4 ms·
For specific impulse. When you have unlimited power, you want the lightest possible molecular mass exhaust for the best total delta V.
by CarVac 3y ago
For specific impulse. When you have unlimited power, you want the lightest possible molecular mass exhaust for the best total delta V.
- andrewla 3y agoI 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.
- idlewords 3y agoFor 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.
- Kim_Bruning 3y agoMostly 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)
- Kim_Bruning 3y agoYes, absolutely: https://xkcd.com/1356/ https://xkcd.com/1356/
- matheusmoreira 3y agoDo it! You'll know you got it when you're maneuvering the rocket on pure intuition without even using maneuver nodes. Just steer clear of the planet Eve. It's been years and I still haven't been able to leave it.
- klyrs 3y agoSo, since beta radiation is even lower mass... just put a block of nickel-63 into a magnetic lens?
- abecedarius 3y agohttps://en.wikipedia.org/wiki/Nuclear_electric_rocket https://en.wikipedia.org/wiki/Nuclear_electric_rocket
- klyrs 3y agoThat'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.
- abecedarius 3y agoYes, just thought you might not know of the related more practical version, and you might be pleased to encounter it.
- deleted 3y ago[deleted]
- ben_w 3y agoShooting 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
- klyrs 3y ago