Pentagon scientists pledged to press on with work on a $308 million futuristic unmanned spaceplane despite the failure Thursday of a second test flight when they lost contact with the vehicle during maneuvers.

“We’ll learn. We’ll try again. That’s what it takes,” said Regina Dugan, director of the Defense Advanced Research Projects Agency (DARPA), which ran both tests.

But some observers said there would now be a question mark over the program’s future.

The Falcon HTV-2, as the experimental weapon is called, blasted off at 10:45 a.m. ET from Vandenberg Air Force Base in California atop a solid-fuel rocket made from a decommissioned ballistic missile.

Just outside the atmosphere, the wedge-shaped plane separated as planned from the rocket and began its glide back to Earth, reaching speeds up to 13,000 mph — almost 20 times the speed of sound. This hypersonic velocity should enable it to reach a target anywhere on the planet within minutes and smash into it with huge explosive force — but it also heats the vehicle’s skin to 3,500 degrees.

Thursday’s plan was for a 30-minute, 4,000-plus-nautical-mile test flight, ending with the Falcon crashing into the ocean just north of a U.S. military test site on the Kwajalein Atoll in the Pacific.

But, instead, engineers lost contact with the plane just a few minutes into the flight. The loss of contact came after the plane entered its glide phase, when the onboard autopilot attempts maneuvers to direct its trajectory toward the target.

“It was not a good day” for DARPA, said Mark J. Lewis, an aerospace professor at the University of Maryland and the former chief scientist of the Air Force, told The Washington Times.

“Obviously, the best outcome would have been for the vehicle to have completed the flight successfully, at the target,” he said. “This has got to be a disappointment.”

DARPA said in a statement an “anomaly” caused the loss of the signal.

“Initial indications are that the aircraft impacted the Pacific Ocean along the planned flight path,” the agency said, adding it would convene an investigation by independent engineers to find out what happened.

The Independent Engineering Review Board for the first test flight in April 2010 concluded that the vehicle began to roll so violently that the autopilot lost control, also during the glide phase. The plane, which employs a controversial new aerospace design, is designed to self-destruct by crashing into the ocean if that happens.

After a series of ground tests, DARPA pushed ahead with a second test flight, saying it would learn from data gathered during the first. DARPA did not say Thursday exactly how far into the second flight they lost contact and did not return phone or email messages requesting comment.

A statement from Air Force Maj. Christopher Schulz, the Falcon program manager, on the agency’s website pledged to learn from the second test flight as well and press on again.

“We know how to boost the aircraft to near space. We know how to insert the aircraft into atmospheric hypersonic flight. We do not yet know how to achieve the desired control during the aerodynamic phase of flight,” Maj. Schulz said.

“It’s vexing; I’m confident there is a solution. We have to find it,” he added.

Nonetheless, the future of the program appeared in doubt Thursday. Mr. Lewis said that the program had been funded for two test flights only, and that scientists were hoping the Air Force would keep the program going. “There’s got to be a question mark over that now,” he said.

The Falcon is part of U.S. effort — known as Conventional Prompt Global Strike, or CPGS — to develop the capability to strike anywhere in the world with a conventional weapon in less than an hour.

CPGS is a new class of weapons that officials hope will address recent threats, such as terrorist nuclear weapons, and help reduce U.S. reliance on nuclear weapons as a strategic option.

“The services are under tremendous budget constraints right now,” said hypersonic flight advocate Richard P. Hallion, the former chief historian of the Air Force.

But he urged the Pentagon to push ahead with other approaches to hypersonic flight, which he called “a critical defense technology.”

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