Decades after the X-15 last flew, hypersonic technology is once again poised to take wing in what some are calling a new space race.
Imagine being able to jet from Miami to Seattle in less than an hour, instead of today’s seven-hour slog. A recent NASA-led study brought that flight of fancy one step closer to reality. How about superfast spy planes that can outpace any intercept, bombers that swoop in before the enemy can escape, or reusable vehicles able to boost satellites into orbit at lower cost than any rocket? Those, too, are on the drawing boards.
After decades of stops and starts, hypersonic research is experiencing something of a renaissance in the United States. Last year, the Defense Advanced Research Projects Agency (DARPA) awarded Raytheon and Lockheed Martin contracts of $174 million and $171 million, respectively, to conduct research into missiles that can travel at up to 10 times the speed of sound, or Mach 10. In July, the U.S. Air Force solicited proposals from Boeing, Lockheed Martin, Northrop Grumman, Raytheon Missile Systems, and Orbital ATK with the aim of awarding a contract by the end of the year for air-launched hypersonic weapons to be carried on fighters and bombers. Meanwhile, engineering academics are developing models for everything from atmospheric turbulence to nanomaterials that can withstand forces associated with traveling at Mach 5 or beyond.
Spurring this revival is research underway by major powers in Asia and Europe—which some have likened to a new space race. China, for example, has tested a hypersonic strike vehicle, the DF-ZF, seven times since 2014, while last year Russia reportedly tested the Yu-74, a hypersonic glider thought to be capable of traveling 7,000 mph armed with 25 nuclear warheads. “In the past, the U.S. was the clear leader in hypersonics, and now that can be disputed,” contends Brian Argrow, chair of aerospace engineering sciences at the University of Colorado, Boulder, noting that some say America “has dropped behind” Russia and China.
From the earliest days of aviation, engineers have aimed to make aircraft go farther, faster, and higher. Supersonic vehicles (Prism, January 2016), a term for any craft that travels faster than the speed of sound, or Mach 1, burst on the scene in the 1960s with the now retired Concorde luxury passenger liner and SR-71 Blackbird reconnaissance plane. The pursuit of hypersonic flight took off at about the same time with the introduction of the X-15. Operated by the U.S. Air Force and NASA, the rocket-powered experimental plane set speed and altitude records for manned aircraft, at one point reaching the edge of outer space. Its speed record, clocked at 4,520 mph (Mach 6.72) in October 1967, remains unchallenged. Although subsequent decades saw many programs get canceled, the United States has managed to sustain funding since the mid-1990s and realize more successes than failures—including the X-43A’s 2004 record for fastest unmanned aircraft.
Superfast flight poses a unique set of design problems, however. “Every single thing we take for granted with commercial aircraft is a new challenge with hypersonic aircraft,” says Colorado’s Argrow. Even physics get funky in the upper atmosphere, where the thin air still can generate enough friction to melt most fuselage materials. “It’s not like building a bridge—we knew more or less how many aspects of that worked 300 years ago,” says Javier Urzay, a senior research engineer at Stanford University who studies hypersonic propulsion systems and turbulence flows. “Hypersonics is a field of engineering that is very dynamic, where very fundamental aspects of it remain unknown. There’s plenty to do there.”
Consider rockets. Though long capable of hypersonic flight, to function that high up they must carry not only fuel but also oxygen or some other oxidizing agent to enable combustion. SpaceX launches like the Falcon 9 carry hundreds of tons of liquid oxygen, and “all that weight costs money,” says Urzay. Hypersonic researchers have long focused on propulsion systems that draw oxygen from the surrounding air, saving weight and expense while creating room for an additional payload, explains Ivan Bermejo-Moreno, an assistant professor of aerospace and mechanical engineering at the University of Southern California. The ideal: a supersonic combustion ramjet called a scramjet.
Unlike typical air-breathing turbine engines, such as those that power commercial jets, ramjets do not need a compressor—a component that squeezes the air entering the engine before it flows into the combustion chamber. Scramjets refine the concept to work at supersonic speeds. “Because they fly at a high speed, they can use the geometry of the engine to compress air,” explains Maj Mirmirani, dean of the college of engineering at Embry-Riddle Aeronautical University in Daytona Beach, Fla. “What really fascinates me is that you can have an engine that produces thrust without any moving parts.” The concept already has demonstrated results. In 2013, the Boeing-built X-51A Waverider flew at Mach 5.1 for 210 seconds—the longest scramjet flight to date.
Many recent successes in hypersonic flight have emerged from a research team called the HyShot Group, led by Michael Smart, chair of hypersonic propulsion at the University of Queensland in Brisbane, Australia. Working with Australia’s Defense Science and Technology Organization and the U.S. Air Force Research Laboratory, HyShot launched the first successful flight of a scramjet engine in 2002. This past July, the group successfully tested a hypersonic glider dubbed HIFiRE 4—for Hypersonic International Flight Research Experimentation—that was designed to fly at Mach 8.
Even so, progress remains unsteady. Smart and his team learned a “hard lesson” in 2015, when an experiment to measure scramjet engine performance at high altitudes went haywire. “The HIFiRE 7 was flying in a lovely straight line, and then 15 seconds before the fuel turned on, our telemetry died, and the payload ended up in the ocean north of Norway,” Smart recounts. The scramjet probably fired, but because one of the components powering the telemetry system’s radio signal overheated and shut down, the team never received any information from the test. “Since then, we’ve had two perfect flights,” he notes. “So you learn from your mistakes.”
Persistence could pay off in numerous ways, from commerce to space exploration to national security. But before tourists can zip from San Francisco to Sydney in a New York minute, engineers must figure out such basic elements as how to launch, land, and build a vehicle that can withstand extraordinary forces and heat. As vehicles hit and exceed Mach 5, thermal effects from the rush of air “become a major issue,” says Smart. Operating in Earth’s thin upper atmosphere can reduce aerodynamic drag, but hypersonic vehicles still go so fast that their surfaces can reach temperatures of 1,500 to 2,000 °C—“hot enough to melt the material,” says Urzay.
Advanced high-temperature composites that can withstand such heat have been a key element in hypersonic technology successes over the past 20 years. Researchers like Smart are fashioning reinforced carbon-carbon materials, which use carbon fibers to reinforce a carbon matrix, into various shapes instead of just flat plates to create the complex 3-D curved shapes needed for supersonic air to mix with the fuel in HyShot’s hypersonic engines. Changhong Ke, an associate professor of mechanical engineering at Binghamton University, sees promise in boron nitride nanotubes (BNNTs). His Air Force-funded research shows the strong, lightweight substance can handle high amounts of structural stress and temperatures of up to 900 °C—more than double the heat resistance of carbon nanotubes currently used in aircraft. If, like that of carbon nanotubes, the price falls from a prohibitive $1,000 per gram to between $10 and $20 per gram over the next two decades, BNNTs could trickle into commercial air travel.
Hypersonic vehicles also must contend with the extraordinary shock waves they create and the rapidly varying impact they can have on the fuselage. That includes causing an aircraft to have “a high probability of becoming unstable in flight” whenever it turns, says Urzay. In addition, he notes, shock waves can accelerate melting over certain spots as well as alter the air molecules in contact with the aircraft, leading to corrosive chemical reactions “that are very difficult to model.”
Engineers also are tackling combustion challenges. “When an airplane is traveling that fast, there is only a very, very short time inside the engine to burn the fuel with oxygen, on the order of microseconds,” explains Urzay. “Also, with all that wind involved, it’s like trying to light a match inside a hurricane.”
Optimally, hypersonic aircraft are “wave riders,” surfing and generating lift from shock waves confined to underneath the fuselage. Those ideal conditions no longer exist, however, if the aircraft’s nose gets pitched up, which can allow the shock wave to enter and turn off the engine in midflight, a condition known as “unstart,” explains Mirmirani. Furthermore, since scramjets work only if they get a hypersonic flow of air into their engines, any attempt to maneuver the aircraft can alter this flow and influence engine power. And since hypersonic vehicles are typically long and slender to minimize friction, “any disturbance they experience can result in vibrations, which can affect engine performance,” explains Mirmirani.
Another challenge is finding a way to accelerate scramjets up to hypersonic speeds so they can achieve combustion. One strategy involves using a rocket booster. Both the X-43 and X-51 were carried high into the atmosphere and released from a B-52, with a rocket booster taking them to speeds where the scramjet engines could start working.
Smart and his team are pursuing an updated version of this strategy, using a rocket motor that can fly back after boosting their scramjet to hypersonic speeds. The rocket has a very simple wing stowed on its back, much like a glider’s, plus a small propeller motor. “Once the booster has done its job, it’s basically just a big tin can,” says Smart. “It’s very light, and it won’t take a lot of power to fly it back to the launch pad.”
Lockheed Martin has taken a different route, partnering with Aerojet Rocketdyne to develop the unmanned SR-72, which combines a normal jet engine with a scramjet engine. Meanwhile, British company Reaction Engines Ltd. is working on a system that uses an air-breathing jet engine to boost its Skylon space plane to Mach 5, after which it switches to rockets. “If it can take off from the runway and go all the way to hypersonic flight, that would be the holy grail,” muses Smart, “but it’s a very complex undertaking.”
Smart’s team seeks a more commercial quarry: developing a reusable spacecraft that could fly to and from low Earth orbit without the need for expensive rocket engines or tons of oxidizing agents. Dubbed Spartan, the aircraft would employ a first-stage, reusable rocket booster, as SpaceX does. The second stage would have the hypersonic aircraft go from Mach 5 to Mach 10. The final stage would involve launching a small satellite from an expendable pad nestled on the back of the aircraft—which then could turn around and fly back to base. “All in all, it’s a way of making a satellite launch system almost 90 percent reusable in terms of its mass,” says Smart. “That’s something people can build a proper business case on.”
Most experts predict that hypersonic technology will first roll out in military applications. Indeed, advances in Russian and Chinese hypersonic nuclear missiles have been widely reported. Some reports claim, for example, that Russia’s Zircon hypersonic cruise missile—which supposedly can hit Mach 8 and has a range of 1,000 kilometers—already is being installed on warships.
Such activity has sparked a flurry of U.S. research initiatives. Stuart “Alex” Craig, an assistant professor of aerospace and mechanical engineering at the University of Arizona, is “definitely seeing an uptick.” He and two colleagues, both fluid-dynamics pioneers, recently won nearly $2 million from the Office of Naval Research to study stability and materials failure in hypersonic aircraft and missiles. At USC, Bermejo-Moreno and his colleagues are working on computer simulations to learn about combustion, turbulence, shock waves, and airflow within scramjet engines and over hypersonic vehicles to enhance their life span and stability. The limited amount of hypersonic flight data makes it hard to build reliable models, however, notes Mirmirani. And while existing models are very advanced, adds Urzay, “there is not enough supercomputing power in the world to predict hypersonic flow around an aircraft.”
Hypersonic wind tunnels offer a workaround. These specialized facilities, used to approximate calm conditions planes encounter in the atmosphere, need highly polished surfaces to prevent bumps or pits from generating unwanted noise, and dust-free air to protect from scratches. Steven Schneider, a professor of aeronautics and astronautics at Purdue University, and his colleagues focus on “quiet” wind tunnels that seek to reduce the level of pressure fluctuations—noise—that arise from the interactions between the airflow and the tunnel walls and can interfere with experiments. Arizona’s Craig plans to test a colleague’s computations in the school’s new low-disturbance, Mach 4 tunnel when it goes online this spring, with a second quiet tunnel slated for 2020.
Instead of flight simulations or tests, Argrow and his colleagues have received a five-year, $7.5 million Department of Defense grant to investigate the stratosphere in which hypersonic vehicles will fly. “If you hit a pocket of turbulence when you are flying at Mach 6 or 7, what will it do to the stability of airflow over the aircraft?” he asks. “What effects might dust have? How do we make weather forecasts for the stratosphere?” His team will launch a series of high-altitude balloons to record wind currents, particulate levels, and other atmospheric conditions during times of expected instability. That includes determining if standard instruments to measure turbulence even work in an environment where the air density is so low that the motion of individual air molecules becomes important. “We may have to develop a whole new class of instruments,” muses Argrow.
While “there’s a lot we don’t know about hypersonic flight, even after working on it for a half century,” says Schneider, developments so far suggest that unmanned spacecraft or missiles could arrive in five to 10 years. Australia’s Smart foresees “practical, commercial” hypersonic flights within a decade. “It’s not that far away.”
By Charles Q. Choi
Charles Q. Choi is a New York-based freelance writer and frequent contributor to Prism.
Design by Francis Igot