[idlewords]

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.

[Kim_Bruning]

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)