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  • Liquid hydrogen is $0.071 g/cm^3$
  • Solid hydrogen is $0.086 g/cm^3$

Since denser fuel is desirable, has solid hydrogen been considered for use in space exploration?

I suspect the very low temperature (hydrogen melting point is 14K) to be impossible to manage.

Maybe hydrogen ice cubes in the tanks would help reduce boil off?

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The melting point of hydrogen isn't much lower than its boiling point (6K), so the temperature isn't necessarily that much of an obstacle. However, using solid fuel requires either melting it, or burning it in place. Trying to burn solid hydrogen would likely result in the whole thing flash-boiling from the radiated heat, so that's a bit of a non-starter. And having to melt fuel out of the tank is a serious problem.

However, Wikipedia notes that there are ideas to use hydrogen slush for fuel, as that will, indeed, increase density a bit. It should also reduce boil-off as long as at least some of the tank is emptied early on; otherwise, the decreasing density of the melting slush is guaranteed to require more space or a much higher-pressure tank.

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  • $\begingroup$ But hydrogen slush should be compatible to valves, turbo pumps and injectors. A high-pressure tank would be much heavier. If there is a extra space in the tank for melting slush, you could use liquid hydrogen only. $\endgroup$ – Uwe Dec 21 '16 at 12:11
  • $\begingroup$ @Uwe: Yes. Point is, slush only saves space if you use some of it quickly. Otherwise it's not all that much better. $\endgroup$ – Nathan Tuggy Dec 21 '16 at 19:59
  • $\begingroup$ If the slush is generated from liquid hydrogen within the tanks of the rocket, vacuum compatible tanks would be necessary and stirring unit in the tank. But these tanks would be very heavy. If the slush is generated within the hydrogen storage tanks it must be pumped through filling pipes and hoses. $\endgroup$ – Uwe Dec 23 '16 at 12:33

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