Any small increase in effective emissivity from a cavity radiator would be more than offset by the greater system mass required. These results, therefore, suggest a flat radiator coplanar with the collector. The heat that needs to be rejected by a 24 percent efficient converter is 3800 W of which only 510 W will be radiated away by the collector surface. Consequently, 3290 W must be transported by the cooling fins. Considering a thin fin with a maximum area of contact between the collector and fin of 20 cm2 (3.06 in2), the associated temperature gradient at that boundary for the thermal conductivity of nickel at 0.55 W/cm-C (see Figure 4-20) is Fig. 4-22. Isometric Cutaway of SPS Thermionic Converter Fig. 4-21. SPS Thermionic Converter Design Fig. 4-20. Thermal Conductivity Data The alternative approach is to employ a heat-pipe concept to provide a uniform radiator temperature and, thereby, achieve uniform heat rejection temperature and high fin efficiency. A number of configurations were considered; the one judged best is shown in cross section on Figure 4-21. Sizing of the emitter is determined by loss of material from thermal vaporization over its thirtyyear lifetime, minimization of resistive power losses, and weight considerations; sizing of the collector depends on resistive and weight considerations. Figure 4-22 presents an isometric cutaway view of this design showing the main features of the converter. which results in an extremely nonuniform temperature of the fin surface (i.e., poor fin efficiency) and, therefore, would require too massive a radiator system. Table 4-3. Heat Pipe Figure of Merit
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