2.8 RADIATOR SYSTEMS The conversion systems which require large concentrating reflectors also require active cooling systems to dissipate waste heat. Two radiator concepts which were considered for this application are the heat pipe radiator (HPR) and the liquid droplet radiator (LDR). A third concept, the moving belt radiator (MBR), was found to be promising but too poorly developed to be considered. The heat pipe radiator was found preferable for all active cooling needs in this study because it is a low technological risk and it would require less non-lunar material than the LDR. 2.8.1 Heat Pipe Radiator (HPR) In the heat pipe radiator, coolant is pumped through a long tube. Heat pipes carry heat from the tube to radiating sheets of aluminum. A major mass component of this radiator is the coolant tube, which must be armored to prevent micrometeoroid punctures. Specific mass of the heat pipe radiator is assumed to be 8 kg/kW at a coolant temperature of 333 K.(l,3) Typically about 98% of the radiator's mass will include pipes or fins which can be manufactured from mostly lunar material. The coolant pumped from the heat source through the ducts might be largely lunar. If coolant temperature is high, and if sodium and potassium can be extracted cost-effectively from lunar regolith, then use of NaX coolant could result in over 99% lunar material in the heat pipe radiator. Likewise, if the coolant temperature is appropriate for water, then lunar oxygen could be used for 89% of the coolant mass. Lunar oxygen could also be used to make heat pipe working fluid, such as water or methanol. There are potential problems with HPRs. Aluminum cannot be used for the radiating sheets if the coolant temperature is near the melting point of aluminum, 934 X. Corrosion and oxidation may limit the useful lifetime of heat pipes to less than the assumed 30 year life of the SPS. 2.8.2 Liquid Droplet Radiator (LDR) In the LDR concept, coolant is sprayed through space as a thin sheet of small droplets which radiate heat to space. The droplets are captured for re-use. Micrometeoroids pose no danger to the droplet sheet, so extensive armor is not needed. Specific mass of the LDR has been estimated to be potentially one third that of HPR at high temperatures where liquid metals would be used as the working fluid, and one sixth that of HPR at low temperatures where silicone oils would be used.(2,3) About 30% of the mass of the LDR would be coolant, which would probably not be lunar. It is possible that lunar NaX could be used if the temperature range were suitable and this material could be extracted from the lunar crust. Coolant would have to be resupplied at regular intervals due to evaporation losses. A complete LDR system has not been demonstrated, but subsystems have been tested.
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