When NASA’s X-43 flight test vehicle separated from its Pegasus rocket booster and accelerated to high-Mach speeds powered by an air-breathing scramjet, the premise and promise of hypersonic flight were forever validated. With a first Mach 7 flight in March 2004, followed by a Mach 10 flight in November 2004, the hydrogen-burning X-43 vehicles were the culmination of nearly five decades of research in hypersonic air-breathing flight.

Sometime in February, if all goes well, the next chapter in hypersonics will be written, as the U.S. Air Force’s X-51 vehicles begin their own series of flight tests.

In many ways, X-51 is a perfectly logical follow-on to X-43. Whereas X-43 was fueled with hydrogen, X-51 uses the somewhat more practical, though less energetic, hydrocarbon Jet Propellant 7, the same fuel that powered the SR-71 Blackbird reconnaissance aircraft. Unlike its hydrogen-powered cousin, the X-51 has an engine that is actively cooled, meaning that it can fly under power for minutes, not seconds. And while the 4-meter X-43 was a subscale version of a future airplane, the X-51 is a missile-type configuration that could evolve directly into a useful hypersonic weapon.

Even if X-51 succeeds, the naysayers will point out that hypersonics has had a checkered history of starts and stops and promises unfulfilled. In fact, from the X-15 to the space shuttle, hypersonic flight has already been a reality, but the remaining challenge is high-lift, low-drag flight, especially using air-breathing engines.

Billions of dollars have been spent on failed programs, ranging from the original 1960s AeroSpace Plane to the U.S. Defense Advanced Research Projects Agency’s (DARPA) recent Blackswift, leading some to question whether hypersonics will ever be practical.

Post-mortem analyses of the failed hypersonics programs usually conclude that they have been too ambitious, linking unrealistic goals to insufficient funds. The poster children for this include the X-30 National Aero-Space Plane, which was to have offered single-stage-to-orbit airplane-like flight, and Blackswift. Yet there are glorious examples of programs that have worked well, including the X-15 rocket plane, the quintessential hypersonic flight test program.

Indeed, despite the quixotic nature of funding and limited successes of previous hypersonic programs, the field has shown amazing resiliency. The prospect of hypersonic flight is simply exciting, and a motivation for the best and the brightest to enter the aerospace field. But hypersonics also offers the enticing promise of practical, useful systems, including weapons that could cover many hundreds of nautical miles in mere minutes, aircraft that could penetrate nearly any hostile airspace, and flexible launch systems that could operate more like aircraft, with aircraft-like costs and support, rather than expensive, labor-intensive rocket-powered vehicles.

Despite ever-limited resources, a survey of activities suggests an optimistic landscape. There is a range of programs and activities in progress in the United States that are filling the gaps in our hypersonic knowledge. The U.S. Navy’s Hypersonic Flight Demonstration (HyFly) program, the DARPA/U.S. Air Force Hypersonic Technology Vehicle (HTV-2) and Mode Transition (MoTr) programs, the U.S.-Australia  Hypersonic International Flight Research Experimentation (HIFiRE) program and the robust basic research programs sponsored by the Air Force Office of Scientific Research and NASA are just a few of the highlights.

Of particular note is the joint nature of many of these ventures, especially between the Air Force and NASA. The HIFiRE flight test program is a marvel of international cooperation, jointly sponsored by the U.S. Air Force and the Australian Defence Science and Technology Organization (DSTO), with strong support from NASA and several companies.

More important, these programs have shown real progress, including the successful tunnel operation of Pratt & Whitney’s flight-weight, actively cooled scramjet engine at NASA’s Langley Research Center in Hampton, Va.; the first flight of a HIFiRE rocket in the Australian Outback; and the recent establishment of three major university centers, cosponsored by NASA and the U.S. Air Force, dedicated to basic research in hypersonic flight. The Air Force’s recent upgrades to the remarkable Hypervelocity Wind Tunnel 9 in White Oak, Md., the world’s premiere long-duration hypersonic test facility, are further testament to the perceived importance of this field. And this year’s formation of a multi-company Hypersonics Industry Team, inspired and led entirely by its members, dedicated to joint planning and advising, is proof of undiminishing industrial interest. Other nations have established ambitious programs as well.

Of course, there is always more that can and must be done in hypersonics. Most lacking is a coherent, robust national research and development plan that could lead to actual acquisition. Such a plan should follow a path that begins with weapon-scale vehicles, expendable but possibly recoverable. The next logical application would be hypersonic aircraft that offer responsive in-theater reconnaissance-strike, followed by transatmospheric craft that exit the atmosphere and return. These can be thought of as the logical 21st-century follow-on to the SR-71. Here it is important to distinguish between practical, achievable hypersonic craft operating in the Mach 6 regime with realistic range expectations  and unachievable pie-in-the-sky “global cruisers,” envisioned to magically reach anywhere on the planet in two hours or less. And finally, the ultimate application of hypersonic research would be access-to-space vehicles, using air-breathing engines to provide mission flexibility and high performance for space launch.

The path forward should build on existing programs. It will be important to resist the temptation to jump to some new platform design when an existing vehicle can fill a role. The X-51 design would serve as an excellent platform to continue hypersonic development, possibly with alternate engine flow paths, different cooling and ignition schemes, and advanced materials, maneuverability and control concepts. The goal should be something closer to the X-15’s envelope-expanding 199 flights, not the X-51’s currently budgeted four. Follow-on craft should be aimed at providing reusability and flexibility, and all reasonable engine options should be on the table.

Similarly, on the fundamental side, the well-conceived HIFiRE program should continue to probe questions of basic science that are relevant to realistic flight systems. It is also essential that ground test infrastructure be preserved. The enormous investments already made in hypersonic facilities, not to mention operator experience, are too precious to lose.

Along the way, several basic rules will be key:

  • Fly. Nothing will beat actual time in the air at hypersonic speeds. This doesn’t mean we should skip ground testing; in fact, it’s quite the opposite: Ground testing must be fashioned in support of flight.
  • Test, don’t demonstrate. That means flights should be aimed at answering meaningful questions, performing experiments that provide useful data following the often-forgotten principles of good science, not just proving things we already know. As in all good research, we must accept risk; hypersonic programs must be allowed to fail or else they will not be pushing the envelope. A key to the X-15’s success was the acceptance of risk. Even the tragic loss of a vehicle and its pilot did not end the program.
  • Keep the operator in mind. Hypersonic programs must lead to practical systems. Unjustified requirements will do nothing to advance hypersonics; they will merely drive up costs, add complexity, and pose risks without benefit.

Aerospace engineering is ultimately about pushing the boundaries, and nothing does that quite like hypersonics. The inexorable march of progress makes it all but certain that there will be hypersonic vehicles operating within, and through, our atmosphere. The only real question is when, and which nations’ flags will adorn them.


Mark J. Lewis is chairman of Clark School’s Department of Aerospace Engineering at the University of Maryland, College Park, and president-elect of the American Institute of Aeronautics and Astronautics. He was the chief scientist of the U.S. Air Force from 2004 to 2008.