Concepts for the Liquefaction of Hydrogen for In-Situ Operations on the Lunar or Martian Surface

While oxygen will probably be the first fluid liquefied from in-situ developed propellants, hydrogen will also be a sought-after commodity on both the Lunar and Martian surfaces. General hydrogen liquefaction processes are not significantly affected by the atmosphere on Mars and similar processes can be applied on both surfaces. However, hydrogen will require different processes than oxygen (which can be directly liquefied into its final use tank) due to the different spin states the hydrogen nucleus (i.e. parahydrogen and orthohydrogen) can have as well as the significant sensible energy that has to be removed prior to the actual phase change. Due to the low efficiency of 20 K class cryocoolers and high sensible heat removal requirements at higher temperatures, significant benefits can be obtained by precooling the hydrogen gas in a variety of manners. The benefits of three concepts are explored to take advantage of different precooling technics as well as accounting for sensible, latent, and para to ortho conversion heating requirements. Process concepts are shown that can reduce the base input power requirements for heat removal by more than 67% and the radiator area by similar amounts. A majority of the technology development for these processes has already begun, both within NASA and externally, with the few gaps being identified.