Development of New X2 Expansion Tube Test Conditions to Facilitate Low Density Non-Equilibrium Testing at 4 to 6 km/s

High-quality air chemistry models are fundamentally important for designing hypersonic vehicles, as post-shock thermochemical non-equilibrium chemistry can have large effects on heat transfer, forces and moments, vehicle control, wake signatures, and vehicle communications. The Modified Marrone-Treanor (MMT) thermochemical non-equilibrium air chemistry model has recently been developed at the University of Minnesota which uses state-of-the-art quantum chemistry calculations to derive the model. This paper describes the start of a project which aims to produce MMT validation data in UQ's X2 free-piston driven expansion tube, an impulse wind tunnel for the study of high-enthalpy hypersonic flight. Expansion tubes like X2 can generate test conditions similar to reflected shock tunnels but with a much lower maximum flow temperature during the processing of the test flow. This reduces chemical freezing effects. X2 conditions also generally have a higher Mach number. This means that the freestream temperature is lower and the bow shock is stronger, which is more similar to flight. While this work is still ongoing, this paper describes the development of new X2 driver conditions to support the project and the development of new low density X2 test conditions from 4 to 6 km/s which are needed for the project.