Hydrogen/Oxygen Liquefaction Subsystem After leaving the drier, the hydrogen and oxygen streams enter liquefaction units. A reversed Brayton cycle is used to liquefy the gases. A diagram of the cycle is shown in Fig. 6. The equipment for the liquefaction of the hydrogen and oxygen streams is similar, except that two refrigeration stages are required for hydrogen due to the extremely low temperature that must be achieved. Approximately 15 kW are removed from the hydrogen stream at an efficiency of 9.5%; 9 kW are removed from the oxygen stream at an efficiency of 12.5%. The liquefaction units are also used to liquefy boiloff from the storage tanks [4], The masses and efficiencies for the 250 kWe hydrogen and oxygen liquefaction subsystems were taken directly from Ref. [4], To determine the mass of the liquefaction subsystems for the 20 kWe system, the 250 kWe subsystem masses were scaled down based on specific power requirements given in the reference along with the mass flows of hydrogen and oxygen to the respective liquefaction units. It should be noted that the in-space liqeufaction technology assumed for this study is based on long-range projections. Current technology addresses cooling requirements on the order of a few watts. Considerable development would be needed to achieve the system described in this study. Reactant Storage Tanks The design of the cryogenic hydrogen and oxygen storage tanks is based on a Beechcroft design [5] as shown in Fig. 7. The tanks consist of a spherical aluminum
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