Free-jet testing of a Mach 12 scramjet in an expansion tube

Abstract

Scramjet technology has the theoretical potential to provide air-breathing propulsion as a more efficient alternative to conventional rocket propulsion. Since the vehicle captures its oxidiser from the atmosphere — as opposed to carrying it like a rocket — the specific impulse can theoretically be increased by an order of magnitude, thereby increasing payload mass fractions. In this context, threestage hybrid rocket-scramjet-rocket launch systems have shown to potentially provide a cost-effective and flexible solution for satisfying the requirements of the small-satellites market. However, to be economically feasible, the proposed scramjet-powered second stage would be reusable. Rectangular-to-Elliptical-Shape-Transitioning (REST) engines have shown to be a viable concept that can be integrated into access-to-space vehicles operating between Mach 5 and 12. The half-scale Mach 12 REST engine is a research scramjet specifically designed to operate in the last part of the ascent trajectory. Previous studies have demonstrated the ability of this design to successfully operate at equivalent flight conditions of Mach 11.6, 30 kPa dynamic pressure. However, tests have never been performed in freejet mode at the design conditions, as the facility which was used — the T4 reflected shock tunnel — was limited, like all RSTs by the extreme total pressure requirements of a Mach 12 flight. These limitations are aggravated by the even higher pressure necessary for pressure-length scaling, used to conserve the flow similarity between the half-scale experimental model and the flight engine. To overcome these limitations and allow freejet testing with pressure-length scaling, the use of expansion tubes has been proposed. These are currently the only kind of facility capable of producing these high-pressure requirements. Currently, the University of Queensland operates the X3 expansion tube, which is one of the few facilities worldwide with the potential to produce both the required total pressures and sufficiently long test times (up to 1.5ms) to test an engine such as the Mach 12 REST engine. The goal of this thesis is to investigate and test for the first time, the Mach 12 REST engine in freejet mode, at fully replicated flight conditions. The study proposed using the X3 expansion tube, however, prior to this study the fastest scramjet test flow produced by X3 was Mach 10. Therefore, a significant part of this work tackles the extension of the X3 tunnel capabilities. A major element of the upgrade to X3 is a new hypersonic Mach 12 nozzle, which has been developed to allow higher Mach number flows and bigger core flow sizes. The nozzle profile has been designed via a parallel implementation of the Nelder-Mead optimiser coupled with a RANS flow solver. The designed, fully contoured nozzle is 2.8m long with an exit diameter of 573mm. A Mach 12 operating condition for the facility has also been developed and tested, replicating the flight condition at Mach 12 and 50 kPa dynamic pressure. Experimental measurements and computation have revealed that the new test condition has a Mach number of 11.1± 0.9, 52.2 kPa dynamic pressure across a useful test time of 1.3ms. The discrepancy to the target Mach number was due to an excessively thick boundary layer in the acceleration tube, which could only be addressed by