Electrostatic Testing of Multilayer Insulation for In-Space Cryogenic Vehicles

Abstract

Multilayer insulation (MLI) can provide large thermal benefits for the storage of cryogenic propellants on-orbit, reducing the heat loads through large areas on storage tanks by two orders of magnitude. Such MLI has flown in space either within contained environments, such as a vacuum jacketed tank (tank in a tank), or external to a spacecraft, but heavily grounded to prevent the build-up of charge on the blanket. The grounding lugs deployed on a traditional spacecraft blanket require each layer to be electrically and thermally grounded to a metallic lug that passes through all the reflective layers. Thus, the metallic lug can have as much heat load through it as several square meters of blanket, significantly reducing the benefit of using the MLI (by a factor as high as 30). As NASA explores the benefits of applying MLI to large cryogenic stages, various environmental effects may change the application of the MLI to those stages. These environmental effects must be both understood and accounted for in the development of MLI, specifically for these applications. Testing was performed to understand the charge build up that multiple different outer covers would generate in both low earth orbit (LEO) and geostationary orbit (GEO). The test set up included an aluminum 6061 baseplate, a 25-mm-thick polyurethane foam, and either 10 or 25 layers of MLI (approximately 2.5 layers per mm) with an outer cover material. Cover materials tested included indium tin oxide (ITO), germanium, and aluminum, all deposited on a 2-mil-thick Kapton substrate. Testing occurred at room temperature and at a temperature of around 170 K. Results indicated that the blankets with Aluminum or ITO outer covers did not arc at voltages less than 250-V negative in simulated LEO and electron beam irradiation of 5.8-keV energy and up to 3-nA/cm2 current density in simulated GEO conditions. These results can be used to minimize the number of grounding points for large blankets or perhaps eliminate them.