to 0.9, is shown in Figure IV-B-l-c-12. The orbital attitude of Figure IV-B-l-c-10a is assumed. This figure shows that for an average radiator temperature of 450°F (assuming radiator fluid enters at 550°F and leaves at 350°F), and an effectiveness of 0.8, the required area per unit of heat rejection is approximately 1.25 x 10-3ft2/Btu hr (one side radiating). This area can be decreased, though not by a direct factor of 2, by using a radiator which dissipates heat from two sides. The effect of surface degradation from solar absorptivity of 0.25 to 0.85 is insignificant at these radiating temperatures, 2.8 The Boeing Brayton System Concept 2.81 System Description Boeing has studied several system concepts including both the photovoltaic and Brayton cycle systems. The basic dynamic conversion concept studied by the Boeing Company involves the use of a closed Brayton cycle system, a solar collector-absorber system, and a space radiator in conjunction with the microwave sending and receiving systems. The system would be boosted into a low earth orbit (LEO), there to be partially assembled for the trip (using solar-powered electric thrusters) to geosynchronous earth orbit (GEO), where final assembly would be completed. Figure IV-B-l-c-13 illustrates the fully deployed system. The four modules each consist of a mainframe structure formed from fold-out trusses. This primary structure supports the reflector, absorber, conversion units, and radiator system. 2.811 Solar Concentrator The reflector, or concentrator, is made up of many individual flat facets, each driven by servoactuators mounted in the hub of each facet. Each facet is controlled about two axes, and control to within t 5° is required. A typical reflecting facet is shown in Figure IV-B-l-c-14. The plastic film reflector material is mounted on a tensioned stretch rack, the free edges of which are scalloped to provide a smoother surface. 2.812 Cavity Absorber The approximately spherical cavity absorber is the highest-temperature component of the system. As such, its walls are covered with a 25 cm. thickness of alumina-silica insulation. This insulation is in turn covered with a titanium skin and stiffening frame. The inner walls are made of columbium and contain tubing for the Brayton cycle working fluid. The hottest temperature in the cavity is 1533K (2300F) and the fluid tube temperature varies from 980K (1300F) at the inlet to 1311K (1900F) at the outlet. In the event of a no-flow situation within the tubes, the facets are automatically turned away from the aperture. The four thermal loss sources of the absorber are re-reflection at the inner walls through the aperture, re-radiation through the aperture, backside radiation from the outer walls, and conduction through the support
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