By Tariq Malik
An experimental aircraft has set a new record for the longest hypersonic flight after streaking across the sky Wednesday for more than three minutes while flying at Mach 5 — five times the speed of sound — the United States Air Force has announced.
The vehicle, called the X-51A Waverider, dropped from a B-52 Stratofortress mother ship while flying over the Pacific Ocean just off the southern California coast. It successfully ignited an air-breathing scramjet engine than accelerated up to Mach 5, Air Force officials said in the announcement.
The entire test flight lasted just over 200 seconds, more than 10 times longer than the previous hypersonic record (just 12 seconds) set by NASA's X-43 vehicle in 2004.
"We are ecstatic to have accomplished most of our test points on the X-51A's very first hypersonic mission," said X-51A program manager Charlie Brink of the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio, in a statement. "We equate this leap in engine technology as equivalent to the post-World War II jump from propeller-driven aircraft to jet engines."
X-51A's new record
Wednesday's test flight was aimed at evaluating the X-51A's scramjet engine, thermal protection, stability and control, and other systems.
With a profile that gives it a shark-like look, the X-51A scramjet cruiser is 14 feet (4.2 meters) long and is virtually wingless. It is designed to ride the shockwave it creates during flight, leading to its nickname "Waverider," the Air Force has said.
Pratt & Whitney Rocketdyne built the SJY61 scramjet engine at the heart of the X-51A cruiser.
The Air Force began Wednesday's test at 1 p.m. EDT, when the B-52 Stratofortress hauling the X-51A took off from Edwards Air Force Base in California. The X-51A test craft dropped from its mother ship while flying 50,000 feet (15,240 meters) over the Point Mugu Naval Air Warfare Center Sea Range on the Pacific Ocean.
Four seconds into the flight, the X-51A's solid rocket booster – actually an adapted Army Tactical Missile booster – accelerated the experimental aircraft to Mach 4.8 before being jettisoned to let the scramjet engine take over.
After the flight, the vehicle was expected to splash into the ocean. There were no plans to recover the craft, according to the Air Force.
"Now we will go back and really scrutinize our data. No test is perfect, and I'm sure we will find anomalies that we will need to address before the next flight," Brink said. "But anyone will tell you that we learn just as much, if not more, when we encounter a glitch."
The test was actually the third flight of the X-51 vehicle; it was the first time it flew independently. It had remained attached to its mother ship on both earlier flights.
Three more X-51 hypersonic tests flights are scheduled for later this year. The Air Force has built four X-51A cruisers in all, with one of them now successfully flown.
The project is a joint effort by the Air Force Research Laboratory, industry teams and the Defense Advanced Research Projects Agency.
Going hypersonic
Hypersonic flight, typically defined as beginning at Mach 5, is more challenging than supersonic flight at lower speeds because of the higher temperatures and pressures involved with the faster flight speed. The speed of sound, Mach 1, is about 760 mph (1,223 kph) at sea level.
Conventional turbine jet engines can't handle such speeds, Air Force officials said.
But scramjets, air-breathing jet engines driven by supersonic combustion, like the one on X-51A have their own challenges too. Air Force project officials compared it to "lighting a match in a hurricane and keeping it burning."
The X-51A is not the U.S. military's only project undergoing tests this year.
The Air Force Research Laboratory and scientists in Australia successfully launched an unmanned suborbital rocket in March during a joint hypersonic flight test that hit Mach 5.5.
In late April, an unmanned hypersonic glider called HTV-2 failed during its test flight after blasting off on a rocket launched from Vandenberg Air Force Base in California. That test, too, was aimed at learning more about the intricacies of hypersonic flight to support the development of future aircraft technology.
The Air Force also launched the secretive X-37B space plane into orbit in late April from the Cape Canaveral Air Force Station in Florida. The exact mission of that reusable space plane, which is smaller than NASA's space shuttles but entirely robotic, is closely guarded. But amateur skywatchers have managed to spot the mysterious X-37B in the night sky using telescopes equipped with video cameras.
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An artist's rendition of the Boeing X-51A Waverider
The    managed to break a hypersonic flight record Wednesday during a test flight.

The United States Air Force (USAF) said that the scramjet was able to fly for 200 seconds, achieving a top speed of around  5 and setting a new record for what the Air Force called "the longest supersonic combustion ramjet-powered hypersonic flight." The previous record of twelve seconds was set by the  in 2004. This hypersonic flight, also the first to use  fuel, was hailed by US government officials as a success. Despite an unknown failure which caused the X-51 to lose acceleration, an X-51 program manager said that the USAF was "ecstatic" about the event's accomplishments.

The X-51 is 14 feet (4.2 metres) long and has no real wings, allowing it to withstand the  created during flight. The aircraft launched around 10 a.m.  (1700 ) from  in California. It was carried by a  to a height of 50,000 feet (15,240 metres) and then released over the  Naval Air Warfare Center Sea Range. The X-51 was then propelled by a solid rocket booster to Mach 4.8.

After about 200 seconds in flight, Boeing said that "something then occurred that caused the vehicle to lose acceleration. At that point, the X-51A was terminated as planned." Although it was expected to fly for about 300 seconds and reach Mach 6, the scramjet only managed Mach 5 at a height of about 70,000 feet (21,336 metres), possibly due to an engine blowout. The aircraft landed in the ocean as planned, and there are no plans to retrieve it.

This test is the first of four planned flights for the X-51 program, with the other three planned for this coming fall. Previously, the X-51 had flown twice, but was attached to the B-52 both times.