Shooting just electrons out the back of your ship is fine for the brief period before your ship gains a positive electric potential comparable to the net potential those electrons experience due to the inside of the particle accelerator you were using, after which your ship's own field is a major source of drag.
If your ship has a positive electric potential of over 1.044 MeV[0], you also start getting positron-electron pairs forming on your hull.
[0] less in practice, because any electrons you're going to encounter were already moving
Don't ionic thrusters typically solve this problem by also emitting oppositely charged particles from a thin rod behind the motor? Maybe that still causes drag, but overall the main problem is that while the ISP is high, the thrust is almost non-existent.
They do indeed; the oppositely charged particles being electrons. But there's no charged particles lighter than an electron, and the only one with the same mass is the positron. If you try to balance it with protons, then what you've really got is a slightly over-complicated hydrogen-based ion drive.
First, you have a way to store an absurd quantity of positrons. For a sense of scale, without shielding the electric fields, 1 picogram of positrons (or electrons) confined within a 10 cm radius is going to trigger positron-electron pairs formation, thanks to free electrons in the area responding to the surface potential.
Second, AFAICT if you can do that then you're either going to want to use them as an energy source to propel your cheap reaction mass even harder than a fission rocket would, or you're going to want to react them with electrons to make a photon rocket.
If you're getting them from radioactive decay, it works, but those examples are 63 and 26 times heavier than just using hydrogen as your reaction mass in the first place :)
- You'd need to account the mass of the copper-63 waste, in addition to the electron
- This math is specifically about thermal propulsion; your 17 keV electron wasn't in thermodynamic equilibrium with anything, it's a different sort of problem
That's not quite what I'm picturing (and let's be honest, the idea is silly). That page describes capturing the heat of a nuclear reactor, converting it to electricity, and then using that electricity to force ions out of a thruster. I'm saying, the radiation itself has momentum; don't bother with the contraption.
I need to play Kerbel Space Program. I have absolutely no intuition about this sort of thing. Some intuition around orbital mechanics from my physics days but rocketry is a bit of a mystery.
For me it helped to remember that the term that matters in the rocket equation is exhaust velocity, not momentum. You want to be shooting the lightest things you can out the back of the rocket, at the highest speed.
Unfortunately with chemical rockets, the energy source and reaction mass are the same thing, so you're kind of stuck with whatever burning your fuel gives you. But when you can separate reaction mass from energy source (like in nuclear or electric rockets), hydrogen is always the best bet.
Mostly true. However, there's tradeoffs qua thrust and specific impulse for different propellants. For pure vacuum work hydrogen is a good candidate. For operating in a gravity field (eg. the moon), you might be able to use a smaller nuclear engine with a denser propellant. Obviously you ARE trading in some range when doing this. But if the question is whether you can get off the ground in the first place, you may have less of a choice. It might also depend on what you can actually lay your hands on qua ISRU (if/when relevant)