Thrust-assisted fast orbit

[blane]

I don't understand why someone would want to spend vast amounts of delta-V to produce gravity which can be obtained for free by simply rotating. :unknown:

It's one thing to develop theories and propulsion that will actually have an advantage (such as to shorten the travel time to some place x) but quite another to actually waste fuel. :blink:

 

[James.Denholm]

The thing is, not all vessels are large enough to produce "gravity" by rotating. Take the Delta-Glider. Simply not large enough. Now, I'm not sure, but I think that a craft would have to be massive in order to be able to rotate fast enough and not rip itself plus the occupants to shreds. Does anyone remember the scene in 2001 where whats-his-face (it's Dave, isn't it?) is running around the interior of the ship he's on... which I can't remember the name of either. Now, I don't know for sure, but to me, the ship dosen't look big enough to produce earth-like gravity. Still, those scenes of people walking on the walls of the various vessels are quite awesome... even if it is obvious how it's done.

 

[Quick_Nick]

I think I understand the idea finally. :lol:
Neat! lol

 

[tblaxland]

With a few small changes to the orbiter autopilot, this would be possible. First, the "horizon level" feature would have to be inverted. Second, the "hold altitude" feature would have to be inverted so that gaining altitude triggered thrust, not losing it.

I suggest you check out [ame="http://www.orbithangar.com/searchid.php?ID=3165"]Attitude MFD[/ame] since it will allow you to hold any attitude you desire relative to the velocity vector (use VELOCITY mode). Also check out [ame="http://www.orbithangar.com/searchid.php?ID=3508"]computerex's addon pack[/ame], it includes a vertical velocity hold autopilot. IIRC, it assumes you are in the correct attitude so in combination with AttitudeMFD you should be able to acheive the effect you are after.

I'm not just doing this for ****s and giggles though. I think it is quite likely space travel will go this direction once engine technology allows for it in the distant future(think fusion and beyond). It allows for what average humans would need for extended space travel without requiring some magical gravity-generator, and does so by providing the benefits of an Earth-sized rotating space station without needing an Earth-sized rotating space station.

I guess it is a race between magical engine technology and magical gravity generator technology then... they may even be the same thing

 

[HiPotOk1978]

I think someone is planning on the rocket racing that was proposed in this thread
http://orbiter-forum.com/showthread.php?t=5490

You could maintain a higher velocity w/o increasing your orbital altitude very simply in a craft like the Delta Gliders in a few ways, it is gonna cost you a crap load of fuel....

A) Get yourself in orbit, orientate your craft so your main engines will apply thrust towards the planet, and your hover engines prograde
or
B) Get yourself in orbit, orientate your craft so your main engines will apply thrust prograde, and your hover engines towards the planet.

With the DGIV I would use hover engines to apply dV prograde, main engines will be able to hold more energy into a lower orbit.
IMO this move has VERY limited usage in racing cause your not going to be hold much energy in a lower orbit for very long and your gonna have to null that extra energy out before you try to reenter... It is also going to take almost ALL of your attention into maintaining a fuel efficient position

if your doing say 1/3 of an orbit, be easier to just apply prograde burn and let your extra energy bubble out on the other side of the orbit, bursts of thrust toward the planet to move that extra energy around as you complete your orbit keeping an average altitude. I can't imagine many orbits as part of a race ever being 0 Ecc, and just quick number crunching in me head tells me that forcing a 0 Ecc orbit would be less fuel efficient than going to a higher Ecc

 

[Coolhand]

I remember reading "The Cassini Division" by Ken Macleod, what he predicted was a craft that was fuelled by ice (argh, I think it was some fusion thing...), which was capable of transferring from Earth to Jupiter in a matter of days. All it did was accelerate at a rate of 9.81 m/s for half the trip, spin around at the half-way point, and then de-accelerate at a rate of 9.81 m/s. Atmittedly, I remember thinking to myself: Whoa, the fuel compartments of that ship must be like the Tardis, but that's science fiction for you. Perhaps a real vessel would be able to do this by convering electrical energy, hence solar energy, into some sort of kinetic energy or something.

indeed, that sort of thing is possible with fusion rockets, since you get way more energy per particle than you would with a boring old chemical reaction.

you could power around the earth easily with such a ship too, the hard part would be tolerating the g's because the faster you go by this method in orbit, the more g's you pull... that would prove to be the limiting factor for your military, racing or courier ships.

The method could be useful for people who don't like 0g, same as you can generate a constant 1g with 'torch ship' in interplanetary transfers, you could point your nose down and thrust to generate g's in orbit. Of course with such drive systems you could possibly generate even more g's with more thrust and get from place to place faster but thats a big strain on your crew who are built for 1 g.

The ships in Joe Haldemans Forever war pull 3 g's constantly at times, but the crew would suffer increased injuries, broken bones, dislocations, blood pooling when lying down, etc.. nasty stuff. Those ships later on were capable of 30+ G manuvers but the crew would use 'tanks' which pressurised them both internally and externally so their bodies could take the strain of combat manuvers - for a limited time anyway. Hopefully we'll have fusion power before too long, seems like we're currently putting our money into the wrong projects though, which will not payout for 50 years, if ever. Fusion will really open space up to us and make everything that has come before look hopelessly primative.

Anyway, sorry for going off on a tangent there... to summarise it seems like your maximum speed (given infinite, or at least sufficient fuel) is limited by the max g tolerance of your ship and crew and the power of your main engines - unlike in interplanetary space where you can keep accellerating to c, eventually in orbit you'll go so fast that your engines cant cope with any further forward thrust and at this point you'd be pointed straight down with your engines firing you into the planet just to keep from gaining altitude. To go any faster you'd need more power or to lose mass.

 

[unussapiens]

Might I point out that you don't necessarily need to use hover engines, but that you can just use your mains while pointing below the horizon. There's a fair chance you've used this technique to get up to orbital velocity without changing your apoapsis

 

[HiPotOk1978]

I have used my approach in trial runs to Ascension from KCS to try to get there faster, The amount of stress trying to use engines to accelerate a lower orbit resulted in me burning lots of fuel and loosing control... didn't have the skill to accomplish it, but all the theories posted are sound, just hard as to pull off and right now I can't think of any real world use for these processes atm

 

[Andy44]

In Jerry Pournelle's sci fi universe the spacecraft were often pulling 5 G's or more for hours on end; the crew being confined to acceleration couches. The genetically-engineered Sauron humans could handle up to 11 G's for sustained periods of time. Even with fusion, I question the ability to burn at such accelerations for such long periods of time.

 

[Linguofreak]

The thing is, not all vessels are large enough to produce "gravity" by rotating. Take the Delta-Glider. Simply not large enough. Now, I'm not sure, but I think that a craft would have to be massive in order to be able to rotate fast enough and not rip itself plus the occupants to shreds.

It's not being ripped to shreds that's the problem, a g is a g however fast you have to rotate to get it. The problem is that the RPM's required to get decent gravity for a ship that is too small are enough to make the crew violently ill.

 

[tblaxland]

It's not being ripped to shreds that's the problem, a g is a g however fast you have to rotate to get it. The problem is that the RPM's required to get decent gravity for a ship that is too small are enough to make the crew violently ill.

Also, the gradient in centripetal force is very high, putting further pressure on the barf bag stocks.

 

[DarkWanderer]

Don't know how practical is it in appliance to artificial gravity, but it surely has one useful application: faster intercepts. Stock DG's engines are able to give ~3.6G's acceleration, so we may achieve 2.1 times faster orbital movement on Earth and up to theoretically unlimited enchancement on smaller planets.

So - a good topic for a new MFD, me thinks

EDIT: I've occasionnaly tried this approach on some scenarios, on the moon intercept scenarios this reduces encounter time more than twice; however, fuel consumption went about triple the normal value. Usable for emergency tasks.

 

[jedidia]

Even with fusion, I question the ability to burn at such accelerations for such long periods of time.

The most obvious problem is heat. A fusion engine able to do this would burn at temperatures that should lie well above the core temperature of earth. (providing the ship it drives has a reasonable mass). Finding an engineering solution to allow an engine to operate at this temperature is improbable enough, and even if it could be done, there's still the problem that you'd need radiators the size of I guess about the mediteranean sea or bigger to get rid of the waste heat.
Even if this could be solved, there remains the fact that you are creating something like a miniature sun inside the solar system. Burning retrograde towards a target planet would quite simply torch it...
It does seem an impracticle thing at least. :lol:

 

[Quick_Nick]

http://orbithangar.com/searchid.php?ID=3661
An addon made just for this purpose.

 

[penlu]

Jedidia:
The containment system of the container for nuclear fusion would not come into contact with the actual fusing material. Only an idiot would let that happen (or a genius who invented a material with a melting point well over 200 million degrees).
Instead, a powerful magnetic field is used to keep the plasma awaaaaay from the walls.

Realize that our experimental fusion reactors did not have radiators the size of the Mediterranean.

And even if they needed them, I'd bet some mad scientist could come up with a way of using a couple of kilos of zeolites to radiate (one gram can have the area of a soccer field!).

 

[Keatah]

I think that addon doesn't maintain pitch in relation to the surface.. or that it doesn't maintain prograde either. I just got it and have to play with it though..

 

[Linguofreak]

Jedidia:
The containment system of the container for nuclear fusion would not come into contact with the actual fusing material. Only an idiot would let that happen (or a genius who invented a material with a melting point well over 200 million degrees).
Instead, a powerful magnetic field is used to keep the plasma awaaaaay from the walls.

Realize that our experimental fusion reactors did not have radiators the size of the Mediterranean.

Yes, but neither do said experimental reactors provide enough power to give a spacecraft a high enough exhaust velocity that it can accelerate at 5 g's for hours on end with any reasonable supply of propellant.

Assuming 5 g's for 5 hours and an engine that masses ten percent of the weight of your ship, you're looking at (for a rough order of magnitude estimate) power densities on the order of 430 gigawatts per metric ton. That's a bit more than 500 million horsepower per short ton (2000 pounds), or about 250,000 horsepower per pound. The total average power consumption of the human world (electricity, cars, etc.) in 2004 was about 15 terawatts. To run a ship at 5 g's for 5 hours, you'd need a fusion plant that could produce the power that humanity consumed in 2004, but weighed no more than a jet fighter.

At such power densities, you're not just worried about the plasma coming into contact with the walls of the reaction chamber. You're also worried about the fact that the plasma is *very* bright, and that just the light coming off of it (which can't be stopped by magnetic fields) is going to heat the reaction chamber walls to their melting point *very* quickly. You also have to worry about the fact that most of that light is in the form of X-rays or other ionizing radiation (maybe hard UV if you have less power in a larger space, maybe hard gamma if you have more power in a smaller space), and that it will quickly give a lethal radiation dose (in layman's terms a very deadly sunburn) to any unprotected living thing within a few hundred or thousand miles. (And "protected" means "behind a fairly good amount of lead").

And that's just a rough sketch to give you an idea of what we're dealing with. The reality is much, much nastier.


-----Post Added-----

Even if this could be solved, there remains the fact that you are creating something like a miniature sun inside the solar system. Burning retrograde towards a target planet would quite simply torch it... :lol:

Not quite that bad (at least, I figure, as long as you stay above geosynchronous orbit or so). Even a drive 100 times as powerful as what we're discussing here (assuming 50% efficiency) would need to be within 750 miles before it was as bright as the sun, but it would still probably be wise not to point the thing directly at a planet.

Still, there's a reason why a certain mailing list I'm on refers to such drives as "Magical Fusion Torches." (And the power densities we're discussing here are one or two orders of magnitude higher than what is considered on that list to be implausible but *maybe,* if we're lucky, possible).

 

[jedidia]

Realize that our experimental fusion reactors did not have radiators the size of the Mediterranean.

Well, unless they are in a vacuum, there's no need for them... You can pass on the heat to the air or to whatever there is around. In space, radiation is the only means you have to get rid of it. That means the maximum rate at which you can cool is the maximum amount of light you can radiate away from your spacecraft. That's why you'd need a huge radiator area.

Not quite that bad (at least, I figure, as long as you stay above geosynchronous orbit or so).

Ok, well possible I was overestimating the luminousence. I was just throwing a few images over in my head, I didn't bother with actual numbers.

 

[dougkeenan]

Using an Eaglet I've been able to use this technique to travel from KSC to Ascension in 885 seconds. Start with 20% fuel, main thrusters only, target altitude ~60km, max G about 8 at max velocity just under 20km/s. Near the turnaround point pitch is down almost 50 degrees. Close attention was paid to the SurfaceMFD, the heading widget on Surface HUD, and the distance to Wideawake on MapMFD. If anyone cares I'll post a playback.

The height of the "super-orbit" (or whatever it's called) is critical. At 45-50km there's still enough drag that the ship barely squeaks to ~10km/s. At 75-90km you can pick up more speed but keeping the ship from ejecting from orbit becomes harder, almost impossible at the upper bound. At turnaround the ship is flying almost straight down and it takes forever just to slow down enough to finally level off (max speed >22km/s).

What is this effect/boundary called in rocketry? Where max thrust of a ship equals v*v/r , beyond which maintaining orbit becomes impossible?

 

[Erupter]

Very interesting discussion indeed!

I have a question for Linguofreak who seams well informed in the argument: if all you say is true, how do tokamak reactors effectively contain their plasma?

X-rays, Gamma-rays, UV-rays should all combine to melt down their toroidal chambers. Thing that doesn't happen as the magnetic containment fields are able to withstand the harsh bites of the plasma.

Then regarding the proposed fast orbit.
An "easy" way of achieving such an orbit would be to modifiy the default "zero-roll" of the aircraft in a way that in "prograde" mode the hover thrusters are pointing away from the planet.
Right now if you engage eighter prograre or retrograde mode in the default DG, the autopilot aligns the wings with the orbital plane. thus haveing the hover thrusters pointing eighter to the north or the south.
This way you could get "easily" thrust forward and compensate with the hovering thrusters.

Another possible way (this should be achievable with an orbitmfd-like mfd) could be to segment the trajectory.
Thrust forward considerably and make steering (correction) burns with main thrusters at predetermined points.
Essentialy flying a curved polygon trajectory.
Like an exagon made of circular arches.
The more subdivisions, the more like a circular orbit it gets... the more you consume fuel.