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*Updated 11/18/11 1:59 pm *

For a test of a hypersonic weapon flying at eight times the speed of sound and nailing a target thousands of miles away, this was a relatively simple demonstration. But it worked, and now the military is a small step closer to its dream of hitting a target anywhere on Earth in less than an hour.

The last time the Pentagon test-fired a hypersonic missile, back in August, it live-tweeted the event – until the thing crashed into the Pacific Ocean. This time around, it kept the test relatively quiet. The results were much better.

To be fair, this was also an easier test to pass. Darpa's Falcon Hypersonic Technology Vehicle 2 – the one that splashed unsuccessfully in the Pacific – was supposed to fly 4,100 miles. The Army's Advanced Hypersonic Weapon, lifted by a 34-foot, 36,000-pound rocket, went about 60 percent as far, 2,400 miles from Hawaii to its target by the Kwajalein Atoll in the South Pacific. Darpa's hypersonic glider had a radical, wedge-like shape: a Mach 20 slice of deep dish pizza, basically. The Army's vehicle relies on a decades-old, conventionally conical design. (See the illustration below.) It's designed to fly 6,100 miles per hour, or a mere eight times the speed of sound.

But even though the test might have been relatively easy, the $200 million Advanced Hypersonic Weapon effort could wind up playing a key role in the military’s so-called “Prompt Global Strike” effort to almost instantly whack targets half a world away. A glider like it would be strapped to a missile, and sent hurtling at rogue state's nuclear silo or a terrorist's biological weapon cache before it's too late.

At first, the Prompt Global Strike involved retrofitting nuclear missiles with conventional warheads; the problem was, the new weapon could've easily been mistaken for a doomsday one. Which meant a Prompt Global Strike could've invited a nuclear retaliation. No wonder Congress refused to pay for the project.

So instead, the Pentagon focused on developing superfast weapons that would mostly scream through the air, instead of drop from space like a nuclear warhead. Those hypersonic gliders may cut down on the geopolitical difficulties, but introduced all sorts of technical ones. We don't know much about the fluid dynamics involved when something shoots through the atmosphere at hypersonic speeds. And there really aren't any wind tunnels capable of replicating those often-strange interactions.

"You have to go fly," says retired Gen. James "Hoss" Cartwright, who helped lead the Prompt Global Strike push as vice chairman of the Joint Chiefs of Staff and as head of U.S. Strategic Command. "You have to open up the envelope of knowledge."

Darpa and the Air Force worked on understanding the aerodynamics of hypersonic flight – that's one of the reasons behind the ill-fated Falcon Hypersonic Technology Vehicle tests. Meanwhile, the Army concentrated on controlling the hypersonic glider, and on thermal management. Moving through the air at Mach 8 generates a huge amount of heat. The military was keen to see if the Advanced Hypersonic Weapon's combination of aluminum, titanium, steel, tantalum, tungsten, carbon fabric, silica, and other alloys (.pdf) could take it. The last thing the Pentagon wants is for its Prompt Global Strike weapon to burn up before hitting its target.

Judging from yesterday's test, it looks like the composite held up. And so the plan to take out enemies from continents away just got a little easier to pull off.

*Photo: Missile Defense Agency; illo: U.S. Army

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