Figure 4 shows another approach to a rover design, where larger arrays are designed to be retractable during high-wind periods or when the rover is moving. An alternative approach to investigating large areas of the Mars surface, and particularly for scouting for long distances and over rough but interesting terrain, is to use an airborne rover. A solar-powered airplane designed for Mars by Tony Colozza at NASA Lewis Research Center [10], shown in figure 5, is designed to be deployed directly into the air and fly continuously, night and day. Unlike the rover, the airplane must have continuous power over the night. Solar panels on the wing, tail, and fuselage charge regenerative fuel cells to allow the plane to stay in the air under cruise power for night flight. With a design criterion of 100 kg of payload and assuming a 25% efficient solar panel, the aircraft had a total mass of 438 kg and a wingspan of 120 meters. For manned missions, two applications are power for life support and operations and power for resource processing (i.e., propellant manufacture). Once on the surface, the first priority will be to provide power. An example power system is discussed by McKissock, Kohout and Schmitz [11]. Their power cycle was to provide 40kW continuous power during the day, and a reduced power level of 20kW during the night. For the night storage capability, they assumed that the hydro- gen/oxygen regenerative fuel cell (RFC) has been developed to technology readiness. The RFC assumed pressurized gas storage. Cryogenic storage of reactants was determined to require too much equipment overhead to justify the slight advantages in storage volume. Roundtrip efficiency for the storage system was 61.5%. The power system is shown in figure 6. The flexible array is visible deployed on the ground behind the lander, while the regenerative fuel cell unit, with spherical pressure tanks for the hydrogen, oxygen, and water reactants, is visible in the foreground. A roll-out thermal radiator provides thermal management for the system. Sample Case: PV Blanket for the "Mars Direct" Scenario Power In-situ propellant generation on Mars is an option for drastically reducing the cost of Mars expeditions, which could be used for both manned missions and for unmanned precursor missions. One scenario for propellant generation on Mars (proposed by Robert Zubrin and David Baker [12, 13]) is to generate 107 tons of methane/oxygen propellant from 5.7 tons of hydrogen brought from Earth plus CO2 from the Martian atmosphere. The following analysis shows how the energy requirements for this processing could be met with a surface photovoltaic array.
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