lunar cargo transit stage is given in Table II. The other lunar transfer propulsion option is nuclear electric propulsion. NEP requires higher delta-V's and much longer trip times, but has a much higher specific impulse. The vehicles for this option are all- propulsive and are fully reusable. One possible configuration for the NEP lunar cargo transit stage is shown in Fig. 1, and a sequential performance summary is given in Table III. Trans-Mars Injection Stage (TMIS) The Trans-Mars Injection Stage is capable of delivery of cargo or piloted vehicle, including trans-Earth injection propellants, to Mars orbit. The baseline propulsion system is LO2/LH2, with other options including NTRs, and nuclear electric propulsion NEP. Comparisons of the mass required in Low Earth Orbit (LEO) with each of these options are shown in Fig. 2. Note how increased trip time can be traded for reduced initial mission mass, and the launch-to-orbit mass savings available with electric propulsion. Data such as this has convinced NASA to move away from split sprint (440 day) missions, towards conjunction class manned missions with NEP cargo deliveries. The baseline LO2/LH2 TMIS is an expendable system that consists of a propulsion stage and modular tanksets for trans-Mars insertion propellant. For a manned Mars surface exploration mission, the LEO-assembled Mars vehicle consists of the TMIS, the MPV, a Mars Crew Sortie Vehicle (MCSV), and a Mars Cargo Lander (MCL) with a 45 metric ton payload (Mars surface habitat and exploration equipment). Following Earth escape, the TMIS is expended, and the other elements separate for the transfer to Mars. During Mars transfer, 1 g artificial gravity is maintained by use of a tether system between the piloted vehicle and Earth return tankage and propulsion system (see Fig. 3). Prior to Mars orbit insertion, the system is de-spun, the vehicle and propulsion module re-attach, and the vehicle performs an
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