These results lead to an unexpected conclusion: that the less efficient (e.g. lower I ) propellants can justify larger factories. This apparent contradiction can be understood as follows. In the hydrogen versus methane case, hydrogen is such a higher performance fuel that the penalty of importing it is much lower than of importing methane, so the benefit of producing it locally is also much lower than that of producing methane. The question is whether it is practical to store liquid hydrogen for fuel use on Mars, which is not addressed in this study. In the case of CO, oxygen is a much lower mass fraction of the propellant. Also note that the methane system requires the least amount of hydrogen. Hence, if terrestrial hydrogen is used, it may be better to use methane rather than hydrogen fuel for the return portion of the mission, even if hydrogen could easily be stored on the surface of Mars. The effect of using Martian water as a means to provide hydrogen locally instead of importing it from Earth is also presented in Table 4. Note that allowable specific plant mass on the basis of oxygen output actually decreases, despite more of the propellant being Mars derived. This indicates that a larger amount of Martian feedstock must be collected, and a larger amount of energy must be invested in it to make the required propellant. This is because excess oxygen is being produced in order to obtain the necessary hydrogen from the Martian water (this need for extra O2 production is stoichiometrically determined by the composition of water). Thus, slightly better theoretical factory performance would be needed to take economic advantage of Martian water. It is important to note, however, that the different chemistries of these processes make a direct evaluation on the basis of required specific output impossible. The numbers in parentheses in Table 4 represent the actual stoichiometrically required production of oxidizer (or fuel), often somewhat in excess of propellant requirements if the simplest possible process is used without regards to producing the desired O/F ratio. Component Description And Power Requirements All Mars processing plants discussed below require certain necessary components for operation. Specific engineering factors regarding some of these elements are discussed here. Power supply systems with adequate capability are covered elsewhere in the literature [1,3], as are heat rejection systems [11]. These components will not be detailed further here. In order to ensure reliable plant operation, dust and any particulates must be removed from the incoming CO2 stream. The filter used here is a membrane type as described by Frisbee [12]. The pressure drop across the filter is assumed to be small (verification of this assumption is beyond the scope of the present work), and the outgoing carbon dioxide pressure is assumed to be 6 mbar in the worst case.
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