I IlVIt (IlVUrAO/ Fig. 4-71. Radiator Fluid Temperature During Occulation causing "water hammer" and "chugging" (possibly destructive) at startup. Since the analysis was conducted, a ternary eutectic alloy of sodium, potassium and cesium has been proposed which has a lower freezing temperature, 197K (-105°F), than NaK. 4.10.6 Heat Pipe/Fin Radiator 4.10.6.1 Panel Design Analysis and Modeling The mass optimized tube/fin radiator with pumped manifolds has a number of inherent drawbacks: • Vulnerability of tubes to meteoroid puncture. • Flexing of panels due to differential expansion of input and output headers. • Freezing of NaK in tubes during occultation. Due to the number of panels required in the tube/fin radiator design, and transporter space limitations many thousands of welds will have to be performed in orbit. Figure 4-72 shows that 14 Fig. 4-72. Radiator Welds Performed in Orbit welds will be required for each 20m x 20m (65.6 ft x 65.6 ft) area. This includes joining the header sections together. Because of the inherent drawbacks of the tube/fin pumped manifold radiator a design analysis of a radiator employing heat pipes was performed. Extensive study of heat pipes has been conducted during the last ten years. Although terrestrial applications have been somewhat limited, heat pipe systems have been proposed for several space applications because of the capability for transporting large amounts of heat in a light system. The theory of heat pipes is fairly well established and criteria for heat pipe design are available. Some advantages of heat pipe radiators over tube/fin radiators are: • Less vulnerable to meteoroid puncture • Can be designed to avoid differential expansion of members • Self starting Each of these items is discussed in this section. 4.10.6.2 Heat Pipe Concept The basic heat pipe is a closed container consisting of a capillary wick structure and a small amount of vaporizable fluid. The heat pipe employs a boilingcondensing cycle with the capillary wick pumping the condensate to the evaporator as shown in Figure 4-73. The vapor pressure drop between the evaporator and the condenser is very small, therefore the boiling-condensing cycle is essentially an isothermal process. Fig. 4-73. Heat Pipe Concept Four typical heat pipe cross section options are shown in Figure 4-74. The first is a simple thin walled round tube with a screen wick. The second
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