Development and validation of an <i>in-situ</i> recession measurement technique for ablative materials

Abstract Thermal protection materials for high-speed flight vehicles are subject to conditions that can cause ablation and recession of the surface. Quantifying the magnitude of this behavior in ground testing facilities is critical for materials selection and design. Existing experimental methods to measure the recession rate in high-enthalpy flows require expensive equipment, extensive post-processing, or tampering with the material sample. In this study, an in-situ non-intrusive technique to measure the surface recession rate of an ablative material is presented. This method, called multipoint laser transmissivity, is based on the transmission of multiple laser beams over an ablative surface. These laser sheets are collected by photodiodes that, when properly calibrated, provide a signal proportional to the recession of the sample surface. The method was tested in the HyperMATE plasma torch facility at the University of Tennessee. Coupon-size PTFE and graphite samples were tested over a range of heat fluxes and exposure times to induce a variety of recession behaviors and magnitudes. Time resolved schlieren imaging and ex-situ 3D scanning with an optical profilometer were used as validation and verification tools. Good agreement between the different measurement techniques was found, with an accuracy of 4% on the total recession and 10% on the recession rate.