Freejet Testing of the HIFiRE 7 Scramjet Flowpath at Mach 7.5

Abstract

Results are presented on the freejet testing of a 75%-scale replica of the HIFiRE 7 scramjet. The HIFiRE 7 scramjet flowpath includes a two-dimensional forebody, a rectangular-to-elliptical shape transition inlet, an elliptical combustor, and a thrust nozzle. Only porthole injectors are used, giving the internal flowpath a clean configuration suitable for high-Mach-number operation with no physical obstructions to the flow. Furthermore, the structure of the shock waves in the scramjet is tailored to introduce a shock-wave/boundary-layer interaction in the combustor to promote fuel ignition. The objective of the experiments is to investigate the performance of this scramjet flowpath at a simulated flight Mach number of 7.5 and an altitude of 29.5 km with gaseous hydrogen as the fuel. Static pressure measurements show that robust combustion can be sustained in the flowpath at a range of equivalence ratios between 0.48 and 0.84. Based on these experiments, a fuel equivalence ratio of 0.8 is recommended for the flight. Corresponding surface heat transfer measurements reveal that, when the fuel–air mixtures ignite and burn, the surface heat transfer levels in the combustor and nozzle increase to as much as three times the fuel-off levels. A quasi-one-dimensional cycle analysis of the tests shows that the overall fuel-based combustion efficiency of this engine is 89% at the fuelling conditions planned for the flight.