Science/physics questions for my Orbiter addons

[NovaSilisko]

I've been thinking a lot about the fictional space program I'm developing for Orbiter, and have a few questions about propellants and general physics:

Firstly, I had a nuclear thermal rocket engine using methane as fuel, so fuel could be siphoned from Titan's lakes (after purification), but then I learned that using methane in an NTR results in a soot buildup on the inside of the nozzle. So I decided to switch to ammonia. Now, the questions I have: Is the storability of ammonia worth the loss in efficiency when compared to hydrogen fuel? And, would methane really cause a soot buildup?

Next, Can hydrogen peroxide be used as an effective monopropellant for RCS?

And lastly, would it be worth it in terms of amount of fuel to send a ship on a hyperbolic trajectory towards saturn, aerobraking in titan's atmosphere before performing another burn to park at titan's L1 point? Otherwise, the trip could take nearly a decade, one way.

 

[Hlynkacg]

You should check out Atomic Rockets if you haven't already.

 

[Urwumpe]

Next, Can hydrogen peroxide be used as an effective monopropellant for RCS?

It can be used, but is even more ineffective in terms of specific impulse than toxic Hydrazine. Around 800-1100 m/s specific impulse are possible.

 

[T.Neo]

Firstly, I had a nuclear thermal rocket engine using methane as fuel, so fuel could be siphoned from Titan's lakes (after purification), but then I learned that using methane in an NTR results in a soot buildup on the inside of the nozzle. So I decided to switch to ammonia. Now, the questions I have: Is the storability of ammonia worth the loss in efficiency when compared to hydrogen fuel? And, would methane really cause a soot buildup?

I don't know about soot buildup in a methane-using NTR, but ammonia doesn't seem like a good fuel to me- it is fairly rare in the solar system. Water (for hydrogen) is best, and methane may be acceptable.

But why launch your propellant all the way from Titan? Perhaps this is simply my aversity to gravity wells talking, but why not purify hydrogen or other volatiles from a small moon or asteroid relatively nearby?

 

[NovaSilisko]

You should check out Atomic Rockets if you haven't already.

That's actually where I learned that Methane produces soot buildup. It's also where I got the idea that ammonia works as a fuel for NTRs, while water is very ineffective.

 

[francisdrake]

Hi, many questions in one thread I will try to give my oppinion on the ammonia, not claiming I am right at all ...

NH3 sounds like the fuel of choice for long term thermo-nuclear missions. It can be conveniently stored liquified in a range of -78 to -33°C, which is easy to achieve in deep space. This compares to -250°C for H2, which would require cooling, either by partial venting or by active refrigeration. Its liquid density is vastly higher than hydrogen, making the ammonia tanks smaller.

And it decomposes into gases. I think the danger of soot build-up from CH4 is real, and you would not want to have soot deposits in your reactor, possibly interferring with the movement of the control rods ...

You could calculate if you are better off with hydrogen or ammonia with the formulas of the atomic rocket page, trading in the lower ISP of ammonia with the larger tank size of hydrogen. The density ratio is 682 kg/m³ NH3 / 68 kg/m³ LH2 = ~10.
The tanks for hdrogen are 10 times bigger in volume, about 4.6 time bigger in surface area. Plus hydrogen needs better insulation (=thicker tank wall) and an active refrigeration unit with fairly large heat radiators if stored for long time.

It could be feasible to do the initial burn (leaving Earth orbit) on hydrogen, as the time from topping off the tanks to the burn can be short, and then use ammonia for deep space maneuvers including the capture burn.

 

[T.Neo]

Anyone operating an in-space NTR really ought to have developed a means of reducing LH2 boiloff to manageable levels. Even if it means you have to vent an amount of LH2 off the vehicle or invest in more massive insulation, the higher ISP of a hydrogen-fueled NTR should well outweigh it. Low-boiloff or zero-boiloff (ZBO) systems are not obscure and doubtful ideas; they are a matter of importance to long-loiter cryogenic stages and propellant depots.

The basic fact is that ammonia is rare in space; nitrogen as a whole is rare in the solar system, even beyond the frost line. It isn't something one would want to have to rely on for propellant. Water on the other hand is abundant, even in NEOs, and though hydrogen needs to be liberated from water, there's plenty of it to go around.