Modified X-51A Waverider ready for next hypersonic test

by Derek Kaufman, 88th Air Base Wing Public Affairs

WRIGHT-PATTERSON AIR FORCE BASE, Ohio (AFNS) -- Air Force engineers currently plan to fly the 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," said Curtis Berger, the director of the hypersonics programs at Pratt & Whitney Rocketdyne, the company that built the X-51A's fuel-cooled supersonic combustion ramjet, or scramjet engine. "We can't wait to get this second vehicle in the air and show what we can do."

Four X-51As were built for the Air Force by teams at 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 Stratofortress. After release, it ultimately accelerated to Mach 5 under scramjet power. The flight 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, Charlie Brink, the Air Force Research Laboratory X-51A program manager, 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," Mr. 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. Mr. 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 at Edwards AFB 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 AFB and is ready to fly, Mr. Brink said. He added the next flight is scheduled for March 22 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.

Mr. Brink said officials at the Defense Advanced Projects Research Agency and at NASA have been critical to the X-51A's development. Those at DARPA provided substantial funding and NASA officials provided strong support via access to their wind tunnels at Langley Research Center and to the supercomputer at NASA Ames Research Center where computational fluid dynamics calculations of drag and other aerodynamic forces were performed.

Mr. Brink said the computations were validated during the first hypersonic test, coming in within 2 percent of actual data observed during the first flight. NASA Dryden aircrews also provide photo and safety chase during the flight tests.

He noted the X-51A was not designed to be a weapon, but its success as a technology demonstrator soon may 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," Mr. Brink said. "We definitely hope it will go longer...and faster than the first."

X-51A Waverider flight planned for May 25

WRIGHT-PATTERSON AIR FORCE BASE, Ohio (AFNS) -- Weather permitting, Air Force officials said the X-51A Waverider will make its first hypersonic flight test attempt May 25 after it is released from a B-52 Stratofortress off the southern coast of California.

The unmanned X-51A is expected to fly autonomously for five minutes, powered by a supersonic combustion scramjet engine, accelerating to about Mach 6 and transmitting vast amounts of data to ground stations before breaking up after splashing down into the Pacific, as planned. There are no plans to recover the flight test vehicle, one of four built.

"In those 300 seconds, we hope to learn more about hypersonic flight with a practical scramjet engine than all previous flight tests combined," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory's Propulsion Directorate here.

The longest previous hypersonic scramjet flight test performed by a NASA X-43 in 2004 was faster, but lasted only about 10 seconds and used less logistically supportable hydrogen fuel, Mr. Brink said.

The X-51A program is a collaborative effort by representatives from the Air Force Research Laboratory and the Defense Advanced Research Projects Agency, with industry partners The Boeing Company and Pratt & Whitney Rocketdyne.

The X-51 will depart Edwards Air Force Base, Calif. and be carried aloft under the wing of a B-52 belonging to the Air Force Flight Test Center there. It will be released at approximately 50,000 feet over the Point Mugu Naval Air Warfare Center Sea Range. A solid rocket booster will accelerate the X-51A to approximately Mach 4.5, before being jettisoned.

Then the cruiser's scramjet engine, remarkable because it has virtually no moving parts, will ignite. Burning the same JP-7 jet fuel once used by the SR-71 Blackbird, it will accelerate the X-51A to Mach 6 as it climbs to nearly 70,000 feet. Hypersonic combustion generates intense heat so routing of the engine's own JP-7 fuel will serve to both cool the engine and heat the fuel to optimum operating temperature for combustion.

A U.S. Navy P-3 Orion will aid in transmitting telemetry data to people at both Naval Air Station Point Mugu and Vandenberg AFB, Calif., before it arrives at its final destination: Ridley Mission Control Center at Edwards AFB.

The May 25 hypersonic test will actually be the third time the X-51 has flown, but in each previous instance it has remained attached to the B-52's wing. The first captive carry flight Dec. 9, 2009, verified the B-52's high-altitude performance and handling qualities with the X-51 attached and tested communications and telemetry systems. The other flight, intended essentially as a dress rehearsal for the hypersonic flight, took place earlier this year.

Program officials said this will be the only hypersonic flight attempt this fiscal year, a change from the original test plan which was to fly in December 2009 then three more times in 2010.

A combination of factors, including access to supporting flight test and range assets, was cited as the reason for the pause. Availability of the Air Force Flight Test Center's B-52, which has been in high demand to support a number of other high-priority weapon system tests and is readying to undergo periodic depot-level maintenance later this year, was noted.

"This is an experimental X-plane and it's a complicated test. We knew the original schedule was aggressive and we would need to be flexible," Mr. Brink said. "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."

Mr. Brink called the test "a major team effort" by AFRL, DARPA, Air Force Flight Test Center, NASA, the U.S. Navy, Boeing and Pratt & Whitney Rocketdyne.

The shark-like shape of the 14-foot long X-51A cruiser gives a hint to the technologies it is designed to explore, Mr. Brink said. Virtually wingless, it is designed to ride its own shockwave, thus the nickname, Waverider. The heart of the system is its Pratt & Whitney Rocketdyne SJY61 scramjet engine, but other key technologies that will be demonstrated by the X-51A include thermal protection systems materials, airframe and engine integration, and high-speed stability and control.

Officials said the X-51A program will pave the way to hypersonic weapons and future access to space. Since scramjets are able to burn atmospheric oxygen, they don't need to carry large fuel tanks containing oxidizer like conventional rockets, and are being explored as a way to more efficiently launch payloads into orbit.

X-51A WaveRider gets first ride aboard B-52

An Air Force Flight Test Center B-52H Stratofortress performs a captive carry flight of the X-51A WaveRider Dec. 9, 2009, over Edwards Air Force Base, Calif. The captive carry mission is a key test milestone to ready the X-51's scramjet engine during the vehicle's first powered hypersonic flight in early 2010. (U.S. Air Force photo/Mike Cassidy)

by Derek Kaufman, 88th Air Base Wing Public Affairs

WRIGHT-PATTERSON AIR FORCE BASE, Ohio (AFNS) -- The X-51A Waverider was carried aloft for the first time Dec. 9 by an Air Force Flight Test Center B-52 Stratofortress
over Edwards Air Force Base, Calif.

The test was a key milestone in preparation for the X-51 to light its supersonic combustion ramjet engine and propel the WaverRider at hypersonic speed for about five minutes, before plunging into the Pacific Ocean.

"This was a great day for the program," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory at Wright-Patterson AFB, Ohio. "The early look is we successfully captured all of our test points without any anomalies. I'm really proud of the AFRL, Air Force Flight Test Center, and Boeing/Pratt Whitney Rocketdyne teams' efforts to move us toward the big event. That flight test is currently planned in about two months, he said.

The Dec. 9 test, reminiscent of the early days of the historic X-15 program 50 years earlier, was conducted entirely in the airspace over Edwards AFB while various systems and telemetry were checked out. The Flight duration was 1.4 hours.

"After takeoff we climbed to 50,000 feet and verified B-52 aircraft performance, handling qualities with the X-51A attached to the B-52, control room displays and software integration with the X-51A," said Lt. Col. Daniel Millman, a B-52 project pilot. "The B-52 handled great and the flight preceded as planned."

The next B-52/X-51 flight test mission is expected in mid-January, and will be a full dress rehearsal, for its first hypersonic test flight, now planned for mid-February, Mr. Brink said. The dress rehearsal flight will depart Edwards AFB and head out over the Pacific to Point Mugu Naval Air Warfare Center Sea Range. Both airborne and multiple ground test assets will monitor all X-51A systems, but it will not be released from the B-52 and its engine will not ignite.

In February, the Air Force Flight Test Center's B-52 will carry the unmanned X-51A to approximately 50,000 feet over the Pacific Ocean then release it. A solid rocket booster from an Army Tactical Missile System will then ignite and accelerate the X-51 to about Mach 4.5. Then the booster will be jettisoned and the X-51A's supersonic combustion ramjet propulsion system will ignite and operate for about 300 seconds, propelling the cruiser to more than six times the speed of sound.

Onboard sensors will transmit data to ground systems before the cruiser plunges into the Pacific. There are no plans to recover it.

Four X-51A cruisers have been built and were developed by officials from the Air Force, Defense Advanced Research Projects Agency, Pratt & Whitney Rocketdyne and Boeing.

Pratt & Whitney Rocketdyne built the X-51's scramjet engine, which is the heart of the system. Engineers expect a great deal will be learned about hypersonic flight during the nearly 300 seconds under scramjet power. The longest-ever previous scramjet test, lasted only about 10 seconds, Mr. Brink said.

As the engine ignites it will initially burn a mix of ethylene and JP-7 fuel before switching exclusively to JP-7. The X-51's fuel-cooled engine design will both heat the JP-7 to an optimum combustion temperature and help the engine itself endure extremely high operating temperatures during the long burn.

"We believe the X-51 will lead the path to practical air breathing hypersonic flight," said George Thum, Pratt & Whitney Rocketdyne X-51A program manager.

Boeing's Phantom Works performed overall air vehicle design, assembly and testing for the X-51's various components.

"We're elated with the success of the captive carry and looking forward to our powered flight tests. The captive carry test was the culmination of many months of hard work by the X-51A team to verify that hardware, electrical and software integration was complete," said Joseph Vogel, a Boeing Phantom Works X-51A program manager.

Mr. Vogel called the mission an essential step to ready for the dress rehearsal, where the team will run through the processes of a complete mission profile in final preparation for the X-51's first powered flight early next year.

Previous important ground tests verified the X-51 and B-52 could communicate electronically and that X-51A can be safely and successfully uploaded and downloaded on the modified B-52H pylon. Wind tunnel tests and modeling to ensure safe separation from the aircraft were other prerequisites.

Program officials said the scramjet motor's great advantage is the ability to capture and burn oxygen in the thin atmosphere, rather than having to carry it in a large tank like the space shuttle or other rockets. Not having to carry the oxidizer needed for combustion means more payload capability.

Beyond scalable scramjet propulsion, other key technologies that will be demonstrated by the X-51A include thermal protection systems materials, airframe and engine integration, and high-speed stability and control.

Boeing officials said they also had to demonstrate the X-51A 's compatibility with the electromagnetic environment of the B-52H aircraft. The test verified that the X-51A can be powered by and communicate with the B-52H using a modified Joint Direct Attack Munition interface.

"Adapting existing, proven hardware, helps us keep program costs down," Mr. Brink said.

Mr. Brink said he believes the X-51A program will provide knowledge required to develop the game changing technologies needed for future access to space and hypersonic weapon applications.