In the language of Beltway defense wonkery, the results of this year’s test launch of the hypersonic unmanned U.S. aircraft designated Falcon HTV-2 might be called sub-optimal.
In plain English, it appears certain that the experimental space plane - a key element of U.S. efforts to develop a conventional weapon that can strike anywhere in the world in less than an hour - disintegrated and burned up in the upper atmosphere in a failure that casts a question mark over the program’s future.
The Defense Advanced Research Projects Agency (DARPA), which conducted the test flight, would say only that an engineering review board is examining data from the test flight.
Contact with the Falcon was lost about nine minutes into the half-hour flight on April 20, DARPA spokesman Eric Mazzacone said.
“The test went well for nine minutes,” former Air Force chief scientist Mark J. Lewis told The Washington Times. “After that, not so well.”
Mr. Lewis, who followed the $308 million project closely in his official capacity and has continued to do so since leaving the Air Force two years ago, stressed that it was “dangerous to speculate” on the causes of the failure.
“Once there’s some sort of failure, the vehicle isn’t going to last very long,” given its enormous speed and altitude, he said. “If anything goes wrong, there’s not going to be anything left of the vehicle,” because the pieces would burn up on re-entry.
Richard P. Hallion, a former chief historian of the Air Force and a self-proclaimed evangelist for hypersonic vehicles, told The Times that the review would be “very carefully and thoroughly examining all possible data about this loss.”
“There are many things that could have gone wrong,” he said. “Those studying this are likely trying to find out whether this is a straightforward fix or something more serious that might endanger any future attempts to fly this vehicle.”
The Falcon is a suborbital vehicle launched on a solid-fuel rocket booster made from a decommissioned ballistic missile. Just outside the atmosphere, in a procedure called “clamshell payload fairing release,” the launch vehicle deploys the plane, which then is supposed to glide back to earth at more than 13,000 mph - more than 15 times the speed of sound.
The test flight was a part of a research program involving two Falcon vehicles, the second of which was slated for launch early next year before April’s failure.
Mr. Mazzacone declined to comment on the program ahead of the review board’s report.
Mr. Lewis noted that the loss of contact and presumed destruction of the Falcon occurred “at the point where the vehicle would have been experiencing its maximum degree of heating” as it began its descent.
“One possibility is, it just got too hot,” he said, adding that NASA had many experiences in which computer modeling and other efforts failed to predict the incredible heat exerted on vehicles as they sped through the atmosphere upon entry and re-entry.
“We really don’t know enough about high-speed fluid dynamics,” he said.
Mr. Hallion said ground testing of the vehicle might have been insufficient. “Before one goes to flight testing, a vehicle should be thoroughly tested and analyzed. The kind of rigorous and incremental approach to preflight testing we saw on earlier hypersonic programs was not followed as rigorously and as carefully as it should have been” with the Falcon.
“What you don’t want to find,” he said of the engineering review board, “is a failure of something that could have been predicted.”
Mr. Lewis said, “I wish the HTV-2 had done some more ground testing.”
But he added that “in flight testing, you have to be allowed to fail.”
“I worry that, culturally, we’re losing the ability to do this kind of high-risk flight testing,” he said, noting that program managers were under pressure “to make everything perfect,” especially in a program like Falcon that had provision for only two flights.
“When you do flight testing … you are extending your knowledge” about what is possible, said Mr. Hallion. “That entails a degree of risk.”
The Falcon HTV-2 is part of the U.S. military’s effort to develop the capability to strike anywhere in the world with a conventional weapon in less than an hour. The capability is known as Conventional Prompt Global Strike, or CPGS.
Supporters say CPGS will address novel threats - such as terrorists with nuclear weapons - and help reduce U.S. reliance on nuclear weapons as a strategic option in more conventional conflicts.
“It’s a way to turn fleeting intelligence into actionable intelligence,” Mr. Hallion said.
“If you know something is about to happen, you’re able to do something about it” with such a capability, he said.
“The ability to engage an enemy very rapidly and over great distances, from outside the reach of their defensive or counterstrike capabilities … is significant,” he said.
Mr. Lewis said CPGS development has several alternative approaches, including a DARPA project called Arclight, and the Air Force’s X-51, which successfully test-flew a hypersonic flight-power technology called scramjet for “supersonic combustion ramjet.”
Earlier this month, DARPA issued a notice to contractors about the Arclight project, calling for proposals on how to launch a small, unpowered aircraft to high altitude and then glide it to earth - a similar concept to that of the Falcon.
But Arclight envisages a vehicle launched from the Navy’s Mk41 Vertical Launch System, which is routinely found on U.S. warships and submarines. In comparison with Falcon, it should reduce the cost substantially and increase the strategic flexibility of the program.