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Scram, Jet, You Bother Me

The scramjet-powered X-51A blew up, and the cause of hypersonic air transport was set back…again

2 min read
Scram, Jet, You Bother Me

The military’s experimental scramjet, the Boeing X-51A Waverider, blew up yesterday during a test flight at a California naval aviation center. Though failure is an orphan, and only success has a thousand fathers, it is only fair to Boeing to note that the X-51A is a joint project with Pratt & Whitney Rocketdyne, NASA, DARPA and the U.S. Air Force. One or all are responsible, because this time, no one can pin the blame on pilot error: the X-51 was unmanned.

The Waverider name refers to a design that flies through the air not in the usual manner but rather on the back of the shock wave created by the aircraft’s hypersonic motion. How hypersonic? Say, six times the speed of sound, or about 6,500 kilometers (or 4,000 miles) per hour, at altitude. The  wing-riding concept dates back to a 1951 paper by Terence Nonweiler of Queens’ University of Belfast.

That long intellectual genealogy is typical of many things technological. Hypersonic flight has only the feel of newness. People have been working on it, and failing at it, for a long time.

The interesting question is why anyone bothers. No executive is busy and important enough to want flight times that save him three hours in the air at the cost of five extra hours on the ground, making out his will.

It’s the military that's driving this one, through an initiative called Prompt Global Strike. The idea is to be able to drop a conventional bomb down a chimney, anywhere in the world, within an hour. You could argue that such a precision approach might supplant the blunderbuss of nuclear retaliation. Especially if the chimney is on the house of a Dear Leader with whom the United States may be having some friction.

But quickly getting a bomb to where you want it need not entail spending billions on monstrous, air-breathing missiles. You could just pre-place your weapons—either right in the chimney, secret-agent style, or at an airfield within an hour’s flight of your likely targets. That’s what allies are for.

And speaking of air-breathing, that brings us to the heart of the X-51: its scramjet engine. The first two letters stand for “supersonic combustion;” the rest is just Ye Olde Ramjet, which rams air into its combustion chamber by the sheer force of its forward movement. But in a scramjet, unlike Ye Olde version, the stream isn’t deliberately slowed at the inlet but is allowed to move supersonically all the way through the combustion and emission phases. That trick allows the jet to work at ridiculously high velocities—say, 12 times the speed of sound or more.

It sounds hard to implement, and it is. The temperature gradients are fierce, the materials requirements high, the fuel injection tricky, the chances of a glitch great. And you can’t light that candle without first getting the aircraft up to high speed, which requires a rocket booster, a jetilicious mothership, a rail gun, or a slingshot.

All these complications make you wonder about that 60-minute deadline. “Delivery times may vary,” the package probably will say, in fine print.

There are other ways to drop an anvil on your enemy. For instance, you could use our existing fleet of ballistic missiles. A Minuteman rocket may seem a rather extravagant delivery vehicle for—what? a hand grenade?—but we already have plenty of those rockets. They’re paid for. They are accurate. And they’re very fast.

The Conversation (0)

Economics Drives Ray-Gun Resurgence

Laser weapons, cheaper by the shot, should work well against drones and cruise missiles

4 min read
In an artist’s rendering, a truck is shown with five sets of wheels—two sets for the cab, the rest for the trailer—and a box on the top of the trailer, from which a red ray is projected on an angle, upward, ending in the silhouette of an airplane, which is being destroyed

Lockheed Martin's laser packs up to 300 kilowatts—enough to fry a drone or a plane.

Lockheed Martin

The technical challenge of missile defense has been compared with that of hitting a bullet with a bullet. Then there is the still tougher economic challenge of using an expensive interceptor to kill a cheaper target—like hitting a lead bullet with a golden one.

Maybe trouble and money could be saved by shooting down such targets with a laser. Once the system was designed, built, and paid for, the cost per shot would be low. Such considerations led planners at the Pentagon to seek a solution from Lockheed Martin, which has just delivered a 300-kilowatt laser to the U.S. Army. The new weapon combines the output of a large bundle of fiber lasers of varying frequencies to form a single beam of white light. This laser has been undergoing tests in the lab, and it should see its first field trials sometime in 2023. General Atomics, a military contractor in San Diego, is also developing a laser of this power for the Army based on what’s known as the distributed-gain design, which has a single aperture.

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