Superorbital re-entry shock layers: flight and laboratory comparisons

Abstract

Hypervelocity re-entry through the Earth's atmosphere surrounds an aeroshell with a radiating shock layer, which has complex and potentially critical interactions with the thermal protection system, the key component to the vehicle's survival. Aerothermodynamic re-entry flight data is rare, but the available radiation data has potential for replication in ground-based experimental and numerical testing, where flight equivalent conditions can be achieved. A greater understanding and ability to demonstrate the behaviour of re-entry vehicles will allow increased confidence in pre-flight laboratory predictions, design iterations based on testing and simulation, and an overall reduction in the thermal protection system (TPS) mass, therefore increasing the mass available for scientific payload. This thesis aims to replicate flight data from the unmanned Hayabusa and Stardust aeroshell re-entries in expansion tube experiments and numerical simulations, and assess the similarities and differences in the results. The flight data is in the form of infrared (IR) and ultraviolet (UV) spectra emanating from the bow shock, as captured by air- and ground-based observation missions. The experiments were performed in the X2 expansion tube, a hypervelocity impulse facility capable of producing flight equivalent conditions over a scaled aeroshell model for a brief test time. Prior to optical imaging tests, the conditions were designed through an iterative combination of analytical, experimental and numerical methods, based on binary scaling and enthalpy matching. Emissions from the radiating shock layers were captured by IR and UV spectroscopy, as well as two-dimensional intensity mapping through a narrow wavelength band filter, but the measurements were oriented differently to flight. The axisymmetry of certain imaging planes enabled the transformation of line of sight integrated intensities into radial quantities, which could describe the forebody shock layer and result in an integrated intensity comparable with integrated flight spectra. Numerical simulations must first model the flowfield before radiation modelling is possible. Compressible flow computational fluid dynamics (CFD) simulations were performed for the Hayabusa and Stardust flight vehicles at the selected trajectory points, and scaled models at idealised and simulated inflow conditions. Radiation spectra calculations were performed along the same lines of sight as imaged in experimental spectra, and over the entire flowfield to be comparable to flight data and experimental 2D imaging. The flight, experimental and numerical data combined for a series of comparisons to test the hypotheses that under binary scaling, spectral radiance along a line of sight is invariant, and radiative intensity scales with the square of the length scale. Spectra comparisons between experiment and CFD, and CFD and flight, tested the first hypothesis, and integrated intensity comparisons between experiment and CFD, and experiment and flight, tested the second hypothesis. Favourable comparisons were found between experiment and flight, full scale CFD and flight, and full scale CFD and experiment. All comparisons to scaled CFD were poor, and possible causes and areas for investigation were identified. Given the uncertainties inherent in each part of the comparison, including observing at a distance, imperfect scaling, shot-to-shot variation, and assumptions in the CFD and radiation calculations, the favourable comparisons are a significant achievement. This project is the first work of its kind to directly compare radiation data from re-entry flights with radiation data obtained in expansion tube tests and CFD simulations at matched conditions. The results show that the hypotheses hold, demonstrating the validity and potential of both ground testing methods in re-creating flight environments.