Design of a 3D shape transitioning nozzle and experimental thrust measurements of an airframe integrated scramjet

Abstract

The scramjet engine provides the means for airbreathing propulsion at flight speeds exceeding Mach 5. As part of a three-stage-to-orbit system it is envisioned to play a major role in sustainable access-to-space. When compared to a rocket engine, scramjets offer a higher specific impulse and increased mission flexibility through the manoeuverability of the vehicle.At the scramjet's operating Mach number range, the aerodynamic drag forces acting on the vehicle require the engines to be highly efficient and to be integrated with the airframe to be able to produce net thrust. Scramjet engines with three dimensional shape transition inlets and nozzles allow for modularity of the propulsion system, engine-airframe integration and high efficiency of the individual engine components. The present study introduces a methodolgy to design three dimensional shape transition nozzles and experimentally investigates an airframe integrated Mach 12 scramjet fitted with a nozzle which was designed with the proposed methodology.The nozzle design method is based on the rectangular-to-elliptical shape transition (REST) inlet design. A parent flowfield with the desired nozzle flow properties was designed first. In the present study the method of characteristics (MOC) was used to create an axisymmetric parent flowfield with a contoured centre body. The centre body creates a second set of expansion waves which ensure horizontal flow at a prescribed neutral streamline within the flowfield. This approach avoids the creation of forward facing surface area in the design; a common problem in similar design approaches. Rao's method of thrust optimisation was used to determine the length of the parent flowfield. In his approach, Rao defines a control surface over which the inviscid thrust of an axisymmetric nozzle is maximised. The maximum can be found by constraining the nozzle's length and the inflow and ambient conditions. Next, the desired combustor exit cross-section is discretised by a suitable number of points and each point is streamline traced through the parent flowfield. By combining the streamlines to a surface, a nozzle can be created that recreates a part of the parent flowfield and interfaces with the combustor of the scramjet powered vehicle. The streamline tracing is repeated for the desired cross-section at the nozzle exit. The last step consists of blending these two nozzles in such a way that a smooth surface is obtained. The final result is a quasi streamline traced nozzle that, in the present study, transitions in shape from an elliptical cross-section at the combustor exit to a rectangular cross-section at the nozzle exit. Cross-sections with different geometries can be used as required.The proposed parent flowfield allows for high design flexibility through the use of the contoured centre body while at the same time avoiding the creation of drag surfaces which were observed in previous work.A shape transition nozzle was designed for an airframe integrated Mach 12 REST scramjet model. Numerical simulations of the parent flowfield and the shape transition nozzle showed that the thrust performance of the shape transition nozzle is similar to that of the parent flowfield.The experimental model was modified to accommodate an on-board fuel system and catch and release mechanisms suitable for free-flight experiments. Fishing line was used to suspend the model initially. Once the facility flow started the strands were pushed against razor blades installed on the model and it was released. An image tracking based force measurement technique was then used to track the motion of the model and from the determined trajectories derive the forces on the free-flying model.The model was tested at a Mach 8 test condition with a dynamic pressure of approximately 80 kPa in the T4 shock tunnel at the University of Queensland. Fuelled experiments at various equivalence ratios with a combined injection scheme through two sets of fuel injectors were conducted; the first set located in the inlet and the second in the combustor. Approximately 40 % of fuel was injected into the inlet and 60 % into the combustor. It could be shown that the engine started successfully, robust combustion was achieved and the model was unrestrained during the test time. The results demonstrate that, despite vibrations of the experimental facility, the net forces in two dimensions were successfully measured. A distinct increase in thrust proportional to the equivalence ratio could be measured. Whether the engine produced positive net thrust during the experiments could not be determined due to the measurement uncertainty. However, a combination of results from cycle anlysis and CFD at the Mach 8 condition suggests that the model does produce positive net thrust. Positive lift was observed at all fuelling conditions. Measurements of the pitching moment were found to be unreliable due to large measurement uncertainties.The present study marked the first time that forces were measured on a free-flying experimental model in the T4 shock tunnel.