[guepe]

That reminds me of a question I was asking myself: instead of using rare gas for ion thrusters… why not ejecting very high velocity electrons ? Their mass is much, much smaller than any atom, but why wouldn’t that lead to a thrust ? Possibly without limit except power - no limit on a “tank”?

[addaon]

> why not ejecting very high velocity electrons ?

This works, initially. Now, you have to balance the charge on your space craft, or else you won’t be able to eject any more electrons.

If you balance this charge by ejecting protons, you have a hydrogen thruster.

If you balance this charge by ejecting positrons, you have a photon thruster (in the far field).

The former is still “tank” limited. The latter requires high energy to get any useful impulse.

[mmooss]

I thought thrust in a vacuum required ejecting mass?

... the ejected propellant mass times its velocity is equal to the spacecraft mass times its change in velocity. [0]

Electrons have very little mass; do we eject lots more of them? Is there away to store that many electrons in a small enough volume? Or does their velocity make up for the lack fo mass? Also, if we must expend mass, I don't see how the 'no limit' idea works; I'm also suspicious of any claim of free, unlimited propellant, if that's what you mean.

There's a very good chance that is all my misunderstanding of something ...

[0] https://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel__cm...

[reaperman]

Your [0] provides the "classical" equation for momentum transfer (p=mv), which is only reasonably accurate up to speeds 40-50% of the speed of light. But TFA talks about accelerating electrons to 80% of the speed of light. Then it's p=γmv where γ=1/sqrt(1-v^2/c^2)

Basically, it's taken as a "fact" that the relative speed of light is the same for two observers regardless of one observer's velocity vs. the other observer's. So if something is going 0.8x the speed of light, but light is still somehow observed to be traveling at the same speed for both (speed of light relative to both the fast-moving observer and the slow-moving observer), this apparent inconsistency is solved by realizing that ("magically") lengths are contracted for the faster observer. So the light appears to go the same distance to both observers, because distance itself is different to the two observers.

The net effect is that from 0.5x to 0.99999x the speed of light, momentum increases asymptotically as an object approaches the speed of light. Theoretically, if your spaceship could (truly magically) capture the momentum from ejecting one single electron to 0.9999....(58 nines in total)... a 1,000kg spaceship could achieve escape velocity to leave the entire solar system just from that one electron!

0: https://library.fiveable.me/principles-of-physics-iv/unit-9/...

1: https://en.wikipedia.org/wiki/Lorentz_factor

[mmooss]

Thank you.

[sbdhzjd]

The thruster would quickly die out once the static electricity on the vessel created an energy well that matches the energy of the electrons thrust.

Instead use a powerful laser. Light carries momentum without the need to preserve charge.

[guepe]

Ah! That “well” is exactly what I couldn’t understand from ion thrusters, where they shed electrons as they expulsé ionized gas. All I could find was “to keep the same charge”, but since there is no absolute ground, what’s the problem?

You just explained it :)

[sbdhzjd]

Np. I remember first coming to that realization.

Fun thought experiment: what happens to a beta emitter in outer space?

[hansvm]

Where do the extra electrons come from?