Quote Originally Posted by Fat Rooster View Post
The main factor in this is what mass fraction of a propellant mix is oxidiser, which hydrogen wins out on handily. The high specific impulse is actually just a bonus.
Hydrogen loses quite badly in terms of bulk, though--they didn't use LH2 in the first stage of the Saturn V, where efficiency gains would likely have mattered the most in terms of total fuel usage, because the required fuel tank would have simply been too big. The point about oxidiser does raise a question, which is: what oxidiser are you most likely to be able to find on the Moon? I don't think we actually have an answer for that. If oxygen is in short supply, but nitrogen and hydrogen are relatively plentiful, then hydrazine becomes a much better option for your rocket fuel than LH2--it might be slightly less efficient, but because it's available in much larger quantities it offsets that. I don't think we'll have a final answer for that until we actually have a moon base set up where people can do mining operations to find out what's beneath the surface, which brings us back to a reason for building the thing in the first place!