“The next step is to marry scramjet propulsion with sensors, and logistically supportable fuel,” Brink says. In place of JP-7, a special jet fuel with a high flash point originally used by the SR-71, he says studies will evaluate if an HSSW could use RP1, RP2 or the denser jet fuel JP-10. “But how much endothermic capability does it have?” he asks. Further work will also explore the use of pyrotechnic cartridges to cold-start the scramjet in place of ethylene, building on recent wind-tunnel tests at NASA Langley Research Center.
Engine improvements are also being studied by Pratt & Whitney Rocketdyne, says program manager George Thum. “This [SJX61-2] architecture was frozen in time, so the future is simplification. We are starting to apply jet and rocket-engine heritage to make it more product-like. The full authority digital engine control (Fadec) is a prime example of the things that are being simplified.” The X-51A used a relatively bulky F119 engine Fadec, but a future HSSW controller could be a simpler processor card integrated with the fuel pump, adds Brink. A future engine is also likely to be controlled more smoothly with a feedback loop linking pressure sensors around the isolator position with the fuel pumps.
As the Air Force expects advanced munitions such as the HSSW to be operating in contested environments in which the GPS signal is either degraded or perhaps even denied, an array of new guidance technologies are being explored. According to Walker, these include “technologies that expand upon our current anti-jam GPS navigation capabilities and novel technical approaches to navigation such as optic-field flow techniques and multisensor fusion.” Brink adds that “if you want to guide over enemy territory, you want some sort of sensor up-front.”
Brink, Vogel and other X-51A team members from partners Pratt & Whitney Rocketdyne, Darpa, NASA, Navair and the Air Force's 412th Test Wing, witnessed the flight from a mission control room at the Kirk Flight Test Center at Edwards AFB, Calif. “It was pretty nail-biting, with a lot of tension,” says Vogel. Adding to the drama, the takeoff of the B-52H mothership carrying the X-51A was delayed owing to fog at the Navy installation at Point Mugu, Calif., the control center for the range over which the vehicle was tested.
The B-52H finally lifted off from Edwards with the demonstrator and its Army Tactical Missile System (Atacms) booster mounted under the port wing.
The flight crew, commanded by Maj. Andrew Murphy, was also dealing with the limitations of a minimum fuel load that was needed to lighten the bomber sufficiently to transport it to the launch point at 50,000 ft. “They had literally one shot to be on condition and pointed in the right direction to get the green light,” says Lt. Col. Timothy Jorris, director of the 412 Test Wing, Hypersonics Combined Test Force.
On reaching the launch point south of the Channel Islands and northwest of San Nicholas island, the X-51A was dropped at Mach 0.8. The Atacms ignited and propelled the entire 25-ft.-long stack—including the booster, inter-stage and X-51A cruiser—for 29 sec. until it reached 63,000 ft. and Mach 4.9. The cruiser separated and coasted to Mach 4.8 before the scramjet was started using ethylene. The scramjet then transitioned to JP-7 hydrocarbon fuel, successfully overcoming the point at which the second flight failed in June 2011, when “we unstarted the engine and we lost control of inlet dynamics,” says Brink. The X-51A flew for another 210 sec. under scramjet power, climbing to 64,000 ft. with a constant dynamic pressure (q) trajectory of 2,200-2,350 lb. per square foot. Peak acceleration was over 0.2g, notes Brink.
The vehicle accelerated to Mach 5.1 from Mach 4.8 and was still accelerating “when the tank ran dry,” says Vogel. “We staged the fuel in flight and were halfway through the second staging and going well when we ran out of fuel,” Brink adds. The initial start sequence involves spraying fuel toward the aft end of the combustor to ensure the shock train from the combusting fuel-air mix is not pushed out of the inlet, causing an “unstart.” As the speed builds, the fuel is injected further forward to match the changing pressure profile in the inlet and to generate a greater rise in thrust. This had not been possible with the first flight, which was cut short when the vehicle suffered a nozzle leak 65 sec. into the flight.
With the May 1 flight, “we were able to not only have the initial spray sequence, but in flight but we saw signs of acceleration out of the vehicle when we staged it. That was to me the one little check-mark we didn't get out of the first flight,” says Brink.