Simulations and diagnostics shed light on complex physics of hypersonic flight

Aerospace engineering has always had a preoccupation with speed. Now researchers at the University of Cincinnati are pushing the boundaries of what is possible, practical, reliable and safe at five times the speed of sound, or more than 3,800 mph.

“The bottom line for aerospace is to go farther, faster with more payload. That’s the fundamental question for anything that flies,” said Prashant Khare, director of UC’s Hypersonics Lab. “And we strive every day to come up with new technologies or science concepts that might lead to improvements.”

Hypersonics is getting increasing attention for military applications in the race to develop faster weapons—and perhaps one day aircraft—capable of evading deterrent technology.

Weapons such as Germany’s V-2 rocket were deployed as early as World War II. More recently, Russia used hypersonic missiles in its attacks on Ukraine.

“And China tested one of its own. So there’s been a lot of investment in improving hypersonic technologies,” said Khare, head of UC’s Department of Aerospace Engineering and Engineering Mechanics.

“From a physics point of view, the limitation is the speed of light,” he said. “The challenge is not only how fast we can go, but do we have materials that will withstand the speeds we want to go?”

The crew of Apollo 10 holds the speed record for human flight, reaching more than 24,000 miles per hour on their return trip from the moon. At such extreme speeds, the friction with the atmosphere generates intense heat. Space modules must withstand temperatures of more than 2,700°F.

Researchers are finding new ways to withstand this inferno, he said.

“Friction creates the heat. Can we come up with new materials or cooling technologies so we can go fast?” Khare asked. “The next question is how do we go that fast? That is more related to traditional propulsion or combustion sciences.”

UC engineering student Jeremy Redding said aerospace engineers push the envelope of what is possible and safe, starting with the first rockets in the 1940s, leading to the first human missions to space in the 1950s and ’60s.

The Hypersonics Lab’s work at Digital Futures has examined how changing surface dynamics alters the fundamental physics of hypersonic flight systems. Engineers also are learning more about the bizarre physics observed at high speeds, Khare said. Fluids behave weirdly at hypersonic speeds as molecules break apart and reform, creating a nonequilibrium state.

“A lot of it is counterintuitive or even mind-bending to think about,” Khare said.

And their collaboration with the Army Research Laboratory is leading to engine improvements to prevent stalling at such high speeds, Redding said.

“This is just the beginning,” Redding said. “Hypersonics is a multidisciplinary field of research that requires the insight of a passionate community intentionally pursuing the knowledge it has to go faster and higher.”

At least for now, hypersonic flight is impractical for commercial aviation because of the massive gravitational or g-forces passengers would have to withstand. But it’s of immense interest in military and cargo applications, Khare said.

In his lab, Khare and his students use computer simulations to study the fundamentals of new propulsion systems for rotating detonation engines, which are more efficient than today’s jet engines, and supersonic combustion ramjet engines.

Engineers are deploying tools such as spectroscopy, Raman scattering and photonic Doppler velocimetry to study hypersonics. Meanwhile, supercomputers allow Khare and his students to record hypersonic variables in fine detail in simulations. Capturing just one millisecond of hypersonic flight in simulation can require millions of computing hours.

“With advances in experimental diagnostics, technology and computing, we are now at a point where we can understand it because we have these new tools,” Khare said.

But there is still much to explore. And with so much external interest in this topic from government agencies and aerospace companies, careers in hypersonic technology are promising, he said.

“When people talk about something simple they’ll often say, “It’s not rocket science,'” Khare said. “But this is. This is rocket science.”