A cryogenic thermal vacuum testing facility at NASA's Goddard Space Flight Center (GSFC) was developed to determine thermal control coating emittance at cryogenic temperature for the JWST/ISIM project. In order to meet ISIM thermal requirements in design and analysis accurate knowledge of material and optical properties as a function of temperature for the assembly are required. The optical property data currently available in this temperature regime have a relatively large (30-50%) uncertainty associated with the measurements. In an effort to reduce these uncertainties the 'ISIM Thermal Control Coating Cryogenic Emittance Tests' (3 performed to date) were developed utilizing this facility. The objective of these tests was to determine the emittance of candidate thermal control coatings and radiator geometric configurations for the ISIM over the temperature range from 30K to 150K while minimizing associated uncertainty. This paper presents the testing approach, experimental methods, and results for the emittance tests performed in the GSFC cryogenic facility to date. Challenges encountered during this testing effort included quantifying parasitic heat losses/gains to a high degree of accuracy in an effort to minimize associated measurement uncertainty.
NASA's James Webb Space Telescope-Integrated Science Instrument Module (JWST-ISIM) radiators and structures operate in the 30 to 40 K range. There is limited emittance data for coatings of interest in this temperature range. Calorimetric emittance tests performed at Goddard Space Flight Center in the past have used a transient technique, which results in large uncertainties (typically > +/-30%) at the lowest temperatures. These large uncertainties would practically require use of overly conservative emissivities in radiator sizing, which would in turn pose unnecessary area and mass penalties. There is thus a strong incentive to make highly accurate emittance measurements. A special liquid helium cryogenic facility was fabricated for this purpose, and a series of thermal balance tests were subsequently performed at NASA/GSFC to measure the emittance of selected ISIM coatings accurately at temperatures down to 25K. This paper discusses the test methodology, and the analytical methods used to calculate the emittance and its accuracy from the measured data. Preliminary results show that for relatively high emittance coatings, typical measurement accuracies at 30 K approach +/- 5%.