An Air Force Flight Test Center B-52 Stratofortress from Edwards Air Force Base, Calif., carries an X-51A Waverider prior to the scramjet’s first hypersonic flight test on March 26, 2010. Air Force engineers have made design modifications to the three remaining X-51A demonstrators and plan to resume the hypersonic flight test program in early spring of 2011. (U.S. Air Force photo)

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Modified X-51A Waverider ready for next hypersonic test

Published March 16, 2011
By Derek Kaufman
88th Air Base Wing Public Affairs

WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- The Air Force currently plans to fly its second X-51A Waverider hypersonic flight test demonstrator as early as March 22, program officials said March 15.

"We are proud of the first flight results, and at the same time we understand the inherent risk in a high-technology demonstrator like the X-51A. We can't wait to get this second vehicle in the air and show what we can do," said Curtis Berger, Director, Hypersonics Programs, Pratt & Whitney Rocketdyne, which built the X-51A's fuel-cooled supersonic combustion ramjet, or scramjet engine.

Four X-51As were built for the Air Force by Boeing and Pratt & Whitney Rocketdyne. The X-51A made history on its inaugural hypersonic flight test on May 26, 2010 when it was launched from Edwards Air Force Base, Calif. tucked under the wing of a B-52, released, and ultimately accelerated to Mach 5 under scramjet power. It was about 10 times longer than any previous hypersonic scramjet flight and "80 to 90 percent" of flight test objectives were achieved, program officials said.

Underscoring the complexity and uncertainty of hypersonic flight testing, Air Force Research Laboratory X-51A program manager Charlie Brink noted that not everything went perfectly on the first flight test. The vehicle failed to accelerate as quickly as anticipated and the flight test had to be terminated after 143 seconds under scramjet power. A perfect flight would have lasted another 100 seconds and accelerated the X-51A cruiser to Mach 6.

After the flight, members of the flight test team independently scoured over telemetry data for a month. Then they conducted a comprehensive "fault tree analysis" to identify every piece of anomalous data to determine the root cause. According to Mr. Brink, two separate fault trees were identified: The vehicle failed to accelerate as rapidly as expected and unexpected temperatures and pressures were observed in internal sections of the cruiser. Engineers examined and walked through 156 different nodes in excruciating detail in search of a cause.

"In a demonstrator you learn things," Brink said. Program officials already knew from wind tunnel engine tests about the intense heat the scramjet engine and hypersonic flight creates. During flight, the scramjet engine actually grows about three-fourths of an inch. The effect complicates design for such things as interface seals.

The Boeing "Phantom Works" and Pratt & Whitney Rocketdyne teams pulled the engines from the three remaining flight test vehicles and focused on the interface between the rear of the fuel-cooled engine and its vehicle mounted nozzle. Brink said the effort identified an "apparent thermal seal breach" at the interface which was not as tight as it needed to be. This caused some of the hot gases that should have provided thrust to leak into the rear of the cruiser.

"We went through a complete critical design review of the interface," Mr. Brink said. In the end, the team made design changes to make it a "much more robust" interface. All of the remaining X-51As have been modified with the new beefed up design.

Mission and weather permitting, a B-52 test crew will take off March 16 from the Air Force Flight Test Center and the flight test team will run through a dress rehearsal for the entire flight profile for the next hypersonic mission, albeit without the X-51A attached.

The second X-51 is waiting in a hangar at Edwards Air Force Base and is ready to fly, Mr. Brink said. "We are shooting for March 22," for the next flight over the Navy's Point Mugu Sea Range but a number of factors with the flight test vehicle, weather, range availability, and supporting test assets could slip the flight date to the right.

Brink said the Defense Advanced Projects Research Agency and NASA have been critical to the X-51As development. DARPA provided substantial funding and NASA strong support via access to its famed wind tunnels at Langley Research Center, and the supercomputer at NASA Ames Research Center where computational fluid dynamics calculations of drag and other aerodynamic forces were performed. He added the computations were validated during the first hypersonic test, coming in within 2 percent of actual data observed during the first flight. NASA Dryden aircraft also provide photo and safety chase during the flight tests.

Mr. Brink noted the X-51A was not designed to be a weapon, but its success as a technology demonstrator may soon enable the transition of technologies to a new class of hypersonic weapon systems. He added that there are a number of initiatives in the works, but none had been decided upon and there currently is no program of record for a hypersonic strike or ISR aircraft based upon the Waverider.

"Right now we are just focused on the X-51's next flight. We definitely hope it will go longer...and faster than the first," Mr. Brink said.