Discussion Powell & Pelligrino's Skytrain (aka ISV Venture Star)

[Hlynkacg]

For those unaware, the ISV Venture Star from the film Avatar was based on Charles Pelligrino and Jim Powell's starship from the book Flying to Valhalla. Inspired by this thread here. I thought I'd open discussion on specifications for a near-future varient (a precursor if you will) of the Venture Star.

The basic design consists of a "locomotive" (aka the biggest nastiest rocket one can build) pulling the payload through space like a water-skier. In a traditionally configured rocket the structure must be able to support the effective wheight of the vessel under thrust. In the Skytrain the payload and crew compartments "hang" from a slender cable allowing a mass ratio not otherwise achievable.

As always the first step in designing a spacecraft is to figure out what exactly we want it to do. As the basic mission remains the establishment of colonies/infrastructure on other planets, let us assume that we need to transfer 1,800,000 kgs (two fully-loaded XR5s) from Earth/Lunar orbit to Mars. This will mandate a minimum DV of 10km/s to get there and back. Lighter payloads and/or additional fuel tanks could be used to reach the outer planets.

Below is a rough sketch of how I imagine the payload/crew compartments;



For scale the Gravity-wheel in the drawing has a radius of 60m and would rotate at 3 RPM to produce 2/3 g. Exact dimmensions for all components are TBD.

What remains to be determined

-Crew size/Requirements

-Mission Length/Required consumables

-The above both in turn will determine the total mass of the spacecraft and Propulsion requirements.

For propulsion I am leaning towards some form of NTR for the ability to re-fuel using whatever fluids are on hand and because Orbiter's navigational tools don't handle low-thrust engines very well. :thumbup: Peligrino's design allows for a very light wheight construction so let us assume that the vessle is comprised primarily of aluminum and carbon-fiber with a mass ratio around 6 or 7.

Comments, suggestions, advice, and criticism are welcome.

 

[Manichean]

The design is good for basic "forward" flight, but I see problems when you want to go anywhere else. The cable you're proposing would be subjected to significant shear strenghts if you just slapped a RCS thruster on either end and began turning the ship, so I imagine a solution would be to consider the cable to be essentially weightless and each "weighted" module to be its own ship, then outfit every weighted module with RCS thrusters and let a computer handle firing those in a way that the overall structure is preserved?

 

[Urwumpe]

The design is good for basic "forward" flight, but I see problems when you want to go anywhere else. The cable you're proposing would be subjected to significant shear strenghts if you just slapped a RCS thruster on either end and began turning the ship, so I imagine a solution would be to consider the cable to be essentially weightless and each "weighted" module to be its own ship, then outfit every weighted module with RCS thrusters and let a computer handle firing those in a way that the overall structure is preserved?

Or you just turn very slowly.

 

[T.Neo]

Your main problem here is thrust dispersion. My calculations (with far higher-performance vehicles) have lead to fried habitat modules. The key is thrust stream collimation. I'm not sure what the thrust stream dispersion of any rocket engine (be it an antimatter beam-core drive or a solid-fuel motor) is. Maybe there is a way to mathamatically deduce thrust stream dispersion, at least vaguely, but I have no idea what it is. VASIMR has been shown to have quite a wide dispersion angle, but it magnetically directs thrust- when using an NTR you will presumably have a physical nozzle, which would probably mean that dispersion of the exhaust products would be due to electrostatic forces.

NTRs also produce a good deal of radiation, like any nuclear reactor, which would usually require a good deal of shielding (which is heavy) to protect against. If you make your tether long enough, you might not need any shielding, or you might need only a relatively small amount of shielding, due to the inverse square law spreading the radiation over a wider area and reducing its intensity. This might result in quite a long tether though. But shielding and distance from a source of radiation work well together. You can even go the Valkyrie route and use a shadow shield that 'eclipses' the engine, though of course everything behind the shield needs to fall in its shadow (which could be difficult with the centrifuge).

If you have a 2000 meter long tether, and a 40 meter wide engine section (say there are several engines, 40 meters apart) and you had a 20 meter wide shield 1000 meters from you, it would have the same angular diameter at half the size.

I am not sure why you have such a large flywheel; spin-up of the centrifuge will spin the vessel in the opposite direction, but this can be counteracted with thrusters. Of course over time friction with the rest of the vessel will slow down the centrifuge and tend to spin up the rest of the structure, but this can also be counteracted with thrusters and even normal CMGs. And that friction will be continuously counteracted by a system to keep spin rate within acceptable parameters.

Or you just turn very slowly.

For more complicated manouvering on-orbit (for example docking with a space station) you can always 'reel in' the engine and propellant tanks and attach them closer to the payload/hab section (though maybe on a relatively short truss to keep residual radioactivity away from the crew and payload), which would make rotation and translation much easier. For a burn, the propulsion section could be reeled out and while the ship is extended, only minor adjustments would have to be made.

 

[Manichean]

Or you just turn very slowly.

Well, yeah, but how is that fun? :lol: (I just had the mental image of holding onto one end of the craft and swinging the whole thing around like, say, a stone on a string. I can't figure out how to make that practical for turning the whole thing, though.)

 

[Urwumpe]

Well, yeah, but how is that fun? :lol: (I just had the mental image of holding onto one end of the craft and swinging the whole thing around like, say, a stone on a string. I can't figure out how to make that practical for turning the whole thing, though.)

Well, many spaceflight agencies already did such stunts with up to 30 km tethers, so it can't be magic. :lol:

You just need to use RCS to slowly change the rotation impulse...if you are careful, the tether remains under tension and all works. Essentially like having something on a rope, that you rotate with your hand, and then change the rotation plane.

 

[Hlynkacg]

I immagined that while in orbit the RCS thrusters on the end would give occasional bursts of retro thrust to keep the cable under tension.

As for the issue of frying the hab modules, the solution is simple. Angle the engines 1 or two degrees away from the cable. Thrust will be reduced by the sine of the angle but with a long enough teather a sizeable shadow zone should exist.

 

[T.Neo]

Yes, you can redirect the exhaust by angling it outwards of course, but the problem is the dispersion of the thrust stream- if the exhaust stays nicely collimated in a straight line, or even if it blooms out at a small angle, it won't be a problem, but if it spreads over a wide angle, then you end up angling the engines outward so much that it results in a marked loss of efficiency.

 

[Sky Captain]

[ame="http://www.youtube.com/watch?v=Q-ci9xIgNZM&feature=relmfu"]YouTube - SpaceX Falcon 9 Flight 2 Launch Webcast[/ame]

If you watch Falcon9 launch you will notice exhaust spread at fairly wide angle when rocket reaches vacuum. I have no idea how wide angle would be exhaust from NTR, but given that exhaust velocity is only 2 - 3X higher it is likely the gasses will spread at wide angle after exiting nozzle. Howewer exhaust from NTR would be more "gentle" than exhaust streams from fusion torch rocket so it should be easier to give exposed structure some protection (ablative paint or something)

 

[Hlynkacg]

So, I one of the questions we neet to answer is how collimated (and dangerous) would the exhaust gasses be and thus how big an angle/how long of a cable do we need to avoid damage to the rest of the space craft?

For the sake of argument let us assume an NTR or "Nuclear Lightbulb" using either ammonia or methane as a reactant.

 

[Izack]

I'm guessing a general rule would be that the cant angle of the engine must be greater than half the angle of the max. exhaust dispersion cone, plus the angle between the acceleration vector and the exhaust nozzle from the most protrusive part of the ship (in this case the rotating crew modules.)

 

[Sky Captain]

A ship like this don't really need centrifuge module. NTR burns would last less than an hour. After burn is done why not spin the ship end over end to generate artificial gravity? No need for complex centrifuge module and less portruding stuff that could be coocked in exhaust.

 

[Hlynkacg]

A ship like this don't really need centrifuge module. NTR burns would last less than an hour. After burn is done why not spin the ship end over end to generate artificial gravity? No need for complex centrifuge module and less portruding stuff that could be coocked in exhaust.

Remember that the Idea is to make a plausible extrapolation of an existing (fictional) design.

That said, you do have a point but I image the the "tumbling pidgeon" method of producing gravity would run into issues in low Earth/Mars Orbit.

I guess the real question is, can you make a centerfuge light enough to justify not having to take spin off the spacecraft every time you want to make a burn?

---------- Post added at 08:20 PM ---------- Previous post was at 08:12 PM ----------


Been giving some thought to crew requirements. I figure that we will want 5 or 6 people at a minimum to allow for some redundancy and ensure that the mission isn't jeopardized by someone getting sick, injured, or pissed off at another crewmember.

Here's what I have for roles so far...

Mission Commander/Navigator: Self explanitory.

Pilot/Assistant Navigator: Actually flies the ship, checks the Captain/Navigator's math.

Chief Engineer (Propulsion/Structural): Self explanitory.

Assistant Enginner (Electrical/Auxillaries): Assists the Chief Engineer on maintenance tasks, and manages subsystems.

Science/Communications Officer: The guy you call to when something strange pops up on radar. Seeing as there is a lot of overlap between Radar and Radios (duh) this guy would also be responcible for baby-sitting the communications gear.

Medical/Psyche Officer: Ship's doctor.

Farmer/Lifesupport technician: Assuming that there's a hydroponics garden/Algae Tank or something similar you're going to want somone onboard who knows can tend to it and make sure the little green things keep growing, and keep converting CO2 into O2.

Anyone have anything to add or subtract?

 

[T.Neo]

Atomic Rockets says that the exhaust velocity of a nuclear lightbulb using LH2 is 20 405 m/s. GUIPEP suggests a temperature of roughly 4000 K if using LH2. Now, the temperature that can be imparted stays roughly the same, but this will result in a lower exhaust velocity for heavier particles (ammmonia, methane). This suggests an exhaust velocity of 18 900 m/s for ammonia, and roughly 19 750 for methane. Keep in mind that there are many factors at play here, so it is probable that both I and GUIPEP are wrong.

Since thrust power is V * F / 2, we can calculate with a thust of 3 MN- enough to move a 500 ton vehicle at 6 m/s^2 or roughly 0.6 G.

This gives us 30 GW with H2, ~28 GW for ammonia, and ~29 GW for methane.

Say that the nozzle diameter for each engine is 5 meters, and the dispersion is 35 degrees. There are four engines. The area of that cone at 2000 meters is roughly 6 184 000 m^2. 30 gigawatts spread over that area is 4850 watts/m^2. 7.5 gigawatts spread over that area is ~1210 watts/m^2.

That can lead to a surface temperature of around 380-540 K (roughly 100-270 C).

Since in the inner system insolation can already heat a surface to around that sort of temperature, it could probably be bearable for a spacecraft structure. A specially designed shield that can take the brunt of the exhaust might be a good idea, especially to shield payloads, more sensitive materials, windows, instrumentation, radiators, panels, etc.

An ablative structure would probably be too much effort, but a radiatively cooled or even partially actively cooled structure would do fine.

At that distance the density of the exhaust stream would probably be so low that interaction would be minimal and you could set up a shield that could protect all the stuff behind it in its exhaust-wake, without gases wrapping or licking around it.

The overall particle energy will be negligible, so the exhaust will not amount to particle radiation... but if there is even a slight amount of radioactivity released from the core and particles embedded themselves into the shield over time, the shield might become (mildly) radioactive. But it isn't like anyone is going to go up to it an lick it.

EDIT:

Anyone have anthing to add or subtract?

I would say that you could whittle that away a lot, because a lot of systems can be automated, or even controlled remotely from Earth if possible. For example the navigator and 'chief operator' position melds in with the 'engineer' position on a spacecraft, though it's worth it to have more than one. The same goes for communications and detecting potential threat objects; the latter should be part of the navigator's job, the former is already the job of the commander on modern spacecraft. The really important individual position you have there, I think, is a dedicated doctor. You can (or should) have the other crewmembers trained in first aid, but that is not the same as an onboard flight surgeon, especially for long-term trips where immediate return is not really possible.

Managing the ECLSS is also part of the spacecraft operator's job, and I wouldn't think farming stuff would be that hard... I'm sure it is a comparatively low skill activity that all crewmembers can participate in. Having the flight doctor performing other jobs on the ship would also be helpful.

After burn is done why not spin the ship end over end to generate artificial gravity?

Because this is my idea, and epic fails aside, I might still be working on it secretly. :shifty:

 

[Izack]

What will they do on Mars? I think you need science personnel (geologists, climatologists, etc.) as well as mission specialists (technicians, structural engineers, construction workers, vehicle operators, administrative personnel, etc.) if you're bringing the infrastructure for a colony. If supplies are flown down with some sort of reusable spacecraft, you would need pilots and technicians and ATC personnel for that, too...

With that much capacity, you have the potential to bring a lot of infrastructure down with you, which would be pointless without the people to use it.

 

[T.Neo]

I thought that was a breakdown of the ship's crew, not every person aboard... there could be passengers, but they could be treated seperately (wouldn't have many onboard tasks beyond cleaning up, etc).

 

[River Crab]

I think the centrifuge could at least be smaller; if they are going to Mars, maybe you should design it to provide 0.38 of Earth's gravity (about that of Mars), so they can get used to it? I think this should be enough to prevent bones from degenerating too much, and they will be accustomed to the gravity once they arrive.

 

[Hlynkacg]

T. Neo, just to make sure I'm following your math, could you please post the equations you used? (show your work)

Also, am I corect that you are assuming a cable length of 2000 meters?

Based on the numbers it seems that a few light wheight exhaust-deflectors/whipple shields shouldn't be too hard to include in the final design. :thumbup:

What will they do on Mars? I think you need science personnel (geologists, climatologists, etc.) as well as mission specialists (technicians, structural engineers, construction workers, vehicle operators, administrative personnel, etc.) if you're bringing the infrastructure for a colony. If supplies are flown down with some sort of reusable spacecraft, you would need pilots and technicians and ATC personnel for that, too...

With that much capacity, you have the potential to bring a lot of infrastructure down with you, which would be pointless without the people to use it.

The crew position I listed are what I imagine to be the bare minimum required for orbital operations. Obviously we'll need more people but how many?

Then again, there's nothing that saying the 1,800,000kgs of cargo can't include a few freeze-dried colonists

---------- Post added at 09:05 PM ---------- Previous post was at 08:45 PM ----------

EDIT:
I would say that you could whittle that away a lot, because a lot of systems can be automated, or even controlled remotely from Earth if possible. For example the navigator and 'chief operator' position melds in with the 'engineer' position on a spacecraft, though it's worth it to have more than one. The same goes for communications and detecting potential threat objects; the latter should be part of the navigator's job, the former is already the job of the commander on modern spacecraft. The really important individual position you have there, I think, is a dedicated doctor. You can (or should) have the other crewmembers trained in first aid, but that is not the same as an onboard flight surgeon, especially for long-term trips where immediate return is not really possible.

I'd prefer to avoid remote operation as communications interfearance and light-lag may become issues. That said you make fair point about automation and shared tasks, so let's say that the basic flight crew consists of 2 Pilots, 2 Engineers, and 1 Doctor. A total of 5 people, not counting passengers, scientists etc.

Obviously a good deal of cross-training among the crew would be encouraged.

 

[T.Neo]

T. Neo, just to make sure I'm following your math, could you please post the equations you used? (show your work)

Ok, let me bear the gory details;

I used GUIPEP to backwards-calculate the temperature needed in an engine to heat LH2 to generate an exhaust velocity of 20 405 m/s.

I then used this calculator to figure out what thrust would be needed to accelerate a 500 ton vessel at 0.6 G (I'm a sucker for online calculators; I'm sure the actual equation there is pretty simple, it's just that I'm lazy at math).

I provided the thrust power equation P = V * F / 2, where V is exhaust velocity in m/s, F is thrust in newtons, and P is the thrust power in watts (thrust power, is essentially the kinetic energy of the mass flow... if that makes sense).

I then used anim8or to deduce the area of a 35 degree frustum starting at a 5 meter diameter, at 2000 meters. I then divided the thrust power by that area.

I then used the equation T = Fourth root of[P/(K*S)], where S is the area, K is Stefan's constant (5.67e-8), P is the incoming power, and T is the temperature in Kelvin to deduce the temperature of the shield.

I did try to calculate the particle density at that distance, not sure if I got close to getting an accurate answer, but it seems it would be pretty low. I've figured out the kinetic energy of exhaust particles from much higher performance engines in the past, and the particle energies have been low- around 700, 800 eV, not even a keV, and this is for an engine that has an exhaust velocity of nearly 400 000 m/s, which is far higher than this, so it's safe to say the particle energy would be negligible.

Also, am I corect that you are assuming a cable length of 2000 meters?

Yes, 2000 meter cable length.

Based on the numbers it seems that a few light wheight exhaust-deflectors/whipple shields shouldn't be too hard to include in the final design.

It would probably be less of a whipple shield and more of a... shield. Just a flat plate. At that range the pressure on it from the exhaust would probably be pretty low too, so it wouldn't need to be majorly reinforced. The temperature parameters aren't all that high, it'd probably be rather easy, engineering wise, to cope with. The only problem is getting a shield large enough to shield the centrifuge, but the whole structure should in general be pretty light. Maybe it can serve double purpose by including radiation shielding, but this of course would increase mass (you need that kind of mass and shielding distribution anyway though).

Then again, there's nothing that saying the 1,800,000kgs of cargo can't include a few freeze-dried colonists

Youch... freeze-drying colonists is difficult, and there could be a large chance that they'd wake up dead...

Out of your 1 800 000 kg cargo, it shouldn't be difficult to accomodate 450 people if 4000 of mass is allocated per person. A start-out of 450 people is already quite a large base, in a few flights you would have enough people to start a genetically viable colony.

Of course all of those people would be in microgravity... it might be possible though to have them excersise in the centrifuge and even sleep there on a shift basis; they'll still have microgravity muscoskeletal and cardiovascular decay, but it might be reduced enough for surface operations at the destination to be practical.

Another thing that might be considered is increasing the dV capacity, to decrease travel time, which could help in general. Already with a mass ratio of 6 and an exhaust velocity of 20 400 m/s, you could have a dV of 36 500 m/s. Obviously that has to be spread over the whole mission (unless there's a refueling at Mars), but it could decrease travel time quite a bit.

I'd prefer to avoid remote operation as communications interfearance and light-lag may become issues. That said you make fair point about automation and shared tasks, so let's say that the basic flight crew consists of 2 Pilots, 2 Engineers, and 1 Doctor. A total of 5 people, not counting passengers, scientists etc.

Obviously a good deal of cross-training among the crew would be encouraged.

Well, remote operation works nowdays because spaceflight is low-volume and a lot of spacecraft are unmanned. Even manned craft that can be controlled from the ground ought to have a good deal of onboard control in case of an emergency.

I would say, merge the pilots and engineers together, so instead of having 2 engineers and 2 pilots, have four engineer-pilots, that all share eachother's jobs and responsibilities (because it should be possible to do so).

Spaceflight is pretty boring, and the chance that a large rock will hit you is quite low, so actually your biggest problem in interplanetary space is keeping your ship running. Pilots would be a waste of mass and air in that case. Actually flying a ship is part physics, part knowing how to work the flight computer, and part knowing the parameters of the systems themselves. All of which an engineer could do.

 

[Eli13]

Well we pilots dont think of ourselves that way.