Free-Jet Testing of a REST Scramjet at Off-Design Conditions

Scramjet flowpaths employing elliptical combustors have the potential to improve structural efficiency and performance relative to those using planar geometries. NASA Langley has developed a scramjet flowpath that is integrated into a lifting body vehicle, transitions from a rectangular capture area to an elliptical throat, and includes an elliptical combustor. This Rectangular-to-Elliptical Shape Transition (REST) scramjet has a design point of Mach 7.1, and is intended to operate with fixed-geometry between Mach 4.5 and 8.0. This paper describes initial free-jet testing of a heat-sink REST scramjet engine model at conditions simulating Mach 5.3 flight. Combustion of gaseous hydrogen fuel at equivalence ratios between 0.5 and 1.5 generated robust performance after ignition with a silane-hydrogen pilot. Facilitymodel interactions were experienced for fuel equivalence ratios above 1.1, yet despite this, the flowpath was never unstarted by fuel addition at the Mach 5.3 test condition. Combustion tests resulting in engine unstart at reduced stagnation enthalpy indicated that the inlet/engine self-started following termination of the fuel injection. Engine data is presented for the largest fuel equivalence ratio tested without facility interaction. These results indicate that this class of three-dimensional scramjet engine can operate successfully at off-design conditions.