Thermal Testing on High Temperature MLI and Sunshields for Solar Probe Sun-Viewing Instruments

Abstract

The Solar Probe science team at NASA Goddard Space Flight Center (GSFC) is considering enhance science observations by adding the nadir viewing Plasma Spectrometer instrument and using heat shields to protect the Plasma Wave Antennas to permit full extension at 3 solar radii (Rs). The main components of the thermal protection system for these nadir viewing instruments are high temperature multi-layer insulation (MLI) blankets and carbon-carbon (C-C) composite sunshields that have an aluminum oxide (Al2O3) coating on the sun facing surfaces. A thermal test on the Plasma Wave Antenna sample was performed in September 2006 using a small and specially designed vacuum chamber and a Vortek Lamp. A 12.7 cm x 12.7 cm low-cost high temperature MLI blanket was built. It consists of 15 layers of 0.01524 mm tantalum foils which are interspaced with a 0.00508 mm carbon fiber mesh. Although the MLI blanket is much thicker than that proposed in the original design, the thermal test results show that the MLI effective emittance is about the same as the 0.03 value assumed for the original design. The highest temperatures of the MLI and hot side C-C measured in the test were 1,000oC and 1,300oC respectively. Higher temperatures were not pursued because the chamber pressure increased as the temperature increased. The pressure increase was caused by outgassing from the test sample and thermocouple connectors. An inspection of the MLI blanket after the test revealed that it had no degradation. The Al2O3 coating on the hot side C-C was discolored, possibly by contaminants from outgassing. When the hottest hot side C-C thermocouple was 700°C or warmer, the C-C warped. The Al2O3 coating delaminated, possibly due to the C-C warping.