The module size shown is approximate to the use of four modules per 10 GW ground output SPS. The appropriate number of modules to use was determined by mass optimization. If the number of reflector facets is held constant (to keep a fixed concentrator efficiency) then the concentrator becomes proportionately heavier as a module is made smaller, since each facet still requires a pointing system and a support frame. As a module is made larger, the radiator system becomes proportionately heavier since addition of radiator area requires more manifold (and NaK) mass for a large module than for a smaller one (since the manifold lines must be longer to reach the added area in a larger module). These two effects act in opposite directions, allowing an optimization to be affected, as shown in Figure 5-13. Since the quantity of four, used in previous studies, is very close to the optimum of six, four modules were baselined. The resultant SPS system is illustrated in Figure 5-14. The four modules are arranged along the north-south axis which, in operation, lies parallel to the north-south axis of the earth. Thus the satellite flies "perpendicular to the orbit plane." Note that the four radiators are each inclined by 11.75°. Power distribution from the four cavity absorber assemblies to the transmitter takes place down the central "spine." The total solar capture area of the system is approximately 62 km^ (24 square miles). Fig. 5-14. Brayton SPS Configuration Table 5-5 is a mass statement for the Brayton SPS. Fig. 5-13. Module Quantity Optimization Operating temperatures for the Brayton System were set by use of an ISAIAH model. This model contained 93 independent and 32 dependent variables, all of which were simultaneously interacted to obtain an optimum. Significant resulting parameters are given in Table 5-4. Table 5-4. Brayton SPS Parameters
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