Knowledge of aerothermally induced convective heat transfer and plume-induced heat transfer loads is essential to the design of thermal protection systems (TPS’s) for launch vehicles. Aerothermal and radiative models are typically calibrated via the data from in-flight heat flux sensors. The most commonly used sensor for this purpose is the Schmidt-Boelter gauge because it is easy to install, gives a rapid transient response, and has an analog output proportional to the incident heat flux. There are errors involved with these instruments, however, that can lead to indicated measurements twice as high as actual environments under common launch vehicle flight conditions. Correcting these errors will reduce thermal protection materials used and associated processing and allow for the use of more materials that cannot meet the current conservative-design thermal loads.

The standard Schmidt-Boelter gauge is made largely of copper or aluminum but also uses epoxy and a thermopile. The sensors are cylindrical, with the sensing end of the cylinder flush-mounted to the exposed surface of the launch vehicle and thus exposed to the external thermal and velocity boundary layers as well as thermal radiation. Schmidt-Boelter gauges take advantage of the one-dimensional Fourier’s law to measure the incident heat flux, with the thermopile analog output proportional to the incident heat flux. When surrounded by low-conductivity insulation, the sensor has an exposed surface temperature significantly lower than the insulation and this leads to two types of measurement errors.

One error is experienced only in convective heat transfer and is induced by the disturbance of the thermal boundary layer. This is caused by the lower-temperature calorimeter surrounded by the higher-temperature insulation and results in a higher-incident heat flux on the gauge than on the surrounding insulation. The second type of error is caused by heat conduction from the surrounding insulation radially into the cylindrical calorimeter. This is manifested in the gauge reading as an indicated heat flux higher than the incident heat flux on the sensing surface.

An effort is currently underway to model the heat flux gauge under typical flight conditions that includes an installation surrounded by high-temperature insulation. The goal is to correct the measurements to reflect the local heat flux on the insulation had the instrument not been present. The three major components of this effort include: (1) a three-dimensional computational thermal math model including the internal conduction heat transfer details of a Schmidt-Boelter gauge; (2) a computational flow dynamics (CFD) analysis to determine the effects on measurement of the rapidly changing thermal boundary layer over the near-step changes in wall temperature; and (3) testing performed on flat plates exposed to an aerothermal environment in the Marshall Space Flight Center (MSFC) Improved Hot Gas Facility (IHGF).

This work improves on previous work in three ways: (1) it models radial energy error components, including both as-built and as-installed factors; (2) its CFD model more precisely accounts for property changes of the supersonic stream and their impact on the dynamic boundary layer; and (3) it provides data that will be used to calibrate the models.