THE HYPERSONIC REALM is out there, just beyond our reach, above Mach 5. And so it has remained for decades. We’ve touched it briefly, even built vehicles—notably the X-15 and the space shuttle—capable of traveling at hypersonic speeds for short periods as they dive down from the edge of space. Yet we always seem “10 years away” from a true aerospace plane that can cruise long distances through the atmosphere at many times the speed of sound without burning up.

The vision of hypersonic flight has seduced aviators, warriors, engineers, and presidents. It was Ronald Reagan who in 1986 pitched the National Aero-Space Plane, the most ambitious hypersonic flight program ever conceived, a vehicle that was supposed to, “by the end of the next decade, take off from Dulles Airport, accelerate up to 25 times the speed of sound, attaining low Earth orbit or flying to Tokyo within two hours….”
It never even came close. And it took the field of hypersonic research years to recover from the letdown. After Reagan’s State of the Union speech, the media immediately branded the National Aero-Space Plane “the Orient Express.” The program was canceled in 1994, never having emerged from the research phase.

“Many post-morta have been done on the NASP program,” says Mark Lewis, a professor of aeronautics at the University of Maryland and currently chief scientist for the U.S. Air Force. “I think most people will agree…that they oversold the program. They bit off much more than we could chew. They were looking to get to Mach 25 with a single-stage-to-orbit the first time out of the hangar!”
Looking back on NASP and the other flameouts, former Air Force historian Richard Hallion sees more than a string of failures, however. “Hypersonics has had this image that it has been nothing but a huge rat hole for money,” he says. “But when you look at it, you can see the value of the research.” In fact, Hallion believes the many less-publicized successes since NASP have put hypersonic research on the verge of a real breakthrough. “To make an historical analogy, this is like 1937 with the jet engine, which appeared in ’39,” he says. “Or it’s like 1944 in supersonic [flight], which we achieved in 1947. We’re right there. We’re starting to close theory and practice. We’re starting to see the reality of what we can achieve in terms of performance prediction and construction and materials.”
In 2000, Hallion participated in a study for the Air Force Scientific Advisory Board, which concluded in its report, “Hypersonics could be the next great step forward in the transformation of the Air Force into a completely integrated aerospace force.” Partly as a result of the study, the Pentagon took the lead in U.S. hypersonic research, though NASA is still involved. “My suspicion is that this is a technology that first and foremost is going to be a military technology, then a space access technology,” Lewis says. “Then maybe down the line it will have some civilian applications.”
So forget about the Orient Express for now. Think hypersonic weapons—Mach 6 missiles, more than six times as fast as today’s cruise missiles. Launched from a distance, such weapons could destroy hardened targets with their high-speed impact alone. The Pentagon wants the capability to reach any place on Earth—say a terrorist’s temporary hideout—within two hours. And unlike an intercontinental ballistic missile, a hypersonic missile could change course in flight or even abort its mission.

That vision has spawned a mini-boom in hypersonic research—this time without the hype. Dozens of projects are under way worldwide, several of which will lead to test flights within the next few years. A trio of inter-related U.S. military projects—HiFire, X-51A, and FALCON—are intended to solve different pieces of the hypersonic puzzle, from propulsion to aerodynamics to the peculiar physics of hypersonic flight.

THE CURRENT BOOM began in the summer of 2002, with researchers at the University of Queensland in Australia launching a small hypersonic test vehicle on top of a sounding rocket. For the first time, the experiment, called HyShot, proved that a key component of hypersonic propulsion, the scramjet, or supersonic combustion ramjet, could work in the atmosphere and not just in wind tunnels (see “Outback Scramjet,” Oct./Nov. 2002). By scooping oxygen from the atmosphere as they fly, scramjets liberate hypersonic vehicles from the need to carry heavy tanks of oxidizer for combustion. Since HyShot’s 2002 launch, international researchers have successfully flown air-breathing engines three times, reaching speeds just short of Mach 10.
In 2004, NASA took the next step by flying a scramjet engine that accelerated a 14-foot-long, surfboard-shaped unmanned vehicle called the X-43A to an astounding Mach 9.8 before the craft made a planned plunge into the Pacific Ocean (see “Debrief: Hyper-X,” June/July 2005). The X-43 was a turning point, says Jim Pittman, principal investigator for hypersonics at NASA. “We learned two things: Scramjets really do work—you really can get positive thrust out of a scramjet—and you really can integrate a scramjet with a vehicle that you can fly and control. And both of those things are huge.”
As a hypersonics engineer at NASA for 30 years, Pittman has been through flush times and lean times. “Living through it is frustrating,” he says. “It’s a cycle, and you just have to tough it out.” Pittman worked on the NASP as well as the X-43, which was part of a larger NASA program called Hyper-X. Ironically, around the time the X-43 succeeded, the agency’s aeronautics budget got slashed, one reason NASA now finds itself playing a supporting role to the Pentagon.

Which is not to say the space agency’s contributions are insignificant. This fall NASA will launch a small experiment on a commercial sounding rocket from NASA’s Wallops Flight Facility, off Virginia’s eastern shore. Called HyBoLT, for Hypersonic Boundary Layer Transition, the wedge-shaped payload should provide valuable data on the fundamental physics of high-Mach flight.

“When you hear the term ‘hypersonics,’ you should always think heat transfer,” Pittman says. “The single most important distinguishing feature of hypersonics is heat—the heat caused by the frictional forces of the air passing over the surfaces. In hypersonic flight the heat transfer is extremely large, and the higher the Mach number, the higher the heat transfer.”
These problems are especially tricky in what’s called the boundary layer, the air that washes over the vehicle’s skin. Though the boundary layer has been studied in wind tunnels and with computer modeling, how it behaves in actual hypersonic flight is still poorly understood. What is known is that as speed increases, the layer goes through a transition, eventually becoming fully turbulent. As that happens, temperatures double or triple. And as the heat ratchets up, so does drag, which can radically affect flight characteristics. “We need to better understand it,” says Pittman. “It’s the most critical thing in hypersonics.”
The HyBoLT test article, which looks like the flat tip of a screwdriver, will be launched on its suborbital rocket to an altitude of 250 miles, while instruments record temperatures and pressures on different parts of the surface. It’s a modest experiment—the kind of basic data collection that supports the sexier test flight programs.

Not far from Pittman’s office at NASA’s Langley Research Center in Virginia, testing is under way for one of those high-profile programs—the X-51A scramjet demonstrator, a $240 million collaboration between the Defense Advanced Research Projects Agency (DARPA) and the Air Force. A scramjet engine for the vehicle has been fired dozens of times at Langley’s 8-Foot High Temperature Tunnel.

With four test flights over the Pacific Ocean slated to begin in 2009, the X-51A will ultimately attempt record-breaking engine burns lasting five minutes, which should propel the craft to about Mach 7. Like the X-43, the X-51A is a wave rider. After being boosted to high altitude, the vehicle will light its engine and surf its own shock wave, compressing the air in front of it and lowering drag. Though the immediate goal is to flight test a propulsion system for a superfast missile, the project received the X-plane designation in recognition of its potential to advance the field of hypersonics generally.

For Mark Lewis, the X-51A is all about the scramjet. “We want to see a scramjet engine work for more than 10 or 11 seconds,” he says, referring to the burn times of the two Hyper-X flights. Engine burns of several minutes would demonstrate to skeptics that long-duration scramjet-propelled flight is feasible.

Skeptics might be forgiven their doubts. Achieving combustion in an air-breathing engine moving at thousands of miles per hour has been compared to keeping a match lit in a hurricane. Hyper-X protected the precious flame in its combustion chamber behind carefully focused shock waves, but only for seconds. The X-51A engine will have to run at least 30 times longer.

To cover their bets, DARPA and the Air Force have two companies, Pratt & Whitney Rocketdyne and ATK, developing two kinds of hypersonic engines. One major difference from Hyper-X is that the X-51A will burn conventional jet fuel instead of the liquid hydrogen that very-high-performance rocket and scramjet engines normally use. It won’t be the first scramjet to do so: In December 2005, a DARPA-Navy project called HyFly launched a missile perched on a booster rocket from Wallops Island in Virginia. The missile’s air-breathing engine, which ran on JP-10 aviation fuel, flew for more than 15 seconds under scramjet power.

Pratt & Whitney’s engine is called the X-1. When flying at hypersonic speeds, JP-7 aviation fuel rushes into the X-1’s three-foot-long combustion chamber at 3,300 feet per second. A closed-loop system cycles the fuel around the engine, using it as coolant to draw heat and pressure off the combustion chamber. In the process, the extreme heat—more than 3,000 degrees Fahrenheit—“cracks” the fuel’s molecular structure. The cracking shortens the molecules and allows the fuel to burn more quickly, which is imperative. If the fuel doesn’t ignite in the microsecond in which it flows through the chamber, it will spew out uselessly, producing zero thrust—and a very fast falling object.

Over the past year, the X-1 engine has worked as advertised in Langley’s test chamber, culminating in a 50-second-plus, simulated X-51A flight at more than Mach 5 last April.

In less than two years, the X-51A will have a chance to prove itself in the atmosphere. Each test flight will begin with a B-52 taking off from Point Mugu, California. The airplane will carry the 14-foot vehicle up to 49,500 feet over the Pacific, where it will be released attached to a booster derived from an Army missile. The booster will get the demonstrator to over Mach 4, whereupon the scramjet engine will fire to propel it to full speed.

With the X-51A attempting to prove that hydrocarbon scramjets can propel hypersonic missiles, it’s up to other projects to sort out how to achieve higher Mach numbers. For some of those answers, Lewis and the Air Force made a long flight down under to work with the Australians who came up with HyShot.

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.

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