Effects of water presence on low temperature phenomenon in porous TPS materials

During the development of the MSL Entry, Descent, and Landing Instrumentation suite, extensive arc jet ground testing was performed at NASA Ames Research Center on PICA models with embedded thermocouples. During these tests, a low temperature phenomena was consistently observed through thermocouple measurements deep within the material. 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 °C. The "hump" temperatures in the MEDLI test series correlate well with the known saturation curve of water when plotted against the stagnation pressure. 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 phenomena, which is observed in multiple different porous TPS materials.