If you have even a passing interest in the science of air travel, you’re probably already familiar with the concept of hypersonic flight. But unless you’re an aerospace engineer, you may be surprised to learn that it remains largely experimental. Even today, the hypersonic regime is the province of spacecraft and demonstration and test vehicles, which travel in the atmosphere at speeds between Mach 5 and Mach 10 for only limited spans of time, mere minutes in most cases.
There’s a good reason. Hypersonic flight is punishing. At hypersonic speeds, the molecules of air around the vehicle break apart and can produce electrical charge (ionization). Aerodynamic heating from the friction of the air causes the temperatures to climb exponentially; temperatures are so hot that literally all known aerospace materials, including the tiles once used on the Space Shuttle, cannot survive long. That doesn’t mean that hypersonic flight over extended periods is impossible to achieve; it just means it’s a very complicated problem to solve.
If we’re going to master reliable hypersonic flight, it’s going to take advanced research into new materials, new cooling systems, and ingenious new aerodynamic designs for hypersonic aircraft that can withstand extreme conditions and perform equally well across a range of operational regimes. At U.S. Naval Research Laboratory, that’s exactly what researchers have been working on.
Now the engineers at NRL’s Space Mechanical Systems Development Branch are looking to an innovation from the earliest days of manned flight, specifically one pioneered by the Wright brothers, as inspiration for their new design for a hypersonic aircraft. They’re calling it a morphing waverider.
Cooling the leading edge
In 2015, Jesse Maxwell and a team of mechanical engineers at the Space Mechanical Systems Development Branch were developing designs for leading edge cooling for hypersonic aircraft as part of a 6.2 Base Program, experimenting with different materials and cooling system architectures for the nose and wings of aircraft that would ensure their survivability under these extremely high temperatures.
“If [the material] can't survive the temperatures, you've got to keep it from heating up that much, and the way you keep it from heating up is by pulling energy away from it,” explains Austin Phoenix, an NRL mechanical engineer who has been developing the morphing waverider design with Maxwell.
Among the options the team conceived (that are pending patent) was one that uses the evaporation and condensation cycles of liquid sodium—a molten metal—to pull heat away from a leading edge. Liquid sodium is created by heating elemental sodium, a silvery metal, past its melting point of 98 degrees Celsius. According to Maxwell, the operating temperatures for these sodium heat pipes are 800 to 1,200 degrees Celsius.
(It should give some idea of the temperatures at hypersonic speeds that they chose using boiling metal to cool the aircraft’s leading edges.)
“[The system uses] a continuous cycle,” Maxwell explains. “We use a porous wick of metal, and the liquid sodium soaks its way toward the heat source, and then, when it evaporates, the vapor moves away from the heat source and re-condenses. Any voids left by that vapor, the liquid then just soaks back up.”
But extreme heat can threaten the structural integrity of an aircraft on places other than its leading edges. Today’s conventional aircraft have wings that remain fixed during flight and for steering employ motorized flaps and ailerons that move on hinges. And while that’s proven to be a reliable and durable system for maneuvering aircraft traveling at subsonic speeds, when you get to hypersonic speeds, the physics are such that those hinges become something of a problem.
“If you have a flap or an aileron, you inherently have a hinge,” Maxwell says. “And any sharp change in hypersonic flow concentrates heating and pressure. [That hinge] adds to your drag and adds to heating. And heating is what really kills you at those speeds. Everything is in danger of burning up.”
That’s where the Wright brother’s innovation could prove useful. The Wright Brothers had observed that birds maneuvered in flight by changing the shape of their wings. With their power-driven, heavier-than-air design, the Wright brothers emulated that method of flight control with the technique of wing warping, pulling on cables to warp their aircraft’s wings to steer it. That’s how the brothers made history in 1903 by achieving controlled and sustained flight at Kitty Hawk, North Carolina.
Maxwell and his team are adopting that simple idea—altering the shape of an aircraft’s wing during flight—but they’re taking it a step further. They’re aiming to achieve a smooth, seamless control surface, one without ailerons, flaps or hinges—to which they can introduce small deviations through morphing—changing the aircraft’s shape, rather than just its wing. They believe that will allow them to control their waverider at hypersonic speeds.
“With the small changes in a surface to give you control capabilities, those small deviations are also smooth,” Maxwell says. “And so you avoid the intense heating that you get with normal control surfaces for low speed aircraft.”
Their designs involve morphing the geometry of the body of the aircraft, changing the shape of the underside of the vehicle to improve lift, reduce drag and provide control capabilities for the pilot, according to Phoenix.
Riding the shock
Back in 2015, when Maxwell was working on leading edge cooling, he was looking for a case study vehicle would allow him to analyze a wide range of conditions for heating. He discovered that the waverider design would serve as an ideal, simple case study vehicle for studying heating and flight conditions. That’s what led to his discovery of concept of a morphing waverider.
But what is a waverider? First proposed by professor Terence Nonweiler of the Queen's University of Belfast in the 1950s, a waverider is a design for hypersonic aircraft that uses the shock waves it produces during flight as a lifting surface, a concept known as “compression lift.”
While flying through the air, the shape of an aircraft moves the air around it, just as a boat moves the water around it. As it moves, air pressure waves form in front of the aircraft and behind it. When an aircraft travels at the speed of sound or faster, those pressure waves don’t have time to get out of the way. So they bunch together and compress into a shock wave on the aircraft, or just plain “shock,” as engineers like Maxwell and Phoenix like to refer to it.
If you have ever seen the cone of vapor that forms around the back of a high speed jet traveling at supersonic speed through moist air—that conical wave of vapor resembles the shape of a shock wave. The cone of vapor is often called a “Mach cone.”
The shock wave will vary according to the speed of an object. For example, a bullet will produce an approximate cone shape, while an object traveling at a higher speed will produce a shock wave with an angle closer to its body.
“Imagine that you have a high Mach number, say Mach 5 or 10,” Maxwell says. “You've got a pretty narrow cone angle. You can design the [aircraft’s] leading edge to touch the surface of that cone angle to create a high-pressure pocket of air underneath the vehicle, which gives you a really great lift-to-drag ratio. You're riding the shockwave.”
To date, the only aircraft based on the design has been the Mach 3 supersonic XB-70 Valkyrie. Developed in the 1960s, only two prototypes were ever flown. Designed to cruise at Mach 3, the aircraft reached Mach 3 speeds a little more than a dozen times. Its triangular delta wing was a variation on the waverider concept, designed for compression lift, with wingtips that could face downwards during flight to trap the shockwave.
“That was the state of the art for the last half century,” Maxwell says. “It turns out that it didn't perform very well. So they stopped making them.”
The chief drawback of hypersonic designs, according to Phoenix, is that, they’re highly efficient, but only at a single operational regime—that is, at a certain altitude, speed and atmospheric density. Most hypersonic designs, Maxwell and Phoenix say, are designed to fly in the near space regime within low-density atmosphere.
“So they produce a lot of lift for that one configuration,” Phoenix explains. “But if they slow down or speed up, which is what most things do, they reduce their efficiency. The ideal morphing waverider would maintain the perfect geometry across the entire flight.”
Maxwell’s solution was simple: If your aircraft design employed a flexible bottom surface, you could push and pull on that surface to create the best geometry for each operational regime. In 2016, he approached Phoenix, who has a background in morphing structures, and asked him the obvious question: “Can we physically make this thing?”
“We had this model we believed was more efficient,” Phoenix says. “But we just didn't know whether it was feasible or whether the performance benefits would justify the additional complexity.”
For the last two years, they have been studying the viability of the idea. Over the summers of 2017 and 2018, they have conducted preliminary low-speed testing of models at the United States Naval Academy wind tunnel. For security reasons, they can’t yet release details on their results (indeed, Maxwell’s dissertation has yet to see public release), though they can say the results were consistent with their predictions.
They are envisioning a design that can travel at hypersonic speeds for extended periods over multiple operational regimes, from the conventional aircraft regime, to the near-space regime, where the atmosphere is low-density, and even into the space regime where spacecraft are orbiting at phenomenal speeds.
Such a design would of course have a number of applications, from hypersonic air travel to hypersonic weapons. But Maxwell’s team is concentrating their work on their design’s potential space applications, in the hopes that they might someday inform the design of a new kind of entry vehicle, or even the next space shuttle.
“Anything you send to or from space can benefit from a high lift body, and that's what we are primarily focused on,” Maxwell says. “Here's a vehicle that can span a wide range of speeds; it has an excellent lift-to-drag ratio, and you can use it to get to space and back with a sufficient propulsion system.”
Hypersonic aircraft have already managed to reach space. In 1963, a rocket-powered aircraft called the North American X-15A-2 set a world record altitude of 354,200 feet (67 miles above the Earth), operating effectively outside the Earth’s atmosphere. (Four years later, the X-15 would go on to set a world record for the highest speed achieved by a manned, powered aircraft by reaching Mach 6.7, about 4,500 miles an hour.)
But getting into space is easy compared to staying there. According to Maxwell, it requires about 16 times the energy to stay in orbit than it takes to just reach the altitude: hence the use of office-building-sized rockets for space launches. Even so, Maxwell and Phoenix believe hypersonics could be another solution, a potentially cheaper and more efficient one.
The gold standard, as Maxwell calls it, would be a vehicle that could fly into space, reach orbit and then fly back.
“The way we do it right now is obviously brute force,” Maxwell says. “You just build a bigger rocket.”
Building the tunnel
In 2017, they scheduled time at the United States Naval Academy’s wind tunnel—just a few days a month to conduct testing on models they had custom made at an external machine shop. Without much experience as experimentalists, they initially relied on an academy professor to walk them through experimental techniques.
“After a few iterations we got better and better at it,” Maxwell says. “We took some pictures, but a lot of the flow fields happen so fast that you only get to see the average—unless you have a really high speed camera.”
Eventually they found that, for their purposes, the range of conditions the tunnel could simulate was too narrow. They needed a wider range and the ability to test a larger model than the Naval Academy tunnel could accommodate. But wind tunnels are expensive—that’s what the team discovered when they conducted some preliminary trade studies.
“Better assets are out there that can reach a wider range of conditions, but they are extremely expensive,” Maxwell says. “We went to see what other tunnels would cost, and in some case they're $20,000 to a $100,000 a day.”
Soon after, they began laying the groundwork for their own wind tunnel at NRL that would allow them to continue their research. Rather than designing the facility from scratch, they opted to go with a company with a pre-existing design. That would allow them to devote the remainder of their funding to essentials like the diagnostic computers, sensors and measurement systems.
“We pitched the idea of a wind tunnel [as a capital procurement project]—a small one and a large one,” Maxwell says. “NRL originally approved the small one in 2015 for the 2017 cycle, but they ended up asking us to wait a year and then doubled our budget so we could buy the big one.”
Work on the new tunnel has only just begun. Currently, it’s just a 4,000-square-foot laboratory that once served as the storage space for a standing machine shop and assorted parts. Over the next few months, the team will conduct the initial design reviews. Hardware is expected to arrive on site in September. Full initial operating capacity is forecast for the end of calendar year 2019.
According to Maxwell, the new wind tunnel being constructed will be ideal for studying models of waveriders that will change shape to adapt to a variety of operational regimes. It will have the ability vary pressure to simulate variations in altitude and wind speed in situ.
“So we can take the model and do control flap deflections; we can pitch up and down angle of attack,” Maxwell says. “We can accelerate and decelerate, climb and descend. We will be able to fly [simulations of] this vehicle anywhere from about sea level up to over 100,000 feet at speeds of Mach 1.3 up to at least Mach 5 early on—and eventually Mach 6 and a half.”
Rather than plotting disparate points noting factors like altitude, height, and speed, this new wind tunnel will give the team the ability to see those factors for a model’s performance along an unbroken spectrum—or as a “million little continuous test points” as Maxwell puts it.
Asked whether that’s a unique capability for a wind tunnel, he replies, “As far as we have found, no one else is capable of doing that.”