An expansion tube with a free piston driver has been used to generate quasi-steady hypersonic flows in argon at flow velocities in excess of 9 km s-1. Irregular test flow unsteadiness has limited the performance of previous expansion tubes, and it has been found that this can be avoided by attention to the interaction between the test gas accelerating expansion and the contact surface in the primary shock tube.
Test section measurements of pi tot pressure, static pressure and flat plate heat transfer rates are used to confirm the presence of quasi-steady flow, and comparisons are made with predictions for the equilibrium flow of an ideal, ionising, monatomic gas. Inviscid equilibrium theory, that does not account for shock reflection at the secondary diaphragm, is found to be somewhat limited in calculating the test flow condition at the end of the expansion. An approximate analytical theory has been developed for predicting the velocities achieved in the unsteady expansion of the ionizing or dissociating test gas.
The results of this work indicate that expansion tubes can be used to generate quasi-steady hypersonic flows at speeds in excess of Earth orbital velocity.