There is a great interest in decreasing the access to space cost by the use of reusable vehicle technologies, and dramatic propellant weight reductions may be produced by the use of Scramjet powered concepts.
Air-breathing trajectories for access to space are characterized by a specified dynamic pressure range, bounded by combustion and structural/thermal requirements. For the later stages of the envisaged flight, meeting this requirement combined with high Mach numbers (10-15) results in total pressures of the order of gigapascals. In addition to that,
ground testing may require sub-scale models (scaled versions of a flight configuration), and an approximate scaling of the flight condition dictates that the product of pressure by length has to be maintained to keep similarity of the flow. As a result of that, the total pressure requirement is increased even further, proportional to the static pressure product. The generation of such flows for ground-testing is restricted to facilities where the gas is not stagnated at any stage, which currently means that only expansion tunnels such as UQ's X2 and X3 free piston driver facilities, are suitable for these tests.
The X3 facility has been configured for Scramjet testing in the Mach 10-15 corridor with the addition of new hardware, a model support, a fuel injection system, and upgrades on the data acquisition system.
A Mach 10 condition 32Km altitude, was developed in X3, which matched those previously tested in T4 and X2 (pressure-length scaled), by McGilvray (McGilvray_2008a). Another high pressure condition was also developed in X3 maintaining the Mach number and increasing static pressure, reaching 1Gpa of total pressure. The same Hycause Scramjet model tested in T4 and X2 (scaled), was tested in X3 for nominal and high pressure condition.
This is the highest total pressure at which a nose-to-tail Scramjet has been ground tested in the open literature, serving as a cornerstone for future exploration of the upper part of the access to space trajectory Mach corridor in X3 facility. Equally importantly is the fact that the results are comparable to those reported for the nominal condition, which is in the
limit of T4 reflected shock tunnel operational capability. This means that the envelope overlap of T4 and X3 facilities have been factually demonstrated, providing strong experimental support to the statement that expansion tubes can extend the envelope of existing hypervelocity facilities beyond their total pressure limits for Scramjet testing.
During the conditions development process, a review of the current experimental, analytical and numerical tools for the analysis of expansion tunnels, was performed. This included the first two dimensional expansion tunnel CFD simulation with coupled piston kinematics, reported yet in the literature. Some interesting flow features were found when analysing the facility that are briefly detailed as follows.
Developing the Scramjet conditions required high piston velocities which may choke the piston launcher section, as it was demonstrated for the X2 facility. This may explain the necessity of adjusting a launcher pressure loss coefficient in the facility one-dimensional previously used model to match the compression tube behaviour.
The piston deceleration creates longitudinal waves that experience multiple reflection between the end of the compression tube and the piston, and when the primary diaphragm opens, they are processed by the primary driver area change generating substantial amount of noise that propagates downstream. The secondary driver reflected wave interferes with the u+a characteristics generated at the piston sending a reinforcement wave downstream that interferes with the test gas. This effect was previously investigated by Gildfind (Gildfind_2012a). The acceleration tube flow behaviour is affected by viscous
boundary layer growth, which reduces test time through the Mirels (Mirels_1963b) effect and also perturbs flow uniformity. In any case, approximately 1msec of usable test time is found, as predicted by McGilvray (McGilvray_2008a).
The Hycause model was tested with the developed conditions. It is a three ramp inlet compression model, with inlet porthole injection, constant area combustor, and ramp expansion nozzle. A cross comparison between X3, X2 and T4 model pressure
distributions for the nominal condition was made, with the aid of tridimensional reacting flow numerical simulations. Good agreement between facilities results was found at the inlet but the combustor show some disagreement attributed to slight model geometric differences and minor differences in facility free stream inflow. Some combustion is found in the numerical model at the inlet, but it may be attributed at his stage to deficiencies of the mixing and combustion numerical models.
Experiments and modelling were repeated for the high pressure condition aiming to compare the differences due to scaling. Good agreement and scalability is found for non reacting cases, but cases with combustion show differences due to combustion scaling, with an important contribution coming from inlet combustion.
Based on these thesis results, it is encouraged the development and upgrade of the facility with a new lightweight piston, reservoir extension, and high Mach nozzle in order to explore the Mach 10-15 access to space corridor.