In its first flight attempt, the X-51A WaveRider successfully completed what is now believed to be the longest supersonic combustion ramjet (scramjet)-powered flight ever recorded. Boeing Phantom Works performed the overall air vehicle design, assembly, and testing for the X-51A's various components. The X-51A program, which Boeing had worked on for the past seven years, is a collaborative effort of the Air Force Research Laboratory and DARPA, with industry partners Boeing and Pratt & Whitney Rocketdyne.
Engineers involved in the program believe that the X-51A has the potential to be the bridge for several applications, including global reach and commercial transportation, but will particularly pave the way to hypersonic weapons and future access to space. Since scramjets are able to burn atmospheric oxygen, they do not need to carry large fuel tanks containing oxidizer, such as conventional rockets, and are being explored as a way to more efficiently launch payloads into orbit.
"The shark-like shape of the 14-ft-long X-51A gives a hint to the technologies it is designed to explore," said Charlie Brink, X-51A Program Manager with the AFRL's Propulsion Directorate.
While it flew on May 26 autonomously for almost three and a half minutes at a top speed of Mach 5, powered by its PWR scramjet SJY61 engine, program managers had hoped, and expected, that the craft would fly for five minutes and accelerate to about Mach 6.
The longest previous hypersonic scramjet flight test performed by an X-43 in 2004 was faster but lasted only about 10 s and used what Brink described as “less logistically supportable hydrogen fuel.”
The X-51A reached the sky under the left wing of a U.S. Air Force Flight Test Center B-52H out of Edwards Air Force Base. It was released while flying at approximately 50,000 ft over the Point Mugu Naval Air Warfare Center Sea Range.
Four seconds later, a solid rocket booster from a U.S. Army tactical missile fired and propelled the X-51A to about Mach 4.5 before it and a connecting interstage were jettisoned. The X-51A's engine ignited on a mix of ethylene and JP-7 jet fuel, and then exclusively on JP-7. The vehicle's fuel-cooled engine design serves both to heat the JP-7, which, due to its flashpoint and thermal stability, was also used by the SR-71 Blackbird, to an optimum combustion temperature and to help the engine itself endure extremely high operating temperatures during the long burn.
The flight reached an altitude of about 70,000 ft, as it transmitted telemetry data to ground stations before breaking up in the ocean, as planned. While development of the vehicle's engine and the test program are complex, controlling costs was a key objective, thus the team elected not to build recovery systems into the flight test program.
Even before analyzing the terabytes of telemetry data transmitted by the X-51A during flight, officials called the test an unqualified success.
"We built four test vehicles to get a successful flight, and we hit many of our goals right out of the gate, the first time around," said Brink.
Onboard sensors transmitted data to both an airborne U.S. Navy P-3 Orion and to ground systems at Point Mugu, Edwards, and Vandenberg Air Force Base before the X-51A was terminated. The team plans to review the data from the test before scheduling additional flights with the three remaining test vehicles. While the heart of the system is the scramjet engine, other key technologies of concern with X-51A engineers include thermal protection systems materials, airframe and engine integration, and high-speed stability and control.
The May 26 hypersonic test was the third time the X-51A has flown, though in both previous flights the aircraft remained attached to the B-52's wing. The first captive carry flight this past December verified the B-52's high-altitude performance and handling qualities with the X-51A attached and tested communications and telemetry systems. The other flight, intended essentially as a dress rehearsal for the actual hypersonic flight, took place earlier this year.
The original plan for the four test vehicles was to fly first in December 2009 and then three more times this year. That changed due to "a combination of factors," according to the program team, including access to supporting flight test and range assets.
"We knew the original schedule was aggressive and we would need to be flexible," said Brink. "It's also expensive to keep a staff of engineers and support staff at the ready and then not be able to fly when supporting assets aren't available. So we elected to make only one hypersonic try this spring and then pause for a few months to conserve funding."