Comparisons of Ablator Experimental Performance to Response Modeling and Effects of Water Phase Transition in Porous TPS Materials

The Mars Science Laboratory Entry Descent and Landing Instrumentation (MEDLI) project performed extensive arc jet tests for development, qualification, and calibration of instrumented heat shield plugs. These plugs each contained several thermocouples for recording near-surface and in-depth temperature response of the Phenolic Impregnated Carbon Ablator (PICA) heat shield. The arc jet test results are entered into a comprehensive database so that broad trends across the test series can be compared. One method of analysis is to compare with ablator material response calculations and solve the in-depth heat conduction equations. Using the near-surface thermocouple measurements as a boundary condition in numerical simulations, comparisons are made with other thermocouple measurements taken deeper within the TPS test article. The work presented here uses this technique to compare test results with model simulations using several metrics, such as peak-temperature difference, maximum difference in temperature, and a total integrated temperature deviation. A significant difference in prediction behavior with respect to the location of source thermocouple is shown based on these comparisons. The temperature prediction accuracy is quantified for the tested material and material response code and is found to be highly dependent on the distance between the boundary condition thermocouple and the deeper reference thermocouple. Based on this test data, it is shown that numerical models can predict in-depth temperature measurements equally well for sensor plugs installed in the arc jet test model with or without a silicone adhesive. It is found that predicted temperatures are consistently greater than measured values, indicating the PICA material model is generally conservative for in-depth temperature predictions. In addition, a low-temperature phenomenon was consistently observed through thermocouple measurements deep within the material during the MEDLI arc jet testing. This anomaly, referred to here as the hump, consists of a change in concavity of the temperature profile well below the maximum temperature and is seen in various TPS materials and atmospheric conditions, and typically occurs around 40 ºC. It is proposed that the observed ``hump is a result of the heat of vaporization during the endothermic phase transition of water within the TPS material. This is supported by the known absorption of water by PICA from the atmosphere prior to testing or flight. The presented material response model captures energy effects of phase transition from a pre-existing water presence. This work shows that water presence currently appears to be the most probable cause for the phenomenon, which is observed in multiple different porous TPS materials