Thermal Vacuum Evaluation of Simulated Spacecraft Radiators with Discretized Emissivity Surface Properties

Abstract

Incorporating variable emissivity electrochromic devices onto spacecraft radiators offers a unique potential for modulating passive heat rejection, but implementation challenges exist. Although high-/low-end states of these devices are readily achievable, uncertainties remain with reproducibility at intermediate states and also with stability in a thermal vacuum environment. An average intermediate heat rejection rate can be obtained by discretizing the radiator surface into mixed high- and low-emissivity values. We emulated this capability by constructing test articles with two fixed-emissivity states divided into four sections. Coupons were constructed from aluminum 6061 and stainless steel 304 to characterize performance variations resulting from differences in thermal conductivity. Tests were devised to maintain either a constant surface temperature or constant heat flux. The results showed that this approach provided a predictable area-averaged intermediate emissivity regardless of the material’s thermal conductivity or the heat rejection scheme used. When operating in a constant flux mode, however, the stainless steel coupon experienced larger lateral temperature gradients across the surface due to its lower thermal conductivity. Performance of a variable emissivity radiator must, therefore, take into account not only surface properties, but also consider the desired system heat rejection control scheme and localized thermal gradient tolerance.