Thus, prior to boil-off, the initial hydrogen loading amounts to 1,250/1,540 = 80% (FI = 0.8) of the total mass according to this model.
Other references126 provide much less optimistic estimates of hydrogen tank mass requirements, but they do not provide enough data to check the details. Another reference127 provides rough estimates of the tank mass to store liquid hydrogen for short periods appropriate for Earth orbit transfers. However, no details were provided on insulation. An aluminum tank to store 5 to 10 mT of hydrogen was estimated to weigh about 10% of the hydrogen mass, so the hydrogen would be about 90% of the total mass excluding insulation.
It appears from such approximate estimates that a rough rule of thumb is that one can store liquid hydrogen in a tank with a mass distribution of roughly 20% tank mass, 75% usable hydrogen mass, and 5% ullage and residual hydrogen (FI ~ 0.75). This does not include other mass effects such as the need for shields, or implications
125 An Updated Zero Boil-Off Cryogenic Propellant Storage Analysis Applied to Upper Stages or Depots in an LEO Environment, David Plachta and Peter Kittel, NASA/TM2003-211691, June 2003, AIAA20023589.
126 "Simulations of Zero Boil-Off in a Cryogenic Storage Tank,'' Charles H. Panzarella and Mohammad Kassemi, 4lst Aerospace Sciences Meeting and Exhibition, January 6-9, 2003, Reno, NV, AIAA 2003-1159.
127 "Evaluation of Supercritical Cryogen Storage and Transfer Systems for Future NASA Missions,'' Hugh Arif, John C. Aydelott, and David 1. Chato, 28th AIAA Aerospace Sciences Meeting, Reno, NV, January 1990, AIAA 90-0719.
for spacecraft structure induced by inclusion of cryogenic tanks. For short-term applications such as lunar transfers, these requirements might be minimal. For long-term applications such as Mars transfers, they could be very significant. For short-term applications, hydrogen storage would appear to be quite efficient mass-wise, and therefore the question of viability for various space applications of various durations will depend on the rate of heat leak into the tank, resulting in boil-off over the duration of the mission. Alternatively, one could possibly use active refrigeration to remove heat from the tank at the rate that heat leaks in, resulting in "zero boil-off" (ZBO). However, the mass and power requirements for ZBO, and issues related to reliability are complex. If ZBO is used, the rate of heat leak will determine the capacity of the required cryocooler system, which is likely to be large in many instances.
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