Spacecraft Thermal Management Via Control of Optical Properties in the Near Solar Environment

The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is currently evaluating multi-functional ceramic coatings for spacecraft operating in the solar environment. Ceramics were selected based on chemical stability; and inertness to radiation damage and hydrogen degradation. Investigations have focused on “white” ceramics such as aluminum oxide, pyrolytic boron nitride, and barium zirconium phosphate which also possess desirable optical characteristics. Engineered ceramic coatings have been found through testing to possess both high visual reflectivity and infrared emissivity. These optical properties make this a prime passive thermal management solution for NASA's Solar Probe spacecraft. This future mission is designed to send a spacecraft within 4Rs (4 solar radii from the center of the Sun) to collect and analyze solar wind and dust. Notionally, the uncooled carbon-carbon spacecraft (αs/ϵir = 1) is predicted to reach equilibrium temperatures of 2100 K. This temperature results in three major problems: carbon-carbon mass loss leading to a compromise of science data; increased thermal protection system mass leading to higher launch costs; and incorporation of immature insulation materials leading to increased mission risks. These ceramics with αs/ϵir < 0.6 have been shown to notionally reduce equilibrium temperatures to between 1300 and 1850 K. Operational requirements for this mission imply that the ceramics must structurally survive launch, thermal cycling, and paniculate impact; be inert to radiation damage; have manageable spacecraft charging; and be chemically stable. Preliminary testing of these ceramics in simulated space environments (including structural and radiation conditions) has yielded promising results. Higher fidelity studies will be continuing focused on reducing the risks associated with implementing these candidate materials.