Expansion Tunnel Experiments of Earth Reentry Flow with Surface Ablation

The thermal protection system of a reentry vehicle must endure extreme heating loads, leading to high surface temperatures and associated phenomena such as thermochemical ablation. Accurate modeling and prediction of these processes are critical for determining heat-shield sizing requirements; however, large uncertainties still remain. Due to the high costs associated with flight tests, ground testing remains an economical option for addressing these uncertainties. In this study, experiments with electrically preheated graphite samples are conducted in the University of Queensland's X-2 expansion tunnel. Calibrated spectral measurements of the shock-layer emissions are taken in the wavelength region of 353 to 391 nm, targeting the carbon-nitrogen violet band for surface temperatures from 1770 to 2410 K. It is observed that carbon-nitrogen concentrations clearly increased within the boundary layer at higher surface temperatures. Numerical simulations fail to reproduce this behavior and instead indicate that production should be relatively constant. Increased levels of carbon-nitrogen radiation with surface temperature are also found throughout the entire shock layer and even the freestream, another observation that is not predicted by the simulations. It is proposed that this effect is due to spallation; however, further experiments are required to verify this.