Fig. 4-83. Occultation Effects: Heat Pipe/Fin Radiator masses for fluid inlet temperatures of 644K (700°F), 820K (1016°F) and 1000K (1340°F). Figure 4-82 shows the basic system configuration used in the analyses. The heat pipe/fin panels shown in Figure 4-82 are each 20m x 20m (65.6 ft x 65.6 ft) with a central NaK manifold carrying thermal energy from the heat source. Analyses were first performed to determine whether one or a greater number of NaK carrying manifolds would provide minimum system mass. Factors considered were NaK mass flow, system pressure drop, manifold mass, manifold material stress allowable, pump mass and SPS mass penalty for the pump power delta. (The pump mass was assumed to be 0.36 Kg/Kw (0.79 Ibm/Kw) and the SPS penalty, 4 Kg/Kw (8.82 lbm/Kw).) No penalty was assessed for additional welds required in space for increasing number of manifolds per panel. However, allowance was made for radiator fin area lost to increasing number of manifolds. Table 4-31 shows the results of the analyses (optimum systems). Table 4-31. Number of Manifolds Per Panel to Provide Minimum Panel Mass (Constant Section Manifolds) Analyses of the three systems was then expanded to include the heat pipes, to determine if the optimum for the number of manifolds per panel is also the optimum for heat pipe mass; i.e., would provide minimum radiator system mass. The results show that this is so. The reason for this is a basic characteristic of the heat pipe—the heat transport capability factor, QL. This factor is a function of the heat flow, Q (watts) and the effective length (L) of the heat pipe. For the radiator with a fluid inlet temperature of 644K (700°F) each 20m x 20m (65.6 ft x 65.6 ft) panel is required to radiate 3100 Kw. QL for a 25.4 mm (1") diameter heat pipe operating in this temperature range is approximately 6000 WM (19680 W ft). For a panel with one manifold, there are two rows of heat pipes, one on each side. The effective length of each heat pipe is approximately 7.5 m (24.6 ft); therefore Q is 0.8 Kw. The number of heat pipes required is thus 1938 per row. For a heat pipe 25.4 mm dia. it is possible to have only 787 heat pipes across a 20m panel. Thus the design with one manifold per panel is not feasible without increasing the heat pipe capability or using more panels, a considerable weight penalty. If, however, the number of manifolds per panel is increased to 5 for the same radiator, there are 10 rows of heat pipes and the effective length of the heat pipe is approximately 1.5m (4.92 ft). Therefore Q is 4 Kw. Only 78 heat pipes per row are required, spaced 256 mm (10") apart. The heat pipes are joined together with fins 0.5 mm (.02") thick. Radiators for each of the fluid inlet temperatures have been analyzed using the model described above to obtain minimum radiator system masses. The system masses are shown in Table 4-32. Table 4-32. Heat Pipe/Fin Radiator Mass (Constant Section Manifolds) The system masses for the heat pipe/fin radiators with constant section manifolds are considerably greater than for tube/fin radiators with the same fluid inlet temperatures and radiator power. Figure 4-84 compares the system masses of the two designs in graphic form.
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