Another big problem is leaking and boil-off. There's a reason that no space mission longer than days uses LH2 as its propellant; it just doesn't last long enou

Another big problem is leaking and boil-off. There's a reason that no space mission longer than days uses LH2 as its propellant; it just doesn't last long enough, because of boil-off caused by solar heating and the hydrogen actually leaking out through the fuel tank. Insulation isn't 100% effective and it's hot in space near the Sun where we are, certainly much hotter than the 20K boiling temperature of LH2. Even if the insulation is 99% effective that's still significant heat flow at a temperature higher than 20K, and you have boil-off. So, yeah, it's fine for uses in stages to orbit, and it was even fine to use for the Apollo missions because it was only 3 days there, but anything longer than that and it's increasingly not suitable. > In general it seems like density should matter less than mass Mass is already incorporated into Isp. So, for the same total mass of propellant being exhausted at the same velocity, you definitely would rather have the denser fuel so that your fuel tank can be smaller and thus lighter.