ACES is effectively dead, although a lot of its ideas ended up getting into Vulcan's upper stage. Long duration in its context means days. It was intended to do something similar to what SpaceX is doing with Starship, using the boiloff gas to pressurize the tanks.
The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites.
However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.
Long term hydrogen storage isn't that bad with the proper architecture. You need a cryocooler which can be powered by the nuke, and thermal shielding for the tank which in vacuum is thin film and of minuscule weight.
Hydrogen leakage and structural embrittlement are overblown, i.e. the Space Shuttle tank is one of the most mass efficient architectures in history and it was full of liquid hydrogen. Terrestrially, you can buy a Toyota hydrogen car today. Materials matter, but people act like the thing needs to be made of 4" plate and will fall apart if you look at it. Scaling helps here too, as volume increases to the third power while wall area increases to the second.
The thing will, if there is any sense in the architecture, be assembled in orbit so gossamer heat shields and the like won't be a problem, nor will an extended assembly program that makes with a separately launched nuclear reactor.
For ISRU Mars return, water is incredibly abundant and there's no concern with "wasting" residual oxygen. For lunar applications, water may be scarce but oxygen is abundant in regolith.
You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input.
In vacuum, the required insulation is cheap and easy: more layers of the famous metallized crinkled plastic foil. The stuff that (with gold-colored metallization) is an iconic part of "the" satellite/space probe design.
The hard part about that insulation is that on earth, you need to sustain a vacuum in the annular space while overall being light due to the LH2 itself being light. Ideas would be to get tension fibers bridging that annular space, the inner tank with the LH2 being slightly pressurized, and thus the outer wall being kept from large-scale buckling (and small-scale buckling is cheap to reinforce for with an isogrid (triangle honeycomb) or other similar reinforcement structure on the outside of it). But in space, the outer wall isn't needed, because space is already a vacuum.
A pure vacuum shuttle should be fine with getting launched and then wrapped with the insulation afterwards.
Launching it empty is likely not really easier than launching it full, due to LH2's low density. But that's not important.
> pace Shuttle tank is one of the most mass efficient architectures in history and it was full of liquid hydrogen.
That being said, it didn't have to last very long while filled with LH2/LOX; a few hours at most prior to launch, and a few minutes during launch. They were never reused, unlike the orbiter and SRB segments.
More's the pity. They actually used extra fuel to keep it from joining the shuttle in orbit. That much raw aluminum pressure vessel in orbit could have been so useful!
> You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input.
This is probably right, but the way you said it made me wonder. Would it be possible to strip electrons from atoms, then use just the electrons as propellant? Or would the ensuing static charge of the spaceship render this infeasible? I imagine it'd pull in electrons from all around itself, but I don't know how the numbers come out.
- "Or would the ensuing static charge of the spaceship render this infeasible?"
Back-of-the-envelope math says a large spaceship will reach 100 kV potential at a charge imbalance of around 1e15 electrons (total mass: 1e-15 kg). So yeah, completely unfeasible.
(It's asking the wrong question though. Electric thrusters aren't thermal systems, and aren't limited by molecular weight as severely as thermal engines are. You can get stupidly high Isp (>200 km/s) out of heavy ions, just by raising the voltage).
And, in fact, you WANT heavy ions, as the thrust/area of an ion engine at a given exhaust velocity scales as the square of the ion mass/charge ratio. The thrust in an ion engine is limited by space charge (where the charge of the ions between the accelerating grids becomes similar to the charge on the grids). Using heavier ions also reduces the ionization energy/mass. There's been work on using molecules or small droplets ("colloidal thrusters") to get even higher mass/charge, but you need to totally avoid generation of fragments with low mass/charge as they will dominate the current.
Funny that you mention this, Ion thrusters do exist. They are a thing but with very limited uses cases. They still need a kind of propellant gas like Xenon or Krypton that gets used.
If you strip the electrons off some atoms and use just the electrons[0] as reaction mass, you will eventually get a large enough electric charge you can no longer throw the electrons away from you. Electric forces behave similarity to gravity, so while it wouldn’t normally be phrased like this, you could say your engine exhaust will eventually no longer have escape velocity from your ship.
For this reason, ion drives do things to neutralise the net charge.
(If you meant using them as a power source rather than reaction mass, it’s technically possible but that’s called a capacitor and they have very low energy density).
You didn't address this part of the parent comment:
> I imagine it'd pull in electrons from all around itself, but I don't know how the numbers come out.
I never thought of it before but it seems like that should work. "Space" is actually a neutral plasma, right, so it should be full of free electrons. Those should neutralize the ship before any significant charge builds up. It seems like you should be able to use space itself (or more accurately the interplanetary medium) as a massive ground plane to complete the circuit for the charged exhaust beam.
Space is pretty empty. From what I can find, the interplanetary medium is around 5 particles/cm^3 compared to the exhaust from the ion thruster which seems to be around 10^6 particles/cm^3 and disperses to 10^4 particles/cm^3 further away.
"As the charge-exchange plasma
density near spacecraft is at least about three orders of magnitude larger than the solar wind plasma density, the plasma environment of DS1 spacecraft is completely dominated by the charge-exchange plasma in the plume."
I'm assuming you're asking about ejecting the electrons and remaining positively-charge hydrogen ions separately, since keeping the hydrogen around would be a waste of mass.
I'm no expert at space propulsion, but I think this would have a few issues:
- Hydrogen has a pretty high ionization energy, even higher than Xenon
- As you said, static charge buildup
- Momentum = mass * velocity. Electrons have 1/1836 the mass of a proton, so for the same momentum you need a much higher velocity
- Imparting velocity is harder for low-mass particles because they tend to zoom off very quickly if not contained
>Scaling helps here too, as volume increases to the third power while wall area increases to the second.
Not really. As surface increases the wall tearing force at the given pressure is increases too, so you have to increase the wall thickness, and thus the mass of the tank also grows close to the third power.
Apparently Bruno has been talking about two orders of magnitude improvement to Centaur V's duration. Seems far fetched to me, but I think months of duration would be necessary to make this engine worthwhile (the PDF is talking about the value of this engine for getting astronauts home from Mars quickly in emergencies; that would only be possible with months of duration at least I think.)
DARPA says they're expecting designs using liquid hydrogen, and as far as I understand liquid hydrogen would be the most efficient propellant for an NTR. What might the best storable alternative be?
From what I understand, Bruno didn't say that Centaur V has those two orders of magnitude improvements, rather that they're aiming to push improvements of that level over the next few years. That said, I don't think it's too far fetched, assuming that the long duration version is separate from the regular version (ie it can be heavier to support denser tanks).
Liquid Hydrogen would be most efficient in a pure physics sense, but due to the mass tradeoffs with storage tech, there may be other propellants that are comparable in a practical sense. I'm not informed enough on the matter to say exactly which would be better, but for a somewhat comparable point of reference,
Hydrogen+Oxygen is the most efficient propellant for chemical rockets but when accounting for the special tanks needed for storing hydrogen, methane can achieve pretty comparable performance due to being perfectly fine in a thin-walled stainless steel tank.
The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites.
However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.