Upon takeoff the vehicle will have a mass of 390 tons. Let's say the vehicle has a L/D ratio of 6. Let's give the vehicle a thrust of 1000 kN in first gear; that is a thrust power of 6.85 gigawatts, if my math is correct.

In terms of mass ratios and variable ISP ships, it's best to work backwards; Let's say that the 'second gear' stage is responsible for 4000 m/s of the overall dV. This requires a mass ratio of 1.03, or 3.9 tons of propellant, if my math is correct.

Let's say that in second gear, the spacecraft needs to accelerate at 6 m/s^2. For a 135 ton ship, that is 810 kilonewtons. At the second-gear ISP, that is a thrust power of ~55.5 gigawatts. Oops. Maybe that can work in your favour, however- it could allow you to have a slightly higher thrust power in first-gear mode, or the engine could be designed as such that it copes better with higher power loads for example, in second-gear.

If you want to keep power production steady at 6.85 GW, you would have a thrust of 100 kN at 137 000 m/s. Which is still pretty good, but only enough to accelerate a 135 ton ship at about 0.74 m/s^2. I don't know if this is enough to maintain a proper trajectory while thrusting to orbit, though it'd probably be desirable to go for the lowest acceleration that can still sufficiently cope with gravity losses, for example.

Anyway, you've still got 5800 m/s to deal with using the first-gear mode. You'll need a mass ratio of about 1.53 for this; that would entail 71.5 tons of propellant (for 130 ton ship + 5 tons of propellant used in the "second stage"). Mass flow should be around 73 kilograms/second.

A little test I did with the Deltaglider and varied thrust levels led to a time of 5-7 minutes to reach mach 5.5 and 27km altitude. Let's say it takes this vehicle 15 minutes to accelerate to 5800 m/s; 900 seconds. At 73 kg/s, that is roughly 65 tons, which equates quite closely with the previous figure of 71.5 tons. In order to recuperate for losses over the atmospheric phase of the flight, an overall propellant mass of 80-90 tons might be suitable, and still definitely within the overall mass limit.

For the first-gear burn you would need to burn roughly 0.0786 grams of D-D fuel per second (or about 70 grams over a 15 minute burn). For the second-gear 55.5 GW burn you would need to burn roughly 0.6370 g/s (or about 430 grams over the entire burn); note that this is not the amount of fuel that has to be fed into the engine, which would be quite a bit more (due to reactor and energy conversion efficiency issues).

From what I understand you basically want an engine that does stuff that no other engine design does (probably). Fusion engines- fusion reactors- fusion anything, is extremely finicky, tricky stuff, and I haven't a clue about how they work beyond saying that the way you're doing it probably won't work- the specific power is too low, the magnets will be too heavy, the engine would be too bulky, it could flood with air or something and not work, all sorts of stuff like that.

It's also doubtful that the ship will mass only 100 tons; you might get all the peripheral stuff under that mass, but the engine and radiation shielding will be the most massive components, and the engine specifically will almost certainly be very heavy. But that's what overkill is important for; managing extra growth. You'll even see it in NASA studies; it is not a good thing to be caught with a ship that can't carry enough mass.

I'm not saying that you should use my figures -my figures are very likely wrong- but the key here is to understand the concept of the ship, to visualise it. Figure out the requirements and the limitations of this ship. And then the key is to understand the relationship between power, and mass, and the rocket equation. If they do not fit, change the requirements- or change the relationship.

You might want to experiment with VTOVL for example. Or a lower second-gear exhaust velocity. Or some sort of airbreathing propulsion, such as a scramjet. The possibilities are endless...

That's a lot of work! :blink: .
I designed the engine first, and tried to make it as capable as possible; simply because I know that I won't be able to make the engine anything I want, I'll only be able to make it perform like X, then I'll be forced to design the vehicle and ascent trajectory around that. I do have the mass ratios, thrust power, thrust, impulse, mass, etc. etc. etc. all planned out, even for different water injection rates (it doesn't have set "gears" or "modes", the water injection rate is throttleable just as much as a chemical rocket engine is throttleable, but unlike a chemical rocket engine; it's much more like VASIMR. The thrust/Impulse ratio is throttleable. You throttle it simply by controlling the water injection rate on the pumps).

I do know of other ways; Scramjets would need either very powerful active cooling or an ablative material; they also don't offer any truly amazing ISP, it's good, but not revolutionary; especially not once you consider you either need to bring active cooling material along or use a non-resuable engine. (Though an ablative engine could have a number of uses greater than 1...)

I remember writing to an Aerospace engineer on the topic, one thing he mentioned, IIRC, was something along the lines of "flash heating". Any known metal is going to melt under the conditions needed for a scramjet to provide thrust. And a rather key point I wish I could remember better was if it was any known "metal" or "material", lol, wonderful :facepalm: .

Fission engines usually involve spraying radioactive death everywhere,
Fusion engines don't have much thrust; which is something I'm trying to address with water injection.

As for VTOVL, a Shuttle or Rocket-style liftoff requires a lot of infrastructure; and Vertical Landing is always more difficult than runway landing; the best vehicle for a low-infrastructure world would be one that can take off from a runway and land on a runway like an airplane; requiring the least infrastructure possible.

Heck, 1 out of every 5 miles of U.S. interstate can be used as an airstrip, just to give an example of how much infrastructure a strip of concrete requires as opposed to vertical launching or landing facilities, towers, etc.

 

[T.Neo]

Why are you still using water? Methane is better.

In this sort of way the design is "hitting" the water with the fusion exhaust, and thus directly transferring kinetic energy by impact rather than heating.

Honestly I don't think that'd be very efficient. I'd imagine that when you impact the propellant with a fusion particle, it's going to lose velocity heat up the propellant anyway; the mass flow of the fusion products is low and it'll be far more efficient to spread the energy to the propellant thermally.

I don't think I've ever seen a design that uses kinetic energy to impart exhaust velocity...

And since it's nothing but a low-mass, high velocity stream of air hitting the high-mass water, I would NOT want to confine it to a chamber. Reason simply being that the fusion products cannot be allowed to impact the walls of the fusion chamber. If the water injection occured in a chamber with a "throat", then the slowing of the flow would backtrack and the fusion products would impact the walls of the fusion chamber.

Best example: Compare the pressure in a watering hose when it's on, as opposed to when you've got your thumb over it. The pressure is much higher with your thumb over it because something is obstructing the flow; the same would happen if the injection happened in a chamber, and the increase of pressure would mean the fusion products would impact the walls of the fusion chamber.

Either that, or you'd need much more powerful, heavy magnets to keep the fusion products from hitting the walls of the chamber.

I'm pretty confused with what you're saying. The plasma will be at a far lower pressure than anything coming out of a garden hose, you can pretty much only redirect it with magnets and those will tend to be very heavy...

Yes, but Deuterium has to be manufactured, too...

Sure tritium has a short half-life... measured in years. I don't need it to last years any more than the space shuttle needs to hold LH-2 in the ET for years.
And it can be bread in a chamber lined with a certain isotope of Lithium, though lithium itself is hard to get, I still think that'd be easier than mining He3 from elsewhere in the solar system and shipping it to the habitable planet.

Deuterium actually exists in nature, Tritium does not. You can isolate deuterium from water. A tritium production facility would likely be far more demanding than a deuterium refinement one.

Tritium decay isn't a problem vessel-wise (though it could be with an interplanetary vessel that spends months or years in space), but it does pose a problem when you're storing tritium on the ground.

And there are still problems like the high neutron flux of D-T fusion.

Here's the science behind Frankenstein: The new systems include a turkey feather nozzle, a short portion of metal for the injection area, and some pipes to transport the water, a pump for the water, with a negligable mass of spray injectors. Really, I doubt that all would weigh 1.8 tons, but I'd prefer to err on the side of caution...

Only 1.8 tons? Those systems could mass around 10 tons or more. And your reactor could mass 100 tons or more...

The concepts are nice, but numbers are required for them to have any validity. You should have seen the 'science' behind HVIPS... :facepalm:

For instance, now I realize I'll need caps over the engine nozzle before ignition. It goes like this: Vehicle is taxiid to the runway the same way an airplane is pulled out from a jetway (hitched on a small utility, truck-like vehicle. I know they have a name...).

Before passengers board, one of the pre-flight ops is to place a small, thin seal over the fusion combustion chamber (which will sort of be like a nozzle behind the injection area and inside the Turkey Feather Nozzle.)
Once it's sealed off, a small pump turns the combustion chamber into a vacuum, allowing ignition to occur. As ignition occurs, water injection is enabled, and the massive force of the fusion products blows the tiny plastic or paper seal off like it's not even there. And that's how you start a fusion engine in atmosphere .

Massive force of the fusion reaction? Those will be pretty high energy particles, but the pressure inside the reactor won't be that high. Also the problem of air getting into the fusion chamber after ignition, or the pressure from the expanding propellant, and that would have to be contained and directed out of the nozzle...

Maybe one option is to have some sort of fusion plasma set up (somehow) and then have a layer of propellant surrounding it, cutting it off from the rest of the engine; the pressure inside the bubble would have to be pretty high- it'd basically be like a nuclear gas core rocket. But keep in mind this is an extremely vague idea that I came up with, without any further number-crunching or qualified opinions its validity is at best dubious.

Same unit the rest of the Engine list uses: Gigawatts.

Ah. I didn't know that, so I got totally confused. :shifty:

Frankly I don't see the point. I'm not designing an engine system as a professional. I'm playing with an idea. I don't pretend to be a professional who can actually design every last detail on something like this.

It isn't about designing it right down to the last nut and bolt. It's about coming up with numbers that could plausibly fit a plausible system.

I'm not trying to be funny here, but as far as I can see your idea is just floating in mid-air...

As for the mass for open-cycle cooling, are those on Atomic Rocket? Those would be interesting to find, I somehow doubt I'll have enough, though there's no reason the water can't help, and let radiators take what heat is left. (The Space Shuttle uses Flash Evaporators until it's payload doors can open in orbit; doubtless I can use a similar system.)

I haven't seen any figures for it, but you can try to figure it out using the specific heat of your coolant, among other things...

I do know of other ways; Scramjets would need either very powerful active cooling or an ablative material; they also don't offer any truly amazing ISP, it's good, but not revolutionary; especially not once you consider you either need to bring active cooling material along or use a non-resuable engine. (Though an ablative engine could have a number of uses greater than 1...)

Difficult, yes. But this is a fusion powered SSTO, after all...

And there's always the possibility of a precooled SABRE-like engine; such an engine is being actively researched and seems far more in the near-future at least, than a SCRAM engine.

Skylon is supposed to get up to Mach 5.14 and 28.5km altitude in airbreathing mode; that could make a big difference to such a vehicle.

I remember writing to an Aerospace engineer on the topic, one thing he mentioned, IIRC, was something along the lines of "flash heating". Any known metal is going to melt under the conditions needed for a scramjet to provide thrust. And a rather key point I wish I could remember better was if it was any known "metal" or "material", lol, wonderful .

Considering the amount of research put into scramjets (as well as the existence of technology demonstrators, I doubt there are such serious stumbling blocks to make them impossible universally...

Fusion engines don't have much thrust; which is something I'm trying to address with water injection.

Propellant injection isn't magic. Water injection isn't even the best form of propellant injection.

You can't just fix the thrust problems of a fusion engine by injecting propellant. There are a whole lot of issues that need to be addressed. It isn't impossible; just difficult.

Fission engines do not spray radioactive death everywhere (as a general rule). Only NSWR, gas core, and liquid core (debatable; you'll probably get some fuel leakage) are truely bad offenders; a properly designed solid-core engine can release a negligible amount of radiation only.

The problem with a fission engine (that might unfortunately extend to fusion engines in some respects) is that the engine becomes radioactive after you've used it, which makes maintainance exceedingly difficult.

As for VTOVL, a Shuttle or Rocket-style liftoff requires a lot of infrastructure; and Vertical Landing is always more difficult than runway landing; the best vehicle for a low-infrastructure world would be one that can take off from a runway and land on a runway like an airplane; requiring the least infrastructure possible.

Heck, 1 out of every 5 miles of U.S. interstate can be used as an airstrip, just to give an example of how much infrastructure a strip of concrete requires as opposed to vertical launching or landing facilities, towers, etc.

Just because you have HTHL does not mean you need minimal infrastructure; you'll still need a large amount of the infrastructure needed for a VTOVL, for example. The only difference is that some aspects of ground handling might be easier.

And the landing area for a VTOVL might be less intensive to build and maintain than a runway, for example. On the other hand there are requirements that the propulsion system on a VTOVL has fulfill, that the propulsion system on an HTHL might not, for example.

Note though that "VTOVL" doesn't mean "use operation akin to the Shuttle". The DC-Y study and the DC-X test vehicle is a good example. That doesn't mean that you could run the whole vehicle with a one man operation though, but hopefully things are a bit better than they are today

 

[Eagle1Division]

Why are you still using water? Methane is better.

Methane it is.

Honestly I don't think that'd be very efficient. I'd imagine that when you impact the propellant with a fusion particle, it's going to lose velocity heat up the propellant anyway; the mass flow of the fusion products is low and it'll be far more efficient to spread the energy to the propellant thermally.

I don't think I've ever seen a design that uses kinetic energy to impart exhaust velocity...

The math I worked earlier worked on the mathematical model of the water taking kinetic energy from the rest of the mass flow. It reduces exhaust velocity, I know, but adds thrust. Like I've said a number of times... :shifty:

Methane breeding shouldn't be very difficult at all; and it probably does expand much more greatly when heated, so it's probably a better choice.

And I did note significant thrust would be added due to transfer of thermal energy, i.e., heating the propellant.

I'm pretty confused with what you're saying. The plasma will be at a far lower pressure than anything coming out of a garden hose, you can pretty much only redirect it with magnets and those will tend to be very heavy...

?

I really need to write up a simple diagram/blueprint...

Massive force of the fusion reaction? Those will be pretty high energy particles, but the pressure inside the reactor won't be that high. Also the problem of air getting into the fusion chamber after ignition, or the pressure from the expanding propellant, and that would have to be contained and directed out of the nozzle...

Maybe one option is to have some sort of fusion plasma set up (somehow) and then have a layer of propellant surrounding it, cutting it off from the rest of the engine; the pressure inside the bubble would have to be pretty high- it'd basically be like a nuclear gas core rocket. But keep in mind this is an extremely vague idea that I came up with, without any further number-crunching or qualified opinions its validity is at best dubious.

You could still cap off the nozzle, then, and get the same effect. Fusion products may be at low pressure, but huge amounts of methane heated to thousands of degrees from a liquid state will NOT be at a low pressure at all.

Deuterium actually exists in nature, Tritium does not. You can isolate deuterium from water. A tritium production facility would likely be far more demanding than a deuterium refinement one.

Tritium decay isn't a problem vessel-wise (though it could be with an interplanetary vessel that spends months or years in space), but it does pose a problem when you're storing tritium on the ground.

And there are still problems like the high neutron flux of D-T fusion.

Only 1.8 tons? Those systems could mass around 10 tons or more. And your reactor could mass 100 tons or more...

The concepts are nice, but numbers are required for them to have any validity. You should have seen the 'science' behind HVIPS... :facepalm:

HVIPS?
The mass used is the mass listed on Project Rho's engine list; I didn't pull it out of the air. As for the mass of the old water injection system; the system is little more than a pump, a few hoses, and a slab of alloy much smaller than the rest of the engine (just large enough for the hoses to attach to), I highly doubt it'd weigh 1/2 as much as the rest of the engine. But for the sake of argument, in the MCF design I'm working below the mass of the water injection system is greater than the mass of the rest of the engine, by 3x.

Finally found what I was looking for... MC Fusion.
(Cylindrical Geometry)

As listed:

Thrust Power:
200 GW
Exhaust Velocity:
8,000,000 m/s
Thrust:
50,000 N
Mass:
600 kg
Thrust/Mass (T/W):
83 m/s^2

The Thrust Power of 200 GW really concerns me in terms of heat radiation. The vehicle in mind will have large wings; perhaps the entire upper wing surface can be used as a radiator? Not that I've actually done the math to find radiating area yet... :shifty:

Anyways, I'm going to arbitrarily choose a mass ratio of 1.2, since we now have an Exhaust Velocity of 8,000 KM/S :blink:.
What I'm going to find here is the exhaust velocity required for orbit with a mass ratio of 1.2 and Delta-Vee of 11,000 m/s.
Since the propellant injection system decreases Exhaust Velocity, this will be the new one...
Dv = Ve * ln[R]
Ve = Dv / ln[R]
Ve = (11,000) / ln[1.2]
Ve (required) = 60,332.96

Now, since I only need an exhaust velocity of 60,333 m/s, and the MCF gives 8,000,000, I can find how much propellant injection would lower my exhaust velocity to 60,333 m/s.

Ve (MCF) = 8,000,000
Sqrt (8,000,000 / 60,332.96) = Sqrt (132.6)
= 11.5

So, on average, the new propulsion system will have a mass flow 11.5x greater, but using the same amount of Fusion fuel, so the mass flow is 11.5x greater but energy is still the original number; making the average performance through the ascent as such:

Propellant Injection Rate:
11.5x normal

Thrust Power:
17.39 GW
Exhaust Velocity:
60,333 m/s
Thrust:
575,000 N
Mass:
2,400 kg (I added 1,800 kg)
Thrust/Mass (T/W):
240 m/s^2

Referring to the earlier math above this, the max Exhaust Velocity of the engine system is 132.6x greater than what's needed. Since the mass ratio is fixed in the design of the vehicle, we can say the Exhaust Velocity is directly proportional to the Delta-Vee. So, by changing the Exhaust Velocity with Methane injection, we're changing the Delta-Vee.

Now in order to make it to orbit, the average delta-vee through the ascent has to be 11,000 m/s, and thus the average Exhaust Velocity through the ascent has to be 60,332.96 m/s.
However, I can increase injection at the beginning of the flight, and decrease it near the end to optimize thrust (similar to how the STS uses SRB's to increase thrust at launch), much greater thrust is needed to overcome aerodynamic drag and to climb to leave the atmosphere and to overcome gravitational dragging through the ascent. However, as the effect of gravity becomes weaker through the ascent, less thrust is needed.

To make the math easier, I'll assume my propellant injection rate is linear; at MECO, I will have no injection, and thus 132.6x the average Exhaust Velocity. At Ignition, I will be at the opposite end of the scale with 1/132.6x the average exhaust velocity, but a much greater thrust.

MECO exhaust Velocity: 8,000,000 m/s.
Average Exhaust Velocity: 60,332.96 m/s.
Ignition Exhaust Velocity: 60,332.96/132.6 = 455 m/s (Holy Crap! Model rockets have an exhaust velocity better than that! :blink: I'll work with this number anyways; the point of this excercise is to find the max thrust I could get out of this system.)

Ignition Engine Propellant Injection Rate:
Sqrt (60,332.96 / 455) = Sqrt (132.6)
= 11.5

Ignition Engine Performance:

Propellant Injection Rate:
11.5 * 11.5
= 132.5x normal

Thrust Power:
1.5 GW
Exhaust Velocity:
455 m/s
Thrust:
6,612,500 N
Mass:
2,400 kg
Thrust/Mass (T/W):
2,755 m/s^2

MECO Engine performance (Same as listed under Project Rho's Engine list)

Propellant Injection Rate:
1x normal

Thrust Power:
200 GW
Exhaust Velocity:
8,000,000 m/s
Thrust:
50,000 N
Mass:
2,400 kg
Thrust/Mass (T/W):
20.8 m/s^2

As you can see, the performance of the engine can be changed drastically... I think I can rule this out, though; simply put that's too wide a range of acceleration and propellant injection for the vehicle to undergo.

It seems no injection would make a great OMS system, assuming it can be ignited easily in a vacuum.

Keep in mind the information above is with a Mass Ratio of 1.2. With a mass ratio of 1.5:

Dv = Ve * ln[R]
Ve = Dv / ln[R]
Ve = (11,000) / ln[1.5]
Ve (required) = 27,129

Ve (MCF) = 8,000,000
Sqrt (8,000,000 / 27,129) = Sqrt (294.89)
= 17.17

Which would give an average ascent engine performance of:

Propellant Injection Rate:
17.17x normal

Thrust Power:
11.65 GW
Exhaust Velocity:
27,129 m/s
Thrust:
858,500 N
Mass:
2,400 kg
Thrust/Mass (T/W):
357.7 m/s^s

Since we've already established that the performance is capable of varying greatly for different injection rates, I think it's safe to assume with 2x the injection the Thrust could easily be raised to 1,717,000 Newtons, which would yield 1/4 the Exhaust Velocity of the average.

I haven't seen any figures for it, but you can try to figure it out using the specific heat of your coolant, among other things...

I think I remember a site for building home made liquid fueled rockets that had the equation. I'll have to dig it up sometime soon...

Difficult, yes. But this is a fusion powered SSTO, after all...

And there's always the possibility of a precooled SABRE-like engine; such an engine is being actively researched and seems far more in the near-future at least, than a SCRAM engine.

Skylon is supposed to get up to Mach 5.14 and 28.5km altitude in airbreathing mode; that could make a big difference to such a vehicle.



Considering the amount of research put into scramjets (as well as the existence of technology demonstrators, I doubt there are such serious stumbling blocks to make them impossible universally...



Propellant injection isn't magic. Water injection isn't even the best form of propellant injection.

You can't just fix the thrust problems of a fusion engine by injecting propellant. There are a whole lot of issues that need to be addressed. It isn't impossible; just difficult.

Fission engines do not spray radioactive death everywhere (as a general rule). Only NSWR, gas core, and liquid core (debatable; you'll probably get some fuel leakage) are truely bad offenders; a properly designed solid-core engine can release a negligible amount of radiation only.

The problem with a fission engine (that might unfortunately extend to fusion engines in some respects) is that the engine becomes radioactive after you've used it, which makes maintainance exceedingly difficult.

Another reason I'm choosing this is because It's the most advanced propulsion system. Okay, true, whatever works best is the most advanced; but this is the one that would offer the lowest mass ratio, and thus the highest payload fraction; which in turn means the easiest access to orbit, assuming it's turnaround time, and costs of maintenance are roughly similar to those of other engine systems.

Just because you have HTHL does not mean you need minimal infrastructure; you'll still need a large amount of the infrastructure needed for a VTOVL, for example. The only difference is that some aspects of ground handling might be easier.

And the landing area for a VTOVL might be less intensive to build and maintain than a runway, for example. On the other hand there are requirements that the propulsion system on a VTOVL has fulfill, that the propulsion system on an HTHL might not, for example.

Note though that "VTOVL" doesn't mean "use operation akin to the Shuttle". The DC-Y study and the DC-X test vehicle is a good example. That doesn't mean that you could run the whole vehicle with a one man operation though, but hopefully things are a bit better than they are today

True, though VTHL does mean "use operation akin to the Space Shuttle". You have to take the entire vehicle, ready it for launch, then rotate it upwards on some sort of giant machine that can lift a multi-hundred ton spacecraft onto it's rear end. Not to mention this also necessitates a launch tower unless you launch within minutes or hours of rotating it.

Taxiing the vehicle to a pad, readying it, climbing in, then sitting in it as a giant mechanical arm points it skywards next to a launch tower does offer a lot of "rule of cool", though .

It's just that's a lot more difficult than taxiing to a runway and doing it the airplane way. Not to mention that you've got wings for HL, anyways, which means you have great cross-range ability, then you've already got wings in place for Horizontal Takeoff. Shuttle can't do that because it has to have the ET and SRB's on during launch, due to it's huge mass ratio.

Also, another added bonus of wings is radiating area. Though cross-range ability still wins as #1 best reason to have wings. Since they're there anyways... might as well use them and not have to build a giant mechanical arm and pad.
Granted, though. A giant mechanical arm is just a motor and some hydraulics; in terms of machine complexity it's far simpler than, say, a truck.
(Honestly rule of cool in terms of the Sci-Fi story is tugging on me; but for now I'm resisting: HTHL still seems more realistic for a colony world like this IMO, though I'm not really sure either way yet.)

One thing VT does have going for it is that undoubtably HT wastes a lot of Delta-Vee turning to the desired course, as any experience in the Delta-Glider will show. Though I'm not entirely sure the Delta-Vee restrictions will be so great when the mass ratio is still less than 1.5, nevermind 2, the mass ratio of some airliners.
Perhaps a few VT pads could be available for a more expensive, but heavier payload?...

(in the story the ISV they came in on struck a micrometeorite larger than planned, depressurizing one of the TransHab sections full of cryo tubes. Current operations are underway to try and restore the module and rescue as many cryo tubes (lives) as possible. Repairing an ISV (and there's a damaged shuttle, too) would probably necessitate launching with as much equipment as possible, which means getting every last kg out of the shuttle's launch capability, which means using the VT pads...)

 

[T.Neo]

The math I worked earlier worked on the mathematical model of the water taking kinetic energy from the rest of the mass flow. It reduces exhaust velocity, I know, but adds thrust. Like I've said a number of times...

Methane breeding shouldn't be very difficult at all; and it probably does expand much more greatly when heated, so it's probably a better choice.

And I did note significant thrust would be added due to transfer of thermal energy, i.e., heating the propellant.

It's a matter of efficiency. Optimising thermal transfer will increase efficiency, it should actually increase your exhaust velocity due to the fact that more energy is being put into the propellant.

You could still cap off the nozzle, then, and get the same effect. Fusion products may be at low pressure, but huge amounts of methane heated to thousands of degrees from a liquid state will NOT be at a low pressure at all.

Yes... but then you have the problem of the propellant invading the fusion space chamber thing...

Maybe you can have an engine design that avoids that, but as always, there is no magic solution.

HVIPS?

High Velocity Interplanetary Passenger Spacecraft.

You will only understand my shame, once you have failed in a similar manner. :facepalm:

The mass used is the mass listed on Project Rho's engine list; I didn't pull it out of the air. As for the mass of the old water injection system; the system is little more than a pump, a few hoses, and a slab of alloy much smaller than the rest of the engine (just large enough for the hoses to attach to), I highly doubt it'd weigh 1/2 as much as the rest of the engine. But for the sake of argument, in the MCF design I'm working below the mass of the water injection system is greater than the mass of the rest of the engine, by 3x.

It's good that you know what the major components would be, but how much do they weigh? How much do they need to weigh? What calculations and/or figures have you used to discern that?

Mass:
600 kg
Thrust/Mass (T/W):
83 m/s^2

I'd be skeptical of those figures. They seem way too light for a fusion engine, especially an MC fusion engine...

Unless my small mind is missing something.

What I'm going to find here is the exhaust velocity required for orbit with a mass ratio of 1.2 and Delta-Vee of 11,000 m/s.

I suggested a dV of 9500 m/s, you stated a dV of 9800 m/s, now you're saying a dV of 11 000 m/s? I'm thoroughly, utterly confused.

The Thrust Power of 200 GW really concerns me in terms of heat radiation. The vehicle in mind will have large wings; perhaps the entire upper wing surface can be used as a radiator? Not that I've actually done the math to find radiating area yet...

That depends on the efficiency of your drive and how much of the waste heat is absorbed in the exhaust.

Odds are your radiator area will have to be big. And there could be problems with wing-mounted radiators, too. (thermal limitations on structure)

200 GW is probably heavily overkill on takeoff. You might want to have low output at takeoff and then crank the power up once you get into high-gear. I don't know why you want an exhaust velocity of 60 000+ m/s at takeoff, far less will probably do, and probably cause far fewer problems.

I think I remember a site for building home made liquid fueled rockets that had the equation. I'll have to dig it up sometime soon...

Q= m*c*Delta-T

Where:
m = mass
c = specific heat capacity
Delta-T = temperature change (in Kelvin)
Q = amount of energy (kilojoules for kJ/kg*k, joules for J/kg*k)

I think. :shifty:

Another reason I'm choosing this is because It's the most advanced propulsion system. Okay, true, whatever works best is the most advanced; but this is the one that would offer the lowest mass ratio, and thus the highest payload fraction; which in turn means the easiest access to orbit, assuming it's turnaround time, and costs of maintenance are roughly similar to those of other engine systems.

"Most advanced" means very, very little. A handheld laser is more advanced than an AK-47, but the latter would be more practical in a realistic war environment.

While a high performance fusion drive might get the lowest mass ratio, a combination of a lower-performance, ruggedised drive, combined with airbreathing propulsion, for example, might be simpler, cheaper to operate, or require less constraints on the vehicle as a whole.

True, though VTHL does mean "use operation akin to the Space Shuttle". You have to take the entire vehicle, ready it for launch, then rotate it upwards on some sort of giant machine that can lift a multi-hundred ton spacecraft onto it's rear end. Not to mention this also necessitates a launch tower unless you launch within minutes or hours of rotating it.

There's a big difference between VTHL and VTOVL. VTOVL craft don't land on runways; you might have to move them around a bit on the ground, but you don't have to prop them up on launchpads or anything like that.

A good example is the DC-X.

I do agree though, hoisting a gigantic spacecraft verticle with a huge crane would be cool, but probably more than a tad impractical.

Also, another added bonus of wings is radiating area. Though cross-range ability still wins as #1 best reason to have wings. Since they're there anyways... might as well use them and not have to build a giant mechanical arm and pad.
Granted, though. A giant mechanical arm is just a motor and some hydraulics; in terms of machine complexity it's far simpler than, say, a truck.
(Honestly rule of cool in terms of the Sci-Fi story is tugging on me; but for now I'm resisting: HTHL still seems more realistic for a colony world like this IMO, though I'm not really sure either way yet.)

How important is crossrange? Do you need to do an AOA from a polar orbit?

Have you calculated the size of the radiators you'd need to get rid of your waste heat? Granted, it depends on various factors (including how much waste heat you actually have), but it's still pretty painful when you find out that your radiator area is not nearly large enough...

One thing VT does have going for it is that undoubtably HT wastes a lot of Delta-Vee turning to the desired course, as any experience in the Delta-Glider will show. Though I'm not entirely sure the Delta-Vee restrictions will be so great when the mass ratio is still less than 1.5, nevermind 2, the mass ratio of some airliners.
Perhaps a few VT pads could be available for a more expensive, but heavier payload?...

Optimise your trajectory and you'll be fine. It could also be a reason to use airbreathing propulsion in the early phases of flight (as well as not exposing ground personnel and facilities to fusion radiaiton).

If VTOVL craft support heavy lifting better, and there's a need for heavy lifting, why not have heavy-lift capable VTOVL craft?

 

[Eagle1Division]

It's good that you know what the major components would be, but how much do they weigh? How much do they need to weigh? What calculations and/or figures have you used to discern that?



I'd be skeptical of those figures. They seem way too light for a fusion engine, especially an MC fusion engine...

Unless my small mind is missing something.

That's what I was thinking, I was shocked when mass (tons) was 0.6! But, that's the given number, and unless I want to design the whole thing myself from scratch, I'll be satisfied with that...

It's a matter of efficiency. Optimising thermal transfer will increase efficiency, it should actually increase your exhaust velocity due to the fact that more energy is being put into the propellant.

Back to what I was saying before about the gardening hose... It's one thing to create a combustion chamber for a chemical reaction. It's another thing for heating methane to tens of thousands of degrees. Although, once again, I don't have the math (once again requires expansion... Now that I'm using methane I could probably get some data from NTR designs?); something of intuition tells me that adding a combustion chamber wouldn't really be worth the weight: Already without any heat transfer it fits the requirements with just a mass ratio of 1.2, in reality there would still be a significant heat transfer with the current design, a very fine spray if methane (which, once again works for the better: low viscosity means smaller droplets) will offer a huge surface area per mass ratio, meaning it should heat up rather quickly.

My intuition says that adding a combustion chamber would mean a huge amount of extra weight, with only diminishing returns on heat transfer. Heat transfer on impact is determined by the size of the droplets, and with a fine enough spray the vast majority of the heat could be transferred as soon as the droplet enters the exhaust stream.

Update (sort of):
Here's an idea:
I'm definitely going to need a magnetic nozzle, unless I want one 120m wide :blink:, but I also need a Turkey-feather nozzle for varying thrust (and thus pressure) the Turkey-feather nozzle will have to be closed while Methane is running through the engine, which will negate the magnetic nozzle...
But
What if some of the propellant is injected into the periphery of the exhaust stream, and still not completely heated as it passes through the nozzle? Then it could act as both a thermal blanket in-between the fusion exhaust and the engine, as well as offering some active cooling to the nozzle (It may be a thousand+ degrees, but compared to the fusion exhaust stream it'll keep the nozzle colder). Granted, it will really hurt the Isp, but having the nozzle not vaporize seems well worth it.

And finally, the figures calculated earlier don't calculate for thermal expansion, and they're already more than good enough. The bonus of thermal expansion will be there no matter what; but it won't be necessary at all...

Yes... but then you have the problem of the propellant invading the fusion space chamber thing...

Maybe you can have an engine design that avoids that, but as always, there is no magic solution.

The force coming from the engine is in excess of 800,000 Newtons. The barrier is made to hold atmospheric pressure back from a vacuum, 14.7 PSI. (65 N per square inch), with a 1 meter radius, the seal could take ~320,000 N of force. If I place the injectors closer towards the seal than the fusion portion, then it could be designed so that the seal blows out and exhaust flows out the engine before it backs up into the fusion portion.

Keep in mind that 1 meter radius is at the back of the propellant injection area, not the nozzle exit.

If that doesn't work, I could always design the seal to be held on with explosive bolts set off at the same moment as engine ignition, so it blows off and doesn't cause backing up of the methane exhaust into the fusion portion.

High Velocity Interplanetary Passenger Spacecraft.

You will only understand my shame, once you have failed in a similar manner. :facepalm:

Took awhile to find where you went wrong... Personally I think you should've kept with it but had a bit of a more modest Delta-Vee.

So, in keeping with this advice; I'm going a little more in-depth on a few things.

Reason why I've been avoiding this thus far is because I'm trying to not spend all the time making the story on making some small background item that so far doesn't even appear in the plot, but I'm designing merely for fun...

However, the fact that it uses Methane for propellant has actually filled a large niche' I needed filled that's closer to the foreground of the story...

I suggested a dV of 9500 m/s, you stated a dV of 9800 m/s, now you're saying a dV of 11 000 m/s? I'm thoroughly, utterly confused.

I remember you stating 9,800 m/s, so I used that. I meant to replace all the 9,800 m/s with 11,000 m/s, that's an old figure I calculated as the Space Shuttle's Delta-Vee a long time ago. It's probably larger than necessary, but it's best to err on the side of caution; and my guess is using a winged ascent will mean a lot of Delta-Vee loss in atmospheric drag.

That depends on the efficiency of your drive and how much of the waste heat is absorbed in the exhaust.

Odds are your radiator area will have to be big. And there could be problems with wing-mounted radiators, too. (thermal limitations on structure)

200 GW is probably heavily overkill on takeoff. You might want to have low output at takeoff and then crank the power up once you get into high-gear. I don't know why you want an exhaust velocity of 60 000+ m/s at takeoff, far less will probably do, and probably cause far fewer problems.

This will take awhile... Take a look at the maths, the Thrust Power during liftoff is far lower. (Hold on a sec... Thrust Power is an average of thrust and exhaust velocity? Dang. I need to rework a few figures here...)

EDIT:
Okay, before I started calculating I realized I misread the equation . Not the average of thrust and exhaust velocity.
So, my assumption was that Thrust Power was the amount of energy per mass of propellant, so it'd be linear with the propellant injection. I was shocked to see that I was right, even though I wasn't using the equation for Thrust Power that Project Rho gave :lol: . Good stroke of luck...

Q= m*c*Delta-T

Where:
m = mass
c = specific heat capacity
Delta-T = temperature change (in Kelvin)
Q = amount of energy (kilojoules for kJ/kg*k, joules for J/kg*k)

I think. :shifty:

I'll work this out... Tomorrow (it's late)... Maybe (have plans) :shifty:

"Most advanced" means very, very little. A handheld laser is more advanced than an AK-47, but the latter would be more practical in a realistic war environment.

While a high performance fusion drive might get the lowest mass ratio, a combination of a lower-performance, ruggedised drive, combined with airbreathing propulsion, for example, might be simpler, cheaper to operate, or require less constraints on the vehicle as a whole.

Assumption I'm working on is that it won't be significantly worse off than other drive systems, and scramjets don't offer a miracle ISP, they offer one that will barely work as an SSTO Not even twice that of LOX/LH2 Now watch the Isp drop even lower as the mach number goes up, off the chart.

There's a big difference between VTHL and VTOVL. VTOVL craft don't land on runways; you might have to move them around a bit on the ground, but you don't have to prop them up on launchpads or anything like that.

A good example is the DC-X.

I do agree though, hoisting a gigantic spacecraft verticle with a huge crane would be cool, but probably more than a tad impractical.



How important is crossrange? Do you need to do an AOA from a polar orbit?

Have you calculated the size of the radiators you'd need to get rid of your waste heat? Granted, it depends on various factors (including how much waste heat you actually have), but it's still pretty painful when you find out that your radiator area is not nearly large enough...

*stickied on to-do list*

Optimise your trajectory and you'll be fine. It could also be a reason to use airbreathing propulsion in the early phases of flight (as well as not exposing ground personnel and facilities to fusion radiaiton).

If VTOVL craft support heavy lifting better, and there's a need for heavy lifting, why not have heavy-lift capable VTOVL craft?

Adding a whole new engine system would almost double the complexity of the craft, something you really don't want on a colony world, if it's a avoidable. As for Fusion Radiation, it's fleeting and small... But the taxi out to the runway might be a long one to keep frequent fliers at the terminal safe

The thing is, going out; they don't know if Cross-Range capability will be important or not. You can't see the surface features and find a good colonization spot from ~10 LY away; this will have to be done in orbit.

Even if they could, the world to be colonized has an atmospheric pressure around that of Earth at 40 kft, except for small basins (caulderas, impact craters, vallies and canyons) ~25kft deep where the air is breathable without a positive-pressure O2 mask. This means all the colony sites are off the equator, and so cross-range capability will be important for the vehicle design.



Like I said before, I'm trying not to spend all the time I should be working on the actual story working on a minor background detail... Though I really should be putting more time into designing a new propulsion system like this. I've been spending way too much time talking about the system than actually working on it; so it may be a bit before I reply - It'll hopefully be because I'll be doing work on it that I should've done before starting a thread, at the moment the whole thing is disorganized and half-built.

Cheers.

---------- Post added at 04:11 AM ---------- Previous post was at 02:45 AM ----------

Hmm... Is 0.05% of Thrust Power absorbed by the engine using magnetic containment a realistic number? 0.1%?

There's so many things here that I can't even find what a realistic range is for...

So far I'm looking at building the radiators out Reinforced Carbon-Carbon so they can operate at up to 2,573 K. At the price of a high density of 1,800 kg/m^3.

Next alternative is LI-900 for operations at up to 1,478 K and a density of 144.2 kg/m³.

I'm looking at Thermal Protection System materials to take advantage of the Rt^4 part. These would be the high-performance radiators used for the engine only, and not for ECLSS systems, those radiators would use more conventional materials to be more reflective...

Now I have no idea what they have as far as emissivity. I know it's extremely high: because they're used as TPS materials because they can re-emit any heat they receive very quickly.

Another note, there may be concern about them absorbing Epsilon Eridani's (The sun's) heat in space, but I note that the Space Shuttle Orbiter usually positions itself so that it's heat tiles face the sun; and it's ECLSS radiators do a fine job of keeping it from overheating despite this, so...

I also wonder if it'd be possible to use the heat shield as a radiator to get rid of the tremendous waste heat generated by the Fusion Engines in high impulse mode, since it's already designed to be extremely emissive, and naturally has a very large area... :hmm:

 

[T.Neo]

That's what I was thinking, I was shocked when mass (tons) was 0.6! But, that's the given number, and unless I want to design the whole thing myself from scratch, I'll be satisfied with that...

Just because the number is there doesn't mean it has to make any sense.

I'd love to see a 0.6 ton fusion drive with that sort of performance, but from the other figures I've seen, it doesn't quite fit. I'd be skeptical.

What if some of the propellant is injected into the periphery of the exhaust stream, and still not completely heated as it passes through the nozzle? Then it could act as both a thermal blanket in-between the fusion exhaust and the engine, as well as offering some active cooling to the nozzle (It may be a thousand+ degrees, but compared to the fusion exhaust stream it'll keep the nozzle colder). Granted, it will really hurt the Isp, but having the nozzle not vaporize seems well worth it.

Maybe a thin film of propellant covering the nozzle/chamber could prevent it from melting, or help prevent it from melting. It's advantageous only if it doesn't take up an exorbitant amount of your propellant.

Gas core nuclear engines seperate the hot uranium plasma from the engine using the propellant, so maybe something similar could be applicable here.

Solid core NTR engines heat the propellant, solid core NTR engines heat the propellant, open and closed gas core NTR engines heat the propellant, fusion engine such as that of the Discovery II heat propellant...

I've never seen an engine that imparts energy to the propellant by impacting something into it. If it was so advantageous, why wouldn't it have shown up before?

It's also important to note that while magnetic nozzles can help, you can't use them to magically miniturise your engine (a la HVIPS).

The force coming from the engine is in excess of 800,000 Newtons. The barrier is made to hold atmospheric pressure back from a vacuum, 14.7 PSI. (65 N per square inch), with a 1 meter radius, the seal could take ~320,000 N of force. If I place the injectors closer towards the seal than the fusion portion, then it could be designed so that the seal blows out and exhaust flows out the engine before it backs up into the fusion portion.

You mean roughly 100 kilopascal.

The problem isn't only high pressure propellant invading during startup, but during operation as well.

But you might be able to avoid that problem.

Took awhile to find where you went wrong... Personally I think you should've kept with it but had a bit of a more modest Delta-Vee.

I tried three or four iterations of the thing. It's just such an intrinsically bad design. Maybe I could pull it off if, I dunno, I used chemical propulsion or even some kind of NTR, but then it would be completely and utterly pointless.

I remember you stating 9,800 m/s, so I used that. I meant to replace all the 9,800 m/s with 11,000 m/s, that's an old figure I calculated as the Space Shuttle's Delta-Vee a long time ago. It's probably larger than necessary, but it's best to err on the side of caution; and my guess is using a winged ascent will mean a lot of Delta-Vee loss in atmospheric drag.

Erring on the side of caution is always a good thing. Adding extra propellant capacity is also insurance against mass increase...

Adding a whole new engine system would almost double the complexity of the craft, something you really don't want on a colony world, if it's a avoidable. As for Fusion Radiation, it's fleeting and small... But the taxi out to the runway might be a long one to keep frequent fliers at the terminal safe

Adding another engine system could mean you could simplify your fusion engine. Fusion engines- fusion anything is complex, finicky stuff, and you want to have the simplest set of requirements possible.

Since when is fusion radiation 'fleeting and small'? I've heard that even the X-ray radiation coming from a reactor could be quite high, and D-D fusion will release roughly 30% of its energy in the form of neutrons, even if you absorb a lot of that in the propellant you're still going to get some neutrons leaving the engine.

Assumption I'm working on is that it won't be significantly worse off than other drive systems, and scramjets don't offer a miracle ISP, they offer one that will barely work as an SSTO Not even twice that of LOX/LH2 Now watch the Isp drop even lower as the mach number goes up, off the chart.

They're still advantageous enough to be researched (apparently). You could also go for something like [ame="http://en.wikipedia.org/wiki/Reaction_Engines_SABRE"]SABRE[/ame]- the performance increase enabled by SABRE is apparently what makes Skylon possible.

I chose a hybrid of airbreathing and NTR propulsion for this resounding failure, to simplify ground handling; if not to be able to take off from any airfield, at least not need to have to deal with nuclear radiation and rocket exhaust at launch.

The thing is, going out; they don't know if Cross-Range capability will be important or not. You can't see the surface features and find a good colonization spot from ~10 LY away; this will have to be done in orbit.

Where's your colony? Epsilon Eridani?

Even if they could, the world to be colonized has an atmospheric pressure around that of Earth at 40 kft, except for small basins (caulderas, impact craters, vallies and canyons) ~25kft deep where the air is breathable without a positive-pressure O2 mask. This means all the colony sites are off the equator, and so cross-range capability will be important for the vehicle design.

The ISS is at 51 degrees inclination from Earth's equator. KSC is at ~28 degrees latitude. The key is to have an inclination equal to or greater than the latitude of your landing site. Cross-range might be important, but keep in mind that if your propulsion system is good enough, you can potentially use it to perform a slight plane change to put your track over the landing site.

Considering the relationship between planet size, ruggidity, and scale height, I'd say your planet has severely wacky topography. Also, requirement of only an oxygen mask actually makes it a pretty nice place to be; more habitable than anywhere in the solar system other than Earth. Granted, weather might be troublesome, but if there is anywhere that's ~7.6 km down that isn't covered in water, it'll likely be a tiny speck of land on planetary terms.

Hmm... Is 0.05% of Thrust Power absorbed by the engine using magnetic containment a realistic number? 0.1%?

Depends on the efficiency, the ionising radiation flux, and the nonionising radiation flux. Thrust power isn't your problem though; waste power is. Out of the total power produced, only some will go into the thrust stream, a large portion will not- this is the energy lost to radiation in the fusion reaction, to heating and ionising the propellant, to the inefficiency of the nozzle design... you might end up with only 60-30% of the total power going into the engine.

The trick of the magnetic nozzle is that it seperates the exhaust from the vehicle structure, and then reflects photons coming from the hot exhaust using a material with a very high albedo.

In terms of radiation, you want it to go through the engine. Beryllium is pretty transparent to X-rays (it's used in 'windows' for X-ray equipment), but it does stuff with neutrons so it might not be ideal. On the other hand, you can try to capture as much of the radiation as is possible with your propellant. Hydrogen is probably pretty transparent to X-rays, but should absorb neutrons well.

There's so many things here that I can't even find what a realistic range is for...

Seconded. It's a horrible lack of information.

So far I'm looking at building the radiators out Reinforced Carbon-Carbon so they can operate at up to 2,573 K. At the price of a high density of 1,800 kg/m^3.

Next alternative is LI-900 for operations at up to 1,478 K and a density of 144.2 kg/m³.

I'm looking at Thermal Protection System materials to take advantage of the Rt^4 part. These would be the high-performance radiators used for the engine only, and not for ECLSS systems, those radiators would use more conventional materials to be more reflective...

The material problems of having the radiators in the wings doesn't relate to the material the radiators themselves are made of as much as it relates to how much of the heat conducts/radiates into the rest of the vehicle structure, which might not be able to handle it.

The silica tiles on STS are used because they have low thermal conductivity. You want something that has high conductivity, so you can effectively conduct heat from the coolant to the radiator so it can be radiated into space. RCC might be better in that regard, but it is also quite brittle, so it couldn't have any sort of load-bearing role in the structure.

Note that the temperature of the radiators is limited by the temperature of whatever you want to cool, and the higher temperature everything is at, it's probably more technologically demanding to construct.

Another note, there may be concern about them absorbing Epsilon Eridani's (The sun's) heat in space, but I note that the Space Shuttle Orbiter usually positions itself so that it's heat tiles face the sun; and it's ECLSS radiators do a fine job of keeping it from overheating despite this, so...

I really should read your whole post before I reply. :facepalm:

It might have something to do with the thermal conductivity of the silica tiles; they keep the airframe from melting during reentry when they're at over 1000C; dealing with ~200C and keeping the rest of the vehicle relatively cool should be relatively easy.

I also wonder if it'd be possible to use the heat shield as a radiator to get rid of the tremendous waste heat generated by the Fusion Engines in high impulse mode, since it's already designed to be extremely emissive, and naturally has a very large area...

One solution might be to place the radiators on the underside of the ship where you'd have darker TPS anyway, have the radiators heat up during reentry but isolate that heat from the rest of the ship with layer of something like LI-900. That way a higher amount of the surface of the ship has a higher albedo.

But using the whole heatshield as a radiator... has... problems. For one, heatshields usually aren't designed with channels for coolant cut into them...

 

[Eagle1Division]

Just because the number is there doesn't mean it has to make any sense.

I'd love to see a 0.6 ton fusion drive with that sort of performance, but from the other figures I've seen, it doesn't quite fit. I'd be skeptical.

Well, I am, but how am I supposed to come up with a better number? It's not like I exactly have the know-how to design my own fusion engine...

Maybe a thin film of propellant covering the nozzle/chamber could prevent it from melting, or help prevent it from melting. It's advantageous only if it doesn't take up an exorbitant amount of your propellant.

Gas core nuclear engines seperate the hot uranium plasma from the engine using the propellant, so maybe something similar could be applicable here.

Solid core NTR engines heat the propellant, solid core NTR engines heat the propellant, open and closed gas core NTR engines heat the propellant, fusion engine such as that of the Discovery II heat propellant...

I've never seen an engine that imparts energy to the propellant by impacting something into it. If it was so advantageous, why wouldn't it have shown up before?

It's just a different design. In the Methane Injection engine, there's really 2 propellants: the "primary propellant": Fusion fuel that's also used as reaction mass; and the "secondary propellant": Methane reaction mass.
Primary propellant is the fuel and is always being used as reaction mass when the engine's on.
Secondary propellant doesn't add any energy and is purely throttle-able, and isn't even used for OMS burns or near MECO.

Those design's don't have two distinct kinds of propellants. This one does.

The advantage of two propellants is that the fusion chamber doesn't have to be built to contain more than 10x as much Deuterium, and doesn't produce more waste heat at a higher thrust; so in that way it's better than just injecting more deuterium.

Because they only have one kind of propellant, they heat it with their reactor; but in this design there's already an exhaust stream for the secondary propellant to flow into; so why not? Free, much faster transfer of energy. My guess is that it'd cause a buildup of pressure and shockwaves to travel back into the fusion chamber; but given that the density in the fusion exhaust stream is extremely low, these shockwaves will be weak and should be able to be handled by the fusion chamber's magnets.

It's also important to note that while magnetic nozzles can help, you can't use them to magically miniturise your engine (a la HVIPS).

Okay, this is what I don't understand entirely...
The assumption I'm using is that the nozzle size comes from how much heat comes from the propellant, but this has 2 exceptions:
#1. Chemical Rocket engines don't use this equation because their nozzles are actively cooled.
#2. Magnetic nozzles: The physical metal frame only has a certain percentage of surface area showing to the exhaust, this is the area that's heated. If only 1% of the area is showing, it only receives 1% of the heat, and can work at 1% the size, or at least that's my assumption...

As for my design, the methane could be used in many ways similar to how chemical fuel is used: open-cycle cool the nozzle and fusion chamber with it. An added advantage is that as it's heated it'll expand rapidly: to the point where if I use check valves to keep it from backtracking, I don't even need pumps; the heat expansion will blow it through the lines with plenty of PSI.

Hmm... But even though I've said that; I should probably add turbopumps before the check valves and cooling areas, to increase the pressure and keep the expanding methane going out the engine; in the same way a chemical rocket engine keeps the combustion from backtracking:

You mean roughly 100 kilopascal.

The problem isn't only high pressure propellant invading during startup, but during operation as well.

But you might be able to avoid that problem.

I just addressed this... Sort of. I've called it backtracking, btw, dunno what the actual technical term is for it. But if the methane is injected at a high enough pressure and at a steep angle (pointing out), then the pressure should be good enough to keep the methane from backtracking the same way a chemical rocket engine does it.

Adding another engine system could mean you could simplify your fusion engine. Fusion engines- fusion anything is complex, finicky stuff, and you want to have the simplest set of requirements possible.

Since when is fusion radiation 'fleeting and small'? I've heard that even the X-ray radiation coming from a reactor could be quite high, and D-D fusion will release roughly 30% of its energy in the form of neutrons, even if you absorb a lot of that in the propellant you're still going to get some neutrons leaving the engine.

The original question was posed to the ground crew and such... If the radiation is a problem for them, how are the people in the vehicle safe? Radiation shielding for folks inside the vehicle, inverse-square law for people outside.

They're still advantageous enough to be researched (apparently). You could also go for something like SABRE- the performance increase enabled by SABRE is apparently what makes Skylon possible.

I chose a hybrid of airbreathing and NTR propulsion for this resounding failure, to simplify ground handling; if not to be able to take off from any airfield, at least not need to have to deal with nuclear radiation and rocket exhaust at launch.

Hybridizing with air breathing is an interesting concept.... I've heard before that the Shuttles on Avatar cycled air around the fusion parts of the engine to heat it and create thrust.

The ISS is at 51 degrees inclination from Earth's equator. KSC is at ~28 degrees latitude. The key is to have an inclination equal to or greater than the latitude of your landing site. Cross-range might be important, but keep in mind that if your propulsion system is good enough, you can potentially use it to perform a slight plane change to put your track over the landing site.

Considering the relationship between planet size, ruggidity, and scale height, I'd say your planet has severely wacky topography. Also, requirement of only an oxygen mask actually makes it a pretty nice place to be; more habitable than anywhere in the solar system other than Earth. Granted, weather might be troublesome, but if there is anywhere that's ~7.6 km down that isn't covered in water, it'll likely be a tiny speck of land on planetary terms.

Big lakes is all there is for water; not a problem though, since atmospheric water is trapped in the Basins anyways, where the lakes are.

Did I mention it was a moon?... Extremely long day/night cylcles (calculation pending, neighborhood of 10 days is estimate). Facial locking is the reason the basins are so deep, tectonic plates don't move over the magma, so features are more extreme, hence the 25kft valleys, canyons, caulderas...

Also, since the day/night cycle is so long, you'll be waiting a very long time if you wait for the planet's turning to put you over your destination again. Either that or make it a very short mission.

Depends on the efficiency, the ionising radiation flux, and the nonionising radiation flux. Thrust power isn't your problem though; waste power is. Out of the total power produced, only some will go into the thrust stream, a large portion will not- this is the energy lost to radiation in the fusion reaction, to heating and ionising the propellant, to the inefficiency of the nozzle design... you might end up with only 60-30% of the total power going into the engine.

The trick of the magnetic nozzle is that it seperates the exhaust from the vehicle structure, and then reflects photons coming from the hot exhaust using a material with a very high albedo.

In terms of radiation, you want it to go through the engine. Beryllium is pretty transparent to X-rays (it's used in 'windows' for X-ray equipment), but it does stuff with neutrons so it might not be ideal. On the other hand, you can try to capture as much of the radiation as is possible with your propellant. Hydrogen is probably pretty transparent to X-rays, but should absorb neutrons well.

Oh dear. You mean 40-70% TP as waste heat? I'm afraid I can't do that... Heat Sinks, to give the radiators time to work it off, maybe? :blink: .
I get a feeling that figure is really far off, actually. That might apply for something like the shuttle, but a fusion chamber is magnetically confined. If a majority of that heat went into the chamber structure, how in the world do they keep modern-day ground-based fusion chambers from melting?
I remember quiet distinctly that the magnetic confinement keeps the chamber from melting, by stopping a vast majority of the heat; and that the interiors of the chambers are highly reflective to try and keep most of the radiating heat in the fusion matter, and off the walls of the chamber.

The material problems of having the radiators in the wings doesn't relate to the material the radiators themselves are made of as much as it relates to how much of the heat conducts/radiates into the rest of the vehicle structure, which might not be able to handle it.

The silica tiles on STS are used because they have low thermal conductivity. You want something that has high conductivity, so you can effectively conduct heat from the coolant to the radiator so it can be radiated into space. RCC might be better in that regard, but it is also quite brittle, so it couldn't have any sort of load-bearing role in the structure.

Note that the temperature of the radiators is limited by the temperature of whatever you want to cool, and the higher temperature everything is at, it's probably more technologically demanding to construct.

Unlike the Torus radiator, the temperature gradient isn't very high in this case. Only a hundred *C or so, that's doable...

As for load-bearing, I was only planning to use ~5mm of it or so anyways, to keep it light.

I really should read your whole post before I reply. :facepalm:

It might have something to do with the thermal conductivity of the silica tiles; they keep the airframe from melting during reentry when they're at over 1000C; dealing with ~200C and keeping the rest of the vehicle relatively cool should be relatively easy.

My point was that them absorbing heat on-orbit shouldn't be too much of a problem

One solution might be to place the radiators on the underside of the ship where you'd have darker TPS anyway, have the radiators heat up during reentry but isolate that heat from the rest of the ship with layer of something like LI-900. That way a higher amount of the surface of the ship has a higher albedo.

But using the whole heatshield as a radiator... has... problems. For one, heatshields usually aren't designed with channels for coolant cut into them...

But, if my demands for cooling are bad enough, they could be designed with channels for coolant... And the heat sensors are in place, anyways already.

An added bonus is that if something goes wrong on re-entry and the metal airframe underneath the TPS starts getting too hot (a sign of damage to the TPS, or an incorrect re-entry trajectory), then a small amount of reserve radiator fluid could be ran through the channels as an emergency active cooling system, giving the pilot and crew time to lessen the descent rate and do a colder re-entry, or abort the re-entry altogether if there's enough propellant to do it.

 

[T.Neo]

Well, I am, but how am I supposed to come up with a better number? It's not like I exactly have the know-how to design my own fusion engine...

I know; I face the same dilemma. :tiphat:

Because they only have one kind of propellant, they heat it with their reactor; but in this design there's already an exhaust stream for the secondary propellant to flow into; so why not? Free, much faster transfer of energy. My guess is that it'd cause a buildup of pressure and shockwaves to travel back into the fusion chamber; but given that the density in the fusion exhaust stream is extremely low, these shockwaves will be weak and should be able to be handled by the fusion chamber's magnets.

You want to accelerate all (most) of your propellant to a good velocity out the nozzle so you can make proper use of it. Remember, you want to spit that propellant out to give you the thrust you need- put a little in the propellantand a little in the fusion plasma, and you'll get high velocity fusion exhaust giving you far too little thrust, and propellant blasting out of the engine at a velocity that's far too low, also not really providing enough thrust.

Okay, this is what I don't understand entirely...
The assumption I'm using is that the nozzle size comes from how much heat comes from the propellant, but this has 2 exceptions:
#1. Chemical Rocket engines don't use this equation because their nozzles are actively cooled.
#2. Magnetic nozzles: The physical metal frame only has a certain percentage of surface area showing to the exhaust, this is the area that's heated. If only 1% of the area is showing, it only receives 1% of the heat, and can work at 1% the size, or at least that's my assumption...

The problem is that the radiation (ionising and nonionising) drops off with the inverse square law, and a given amount of radiation (ionising and nonionising) will heat a surface up a given degree. So you're still physically constrained even if your exhaust is seperated from the structure, for example. The way you get around this then, is to use the most reflective surface possible to reflect away most of the nonionising radiation, and then build the engine out of materials that the ionising radiation will just pass through.

I've heard that you have to have a physical 'shell' around your magnetic nozzle (for some reason, I'm not actually sure why and I've never heard it explained), something to do with higher mass flows...

Your magnetic nozzle would also have to work in the atmosphere, with a high mass flow. It could probably work though.

Hybridizing with air breathing is an interesting concept.... I've heard before that the Shuttles on Avatar cycled air around the fusion parts of the engine to heat it and create thrust.

Might be a possibility, but I think air is relatively difficult to heat (see the differences between nuclear ramjet studies using air and nuclear rocket studies using LH2).

Big lakes is all there is for water; not a problem though, since atmospheric water is trapped in the Basins anyways, where the lakes are.

This place is a serious desert them. These depressions are practically deeper than anything found on Mars and they only contain a few lakes?

Only two places on Mars nearly approach that depth; Hellas planitia, and Valles Marineris. And their deepest parts don't account for that much of the planetary surface area.

Did I mention it was a moon?... Extremely long day/night cylcles (calculation pending, neighborhood of 10 days is estimate). Facial locking is the reason the basins are so deep, tectonic plates don't move over the magma, so features are more extreme, hence the 25kft valleys, canyons, caulderas...

Tidal locking doesn't necessarily spell the end for tectonics. Europa is a (meta) example, and Ganymede also has tectonic features. It has been theorised that a tidally locked world orbiting a red dwarf star would still maintain plate tectonics.

Water might be necessary though, to lubricate the crust and reduce its viscosity. This place doesn't have much of that, though I suppose it could seep into the crust. Mind you if you don't have any water in the crust, you might end up building up heat and then releasing it in massive resurfacing events, like Venus... the planet that is also covered in over 90 atmospheres of CO2...

Unlike the Torus radiator, the temperature gradient isn't very high in this case. Only a hundred *C or so, that's doable...

Aren't you still breaking the law of thermodynamics?

As for load-bearing, I was only planning to use ~5mm of it or so anyways, to keep it light.

It does have to accomodate coolant pipes with enough space to handle the needed flow though.

But, if my demands for cooling are bad enough, they could be designed with channels for coolant... And the heat sensors are in place, anyways already.

Adding the required infrastructure would likely increase mass dramatically; there's a big difference between "TPS" and "radiator", specifically in the aforementioned coolant pipe accomodation.

An added bonus is that if something goes wrong on re-entry and the metal airframe underneath the TPS starts getting too hot (a sign of damage to the TPS, or an incorrect re-entry trajectory), then a small amount of reserve radiator fluid could be ran through the channels as an emergency active cooling system, giving the pilot and crew time to lessen the descent rate and do a colder re-entry, or abort the re-entry altogether if there's enough propellant to do it.

Maybe; there could be undefined issues that make it too much trouble to be worth doing.

If your TPS is damaged, you have a serious problem. If your crew is using an incorrect trajectory, they're obviously level 2 Orbinauts, and not the level ~50 Astronauts that should actually be piloting such a vehicle.

Such an occurance should be rare; an improper trajectory has not caused any problems in manned spaceflight as far as I know, and an experienced orbinaut can pretty much land DG-type vehicle (not realistic, I know, but it's the flying that counts, regardless of even the TPS attributes) with more-or-less unfaltering consistency.