Mach 20 or Bust

Weapons research may yet produce a true spaceplane.

Target date 2025: A pilotless, Mach 20 Hypersonic Cruise Vehicle. (Paul DiMare)
Air & Space Magazine

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“HCV is the vision vehicle,” says Steven Walker, who manages DARPA projects related to hypersonic flight, including FALCON. A four-year veteran of the agency with degrees in aerospace engineering, he knows he’s working against physics as well as skepticism in the military ranks. Many Pentagon strategists would rather extend the capability of conventional missiles like Tridents than pursue a notoriously elusive and costly technology. “We need to fly some hypersonic vehicles—first the expendables, then the reusables—in order to prove to decision makers that this isn’t just a dream,” he says. “We won’t overcome the skepticism until we see some hypersonic vehicles flying.”
Walker and DARPA, working with the Air Force, NASA, and Lockheed Martin, hope to commence the airshow in December 2008, launching a series of small, expendable Hypersonic Test Vehicles (HTVs) to demonstrate sustained flight between Mach 10 and 20. One long-standing problem FALCON hopes to solve is how to build an aeroshell that won’t self-destruct in long-duration, high-temperature flight. Easier said than done. Before it had even been assembled, let alone flown, the first test vehicle, the HTV-1, hit a rough patch—literally a bunch of bubbles. The subcontractor for FALCON’s aeroshell was laying up the carbon-carbon prototype material in small sections to provide samples for aerodynamic and thermal testing. Each piece was made of six or seven layers, and as the technicians applied each layer, the material would stretch and pull the layer beneath it, creating voids and air pockets, particularly around curves. It was a potential showstopper. In flight, intense heat would cause the bubbles to burst, destroying the airframe.

With advice from experts, Walker and the team made a tough decision: abandon the highly curved HTV-1 design and go straight to HTV-2. That meant the first vehicle would not fly as planned this fall. “When you’re dealing on the edge of what’s been done before, it’s never going to be perfect the first time,” says Walker, trying to make the best of the schedule slip. “Dash-2” is now being assembled by Lockheed Martin’s legendary Skunk Works in Palmdale, California. Like Dash-1, HTV-2 is an expendable vehicle, but with a narrower delta shape, a dagger tip, and sharper leading edges for sleeker aerodynamics. With fewer curves, it should be easier to construct.

In December 2008, the 10-foot-long HTV-2 will launch from Vandenberg Air Force Base in California. As a boost/glide vehicle, it carries no power of its own, but will be accelerated to over Mach 10 on top of a rocket booster. On the downslope, the vehicle will glide at Mach 20 over the 4,800-mile stretch between California and Kwajalein Atoll in the Marshall Islands, home to the Ronald Reagan Ballistic Missile Defense Test Site.
As it pushes up through the upper atmosphere and begins its glide path down, Dash-2 will generate more than 3,000 degrees of heat, burning off, or ablating, layers of carbon-carbon from its aeroshell. FALCON engineers will study the test data carefully to see how the shape changes affect the aircraft’s aerodynamics. The second flight, in June 2009, will be a more circuitous course, with the craft attempting a sharper angle of attack while performing pitch and yaw maneuvers.

The last of the proposed FALCON test vehicles is HTV-3, which would add vertical and horizontal stabilizers for maneuvers at lower, but more sustained, speeds of around Mach 10. Originally scheduled to fly in 2010 as a recoverable boost/glide vehicle, Dash-3 may instead fly two years later in a different mode—taking off and landing like an airplane, under its own power, using an engine developed by DARPA under another project, called FaCET, for FALCON Combined-cycle Engine Technology. The FaCET engine combines a turbojet (to get up to around Mach 4) with a hydrogen-fueled scramjet (to reach Mach 10). The turbojet is itself a challenge; the fastest turbojet yet flown, the J58 used on the SR-71 Blackbird, could only manage Mach 3.2. Like the Australian engine, FaCET has a fancy 3-D air inlet—a good example of how the different hypersonic research programs feed one another. If successful, the flights would prove by 2012 that a reusable thermal protection system works in actual hypersonic flight. And that would be a big step toward building Walker’s hypersonic-cruise “vision vehicle.”
“If the country wants to put a real operational system together, we’ll be in a position to do that in 2020,” he says. “If we don’t do these demonstrations now, then we’ll never get there.”
While there’s less hype associated with the current hypersonic boom, there’s still plenty of hypothetical. One wild card is politics—how this technology will play in the policy arena. Richard Hallion is certain that missiles capable of flying at speeds between Mach 5 and Mach 7 will transform global warfare. “I would not be surprised at all to see somebody in the next decade unveil a hypersonic weapon that they are able to put into service,” he says. Though he declines to say which somebody he has in mind, many nations other than the United States—allies, foes, and neutrals—are known to be working on the problem. The first weapons, Hallion says, are likely to be small missiles, like the X-51A, fitted with efficient scramjets, able to be fired from mobile transports on land, sea, or air. He further predicts that hypersonic technology will become “common currency,” like the jet engine. Everyone will have it.
Click here to see how artist Paul DiMare created the illustrations for this article.

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