The optimum radiator configuration in hot lunar thermal environments is one in which the radiator is parallel to the ground and has no view to the hot lunar surface. However, typical spacecraft configurations have limited real estate available for top-mounted radiators, resulting in a desire to use the spacecraft's vertically oriented sides. Vertically oriented, flat panel radiators will have a large view factor to the lunar surface, and thus will be subjected to significant incident lunar infrared heat. Consequently, radiator fluid temperatures will need to exceed ~325 K (assuming standard spacecraft radiator optical properties) in order to provide positive heat rejection at lunar noon. Such temperatures are too high for crewed spacecraft applications in which a heat pump is to be avoided.
A recent study of vertically oriented radiator configurations subjected to lunar noon thermal environments led to the discovery of a novel radiator concept that yielded positive heat rejection at lower fluid temperatures. This radiator configuration, called the Upright Lunar Terrain Radiator Assembly (ULTRA), has exhibited superior performance to all previously analyzed concepts in terms of heat rejection in the lunar noon thermal environment. A key benefit of the ULTRA is the absence of louvers or other moving parts and its simple geometry, which consists of a radiating surface and a shielding surface joined to form a horizontally oriented vee. Subsequent analyses investigated optimizing the original concept by varying the orientation of these surfaces.
The benefits of the ULTRA was studied for two applications, a lunar extravehicular activity (EVA) portable life support system (PLSS) and a lunar lander active thermal control system. Analysis of the ULTRA for a lunar EVA PLSS is shown to provide moderate heat rejection, on average, at all solar incident angles assuming an average radiator temperature of 294 K, whereas prior concepts exhibited insignificant heat rejection or heat absorption at higher incident angles.
ULTRA performance at mid-solar angles during an EVA is shown to degrade slightly as a result of a localized increase in lunar surface infrared thermal radiation (IR) due to solar reflections off of the ULTRA. Since the analysis assumes a PLSS with a flow-through fluid loop, heat absorption reduces the overall performance of the radiator. To prevent heat absorption from affecting the total heat rejection, a diode heat pipe system is also considered and compared at all solar angles. The performance of the ULTRA for a lunar lander is also discussed and compared to the performance of a vertically oriented, flat panel radiator at various lunar latitudes. Again, the ULTRA is shown to outperform the traditional vertically oriented radiator in a lunar noon thermal environment. However in this analysis, the radiators are assumed to be unnaturally unidirectional, and therefore the heat rejection is not averaged out between surfaces rejecting and absorbing heat at mid-solar angles. As a result, the ULTRA appears to be less advantageous. To help reduce the overall absorption of lunar surface IR, a future study will re-examine the ULTRA assuming specular surfaces.