Dreams of 14-hour flights cut to a mere hour or two via commercial airlines equipped with hypersonic engines are likely to remain a mirage for the next decade at least. But there does appear to be new emphasis on tackling the challenges of high altitude, hypersonic flight.
NASA’s aeronautics division is coupling with the US’ Argonne National Laboratory outside Chicago to speed up hypersonic flight research in order to make both commercial and military aircraft a commonplace reality.
Hypersonic flight defines any aircraft or drone which can reach speeds of Mach 5 or more, or 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, notes Argonne. CFD is a scientific field devoted to numerically expressing the behavior of fluids such as air and water, the lab notes.
But as any aerodynamicist will readily admit, we simply don’t fully understand airflows at such high speeds and high temperatures. Argonne is lending their supercomputing capacity to understanding these extreme conditions using computer 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 pioneering the use of artificial intelligence to streamline computer simulations and accelerate the development of barrier-breaking aircraft, says Argonne.
“Our role is to model how this works,” Sinan Demir, the paper’s lead author 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 the designing and optimization, computational modeling brings many challenges as the physical flow process is very complex in hypersonic engines, he says.
CFD simulations must account for major shifts in air, not only around the plane, but also as it moves through the engine and interacts with fuel, notes Argonne. Air-breathing jet engines, as they are called, draw in oxygen to burn fuel as they fly. In a conventional plane, fan blades push the air along, the lab notes. But at hypersonic speeds, the movement 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 plane, says Argonne.
A NASA hypersonic CFD code processes multidimensional flamelet tables, where each flamelet represents a one-dimensional version of a flame, notes the lab.
The flamelet table, generated by Argonne-developed software, was used to train an artificial neural network, notes the lab. In an artificial neural network, which is a subset of machine learning, a computer derives insights from data the way a human brain would, says Argonne.
The key to hypersonics is to fly at high altitudes in order to minimize friction and high temperatures. Otherwise, says Demir, there would be so much pressure on the aircraft that it would actually fall apart.
Demir and his colleagues’ Argonne simulations enable NASA aerodynamicists to better understand how these high temperatures, high pressures and turbulent air flows affect the aircraft at hypersonic speeds. That is, without having to risk experimental aircraft in real life test situations at high altitudes. Such altitudes typically range from 100,000 to 300,000 feet at the very edge of outer space.
But the benefit of an air-breathing engine is that the vehicle doesn’t need to carry compressed liquid oxygen (required for the combustion process) on board, as it is readily available in the atmosphere, says Demir.
What’s the biggest challenge in scaling up scramjets for commercial passenger transport?
“Understanding the complex mixing and combustion phenomena in the scaled-up scramjet combustor at extreme flow conditions,” said Demir. “At hypersonic speeds and altitudes, flying is incredibly unstable.”
When do you expect the world to have hypersonic aircraft in practical commercial use?
This is difficult to answer as the science has been progressing so rapidly, the time needed to make the commercial hypersonic dream come true has been shortening, says Demir.
Will Demir fly hypersonic?
“If it is made one day, I would not be among the first ones who fly in it until I am sure that it is safe,” said Demir.