2010 X-51A Scramjet Engine Test Team

Nomination for SFTE James S McDonnell Award

The X-51A Scramjet Engine Demonstration Integrated Test Team is nominated for the James S. McDonnell Flight Test Team Award for accomplishing the first long duration scramjet powered flight in the hypersonic regime. To accomplish the X-51A flight test, the team integrated resources from the Air Force Flight Test Center (AFFTC), NASA Dryden Flight Research Center (DFRC), and Naval Air Warfare Center (NAWC) at Point Mugu NAS. The Integrated Test Team consisted of Air Force Research Laboratory (AFRL), Boeing, Pratt & Whitney Rocketdyne, AFFTC, NAWC, NASA DFRC, Air Force Seek Eagle office, and B-52 SPD personnel.

The X-51A is an expendable hydrocarbon fueled, hypersonic scramjet engine demonstrator with the primary objectives of demonstrating hypersonic propulsion technology and collecting hypersonic flight data. The test vehicle consists of a scramjet powered cruiser, interstage, and modified Army Tactical Missile System (ATACMS) booster. The X-51A is released from a B-52H at 50,000 feet and Mach 0.8. The booster then ignites accelerating the X-51A to 60,000 feet and Mach 4.5+ where the booster and interstage separate from the cruiser. The scramjet then ignites and burns for 4 1â2 minutes accelerating the cruiser to hypersonic speeds. After depleting its fuel, the cruiser impacts the ocean without recovery.

This program was complex and challenging to execute, being high risk for both technical and safety concerns. Balancing technical risk and programmatic pressures, as well as finding ways to mitigate safety concerns within the budgetary and technical constraints were key elements in executing this test. The balancing act of this test was achieved with thorough planning, an incremental build up process, and flexible execution which culminated in a successful test.

Test planning began years ahead of time and included extensive simulation, computational fluid dynamics analysis, and wind tunnel testing. Ground testing and analysis identified the desired engine start conditions and specified minimum temperatures and pressures for the ethylene, JP-7, and ATACMS booster. The detailed timeline of ground and flight events required to meet the test constraints was extremely aggressive. Further planning and analysis targeted the X-51A release and booster trajectory to place the scramjet engine at the most optimum point in the sky for igniting the scramjet engine, ensuring a technically feasible profile while assuring the safety of the B-52 and chase aircrew.

Safety concerns of the program were addressed through an exhaustive AFFTC safety process, along with a buildup approach to the flight test. The launch conditions were at the edge of the flight envelope for the B-52 where it is susceptible to engine stalls and above its engine re-start altitude. It is at the edge of its environmental capabilities and above the rating of the crew’s oxygen system should the B-52 environmental system fail and the cabin depressurize. Moreover, the B-52 is required to takeoff at minimum fuel in order to be able to climb to the launch conditions where it has limited maneuverability which complicates meeting the tight positional tolerances required for launch. The chase aircraft required afterburner to stay in position making fuel planning for the chase aircraft critical. These challenges were met by aircrew proficiency flights, a captive-carry of the X-51, and several a dress rehearsal flights of the mission in preparing for the test.

The buildup process worked and several key lessons were learned. The original F-16 chase aircraft were replaced by NASA F/A-18s which had more capable environmental systems at the launch conditions. The B-52 aircrew workload was addressed by tabletop exercises and coordination meetings with the controlling agencies to eliminate crunch points for the crew.

Flexibility was a key element of executing this test within the fiscal and schedule constraints of the program. On the captive carry sortie, the team was able to reschedule the critical telemetry and range assets to allow the flight to slip for six hours for a maintenance delay. Saving this flight precluded a month long delay before the test assets would be available again. On two other occasions the team was able to reschedule range assets to repeat a test following mission aborts, saving six weeks of schedule on each occasion. Finally, when flying the final dress rehearsal flight, the team developed a plan to accomplish the testing requirements without carrying the X-51. Eliminating the X-51 saved the program over $500K per flight because of the reduced manpower requirements to prepare and support the X-51.

After losing the primary launch date to a range incursion the following day was the only backup available. Losing the flight would have resulted in delaying the program six months due to programmatic fiscal constraints. After some initial problems on the range, Pt Mugu was able to successfully clear the range and allow the test to proceed. During the test, the planning and training paid off and the timeline executed extremely well. Training once again paid off as the aircrew encountered adverse winds on the range which pushed the B-52 over 4 miles off course in setting up for the launch. In spite of the limited maneuverability of the B-52 at the launch conditions, the aircrew was able to achieve the launch point on conditions within 600ft of the intended location.

The launch of the X-51A went as the planning and analysis predicted it would be. The booster accelerated the vehicle at over six g’s to the engine start envelope where a nominal stage separation and scramjet ignition occurred. Transition to JP-7 was as expected although vehicle acceleration was slightly less than hoped. The vehicle had nominal performance except for isolated temperatures which were beginning to rise. After 144 seconds of the planned 270 second burn, the vehicle began to decelerate and descend in what appeared to be a glide. Telemetry was lost and shortly after as planned, Pt Mugu terminated the vehicle.

The mission was an unqualified success. Analysis of the X-51 flight has been ongoing, but the goal is to make the X-51 fly further and faster on subsequent flights. The X-51A Waverider cruiser achieved approximately 4.85 Mach and cruised at that that condition for 144 seconds. This is over 10 times longer than any previous scramjet operation in free flight within earth’s atmosphere. The flight has already gone down in history as a major historical event, being compared to the first flight of the jet engine and igniting serious interest in applications for future operational vehicles.