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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.

Would that suggest a staged approach where the long-range vehicle is fueled up in orbit?

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.

That seems to be why GP suggested wrapping it after reaching orbit.

> 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.

You can't beat it in terms of exhaust velocity, but you can often definitely beat it in terms of whole-system performance.

> 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.

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

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.

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.

Source: 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."

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

- "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).

https://en.wikipedia.org/wiki/Ion_thruster#Comparisons

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.

Or just photons. That's what the genie gives you when you ask for a torch drive—should've been more specific.

>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.

The scaling benefit is that you save insulation.