Several factors are important in the development of active thermal control systems for planetary habitats. Low system mass and power usage as well as high reliability are key requirements. Ease of packaging and deployment on the planet surface are also important. In the case of a lunar base near the equator, these requirements become even more challenging because of the severe thermal environment. One technology that could be part of the thermal control system to help meet these requirements is a radiator shade. Radiator shades enhance direct radiative heat rejection to space by blocking solar or infrared radiation which lessens the performance of the radiator. Initial development work, both numerical and experimental, has been done at the Johnson Space Center (JSC) in order to prove the concept. Studies have shown that heat rejection system mass may be reduced by 50% compared to an unshaded low-absorptivity radiator.
Several different shade geometries have been evaluated using Thermal Synthesizer System (TSS) math models. These models have pointed to the most promising shade geometries by providing an estimate of their expected performance. The models have also been used to study the effects of different optical properties in order to understand how the system will perform over time. Models have been used to optimize designs by considering such factors as end-effects, height-to-length ratio and shade height. Thermal math model predictions indicate that a parabolic shade will reduce the effective environment sink temperature of the radiator by more than 100 K compared to an unshaded vertical radiator.
In order to verify numerical predictions, testing of the parabolic radiator shade concept has also been done. A proof of concept thermal vacuum test has been carried out on a small scale rigid parabolic shade test article under a variety of operating conditions. The rigid shade used a section of a cylinder to approximate a parabola and had non-ideal optical properties. Still, the shade lowered the effective sink temperature of the radiator by 70K compared to the unshaded radiator in the thermal vacuum test.
One shade design which is under study is an inflatable shade. In this design, gas pressure is used to hold the parabolic shape of the shade which is covered with a clear cover to form a long tubular enclosure. A vertical radiator is supported inside the enclosure. Analytical studies indicate that shade performance is reduced due to the transparent cover, but that overall system mass may be reduced over other flexible parabolic shade designs due to the elimination of a support structure and the use of light weight materials.
Future plans include construction of an inflatable shade test article and construction of a flexible parabolic shade deployment test article. Eventually these test articles could be used in a full scale thermal vacuum test.