Arcjet Evaluation of Thermocouple Performance 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 vehicle was 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 arcjet testing that was performed to examine the potential error sources associated with instrumenting NASA’s FTPS with different thermocouple Types (Type K versus Type N), wire gauges (30 versus 28 American wire gauge), insulations (glass versus ceramic), and barriers (mica versus no mica). Several error sources were identified, including 1) conductive deposits on the thermocouple insulation electrically shorting the thermocouple leads; 2) melting of the glass thermocouple insulation, allowing for electrical shorting of the thermocouple leads; 3) thermocouple leads melting, resulting in loss of signal; and 4) oxidation of the positive thermocouple lead (known as green rot), generating a drift in the measured output. A mica tape wrap around each individual thermocouple lead was successfully used to mitigate the first error source, and switching to ceramic insulation eliminated the second error source. The third and fourth error sources were examined by exploring the effect of the thermocouple type and wire gauge. Thermocouple solutions that minimize the potential error sources at each layer in the FTPS are presented. Finally, the arcjet test results are compared to the previously reported ground-based tube furnace test results, and their applicability to the flight environment is discussed.