Comparison of a Direct-Connect and Freejet Dual-Mode Scramjet

Testing of identical dual-mode scramjet flowpath geometries in the freejet and direct-connect configurations were conducted in the ATK GASL Test Bay IV and the University of Virginia Supersonic Combustion facilities, respectively. These tests enabled a comparison study between the two configurations which was directed towards the determination of the effects of inlet distortion and back pressure on the performance and operability of a dual-mode scramjet. Bulk flow conditions were matched between the two facilities at the isolator entrance plane to simulate Mach 4.8 flight, and a series of metrics were established to quantify the similarities and differences of the two configurations. The effects of flowpath back pressure in the direct-connect case were seen to be isolated to regions close to the exhaust. Performance of the scramjet was generally well matched between the two configurations. An approximate 10% decrease in combustor pressure rise and consequently integrated pressure force were observed in the freejet configuration, however shock train length was unaffected. In terms of operability, it was determined that the decreased pressure rise led to a change in mode transition from an equivalence ratio of ~0.5 in the direct-connect to ~0.7 in the freejet configuration. In addition, ignition difficulties were experienced in the freejet tests which were not encountered in those of direct-connect, leading to a limited set of equivalence ratios available for comparison. Though somewhat limited in scope, this work represents the first detailed comparison in the literature of an identical direct-connect and freejet dual-mode scramjet. This provides important insight into the effects of inlet distortion and back pressure on performance and operability, the understanding of which are necessary for the effective extrapolation of ground test data to flight.