launch mass required in LEO would be 75,380 kg, using hydrogen as the fuel for the outgoing trip. If the propellants were produced on Mars, only 4200 kg would have to be landed, and the launch mass at LEO would be 10,900 kg. In the above examples where ISMU was used, the landed masses did not include the mass of the propellant production facility, and it was assumed that all propellant, oxidizer andfuel, was manufactured on Mars. In actuality, to produce methane fuel on Mars, hydrogen would most likely be imported from LEO. This would change the above numbers, but the launch mass savings would still be substantial (A mass savings of up to 80% in LEO could still be realized). While it may be appropriate to compare the complexity of ISMU to that of using a Mars orbit rendezvous without ISMU, the basic result for all reasonable assumptions is still that direct ascent ISMU is cost effective. Debating and selecting the most appropriate non-ISMU mission strategy to compare with ISMU was beyond the scope of this paper. The relative amounts of propellant required in various instances for a Mars sample return mission are shown in Figure 1. The bars labeled A show the amount of propellant needed to send the sample return vehicle to Mars, and to bring it back to Earth, if terrestrial hydrogen and oxygen were used for both legs. B shows the amount of propellant required to return a methane fueled vehicle from Mars if all the propellant is terrestrially derived. C shows how the mass is reduced if carbon
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