Emissivity estimation of spacecraft thermal control surfaces at cryogenic temperatures – a novel experimental approach

Painted surfaces are useful for thermal control in spacecrafts. Knowledge of emissivity of such painted surfaces over the range of working temperatures is critical in eventually deciding the adequacy of any thermal control strategy. In view of the very low temperatures involved and also because of the variation of temperature in the spacecraft, the emissivity is invariably a function of temperature, which makes its determination formidable and challenging. A “straight forward” technique to measure or estimate emissivity at cryogenic temperatures is to use a steady state calorimetric technique coupled with energy balance. This involves heat transfer by radiation under steady state conditions between an evacuated cryogenic shroud and a heated test sample, centrally suspended inside the shroud. Apart from the heat supplied, knowledge of temperatures, the internal surface area and emissivity of the shroud is required to estimate the emissivity of the unknown test sample. The view factor can be taken care of, if the test sample is much smaller compared to the shroud, resulting in a view factor of 1 between the sample and the shroud. The key difficulty associated with the seemingly simple procedure is the shroud emissivity, which is often unknown at very low cryogenic temperatures. This paper presents a novel experimental approach for the in-situ estimation of cryogenic shroud (coated with ECP2200 paint) emissivity using measured data from experiments on a plane surface substrate and a cavity surface substrate (a surface with an array of hexagonal cavities) coated with an unknown test sample (Aeroglaze Z307 paint). For an average cryogenic shroud temperature of 32.53 K, the shroud emissivity has been estimated to be 0.355. Following this, the emissivities of the plane surface substrate and the cavity surface substrate coated with Aeroglaze Z307 are estimated. The associated measurement uncertainties are systematically quantified and presented.