In situ ablation recession sensor for ablative materials based on ultraminiature thermocouples

This study involved the development of an in situ ablation sensor designed to evaluate the recession rate for an ablative thermal protection system exposed to a hyperthermal environment. Design requirements specify that the sensor be constructed using multiple thermocouples at staggered depths, drilled and mounted into carbon/carbon composite and carbon/phenolic ablatives. During erosion, the thermocouples record thermal data and then melt when the flame front approaches the thermocouples. The position of the recession front and breaking time of the thermocouple can determine the ablation rate. Earlier designs developed by the authors' research group involved the use of 0.25-mm-diam thermocouples and electric discharge machining. Later designs used 0.5-mm-diam thermocouples using conventional microdrilling: This approach can be used in all applications in which the intrusiveness of the thermocouple embedded in the thermal protection system is not a primary requirement and allows 0.5 mm thermocouples to be used as a replacement for ultrasmall and costly 0.25 mm thermocouples.