of these members are the truss beams connecting the solar concentrators to the solar absorbers. Ideal cross sections were derived to provide a minimum sum of beam mass and generator penalty. A family of curves was derived for beams configured as shown in Figure 4-10. The spacing between the tubes, the tube diameters and thicknesses was varied, and mass per beam length plotted against beam length for given load. The dotted line is an estimate of the locus of minimum mass. However, since the tubes of the beam are designed to carry current and heat loss (I^R) has to be dissipated, there is a minimum cross section of the beam capable of carrying both the current and the applied load. This is indicated in Figure 4-10 for a typical SPS truss. 25.4 M (83.3 ft.) long are inserted into the clamps at the ends of the diagonals. The sections of tube are welded together and to the clamps where they butt, and the snatch clamps are secured to the tubes. 4.4 CAVITY SOLAR ABSORBER Solar heat flux from the solar concentrator is reflected into the cavity absorber. The cavity is a spherical structure with an aperture for receiving solar radiation as shown by Figure 4-12. (A cylindrical absorber is used for the thermionic SPS.) Fig. 4-10. Derivation of Ideal Beam Dimensions Typical primary structure (trusses) of the SPS consists of three tubes equispaced as shown in Figure 4-11. The tubes are supported by diagonals which are hinged together. Since the tubes carry the primary satellite power the diagonals are insulated as shown. Prior to assembly in low earth orbit the diagonals are folded together tightly. On assembly the diagonals are unfolded and tubes Fig. 4-12. Cavity Solar Absorber Solar energy flux into a cavity absorber is for the most part absorbed into the walls. This is because multiple reflections must in general take place before reflection back out of the aperture can occur. Once absorbed, the energy is available for removal by the energy converter (Brayton cycle or thermionics). The hot walls radiate thermal energy back and forth between them; some of this energy escapes through the aperture. Insulation and a low emissivity exterior coating are used to limit energy loss through the walls. Thermal energy loss by radiation is influenced by the emissivity of the surface and the fourth power of its absolute temperature. Thermal engine efficiency requires high cavity temperatures, therefore reradiation must be controlled if cavity efficiency is to be high. The loss by reradiation is a function of the cavity aperture area. All energy passing through the walls must eventually be reradiated from the cavity exterior. Therefore, a low emissivity coating (gold is baselined) is used. To provide a low exterior temperature, thermal insulation is provided. Fig 4-11. Typical Power Satellite Conducting Primary Structure (Example, Size Varies)
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