Mars Return Mission Based on In Situ Propellant Production with either Cryogenic or Storable Imported Fuel

An investigation is carried out about the different options available for a return mission to Mars envisaging, for the return leg, the use of propellant produced, in whole or in part, on the planet surface. Of particular interest is the option of importing part of the propellants from Earth, in order to achieve greater performance. However, the solution considered so far in the literature focuses on liquid hydrogen (LH) as imported propellant. LH can be used as such in combination with liquid oxygen (LOX) processed from the Martian atmosphere, or otherwise transformed to liquid methane (LCH4) to get a more favourable payload ratio. However, LH clearly poses serious storage problems in relation to the long duration of the mission. Moving on these grounds, in the present paper an alternative is investigated, based on importing a storable propellant, i.e., monomethylhydrazine (MMH), to be burned in combination with LOX. The comparative performance are evaluated first for a sample return mission, and then on a more general basis as a function of the payload mass to be injected into the return trajectory to Earth. It is found that the proposed solution gives performance only moderately worse with respect to the LOX/LCH4 and LOX/LH combinations, but with the remarkable advantage of avoiding all problems related to long–term storage of LH.