Even as the scramjet-powered X-51A Waverider ran out of fuel at 64,000 ft., sending the high-speed demonstrator hurtling into the Pacific to end the program's final flight on May 1, the U.S. Air Force Research Laboratory (AFRL)-led team already knew it had made hypersonic history.

In the 6 min. it took the diminutive vehicle to travel at up to Mach 5.1 over 230 nm to its watery grave, the X-51A program smashed every time and distance record for sustained, air-breathing hypersonic flight. The achievement means that nine years after starting the program, and two years after first flight, the X-51A team has finally proved the viability of a free-flying, scramjet-powered, endothermically fueled vehicle.

Now, as the business of data analysis begins, hypersonic planners are turning to what comes next. Although the X-51A success marks a first step to the potential use of hypersonic propulsion for long-range reconnaissance, transport and even the first air-breathing stage of a space-access system, the near-term application will be a missile. Initial follow-on steps will therefore be guided primarily by the requirements of the Air Force's high-speed weapon development road map and support of the High Speed Strike Weapon (HSSW), which is expected to be demonstrated at a baseline level around 2020. Theoretically, given this timescale, such a weapon could be available by the mid-2020s.

However, as history shows, the road to hypersonic development is littered with the remains of failed efforts and abandoned projects. Given recent test flops ranging from the Defense Advanced Research Projects Agency's (Darpa) HTV-2 and the Hypersonics Flight Demonstration (HyFly) vehicles to back-to-back failures of the X-51A in 2011 and 2012, the success of the May 1 flight could not have come at a better time for hypersonic propulsion.

“It kind of nudges the rock over the hill,” says AFRL X-51A program manager Charlie Brink. He notes that in the build-up to the test, “I started to see the tipping point was getting pretty close. The warfighter was starting to see the potential use of it and, with this test, the science and technology leadership is starting to get the message.” Joseph Vogel, director of hypersonics at Boeing Phantom Works and program manager for the Boeing-built X-51, says, “we realized we cracked the problem when we flew the vehicle [the first time in May 2010], and to get credit we needed to have real success—and that was this mission.”

Guided by results from the X-51A, Brink says researchers have a hit list of potential enhancements and improvements that will be used to develop the concept into a tactically relevant hypersonic weapon. The baseline speed and approximate size of the X-51A will continue to form the model for the HSSW, which will be compatible with the B-2A internal weapon bay and F-35 Joint Strike Fighter. Testifying to Congress in April, Deputy Assistant Air Force Secretary David Walker said, “it will also include a tactically compliant engine start capability and launch from a relevant altitude. The flight demonstration will be the first tactically relevant demonstration of Mach 5.0 plus air-breathing missile technology.”

“We foresee scramjet technology could be brought to bear to propel a light vehicle like X-51 in size anywhere between Mach 5 and 6 against targets 500 to 600 nm away within 10-12 min. It brings a whole set of responsiveness for the warfighter,” says Brink. The vehicle's operational altitude of 60,000-80,000 ft. “brings in a new aspect of survivability,” he adds.

“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.

The X-51A was originally aimed at testing in excess of Mach 6, but none of them exceeded Mach 5.1. Brink says early in the flight-test program, “we had a real long discussion of flight time versus the Mach 6 number, and the consensus was it was more important to show we could control the engine acceleration and fly it on out, run it dry and control it at hypersonic speed. I'm not saying we shot for 5.1. We thought it might be in the mid-Mach 5 range, but we found changes to inlet geometry [which was changed to a stiffer columbium-based alloy] made the wedge much thicker. Plus there were changes to cowling to incorporate the fourth engine. So we knew we had added drag counts.”

Following engine shutdown, the unpowered vehicle was commanded to perform various “parameter identification” maneuvers to characterize its aerodynamic handling and controllability. Three sets of data were collected at decreasing Mach numbers as part of evaluations which will help pave the way for future hypersonic testing and see “what it takes to contain and safely contain these vehicles,” says Jorris. This testing takes place during the unpowered phase of the flight because “we understood the engine parameters. Now we have to understand the vehicle itself, so with the engine off we can isolate the pure aerodynamic phenomenon.” The vehicle's response to specific pitch, roll and yaw inputs will be compared to pre-test predictions made by NASA Langley.

Testing was monitored until telemetry was lost at 20,000 ft., which was caused by loss-of-signal dropout between radar-tracking and telemetry-monitoring sites at Point Mugu and Vandenberg AFB. However a U.S. Navy NP-3D relay aircraft positioned down range was still receiving signals from the vehicle. “We believe we'll get more data all the way down to splashdown, but we still need to reduce that data,” says Brink. “We got all the data we wanted—we were ecstatic with the results.”