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Comment by Filligree | Hacker News Reader

> 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.

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- "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.

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

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