serve as radiators they will have a relatively high operating temperature and will therefore be subject to a fair amount of thermal expansion. In addition, due to the occasional eclipsing of the SPS, there will be thermal cycling. This combination will result in stress on the secondary reflector as well as the SPS structure. This is expected to be one of the more severe problems to be dealt with in the mechanical design of the system. The design used here assumes that the primary reflector has constant thickness for ease of manufacturing. A tapered disk would be more efficient as a radiator, reducing total mass by over half. Members of the Advisory Committee believe that an effective means of manufacturing tapered primary reflectors could be found. Thus, the total mass estimated here is probably conservative by a factor of two. 2.2.4.5 Optimization The thickness of the parabolic dish was the limiting parameter for optimization of the concentrator size. System mass appeared to increase almost linearly with thickness, suggesting that thickness should be as small as practical. It was also necessary, however, to make the thickness large compared to that of the high emissivity coating. As a result the thickness was set at 0.25 mm (10 times that of the coating). With thickness fixed, the maximum concentration ratio (CP) at which the cell can be effectively cooled is a function of disk radius. The maximum CP was found to increase as disk radius decreased. High concentration ratio corresponds to low non-lunar mass, so a small disk size is preferred. Scanty data made it infeasible to optimize the solar cell efficiency with respect to concentration ratio and operating temperature. It was assumed that reasonable progress in technology could produce a GaAs cell 50 microns thick that would have an operating efficiency of 15 percent after all losses (see Table 2.1-2) at 470 K and with a concentration ratio in the 100 - 500 range. Hot spots within the solar cell were not considered in detail so an intermediate value was thought to be best for the concentration ratio. A CR of 260 was chosen as it was in the middle of the range and proved to be convenient for calculations. If the desired efficiency can not be achieved with this value a sub-optimum concentration ratio could be used. The mass of the solar cell is a very small fraction of the concentrator mass and if the concentration ratio was decreased by a factor of two or three the non- lunar mass would still be very good. Furthermore it would probably still be within the error bars of the values shown.
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