Hypervelocity shock layer emission spectroscopy with high temperature ablating models

Abstract

During reentry into the atmosphere, a vehicle can encounter extreme convective and possibly radiative heating loads. Such spacecraft must rely on a carefully designed thermal protection system to maintain the integrity of the underlying structure, thus ensuring the survival of any cargo or crew it may contain. Despite many past developments and research efforts, to this day there remain large uncertainties associated with the prediction of heat shield performance. In particular, accurate modelling of ablation phenomena is critical to minimising uncertainties, reducing excess weight, and increasing safety. This includes thermochemical ablation by oxidation, nitridation, and sublimation, as well as the mechanical removal of material via spallation. Due to the high costs associated with in-flight testing, and the difficulty of mounting precision instruments and advanced diagnostics on flight vehicles, ground-based experiments stand as a cost-effective method for reducing modelling uncertainties. Ablation testing is generally conducted in long-duration facilities such as arc jets, which are well suited to investigating in-depth material response due to their ability to provide representative heating rates for extended periods. They are, however, unable to reproduce a realistic in-flight shock layer due to low Reynolds numbers and, depending on the facility type, a high degree of thermal and chemical nonequilibrium in the free-stream. In contrast, impulse facilities such as shock tubes and their variants are well suited to reproducing realistic flight conditions with free-streams that are a closer match for flows dominated by binary processes in terms of Reynolds number, temperature, and chemical composition. Due to their extremely short test times, however, ranging from the order of 10 us to several milliseconds, they are unable to aerothermally heat test models up to representative in-flight temperatures, or recreate the quasi-steady heat and mass flux balance of flight. This key limitation of impulse facilities was recently overcome in experiments using the University of Queensland’s X2 expansion tunnel by electrically pre-heating samples to temperatures approaching 2500K immediately prior to a test. In this work, the pre-heating methodology was enhanced to generate maximum surface temperatures approaching 3300K in order to study sublimation phenomena. It was found that although sublimation-regime temperatures could be achieved, the relevant surface processes themselves were suppressed due to the high pitot pressures of the conditions used. Instead, the nitridation process was studied by observing air shock layer CN emissions with pure graphite models heated to surface temperatures ranging from 1770–3300K. These results were compared with numerical simulations which predicted a monotonic increase of emissions with surface temperature for all models considered, whereas the experiments showed an initial increase from 1770–2500K and then remained relatively constant from 2500–3300K. The simulations also suggested that a significant portion of the observed CN emissions were not a result of direct nitridation, but instead due to surface CO production and subsequent gas-phase interactions to form CN. A final series of experiments was conducted using a novel model design whereby the heated graphite strip was embedded into a steel base structure in order to relieve edge effects, with the aim of producing quasi two-dimensional flow along the strip. The degree of flow two-dimensionality was found to be questionable due to visible cross-flow and a shock stand-off 10% smaller than a nominal two-dimensional case, although it was much closer than a heated strip on its own. These tests were conducted in both air and nitrogen flows to further investigate the coupling effect of surface CO production on the levels of CN emissions via gas phase reactions, as suggested by the previous comparisons with numerical simulations. Based on the relative levels of CN emissions between the two test conditions, it was concluded that although CO interactions were a non-negligible contributor to CN production, the majority was in fact due to direct surface nitridation. In terms of overall trends, the nitrogen condition results showed continually increasing CN emissions with surface temperature. For air, however, they reduced with temperature above 2500K which was consistent with the previous experiments in air. The combination of these observations suggests that the oxidation rate is decreasing with temperature above 2500K. This phenomenon is well known; however, it is the first such observation for an ablating body with the additional influence of a full realistic hypervelocity shock layer. Improved magnification in the high-speed video recordings for these experiments also allowed spallation phenomena to be observed in much greater detail than in previous work. It was clearly seen that spallation can significantly alter the flowfield, and is a phenomenon which must be given greater consideration in future studies.