Updated 8/10/11 11:24 a.m. EDT: Story changed to reflect new launch time.

The Pentagon has been working for nearly a decade on an audacious plan to strike anywhere on the planet in less than an hour. Thursday could prove to be the do-or-die moment for that plan.

At approximately 7 a.m. PDT, a three-stage Minotaur IV Lite rocket is scheduled to lift off from Vandenberg Air Force Base in California. It will puncture the atmosphere, and then release an experimental aircraft. That aircraft, known as the Falcon Hypersonic Technology Vehicle 2, will then come hurtling back to Earth at nearly 20 times the speed of sound, splashing down near the Kwajalein Atoll in the Pacific Ocean, approximately 4,100 miles away. Total flight time: about 30 minutes.

That is, if the flight goes as planned. The first HTV-2, launched by Pentagon bleeding-edge research arm Darpa in April of 2010, disappeared over the Pacific after just nine minutes of flight. The vehicle was never recovered.

It was a loss not just for this single effort, but for the entire concept of "Prompt Global Strike," the Pentagon plan to eliminate its enemies anywhere around the world.

The Defense Department is pursuing three different families of technologies to accomplish the task. One is to rearm nuclear intercontinental ballistic missiles with conventional warheads. But that runs the risk of accidentally triggering a response from another atomic power, who might mistake it for a nuke. Oops.

A second effort is to build shorter-range cruise missiles that can fly at five or six times the speed of sound. But that X-51 Waverider project is running into complications: A June flight test ended prematurely, for reasons that are still unclear.

Some variation of the HTV-2 is the third choice; to some in the defense community, it's the most appealing. The HTV-2 spends most of its time flying through the atmosphere, before it dives down to hits its target. That means it's unlikely to be mistaken for a nuclear missile, which spends most of its time above the atmosphere. World War III averted.

Yet despite the aircraft's potentially revolutionary role – and despite the tens of millions of dollars poured into the Falcon program every year – Darpa was somewhat cavalier in its first test of HTV-2. The HTV-2 has an unusual, wedge shape; think of it like a 13,000 mph slice of deep-dish pizza. In theory, the shape should provide more lift. But few people had flown anything quite like the HTV-2 at these intense speeds before.

Nevertheless, the agency conducted just a few ground tests of the aircraft's design, and those few tests were only run after pressure from the Air Force, its partner in the Falcon effort. Lockheed Martin was paid to build two copies of the aircraft before the first was even flown. If there turned out to be a flaw in the design, correcting it might've been near-impossible.

"The thing that had everyone panicking was that, if you had to change the shape of the vehicle, it might've meant the cancellation of the entire program," a source familiar with the effort tells Danger Room.

Darpa got serious about ground testing after the April misfire, with month after month of trials. A scale model of the HTV-2 was covered with a temperature-sensitive paint that glowed brighter, the hotter it got. It was brought to the Air Force's hypersonic wind tunnel, near Silver Spring, Maryland, for trials.

What engineers discovered was that the HTV-2 had a problem stabilizing itself in flight. When it tried to correct its yaw, it went into a roll. That triggered the HTV-2's malfunction in April, 2010, leading to "a self-destruct sequence that sent the missile tumbling like a football into the ocean," as Danger Room co-founder Sharon Weinberger puts it.

Fortunately for Darpa, those problems appear to be correctable on the vehicle they've got. "For its second test flight, engineers adjusted the vehicle’s center of gravity, decreased the angle of attack flown, and will use the onboard reaction control system to augment the vehicle flaps to maintain stability during flight operations," the agency notes in a statement.

The stakes are huge for the upcoming flight. Darpa has no plans to build a third vehicle. And, unless this test goes well, it's unlikely that the Air Force or any other branch of the military will pick up on the agency's work.

"More than 20 land, air, sea and space test assets" will be collecting data during the flight, Darpa contends. All that information will inform "future hypersonic flight vehicle performance, ultimately leading toward the capability of reaching anywhere in the world in under an hour."

How direct that path will be depends on how the HTV-2 flies on Thursday.

Image: Darpa

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