system is much like the chemical system, except that the NTR stage continues to Mars and is used for trans-Earth injection following Mars exploration completion. This allows the MPV propulsion system to be smaller and also allows reuse of the NTR stage. Trip time for the NTR system is comparable to the chemical system, as both systems require similar AV's for Mars transfer. A candidate configuration for the NTR manned Mars exploration option is shown in Fig. 4, and a sequential performance summary is given in Table V. The other TMIS propulsion option, NEP, requires higher AV's and much longer trip times, but has a much higher specific impulse. For this option, all orbit capture maneuvers of the main stage with MPV are done all-propulsively, so the MPV does not require a propulsion system or an aerobrake. A candidate configuration for the NEP manned Mars exploration option is shown in Fig. 5, and a sequential performance summary is given in Table VI. The vehicles for this option are all-propulsive and are fully reusable. Note that the NEP vehicle is unmanned during the spiral out from LEO, with the crew arriving by STV ‘taxi' just before the NEP reaches Earth escape. For an unmanned cargo mission to Phobos (propellant production plant delivery), the LEO-assembled vehicle consists of the TMIS, a Phobos cargo delivery stage, and a Mars Cargo Lander with payload. The TMIS provides the Earth escape delta-V, and the cargo stages provide for Mars capture (aerobraked) and rendezvous/landing. In this case the Phobos cargo is an 86 metric ton propellant production plant and the Mars surface cargo is a 45 metric ton logistics payload. A sequential performance summary is given in Table VIL The Block II Phobos Cargo Vehicle, shown in Fig. 6, will include NEP to reduce the costs of supporting Mars Base. The configuration shown generates 5 MW of electrical power and uses argon ion thrusters. A properly designed NEP core unit could be taken to the surface of Mars and converted for ground power after its transportation duties are done. 4 Payload Mass Sensitivities Using the vehicles described above in Section 3, payload mass ratios were calculated for both the lunar and Mars evolutionary missions. These ratios include orbit-
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