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.iii 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. Favourabl...

State-Specific Study of Air in the Expansion Tunnel Nozzle and Test Section

“…The value of dci/dx is proportional to ρ for the VT and VVT reactions, and the value of dci/dx is proportional to ρ 2 for the recombination reactions. Since p is proportional to ρ according to the ideal gas law, the aforementioned trends for ρ can also be said for p. In the expansion tunnel, the nozzle inlet pressure typically ranges from around 10 kPa to 100 kPa [94][95][96][97], while the nozzle inlet velocity typically ranges from around 4 km/s to 11 km/s [95,98,99]. Based on these conditions and the discussion on the dependencies of dci/dx, we can see that it is the range of the inlet pressure (an order of magnitude, which is much higher than the factor of 2-3 for the range of the inlet velocity) which would determine the variety of different thermochemical behaviors (equilibrium, frozen, nonequilibrium) that are possible in the expansion tunnel nozzle.…”

Influence of thermochemical nonequilibrium on expansion tube air test conditions: A numerical study

“…The post-shock nonequilibrium in the shock tube is often ignored in previous studies, and the equilibrium post-shock state is usually the initial condition of the unsteady expansion. 5,6,[38][39][40][41][42] The basis for this assumption is that the post-shock relaxation distance may be short enough such that the slug of test gas in the test flow mostly originates from the equilibrium post-shock region. This could be true when a high shock tube fill pressure is used; as shown in Fig.…”

Estimation of the error in the electron temperature determination of the plasma

“…Plasma formation occurs due to high-temperature aerodynamic heating of the air surrounding the return spacecraft [1]. The impact of plasma on radio communication on the route aboard the spacecraft-Earth leads to a violation of radio communication due to absorption of electromagnetic radiation power in plasma and reflection from its boundaries [2][3][4]. To develop methods to combat the violation of radio communication, it is necessary to know the electrical characteristics of plasma for the conditions of spacecraft flight in the atmosphere.…”