The Hollow aErothermal Ablation and Temperature (HEAT) sensor is a multifunction sensor designed to track an isotherm by making an independent transient measurement at a defined location in the sensor that is equal to the temperature at which its constituent materials char. By this same operating principle, the HEAT sensor tracks the transient char depth progression within a thermal protection system (TPS) material. In the case of a material that sublimes (such as Teflon), or when the aerothermal environment induces steady-state ablation, the HEAT sensor measures material ablation directly.
The sensor design consists of two identical resistive Pt-W wires wound around a hollow Kapton tube. The wires are electrically insulated from each other with a polyimide coating that chars at higher temperatures. The two wires are welded together at the tip of the sensor to provide an initial electrical connection. At higher temperatures, the electrical connection between the two resistive Pt-W wires is formed by the electrically conducting char. The wires become shorter as ablation proceeds, and the time history of wire resistance is continuously monitored to relate the measurement to sensor length. Another key feature is a thermocouple or other temperature measuring device that can be installed into the hollow portion of the HEAT sensor due to its extremely small size.
Novel features include the hollow structure of the sensor, identical conductors, conductors encapsulated in a consistent monolithic insulator, minimal and constant 0.001-in. (≈25-μm) gap between conductors with deviation ±0.0005-in. (≈±13-μm), and just two wires come out of the sensor. Advantages are smooth and reliable performance; no limitation on use with any TPS material having different properties such as different density, electrical conductivity, or non-conductivity; single channel requirement to connect with flight electronic support system; easy manufacturing; and cost reduction.
The sensor is suitable for industries involved in the continued demonstration of heat shield material performance for manned and planetary exploration.
This work was done by Edward Martinez and James Scott of Ames Research Center; Johnny Fu, Joseph Mach, and Jose Santos of Sierra Lobo; and Sergey Gorbunov and Tomomi Oishi of Eloret Corporation. This is a NASA-owned technology protected by U.S. Patent No. 8,069,001.