configuration to a large area, lightweight, reflecting structure having a weight - to-area ratio comparable to the SPS that has been flown was the ECHO II 41 m diameter, gas pressure inflated, rigidized sphere. It has been suggested that a better low curvature reflector, as required for SPS, would be one made up of a composite, laid up on a mold on the ground, and impregnated with an ultraviolet curing epoxy. The structure would be gas pressure deployed and would be allowed to cure in the ultraviolet space environment. One proposal is to create the required concentration by overlapping the images from thousands of flat facets. Construction of the individual facets can be simpler than construction of the double curved reflectors usually used. A large number of facets are required for a high concentration ratio. Large module sizes are desirable to reduce the total number of facets required to collect a given quantity of energy. This concept does not scale readily to small power module sizes. 7.2.8.2 ABSORBER The absorber required for the solar thermal version of SPS has not been developed beyond small scale (small aperture) test modules. A solar absorbing cavity was tested as part of a solar Brayton program in the 1960's [20]. This program did not progress beyond the component test phase. Present efforts to develop large solar absorbing cavities are concentrated on ground solar power applications. The operating temperatures of the ground systems will be lower than those required for SPS, but the analytical tools developed will be applicable to the space system analysis. Small laboratory models of light pipes have been built and used to provide uniform illumination in laboratory testing [21—23]. However, alight pipe, with the selective surface properties required for SPS, has never been constructed. 7. 2. 8. 3 THERMIONIC-BRAYTON CONVERSION SYSTEM The conversion system equipment technology required for this system is similar to that required in other space and ground power systems. Consequently, a reasonably consistent technology development program already exists for the basic components. Thermionic diodes for use with a reactor source and Brayton equipment for use with isotope or reactor sources have been under development for several decades. Extensions from this base required for SPS encompass increasing the operating temperature and efficiency of the Brayton equipment and increasing the efficiency of the thermionic doides. The diodes are restricted from using tungsten for the emitters because of a limitation of available tungsten, and molybdenum is being substituted for SPS use.
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