Expansion Tube Magnetohydrodynamic Experiments with Argon Test Gas

Abstract

In this study an expansion tube is used to generate an experiment to study and evaluate the concept of magnetohydrodynamic aerobraking for planetary entry spacecraft. An expansion tube can theoretically generate the required hypersonic flowfield within which ionisation is confined to the shock layer; this is an important characteristic of the true flight case which previous experiments with arcjets have failed to achieve. The first part of this paper explores the operating envelope for The University of Queensland’s X2 expansion tube facility, to identify flow conditions which should produce a significant interaction between the shock layer and an applied magnetic field. Argon test gas was selected for initial experiments due to its simple chemistry and low ionisation enthalpy, and equilibrium chemistry was used to compute the expected properties of the shock layer around a blunt body model. A candidate flow condition was identified for further analysis with finite rate reacting argon CFD, which indicated that relatively short shock layer length scales for the X2 expansion tube would limit the degree of argon ionisation that was generated, however, sufficient ionisation was still expected. Three sphere models were tested: a steel ball, a plain magnet, and a magnet coated with an electrically insulating ceramic paint. It was found that shock stand-off is unchanged between the steel ball and the plain magnet, but increases by approximately 10% for the coated magnet. Spectroscopy revealed that radiance along the stagnation streamline for some, but not all, of the relevant argon wavelengths approximately doubles when a magnet is used, either coated or uncoated. This increase in radiance does not appear to be associated with changes to shock stand-off. Further work is now required to explain the observed phenomena, and also to address some key experimental challenges, which include accurate measurement of shock stand-off and reducing the significant shot-to-shot variability observed for the coated magnet experiments.