Figure 1.1-1 compares total mass and non-lunar mass of the Boeing SPS design, the General Dynamics SPS design, and the Space Research Associates SPS design. Figure 1.1-2 compares the estimated cost per SPS for each design, based on the assumption that non-lunar material is fifty times more costly than lunar material. The cost units are equivalent to thousands of metric tons of lunar material. Growth and contingency analyses were outside the scope of this study, so the mass estimate for the SRA design assumes the same growth factor, 26.72, as the Boeing design. FIGURE 1.1-2, ESTIMATED SPS COST COMPARISON An alternative to silicon cells is to use gallium arsenide cells with solar concentrators. This roughly triples total SPS mass, but non-lunar mass remains less than two percent of that for the Earth baseline. Four other pox^r conversion systems were investigated: thermophotovoltaic (TPV), Brayton, Rankine, and Stirling. These were found to use significantly more non-lunar material than either the silicon system or the gallium arsenide system. It appears that a lens made of aluminum mesh could economically increase the effective aperture of the SPS antenna. If this proves to be correct, the lower limit on SPS power could be as low as 200 MW per satellite. This is an area for further study.
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