Electrical Shorting of Thermocouples in Flexible Thermal Protection System Materials

Abstract

The flexible thermal protection system (FTPS) on NASA’s Low-Earth Orbit Flight Test of an Inflatable Decelerator (LOFTID) vehicle will be instrumented with thermocouples to measure the in-depth thermal response during entry into Earth’s atmosphere. Accurate flight temperature measurements are critical for verifying vehicle performance during the flight test and reducing uncertainties in the thermal models, but the deployable nature of inflatable decelerator technology presents challenges. The thermocouples need to be compactable, cannot damage the FTPS or the inflatable structure, and need to be able to withstand high temperatures and large thermal gradients. The present paper details the developmental testing that was performed using a tube furnace to examine the potential error sources associated with instrumenting NASA’s FTPS with thermocouples insulated with either glass braid or ceramic braid. Large measurement errors occurred with both thermocouple insulation materials due to conductive deposits on the insulation electrically shorting the thermocouple leads. Two mechanisms were identified that contribute to this phenomenon: 1) the organic binder on the thermocouple insulation carbonizes in a high-temperature, low-oxygen environment and 2) decomposition products from the FTPS are able to permeate the braided insulation. The effect of heat cleaning the thermocouples to eliminate the first mechanism is presented, and the possibility of using a mica wrap as an impermeable barrier to mitigate the second mechanism is explored. Finally, the applicability of the ground-based tube furnace test results to the flight environment is discussed.