A wind tunnel study shows that the flow of a hypersonic jet engine can be controlled optically. Researchers at the University of Virginia are exploring the potential of hypersonic jets for space travel using innovations in thrust control and sensing techniques. The NASA-supported study aims to improve the performance of GVRPDs with adaptive control systems and optical sensors, potentially leading to safer and more efficient space access vehicles that operate like aircraft.

The future of space travel: hypersonic jets

What if the future of space travel looked less like the Space-X rocket ship and more like NASA’s Hyper-X, a hypersonic jet that flew faster than any aircraft before or since 20 years ago? In 2004, NASA’s final tests of the X-43A unmanned prototype marked a major turning point in the final era of jet aircraft development, the transition from ramjets to faster, more efficient ramjets. The final test, in November of that year, broke a world record that only a rocket has ever achieved: Mach 10. That’s 10 times the speed of sound.

NASA collected a lot of useful data from the tests, as did the Air Force six years later during similar tests of the X-51 Waverider before the prototypes hit the ocean. Although the hypersonic proof of concept was successful, the technology was far from operational. Since the technology has relied on sensor approaches for decades, the challenge was to achieve motor control.

Breakthrough in hypersonic engine control

But this month has brought hope for potential successors to the X-plane series. As part of a new NASA-funded study, researchers at the University of Virginia School of Engineering and Applied Science report the data in the June issue of the journal. Aerospace Science and Technology It demonstrated for the first time that airflow in supersonic jet combustion engines could be controlled using an optical sensor. This discovery could lead to more effective stabilization of hypersonic jets.

In addition, the researchers achieved adaptive control of the scramjet, another first for hypersonic propulsion. Adaptive engine management systems respond to changes in dynamics to keep overall system performance optimal.

“One of our national aerospace priorities since the 1960s has been to develop single-stage orbital aircraft that fly into space with a horizontal takeoff like conventional airplanes and land like conventional airplanes,” said Professor Christopher Goin, UVA Director of the Aerospace Research Laboratory. the place where the research was conducted.

“The most modern vehicle right now is the SpaceX Starship. It has two stages, vertical launch and landing. But to optimize safety, comfort and reusability, the aviation community wants to create something more like the 737.”

Goin and UVA Engineering assistant professor Chloe Dedick believe optical sensors could be a big part of the control equation.

“It made sense to us that if the aircraft was operating at hypersonic speeds of Mach 5 and above, it would be better to include sensors that operate closer to the speed of light rather than the speed of sound,” Goin said.

Additional team members included doctoral student Max Chern, first author of the paper, as well as former graduate student Andrew Wanczek, postdoctoral researcher Lori Elkowitz, and UVA senior research associate Robert Rockwell. The work was supported by a NASA ULI grant under the direction of Purdue University.

Scramjet engine performance increase

NASA has long tried to prevent what are called “shutdowns” on GVRPD engines. The term refers to a sudden change in airflow. The name comes from a special testing facility called a supersonic wind tunnel, where “snap” 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 the engine of a hypersonic vehicle traveling at five times the speed of sound.

“We can run test conditions for hours, allowing us to try new flow sensors and control approaches on realistic engine geometries,” Dedick said.

Goyne explained that “RAVD,” short for supersonic direct flow combustion engines, has been a direct flow jet engine technology that has been widely used for many years.

A ramjet engine essentially “kills” the air entering the engine by using the forward motion of the aircraft to create the temperature and pressure needed to burn the fuel. They operate in the range of about Mach 3 to Mach 6. As the inlet at the front of the craft narrows, the internal airspeed in an internal combustion jet engine slows to subsonic speed. But the aircraft itself does not.

But Scramjets are a little different. Although they also “breathe air” and have the same basic layout, they must maintain superfast airflow through the engine to achieve hypersonic speeds.

“If something happens inside a hypersonic engine and you suddenly hit subsonic conditions, that means it won’t work,” Goin said. “The thrust will drop off suddenly, and it can be difficult to restart input at that point.”

Testing a dual-mode Scramjet engine

Currently, just like jet engines, scramjets need a booster to reach a speed where they can consume enough oxygen to operate. This could include a rocket booster as well as a ride-on vehicle attached to the underside of the carrier aircraft.

The latest innovation is the dual-mode jet combustor, the type of engine the UVA-led project is testing. The twin engine starts in jet mode at lower Mach numbers and then transitions to full supersonic combustion chamber airflow above Mach 5. Preventing shutdown is critical as the engine makes this transition.

The incoming wind interacts with the inlet walls in a series of shock waves known as a “shock train.” The leading edges of these waves, which traditionally could damage the integrity of the aircraft, are monitored by pressure sensors. The machine can be adjusted, for example, by changing the position of the shock absorber.

However, if flight disturbances change airborne dynamics, the position of the shock’s leading edge can change rapidly. The impact mechanism can create triggering conditions by creating pressure at the inlet. “If you’re sensing at the speed of sound but your motor processes are moving faster than the speed of sound, you don’t have a lot of time to react,” Goin said.

He and his colleagues wondered if it might be possible to predict the expected shutdown by observing the characteristics of the engine flame instead. The team decided to use an optical emission spectroscopy sensor to provide the feedback needed to control the front end of the shock chain.

No longer limited to information obtained from the engine walls like pressure sensors, the optical sensor can detect subtle changes within both the engine and the flow. The device analyzes the amount of light emitted by the source (in this case, the reacting gases in the combustion chamber of a scramjet) as well as other factors such as flame position and spectral content.

Elkowitz, one of the doctoral students, said, “The light emitted by the flame inside the engine results from the relaxation of molecular forms excited during the combustion processes.” “Different types emit light of different energies, or colors, providing new information about the condition of the engine that is not detected by pressure sensors.”

The wind tunnel test was actually the world’s first demonstration that adaptive control of this type of twin engine could be achieved using optical sensors.

“We are excited to demonstrate the role of optical sensors in guiding future hypersonic vehicles,” said first author Chern. “We continue to test sensor configurations while working on a prototype that optimizes the volume and weight of the package for flight conditions.”

Building for the future

Although much work still needs to be done, optical sensors could be one component of the future that Goin believes will happen in his lifetime: travel to space and return by plane. Dual-mode jets will still need some boost to accelerate the aircraft to at least Mach 4. But there will be additional safety if we do not rely solely on rocket technology, which requires transporting large quantities of chemicals as well as flammable fuels. oxidizer for burning fuel. Reducing weight will provide more space for passengers and cargo.

Such an all-in-one flying vehicle, capable of gliding back to Earth like the old space shuttles, might even provide the perfect combination of economy, safety, and reusability.

“I think it’s possible, yes,” Goyne said. “While the commercial space industry has managed to reduce costs through reusability, it has yet to embrace aircraft-like operations. Our findings could potentially build on Hyper-X’s long history and make its access to space safer than today’s rocket technology.”