May 20, 2013
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.