Fig. 4-51. Radiator Heat Rejection Helium Fluid Fig. 4-52. Radiator Pane! Mass Helium Fluid Table 4-27. Optimum Configurations, Helium Radiator Fluid Figure 4-53 shows that a much greater portion of the total radiator mass is allocated to the panels with the low temperature "A" radiator system. This results from the substantially greater radiating area required with the low temperature system since heat rejection is proportional to the fourth power of the absolute surface temperature. Fig. 4-53. Radiator Mass Distribution (Helium) Table 4-28 shows the relative mass of radiators designed for the year 1990 and year 2000 powersats. Table 4-28. Masses of High and Low Temperature Helium Radiators Substantial reduction in radiator surface area and panel mass results with "B" (high temperature) components due to the higher operating temperature. A lesser mass reduction occurs in the manifolds of the high temperature configuration, because, although the headers are shorter, greater wall thickness is necessary due to lower allowable stresses. A trade study was conducted to compare a gaseous helium radiator concept with a liquid NaK radiator. The use of liquid NaK will require an additional gas-liquid heat exchanger and a circulating pump. Figure 4-54 shows flow diagrams for the two systems. Pressure drop in the helium loop will be reduced with the NaK system with a resultant improvement in engine efficiency and the denser fluid allows smaller headers.
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