HIFiRE-6 Unstart Conditions at Off-Design Mach Numbers

HIFiRE 6 is a test flight to validate the use of adaptive flight control on a hypersonic vehicle. The Air Force Academy Department of Aeronautics conducted wind tunnel testing of the 10.5-inch HIFiRE 6 model at the off-design Mach numbers 3.0 (Re=9,765,000), 3.5 (Re=9,975,000), and 4.3 (Re=9,765,000) to support the development of the flight control algorithm for the test flight. By examining schlieren images, and pressure data from both the wind tunnel and inside the scramjet engine inlet and isolator, engine unstart of the three dimensional inlet was characterized for ranges of both sideslip angles and angles of attack: -5.4° to 5.7° and -5.4° to 14.8°, respectively. The pressure data that indicates the start and unstart conditions were captured by five pressure ports along the inlet and four more pressure ports along the isolator of the vehicle. The combined schlieren and pressure data indicated that the HIFiRE 6 scramjet would only start at the highest Mach number tested. At Mach 4.3 the scramjet design was found to start at -5.4° angle of attack and remain started until positive 6.0° degrees with no sideslip. However, once the vehicle became unstarted it remained unstarted until it was lowered again to -5.4° angle of attack. HIFiRE 6 was able to start at 0° angle of attack when a sideslip of -5.4° was applied and stayed started until positive 5.7°. Using the unstart results a performance map was created for Mach 4.3 test to indicate the flight orientations where unstart could be expected. The Kantrowitz limit was applied to the results to show the low Mach number limit is driven by the choking of the inlet and can be predicted for the three dimensional scramjet inlets.