In-situ Ablation Recession Sensor Based on Ultra-Miniature Thermocouples - Part A: 0.25mm Diameter Thermocouples

Abstract

In this research, a break-wire like recession sensor based on commercial, ultra-small thermocouples (250 μm diameter) was designed and tested. A series of thermocouples (TCs) are embedded in the thermal protection systems (TPS), perpendicularly at the ablator surface. Each sensing head of the TC is positioned at a well-established depth from the surface. During the heating of the ablator, the TCs would first work as a temperature sensor providing precious information about the state of the TPS. When the temperature rises above the melting point of the metal sheath and of the Seebeck junction, the TC would break. Due to this double nature of the sensing heads – as a traditional Seebeck junction and as a position marker it would be possible to obtain a wide range of data on the recession state of the TPS. The proposed ablation recession sensor was tested on carbon/carbon (CC) composites and it was tested under a severe hyperthermal environment produced using an oxy-acetylene test bed. Two types of CC materials with different densities (LDCC=1.34 g/cc and HDCC=1.70 g/cc) were considered. At the same time, two different configurations were tested (4 levels and 8 levels). The obtained results were quite encouraging showing the TCbased approach can provide accurate data on the recession rate of the TPS material. Due to the uniqueness of the proposed sensing technique, it can be applied both on TPS of spacecraft as well as on rocket nozzles. Moreover, the proposed approach uses low cost, commercially available TCs and processing techniques.