4 ms·

Shouldn't that scale well in space as the shell of the H2 tank scales at the square whereas the volume scales at the cube? In general it seems like density shou

Shouldn't that scale well in space as the shell of the H2 tank scales at the square whereas the volume scales at the cube? In general it seems like density should matter less than mass, but I've never attempted to contain and accelerate large quantities of liquid H2. Is insulation that much of a problem in the vacuum of space? I would think you could just cover it in a reflective material and call it a day.

bryanlarsen 6y ago
Yes, I've been assuming that we're not getting our H2 from the bottom of a gravity well. If not, the trade-offs do change dramatically.
sudosysgen 6y ago
The mass-fraction actually doesn't scale like that. While yes if you just wanted to cover your H2 tank it would scale with the square of side length, it's not just about covering it but mostly about structural matters. If you were to keep the same wall thickness your rocket would not be structurally sound.
CydeWeys 6y ago
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.

bryanlarsen 6y ago

Yes, I've been assuming that we're not getting our H2 from the bottom of a gravity well. If not, the trade-offs do change dramatically.

sudosysgen 6y ago

The mass-fraction actually doesn't scale like that. While yes if you just wanted to cover your H2 tank it would scale with the square of side length, it's not just about covering it but mostly about structural matters. If you were to keep the same wall thickness your rocket would not be structurally sound.

CydeWeys 6y ago

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.