header. Contact between the NaK and the heat pipe evaporator imposes a design problem which tends to limit the overall heat pipe length. Figure 4-80 illustrates a significant heat pipe limitation. "DESIGN A" shows heat pipes radiating in a single plane from a heat source. This plane will be coincident with the ecliptic plane to avoid direct sunlight and to minimize damage from meteoroids. The radiating area in this design is limited by the heat pipe length "L" and is thus limited to approximately 27rL^. In "DESIGN B" a much larger radiating area is possible since dimension "X" can be made as large as required. The limit to this design will probably be when the pumping power required and the mass of the fluid and hardware outweigh the gain in radiating capability. Thus, the energy to be dissipated by heat source in "DESIGN A" is limited by heat pipe length; this is not true of "DESIGN B." Fig. 4-81. Radiator: Pumped Manifolds/Heat Pipe/ Fin Fig. 4-82. Heat Pipe/Fin Radiator Panel Concept 4.10.6.7 Occultation Effects: Heat Pipe/Fin Radiator During occultation the temperature of the NaK in the header will drop (flow has ceased). The heat pipe temperature will also have dropped and if properly designed the heat pipe can be made to "stop" while the header temperature is well above the NaK freezing point. The NaK will stay liquid since the header can be well insulated by a low emissivity coating on its meteoroid bumper as illustrated in Figure 4-83. After occultation ends, heat addition to the headers will bring the heat pipes back into operation. Heat pipes with the necessary stop-start characteristics may require the use of a noncondensable gas such as helium in addition to the metallic working fluid. 4.10.6.8 Mass Optimization: Heat Pipe/Fin Radiation For Brayton Systems Heat pipe/fin panel radiators with pumped manifolds were analyzed to provide radiator system Fig. 4-80. Heat Rejection Area and Capability is Limited by Heat Pipe Length Unless Pumped Manifolds are Used A liquid metal (NaK) heat pipe/fin radiator concept is shown in Figure 4-81. This concept is similar to the tube/fin radiator concept in that heated liquid metal is pumped from the heat source through feeders and headers into the radiator panels. The cooled liquid metal is returned through headers and feeders to the heat source completing the cycle. An accumulator provides a positive pressure at the pump inlet. The maximum area possible for the heat pipe radiator panel is 20m x 20m (65.6 ft x 65.6 ft) which is the optimum size for transportation to low Earth orbit for assembly. However, this is not necessarily the size for optimum thermal efficiency. The panel consists of a central header for the radiator fluid (NaK). Heat pipes extend on either side of the header in the same plane as each other. The heat pipes are attached to each other through Fins to enhance their heat radiating capability. The radiator panel concept is shown in Figure 4-82.
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