Changes in the concentration schemes and concentration ratios have been studied. Preliminary analyses indicate that the overall mass of the SPS could be reduced significantly by using the reflectors as passive radiators for the trough concentration concept. This would require using blanket widths of approximately 1 cm to dissipate the heat rather than the current width of 200 m. There are still some areas where analysis is incomplete. The impact of adding secondary structure to support the small width troughs is currently under investigation. The effect on orbital assembly operations and the impact for orientation perpendicular to the orbital plane has yet to be completed. There are several other concentration schemes also under investigation, some of which are parabolic and four-sided reflector configurations. However, the impact of X-POP orientation on the concepts has not been completed. One of the original baseline concepts studied included one 10 GW antenna. It was noted that this resulted in large microwave power density levels of 50 to 60 mW/cm2 near the rectenna. The power density level at which microwave beam ionosphere interactions are expected to occur (approximately 20 mW/cm2) was then used as an upper limit in the design of the microwave system. This limited the output power level at the power interface to approximately 5 GW for each antenna. Recent analyses have been conducted comparing the mass of two 5 GW photovoltaic SPS's to that of one 10 GW SPS if both are equipped with 5 GW antennas. This comparison assumed that the antenna and rectenna designs were identical for both power levels and each 5 GW SPS had one center-mounted antenna and the 10 GW SPS had two center-mounted antennas, each pointing to a different rectenna. The dry weight of the 10 GW system with two antennas was 7 percent heavier primarily because of the de power distribution efficiency losses. In addition, the attitude control propellant requirements for the 10 GW SPS were approximately twice that of the two 5 GW SPST s. The result of this analysis, which indicated some merit for the 5 GW concept over the 10 GW concept, should not be interpreted as assuming that four 2. 5 GW or three 3. 3 GW SPS's would be better than two 5 GW SPS's. The additional antenna and rectenna cost would probably affect any additional savings in power distribution efficiency and attitude control propellant. However, there are plans to study various output power levels to determine the best design point. Also, the limitation of available space in synchronous orbit and its impact on the upper bounds of total program output for various SPS power levels is to be studied. In some of the completed sensitivity analyses it was noted that the total SPS mass could be reduced by approximately 12 percent if the de power distribution voltage was increased from 20 to 40 kV. The impact on the amplitron design and plasma interactions on the solar array are under study. Also, the previously described comparison of 5 to 10 GW SPS's might change if 40 kV rather than 20 kV was used for power distribution.
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