Dreams of 14-hour flights cut to an hour or two via commercial airlines with hypersonic engines will likely remain a mirage for the next decade at least. But there seems to be a new focus on solving the challenges of high-altitude hypersonic flight.
NASA’s Aeronautics Division is partnering with the US National Laboratory at Argonne outside of Chicago to accelerate research into hypersonic flight to make commercial and military aircraft a mainstream reality.
Hypersonic flight defines any aircraft or drone that can reach speeds of Mach 5 or more, five times the speed of sound at sea level. Researchers have long used computational fluid dynamics (CFD) to predict, among other things, how an aircraft in flight will interact with the forces around it, Argonne notes. CFD is a field of science devoted to the numerical expression of the behavior of fluids such as air and water, notes the lab.
But as any aerodynamicist will readily admit, we just don’t fully understand airflow at such high speeds and temperatures. Argonne lends its computationally intensive capability to understand these extreme conditions using computational fluid dynamics or CFD.
A paper detailing their research was presented at an American Institute of Aeronautics and Astronautics (AIAA) forum earlier this year. The lab is a pioneer in using artificial intelligence to streamline computer simulations and accelerate the development of barrier-breaking aircraft, says Argonne.
“Our role is to model how it works,” Sinan Demir, lead author of the paper and a mechanical and aerospace engineer at Argonne’s Advance Energy Technologies (AET) division, told me. Although CFD has the potential to be a powerful tool for design and optimization, computer modeling poses many challenges because the physical flow process is very complex in hypersonic engines, he says.
CFD simulations must account for major changes in the air, not just around the plane, but also as it moves through the engine and interacts with fuel, Argonne notes. Aerobic jet engines, as they are called, draw in oxygen to burn fuel during their flight. In a conventional airplane, fan blades push air, the lab notes. But at hypersonic speeds, the motion of the jet itself compresses the air. Such aircraft designs, known as scramjets, are key to achieving levels of fuel efficiency that rocket propulsion cannot, says Argonne.
However, at hypersonic speeds, the air friction created is so strong that it could melt parts of a conventional commercial aircraft, says Argonne.
A NASA hypersonic CFD code deals with multi-dimensional flamelet tables, where each flamelet represents a one-dimensional version of a flame, the lab notes.
The flamelet table, generated by software developed by Argonne, was used to train an artificial neural network, the lab notes. In an artificial neural network, which is a subset of machine learning, a computer derives information from data like a human brain would, Argonne explains.
The key to hypersonics is to fly at high altitudes to minimize friction and high temperatures. Otherwise, Demir says, there would be so much pressure on the plane that it would collapse.
Argonne simulations by Demir and his colleagues allow NASA aerodynamicists to better understand how these high temperatures, high pressures and turbulent airflows affect the aircraft at hypersonic speeds. That is, without having to risk experimental aircraft in real-life test situations at high altitudes. These altitudes generally range from 100,000 to 300,000 feet at the edge of outer space.
But the advantage of an air-breathing engine is that the vehicle doesn’t need to carry compressed liquid oxygen (needed for the combustion process) on board, because it’s readily available in the atmosphere, Demir says.
What is the biggest challenge in scaling up scramjets for commercial passenger transport?
“Understanding the complex mixing and combustion phenomena in the large-scale scramjet combustor under extreme flow conditions,” Demir said. “At hypersonic speeds and altitudes, flight is incredibly unstable.”
When do you think the world will have hypersonic aircraft in practical commercial use?
It’s hard to answer that question because science is advancing so rapidly that the time needed to realize the commercial hypersonic dream has shortened, says Demir.
Will Demir fly hypersonic?
“If it’s ever made, I wouldn’t be among the first to fly there until I’m sure it’s safe,” Demir said.