Continued Development of a Highly Reflective Solar Coating for Cryogenic Liquid Storage in Space

Abstract

State-of-the-art solar reflectors absorb 6-8 % of the sun's irradiant power, which is adequate for spacecraft thermal control; but not for storage of cryogens in tanks exposed to the Sun. A solar reflector must absorb less than 0.4% of the Sun's power, while radiating effectively in the far IR, to allow LOX storage. Hibbard realized this in 1960's, at about the same time that nearly perfect optical reflectors were developed based on particle scattering. In 2015, under funding from the NASA Innovative Advanced Concepts Program (NIAC), these breakthroughs were rediscovered. Theoretical analysis showed that a tank coated with a thick (1 cm) layer of 150 nm particles, composed of a broadband material, could allow the tank to chill to cryogenic temperatures, even in the presence of 1 AU solar irradiance. Subsequent work lead to the demonstration and testing of BaF2 based rigid coupons. Placing one of these in a cryocooler-based simulated deep space environment and irradiating with simulated sunlight has shown about 1% absorption. This is better than the state-of-the-art but not yet sufficient for LOX storage.This paper will review the background material presented above but will concentrate on work performed over the last year, including the design and build-up of the testing apparatus. Since our last publication, we have changed materials from BaF2 to Y2O3. Y2O3 has slightly higher solar absorptance, but is hydrophobic and higher index