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This is an artist’s rendering of a Hyper-X research vehicle under sramjet power in free flight after separation from its booster rocket. New research into hypersonic jets could transform space travel by making jet engines more reliable and efficient, leading to aircraft-like spacecraft. Credit: NASA
Wind tunnel study reveals hypersonic jet thrust can be controlled optically
Researchers at the University of Virginia are exploring the potential of hypersonic jets for space travel, using innovations in engine control and sensing techniques. Work, supported by NASAaims to improve scramjet performance through adaptive control systems and optical sensors, potentially leading to safer and more efficient space access vehicles that operate like jets.
The Future of Space Travel: Hypersonic Jets
What if the future of space travel looked less like Space-X’s rocket-based Starship and more like NASA’s Hyper-X, the hypersonic jet plane that, 20 years ago this year, flew the fastest than any other aircraft before or since. ?
In 2004, the final tests of NASA’s X-43A unmanned prototype were a milestone in the latest era of aircraft development—the leap from ramjets to faster, more efficient aircraft. The last test, in November of that year, set a world record speed that only a rocket could achieve before: Mach 10. The speed is equal to 10 times the speed of sound.
NASA extracted a lot of useful data from the tests, as did the Air Force six years later in similar tests on the X-51 Waverider, before the prototypes were put into the ocean.
Although the hypersonic proof of concept was successful, the technology was far from operational. The challenge was achieving motor control because the technology was based on decades-old sensor approaches.
NASA’s B-52B launch vehicle navigates a test range over the Pacific Ocean carrying the third and final X-43A vehicle, attached to a Pegasus rocket, on November 16, 2004. Credit: NASA / Carla Thomas
Advances in hypersonic engine control
This month, however, brought some hope for potential successors to the X-plane series.
As part of a new NASA-funded study, researchers at the University of Virginia’s School of Engineering and Applied Sciences published data in the June issue of the journal Aerospace Science and Technology which showed for the first time that the airflow in supersonic combustion jet engines can be controlled by an optical sensor. The discovery could lead to more efficient stabilization of hypersonic jets.
In addition, the researchers achieved adaptive control of a thruster, representing another first for hypersonic propulsion. Adaptive engine control systems respond to changes in dynamics to maintain optimal overall system performance.
“One of our national aerospace priorities since the 1960s has been to build single-stage-to-orbit aircraft that fly into space from horizontal take-off like a traditional aircraft and land on the ground like a traditional aircraft,” Professor Christopher Goyne said. , director. of the UVA Air Research Laboratory, where the research took place.
“Currently, the most advanced craft is that SpaceX Starship. There are two stages, vertical launch and landing. But to optimize safety, comfort and reusability, the aerospace community would like to build something more like a 737.
PhD student Max Chern takes a closer look at the wind tunnel setup where University of Virginia School of Engineering and Applied Sciences researchers demonstrated that control of a dual-mode scramjet engine is possible with an optical sensor . Credit: Wende Whitman, UVA Engineering
Goyne and his co-investigator, Chloe Dedic, a UVA associate professor of engineering, believe optical sensors can be a big part of the control equation.
“It seemed logical to us that if an aircraft is operating at hypersonic speeds of Mach 5 and above, it might be preferable to deploy sensors that operate closer to the speed of light than the speed of sound,” Goyne said.
Additional team members were doctoral student Max Chern, who served as the paper’s first author, as well as former graduate student Andrew Wanchek, doctoral student Laurie Elkowitz, and UVA senior scientist Robert Rockwell. The work was supported by a NASA ULI grant led by Purdue University.
Scramjet engine performance boost
NASA has long sought to prevent something that can happen in scramjet engines called “unstart.” The term indicates a sudden change in air flow. The name is derived from a specialized test facility called a supersonic wind tunnel, where a “start” means that the wind has reached the desired supersonic conditions.
UVA has several supersonic wind tunnels, including the UVA Supersonic Combustion Facility, which can simulate engine conditions for a hypersonic vehicle traveling at five times the speed of sound.
“We can run the test conditions for hours, allowing us to experiment with new flow sensors and control approaches in a realistic engine geometry,” said Dedic.
Goyne explained that “scramjets,” short for supersonic combustion ramjets, are based on ramjet technology that has been in common use for years.
This computational fluid dynamics image from the original Hyper-X tests shows the engine running at Mach 7. Credit: NASA
Ramjets essentially “ram” the air into the engine using the forward motion of the aircraft to generate the temperatures and pressures needed to burn the fuel. They operate in a range of about Mach 3 to Mach 6. As the inlet to the front of the craft narrows, the internal air velocity slows to subsonic speeds in a ramjet combustion engine. However, the aircraft itself does not.
However, Scramjets are a little different. While they are also “air breathers” and have the same basic configuration, they must maintain that super-fast airflow through the engine to reach hypersonic speeds.
“If something happens inside the hypersonic engine and subsonic conditions are suddenly created, it’s a no-start,” Goyne said. “The break will suddenly decrease and it can be difficult at that point to restart the entry.”
Testing a Dual Mode Scramjet Engine
Currently, like ramjets, scramjet engines need a boost to get them up to a speed where they can get enough oxygen to function. This can include a ride attached to the underside of a carrier aircraft as well as a rocket thruster.
The latest innovation is a dual-mode scramjet combustor, which was the type of engine the UVA-led project tested. The twin engine starts in ramjet mode at lower Mach numbers, then switches to receiving full supersonic airflow into the combustion chamber at speeds exceeding Mach 5.
Preventing misfires after the engine makes that transition is essential.
Christopher Goyne, professor and director of the UVA Aerospace Research Laboratory, and Chloe Dedic, associate professor. Credit: Wende Whitman, UVA Engineering
The incoming wind interacts with the inlet walls in the form of a series of shock waves known as “shock trains”. Traditionally, the leading edge of those waves, which can be destructive to the integrity of the aircraft, have been controlled by pressure sensors. The machine can adjust, for example, by shifting the position of the shock train.
But where the leading edge of the shock train is located can change quickly if flight disturbances change the air dynamics. The shock train can pressurize the inlet, creating the conditions to misfire.
So, “If you feel the speed of sound, but the motor processes are moving faster than the speed of sound, you don’t have much response time,” Goyne said.
He and his collaborators wondered if a pending misfire could be predicted by observing engine flame features.
Sensing the spectrum of a flame
The team decided to use an optical emission spectroscopy sensor for the feedback needed to control the leading edge of the shock train.
No longer limited to information received at the engine walls, such as pressure sensors, optical sensing can identify subtle changes both within the engine and within the flow path. The tool analyzes the amount of light emitted by a source—in this case, the reacting gases inside the scramjet combustor—as well as other factors, such as flame location and spectral content.
“The light emitted by the flame inside the engine is due to the relaxation of the molecule species that are excited during combustion processes,” explained Elkowitz, one of the Ph.D. “Different species emit light of different energies or colors, providing new information about the state of the engine that is not captured by pressure sensors.”
Current UVA Mechanical and Aerospace Engineering doctoral students Laurie Elkowitz and Max Chern were among the influential members of the team. Credit: Wende Whitman, UVA Engineering
The wind tunnel team’s demonstration showed that engine control can be both predictive and adaptive, seamlessly switching between scramjet and ramjet operation.
The wind tunnel test, in fact, was the first proof in the world that adaptive control in these types of dual-function motors can be achieved with optical sensors.
“We were very excited to demonstrate the role that optical sensors can play in the control of future hypersonic vehicles,” said first author Chern. “We are continuing to test sensor configurations as we work toward a prototype that optimizes package volume and weight for flight environments.”
Building towards the future
While much more work remains to be done, optical sensors could be a component of the future that Goyne believes will be realized in his lifetime: airplane-like trips to space and back.
Dual-mode scramjets would still require some sort of boost to get the plane to at least Mach 4. But it would have the added safety of not relying exclusively on rocket technology, which requires highly flammable fuel to be transported along with large quantities of chemicals. oxidizer for fuel combustion.
This reduced weight would allow more space for passengers and cargo.
Such an all-purpose plane, which would glide back to Earth like spacecraft once did, might even provide the ideal combination of cost efficiency, safety and reusability.
“I think it’s possible, yes,” Goyne said. “While the commercial space industry has been able to reduce costs through some reusability, they have yet to catch up to aircraft-like operations. Our findings could potentially build on the history of Hyper-X and make its access to space safer than current rocket-based technology.
Reference: “Control of a dual scramjet flow path using optical emission spectroscopy” by Max Y. Chern, Andrew J. Wanchek, Laurie Elkowitz, Robert D. Rockwell, Chloe E. Dedic, and Christopher P. Goyne, April 18 2024, Aerospace Science and Technology.
DOI: 10.1016/j.ast.2024.109144