Building hypersonic planes won’t demand a significantly different design approach, study suggests

Hypersonic planes could fly from Sydney to Los Angeles in just an hour. What’s standing in the way of such ultra-fast planes becoming reality is our understanding of how the turbulence they generate as they fly at such high speeds impacts their design. Now, a new study suggests that building hypersonic planes may not require a significantly different design approach.

If it were to become a reality, hypersonic flight, long the realm of science fiction, could revolutionize global travel, transforming day-long international flights into brief commutes no longer than a feature-length movie. The duration of a long-haul route, such as Sydney to Los Angeles, might drop from 15 hours to just one.

“It really shrinks the planet,” says Professor Nicholaus Parziale, whose research focuses on making such hypersonic flight a reality, and who is a recent recipient of the Presidential Early Career Award for Scientists and Engineers for his research into the fluid mechanics that affects high-speed flight. “It will make travel faster, easier and more enjoyable.”

How hypersonic speeds challenge design

Crossing half the planet in an hour may sound untenable, but such planes may be closer to reality than you think. Military planes already fly at double and triple the speed of sound, which engineers refer to as Mach 2 or Mach 3, where Mach 1 stands for the speed of sound or 760 miles per hour. To cover the distance from Los Angeles to Sydney in an hour, the planes would need to fly at Mach 10—ten times the speed of sound. What’s standing in the way of such ultra-fast planes becoming reality is the turbulence and heat they generate as they fly.

There’s a difference in how the air behaves around the aircraft at low speeds versus high speeds. Aerospace engineers have special terms for this phenomenon: incompressible and compressible flow. In incompressible flow, which occurs at low speeds (below about Mach 0.3 or 225 miles per hour), air density remains nearly constant, which simplifies airplane design. However, at higher speeds, especially above the speed of sound, it switches to compressible flow. “That’s because a gas can ‘squish,'” explains Parziale, or, to put it in scientific terms, compress.

Compressing means that air density changes significantly due to variations in pressure and temperature, which affects how an aircraft flies. “Compressibility affects how the airflow goes around the body and that can change things like lift, drag, and thrust required to take off or stay airborne.” All of which is important for plane design.

Morkovin’s hypothesis and turbulence explained

Aerospace engineers have a pretty good idea how such airflow works with planes that fly below or close to the speed of sound—also known as “low Mach” numbers. To build hypersonic planes, they must understand how airflow works at greater Mach numbers—like five or ten times the speed of sound. And that remains a bit of an enigma, save for the so-called Morkovin’s hypothesis.

Formulated by Mark Morkovin in the mid-20th century, the hypothesis postulates that when air moves at Mach 5 or Mach 6, the turbulence behavior doesn’t change all that much from slower speeds. Although air density and temperature change more in faster flows, the hypothesis states that the basic “choppy” motion of turbulence stays mostly the same.

“Basically, the Morkovin’s hypothesis means that the way the turbulent air moves at low and high speeds isn’t that different,” says Parziale. “If the hypothesis is correct, it means that we don’t need a whole new way to understand turbulence at these higher speeds. We can use the same concepts we use for the slower flows.” That also means that hypersonic planes don’t need a significantly different design approach.

Testing the hypothesis in the lab

Yet, so far no one has been able to provide sufficient experimental evidence to support Morkovin’s hypothesis. That became the subject of Parziale’s new study, titled Hypersonic Turbulent Quantities in Support of Morkovin’s Hypothesis, which was published in Nature Communications on November 12.

In the study, Parziale’s team used lasers to ionize a gas called krypton which is seeded into the air flowing inside a wind tunnel. That temporarily made krypton atoms form an initially straight, glowing line. Then researchers used ultra-high-resolution cameras to take pictures of how that fluorescent krypton line moves, bends, and twists through the wind tunnel’s air—akin to how a leaf swirls through the little eddies in a river.

“As that line moves with the gas, you can see crinkles and structure in the flow, and from that, we can learn a lot about turbulence,” says Parziale, adding that he spent 11 years building that clever setup. “And what we found was that at Mach 6, the turbulence behavior is pretty close to the incompressible flow.”

What this means for future flight

Although the hypothesis isn’t fully confirmed yet, the study brings us one step closer to hypersonic flight because it suggests that planes don’t need an entirely new design to fly at hypersonic speeds. And that simplifies things.

“Today, we must use computers to design an airplane, and the computational resources to design a plane that will fly at Mach 6, simulating all the tiny, fine, little details would be impossible,” says Parziale. “The Morkovin’s hypothesis allows us to make simplified assumptions so that the computational demands to design hypersonic vehicles can become more doable.”

The study findings also hold promise for changing how space transportation is done, Parziale explains. “If we can build planes that fly at hypersonic speed, we can also fly them into space, rather than launching rockets, which would make transportation to and from low Earth orbit easier,” he says. “It will be a game-changer for transportation not only on Earth, but also in low orbit.”