Defense Department scientists are set to conduct a second test launch next year of the Falcon HTV-2 experimental superweapon after the first flight this year ended when the autopilot deliberately crashed the unmanned glider into the ocean as a safety measure.

The Falcon Hypersonic Test Vehicle is designed to skim the top of the atmosphere just below space, and is a key element of the Pentagon’s Conventional Prompt Global Strike (CPGS) capability — a program to build non-nuclear strategic weapons that can strike conventionally anywhere in the world in less than an hour.

In a statement last week, the Defense Advanced Research Projects Agency (DARPA) revealed for the first time that the first test flight April 20 ended when the autonomous onboard control system — the computer autopilot flying the futuristic superweapon — “commanded flight termination.”

“When the onboard system detects [undesirable or unsafe flight] behavior, it forces itself into a controlled roll and pitchover to descend directly into the ocean,” DARPA spokesman Eric Mazzacone explained in e-mail to The Washington Times.

The DARPA statement said that an independent engineering review board found that the flight was terminated after the plane began to roll so violently that it “exceeded the available control capability” of the onboard autonomous piloting system.

Specialists say such problems are expected in test flights.

DARPA said the board “reviewed and concurred with” a series of remedial measures proposed for a second test flight next year, but some analysts said the results of the first one raise questions about the way the program has been run.

The $308 million Falcon HTV-2 is a suborbital near-space vehicle launched on a Minotaur rocket, a solid-fuel booster built from a decommissioned ballistic missile. On the very edge of the atmosphere, in a procedure called “clamshell payload fairing release,” the launch missile deploys the plane, which is then supposed to glide above the Earth at more than 13,000 miles per hour — more than 20 times the speed of sound.

The Pentagon is developing a generation of such hypersonic weapons as a way of being able to strike quickly at urgent threats — such as preparations by terrorists or rogue states to use nuclear weapons.

The issue has been lent urgency by the recent nuclear arms treaty negotiated with Russia. Specialists say the new generation of hypersonic strike craft would not count against the limits the treaty places on strategic weapons, although in treaty negotiations, U.S. officials promised to discuss the new weapons in a treaty consultation commission.

But some other proposals for CPGS systems, such as putting conventional warheads on existing submarine-launched ballistic missiles, would count against the caps set by START.

Falcon, being developed jointly with the Air Force, is just one of a series of conventional long-range strike programs, including another DARPA project called Arclight, and the Air Force’s X-51, which successfully test-flew a hypersonic powered flight technology called scramjet — for “supersonic combustion ramjet.”

A Congressional Research Service report on prompt global strike stated that the program will develop weapons that can “strike globally and rapidly with joint conventional forces against high-payoff targets” using “attacks in a matter of minutes or hours — as opposed to the days or weeks needed for planning and execution with existing forces.”

The report said the HTV-2 was being produced by Lockheed Martin, using technology developed for a precision-guided, maneuverable warhead called ER.

A leading authority on hypersonic flight, Richard P. Hellion, former chief historian of the U.S. Air Force, told The Times that the fiery end to the Falcon’s April test flight “raises serious questions about how well DARPA conceived and executed the project.”

The board found that the Falcon encountered a “higher-than-predicted yaw” — when the nose of an aircraft moves from side to side. Because aircraft get their lift from their wings, yaw creates roll, when the plane starts to rotate around its horizontal axis as first one wing, then the other, is pushed forward, generating lift first from one side, then from the other.

According to the DARPA statement, the board found the Falcon had experienced “a slow divergence about the longitudinal axis (in roll) which continued until the roll rate reached a threshold where the autonomous flight system commanded flight termination.”

Inertial coupling, as flight scientists call the process by which yaw generates roll, “is a very old problem,” Mr. Hallion said, “It was a killer in the early days of test flights” of the first supersonic aircraft in the 1950s.

“It clearly remains a problem in hypersonic flight,” he added, noting that the Falcon “is basically a delta wing type vehicle,” lacking the large vertical surfaces like an aircraft tail that can be used to control roll.

Mr. Hallion questioned whether DARPA had “made certain they had adequate design analysis and [ground] testing before” the test flight.

“There are many unknowns” about hypersonic flight, said Mr. Mazzacone, when asked whether DARPA had rushed into a test flight. “A significant amount of preflight analysis was conducted. Which is in essence why we need to fly again. … There’s more to learn in this area,” he added.

Mark J. Lewis, former chief scientist at the U.S. Air Force and a professor at the University of Maryland, said that since the first flight, the DARPA team had conducted “extensive post-flight testing” of the Falcon at the world-renowned hypersonic wind-tunnel facility called T-9 at the USAF Arnold Engineering Center in White Oak, Md.

“They have really tried to learn the lessons” of the failed ending to the first test, he said, acknowledging that some might see this as “closing the barn door” after the horse is already gone.

Mr. Lewis said such failures are to be expected when testing technology — such as hypersonic planes — that pushed the limits of engineering and of human understanding of aerodynamics.

“There cannot be 100 percent confidence in the outcome,” he said, “That’s the nature of flight testing.”

DARPA said in its statement that the independent engineering board had “reviewed and concurred with” a series of remedial measures for the next test flight.

“Engineers will adjust the vehicle’s center of gravity, decrease the angle of attack flown and use [a system of small onboard maneuver rockets] to augment the vehicle flaps when HTV-2 flies next summer,” said David Neyland, DARPA Tactical Technology Office director.

Arms control advocates fret about the impact of the new generation of CPGS weapons. Congressional and other opposition killed a previous proposal to achieve CPGS capability by fitting conventional warheads to U.S. submarine-launched ballistic missiles. The concern was that other nuclear-armed nations might mistake the launch of such a weapon for the onset of a nuclear war.

Supporters of hypersonic weapons say that they would not be launched from an ICBM site, and that the trajectory would be different from that of a nuclear missile.

“It’s a totally different flight profile,” said Mr. Lewis.

But arms control advocates say the risk of a deadly mistake is still too high.

“There is so much potential for confusion,” said Matthew Hoey, a space weapons specialist and arms control advocate, citing what he called “the very degraded state” of Russia’s space-based early warning system.

Nonetheless, Mr. Hallion said, long-range conventional strike weapons will address novel threats — such as terrorist 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.

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