Non-Intrusive Sensor for In Situ Measurement of Recession Rate of Heat Shield Ablatives

The development of a new sensor for in-situ non-intrusive measurements of recession rate of heat shield ablative materials is described. The sensor utilizes a focused ultrasound approach to non-intrusively detect the ablative material's surface loss while simultaneously correcting for acoustic velocity dependencies on temperature. The latter correction is done via a closed loop feedback approach that yields the average acoustic velocity through the ablative material. The multi-source focusing approach is atypical of current ultrasound based sensors used for ablation recession rate measurement, which require a-priori knowledge of temperature distribution within the ablative to yield accurate data on recession rate. The paper describes the early development of the sensor system resulting in a proof-of-concept breadboard system that demonstrates its unique operational aspects and possibilities as a heat shield health monitoring system for future spacecraft.