modules are attached. The modular system permits utilization of the self power concept, wherein the modules are fabricated in low orbit and are flown to geosynchronous orbit by electric thrusters energized by the output of the module itself. In this orbit transfer mode, only 20% of the cells are in place (the remaining 80% are in radiation shielded canisters). The exposed cells lose 55% of their output capability as a result of exposure to the trapped radiation environment. Figure 5-22 gives the dimensions of a single power module. The basic power element is a one megawatt strip 380 M (1247 ft.) long and 20 M (65.6 ft.) wide. This module is supported on its own frame, including an interface (or "docking") pad by which it connects to the main frame of the satellite. Ideally, the compound parabolic concentrators need be only thick enough to support themselves against installation loads (perhaps 2 micrometers, i.e., 0.0008 inch thick). A possible means of fabrication might be vapor deposition upon a smooth mandril, turning out many CPC's at once. However, the capability to do this is not assured. Therefore a conventional fabrication method, impact extrusion, was baselined. The thickness was set at 75 micrometers (0.003 inch). Consequently the concentrators are the largest item in the mass table (Table 5-5). Table 5-5. Silicon Photovoltaic SPS Mass Statement 5.7 SOLAR GALLIUM ARSENIDE PHOTOVOLTAIC (CONCEPT 6) As with the silicon system, an optimization was performed to determine the best concentration ratio. For this optimization the cost of GaAs arrays was considered to be the same for identical construction volumes as silicon arrays. Due to the better temperature coefficient of GaAs as compared to silicon, the resultant concentration ratio was 7.5 (see Figure 5-16). As with silicon, the minimum cell thickness was taken as 100 micrometers (0.004 inch). Due to the higher concentration ratio, the compound parabolic concentrators are larger for a given cell size than with silicon. This is illustrated in Figure 5-24. The lower radiation damage rate of gallium arsenide cells (only approximately one third that of silicon) means not as much array must be added to the SPS over its life to maintain the required 10 GW output. Fig. 5-22. Silicon SPS Power Module The solar arrays of the power module are as shown in Figure 5-23. The total silicon array thickness is 410 micrometers (0.01614 inches). In this figure, thickness (relative to width) has been multiplied by a factor of 100 for visibility. A cell thickness of 100 micrometers (0.004 inch) was baselined as the minimum that could with assurance be considered practical. Figure 5-23. Photovoltaic Array
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