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.