sodium as the best fluid. In the absence of oxygen this element is compatible with a structure made of nickel. The heat transfer of sodium at the limiting sonic velocity flow where 71 = 3.18 cm (10~3) (for a mesh size of 400), q0 is the required heat transfer, leff is effective path length, Ay/ is the area of wick required, and K is determined from the Blake Koseny equation where E is the volume fraction of solid phase of the wick (0.314), and d^ is the wick's wire diameter (0.025 mm). Solving equations 16 and 17 for A\y provides the required wick areas shown in Table 4-4. Also presented in this table is the wick thickness, t, which is determined from the wick area and circumference of the collector. Solium is seen to be the optimum working fluid. Design of the heat pipe nickel housing was based on minimizing the mass for the total radiating area required (Figure 4-19) and taking into account the structural strength necessary to contain the heated sodium vapor. Because of the large mass penalty which exists if such a structure must tolerate atmospheric pressure loading, it was decided to include a pinch-off port in the heat pipe to allow equalization of pressure during assembly and launch. Once in orbit the heat pipes are charged with the required amount of sodium, and the opening is pinched off. Under this mode of assembly the heat pipe internal pressure is approximately 0.89N/cm2 (1.29 psi). A wall thickness of 1 mm (0.0254 in) was judged sufficient to tolerate this loading. Thin-plate analyses indicated that maximum deflection for a 1 mm thick wall was 0.7 cm (1.78 in). To reduce such distortion, small, light struts were included, as shown in Figure 4-21. Although the electrodes are circular in design, the heat pipe has been configured hexagonally in order to optimize the use of the satellite's surface area. A thin, ten-layer, covering of MULTI-FOIL thermal insulation (Figure 4-23) protects both the Fig. 4-23 Multifoil Thermal Insulation Table 4-4. Heat Pipe Wick Dimensions electrical busbars, as well as the heat pipe from direct solar radiation. MULTI-FOIL insulation, developed at Thermo Electron Corporation, consists of a number of layers of thin refractory metal foils spaced in a vacuum by oxide particles. The oxide is selected on the basis of low thermal conductivity and foil compatibility, and the particles are optimized with regard to size and coating
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