Heat Flux Sensors and Reconstruction Techniques for Atmospheric Entry Spacecraft

Abstract

Characterizing the heat loads on spacecraft during atmospheric entry is essential to evaluate the performance of thermal protection systems (TPS), investigate aerothermal phenomena, and validate computational models. As the result of a growing push to obtain thermal diagnostic data during entry, descent, and landing (EDL), recent atmospheric entry spacecraft have hosted extensive instrumentation suites embedded in the TPS, including heat flux sensors and temperature probes. Current commercially available heat flux sensors, however, often have survivability limitations which constrain their use to less extreme regions of the spacecraft. Furthermore, the post-flight analysis techniques used by the space exploration community to reconstruct atmospheric entry heating conditions are often resource intensive, and impractical to implement in analyses that are either computationally expensive or time-sensitive. To address these engineering challenges, this thesis details the development of heat flux sensors designed for extreme thermal environments and low cost analysis techniques to reconstruct the surface heating conditions on spacecraft during EDL.

Heat flux sensors tailored for moderate and high temperatures are fabricated using the transverse Seebeck effect (TSE) in antimony and rhenium single crystals. The thermoelectric properties and commercial availability of single crystal antimony make it an ideal platform to use for investigating the TSE operating mechanism in a robust sensor package, whereas the high-temperature compatibility of single crystal rhenium positions it as an optimal transducer material for extreme environment sensors. The single crystal antimony sensor demonstrates a linear heat flux-to-voltage transduction and is used to probe the effects of the package configuration on sensor performance characteristics. The rhenium-based heat flux sensor exhibits a responsivity that both increases in magnitude as the conditions become more challenging and remains stable after long term exposure to temperatures as high as 1000 °C. The temperature-dependence of the rhenium sensor's responsivity, characterized using a newly-developed calibration facility, agrees well with analytical predictions, and underscores how calibrations of heat flux sensors at application-relevant temperatures are necessary for accurate measurements.

An efficient Green's function-based inverse heat transfer (IHT) algorithm is developed to reconstruct the heating conditions on spacecraft during atmospheric entry. The Green's function formalism is adapted to accommodate nonlinear heat conduction phenomena in ablative TPS materials, including the effects of pyrolysis and internal gas transport. To improve the stability of the reconstructed heating conditions, a sensor fusion-based regularization methodology is proposed whereby measurements from collocated heat flux sensor and temperature probe instrumentation are embedded within the IHT formulation. The Green's function IHT approach is applied to the reconstruction of the heating conditions on the backshell of the Mars 2020 atmospheric entry spacecraft. The reconstructed heating conditions are in good agreement with results from a high-fidelity IHT code used by the space exploration community, but are achieved with a greater-than 2 order of magnitude reduction in computation cost. These results demonstrate the Green's function framework as a promising IHT approach to reconstruct atmospheric entry heat loads from thermal measurements embedded in ablative TPS, and highlight how these techniques can enable certain post-flight analyses, such as uncertainty quantification, that have been hindered by the prohibitive cost of current methods.