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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 ta

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.

trhway 4y ago
>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.
- namibj 4y ago
  The scaling benefit is that you save insulation.
Filligree 4y ago
> 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.
- mLuby 4y ago
  Or just photons. That's what the genie gives you when you ask for a torch drive—should've been more specific.
- toopok4k3 4y ago
  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. https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster
- ben_w 4y ago
  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). [0] or, by symmetry, just the nucleus.
  - tgflynn 4y ago
    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.
    - e_y_ 4y ago
      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. Source: https://trs.jpl.nasa.gov/handle/2014/15643 https://trs.jpl.nasa.gov/handle/2014/15643 "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."
jhgb 4y ago
> You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input. You can't beat it in terms of exhaust velocity, but you can often definitely beat it in terms of whole-system performance.
namibj 4y ago
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.
- foobarian 4y ago
  Would that suggest a staged approach where the long-range vehicle is fueled up in orbit?
  - namibj 4y ago
    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.
- ncmncm 4y ago
  That seems to be why GP suggested wrapping it after reaching orbit.
messe 4y ago
> 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.
- ncmncm 4y ago
  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!

trhway 4y ago

>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.

namibj 4y ago
The scaling benefit is that you save insulation.

Filligree 4y ago

> 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.

mLuby 4y ago
Or just photons. That's what the genie gives you when you ask for a torch drive—should've been more specific.
toopok4k3 4y ago
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. https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster
ben_w 4y ago
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). [0] or, by symmetry, just the nucleus.
- tgflynn 4y ago
  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.
  - e_y_ 4y ago
    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. Source: https://trs.jpl.nasa.gov/handle/2014/15643 https://trs.jpl.nasa.gov/handle/2014/15643 "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."

jhgb 4y ago

> You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input. You can't beat it in terms of exhaust velocity, but you can often definitely beat it in terms of whole-system performance.

namibj 4y ago

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.

foobarian 4y ago
Would that suggest a staged approach where the long-range vehicle is fueled up in orbit?
- namibj 4y ago
  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.
ncmncm 4y ago
That seems to be why GP suggested wrapping it after reaching orbit.

messe 4y ago

> 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.

ncmncm 4y ago
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!