Mach 20 or Bust
Weapons research may yet produce a true spaceplane.
- By Geoffrey Little
- Air & Space magazine, September 2007
(Page 3 of 3)
EVEN AT 500 MPH, it takes a long, long time to reach Australia. “Just eight movies and you’re there,” Australians joke. The country’s remoteness may account for its fascination with hypersonic flight; someday the travel time from London to Sydney may come down to one movie.
The Australian hypersonics program has been making steady progress for a decade, but it really took off in 2002, when HyShot fired the world’s first scramjet engine in flight. Building on that accomplishment, the Australian Department of Defense joined its U.S. counterpart, along with NASA, Boeing, and other partners, in an innovative international project called HiFire, for Hypersonic Flight International Research Experimentation. Funded with $54 million, HiFire includes a series of experiments and at least 10 test flights to be conducted over the next six years. Mark Lewis, who signed the agreement for the United States last November, says that the project complements the X-51A and other U.S. hypersonics efforts. “In HiFire, we’re looking at very fundamental science: all the problems we think we would anticipate in hypersonic flight.”
Next year the HyShot team will test a new free-flying vehicle as part of the HiFire program (earlier HyShots stayed attached to their booster rockets). One research goal is to try different shapes for scramjet engines in the search for greater efficiency, starting with the air inlet. Instead of a simple rectangular slot, shaped like the front of a Dustbuster vacuum cleaner, the inlet for the REST (rectangular-to-elliptical shape transition) engine is three-dimensional and more complex. The opening is still generally rectangular, but it includes faces that slant in toward the combustion chamber. Michael Smart, an associate professor in the HyShot group at the University of Queensland, explains: “The reason these 3-D inlets are more efficient is that the air is compressed by all surfaces of the inlet. A 2-D inlet only compresses the air in one plane: The side walls create drag, but don’t do any compression.”
The outer rectangular shape of the inlet offers an advantage: Stacking engines side by side is easier. But inside the vehicle, the inlet connects to an elliptical combustion chamber. Joined together, the pieces look like the different sections of a car’s exhaust system. This is a departure from the X-43A, which had a rectangular combustion chamber. Elliptical combustors are better, says Smart, because “round shapes are inherently stronger than rectangles. This leads to thinner walls and less weight. They have less surface area for the same amount of air flow through the engine. Less surface means less drag, less heating, and less weight.” And since energy tends to ebb in rectangular combustors’ corners, getting rid of the corners can increase overall thrust.
HiFire flights will launch from southern Australia’s Woomera test range, the largest testing grounds in the world. The size of the range, its isolation, and the chance to fly frequently are real benefits, says Lewis. “The costs are low enough that if the things break, if they don’t work, if they crash into the Australian Outback, we’ll keep the program going. We’re not going to give up because of one failure.” It’s a small-is-beautiful approach. “When you go to really, really expensive demonstrators, suddenly you’re so terrified of things not working or not flying that you paralyze your flight test program,” he says. “And that’s one of the things we’re trying to avoid.”
Of the three major hypersonic programs under way, the most ambitious is FALCON. HiFire’s short, up-and-down flights will reach Mach 10 or so. FALCON aims to fly up to Mach 20 over a distance of thousands of miles.
Led by DARPA, FALCON is short for Force Application and Launch from CONUS (continental United States). As the name implies, FALCON was conceived as both a potential weapons system with global reach and a capability to launch military space payloads as a quick response. The distant goal of the program is to develop, by 2025, an unmanned, reusable Hypersonic Cruise Vehicle (HCV) approximately the size and weight of a B-52. Taking off and landing like an airplane, the HCV would be able to deliver a 12,000-pound payload 9,000 miles from the continental United States within two hours. It’s the Orient Express turned into a bomber, without the pilot or passengers.
“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.