A wind tunnel study shows that the jet of a hypersonic jet engine can be controlled optically

What if the future of space travel looked less like a starship based on Space-X rockets and more like the “Hyper-X,” a hypersonic jet that 20 years ago this year flew faster than any aircraft before or since? ?

In 2004, the final tests of NASA’s X-43A unmanned prototype marked a milestone in the latest era of jet development—the leap from ramjets to faster, more efficient scramjets. The last test in November of the same year achieved a world record speed that only a rocket could previously achieve: Mach 10. The speed corresponds to 10 times the speed of sound.

NASA gleaned much useful data from the tests, as did the Air Force six years later in similar tests on the X-51 Waverider before the prototypes hit the ocean.

Although the hypersonic proof of concept was successful, the technology was far from operational. The challenge was to achieve motor control as the technology was based on decades old sensor approaches.

However, this month brought some hope for a potential successor to the X-plane line.

As part of the new study, researchers from the University of Virginia School of Engineering and Applied Science published data in the June issue of the journal Aeronautical Science and Technology which show for the first time that the airflow in supersonic internal combustion jet engines can be controlled by an optical sensor. The finding could lead to more effective stabilization of hypersonic jets.

In addition, the researchers achieved adaptive control of the scramjet engine, 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 orbital aircraft that fly into space from a horizontal takeoff like a traditional aircraft and land on the ground like a traditional aircraft,” said Professor Christopher Goyne, Director . from the UVA Aerospace Research Laboratory, where the research was conducted.

“Currently, the most advanced craft is the SpaceX Starship. It has two stages, with vertical takeoff and landing. But to optimize safety, comfort and reusability, the aviation community would like to build something more like the 737.”

Goyne and co-investigator Chloe Dedic, associate professor of UVA Engineering, believe optical sensors could be a big part of the control equation.

“It seemed logical to us that if the aircraft is operating at hypersonic speeds of Mach 5 and above, that it might be more appropriate to build in sensors that operate closer to the speed of light than the speed of sound,” Goyne said.

Other team members included 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 research scientist Robert Rockwell. The work was supported by a NASA ULI grant led by Purdue University and principal investigator T. Pourpoint.

Stopping the “launch” to stay in control

NASA has long tried to prevent something that can happen to scramjet engines called “unstart”. The term refers to a sudden change in air flow. The name comes from a specialized test facility called a supersonic wind tunnel, where “start” means that the wind has reached the required 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 test conditions for hours, allowing us to experiment with new flow sensors and control approaches on realistic engine geometries,” said Dedic.

Goyne explained that “scramjets,” short for supersonic combustion thrust jets, build on scramjet technology that has been in common use for years.

Thrusters essentially “slam” air into the engine using the forward motion of the aircraft to create the temperatures and pressures needed to burn the fuel. They operate in a range from about Mach 3 to Mach 6. As the inlet at the front of the craft narrows, the internal air velocity slows to subsonic speed in a thrust combustion engine. But not the plane itself.

However, scramjets are a little different. Although they also “breathe air” and have the same basic setup, they need to maintain super-fast airflow through the engine to reach hypersonic speeds.

“If something happens in a hypersonic engine and it suddenly creates subsonic conditions, that’s a launch,” Goyne said. “The thrust suddenly decreases and at this point it can be difficult to restart the supply.”

Dual-mode scramjet engine testing

Currently, like thrusters, scramjets need to increase speed to get to a speed where they can take in enough oxygen to operate. This may include a ride attached to the underside of the Carreir aircraft as well as a rocket boost.

The latest innovation is a dual-mode scramjet combustor, which was the type of engine tested by the UVA-led project. The twin engine starts in thruster mode at lower Mach numbers, then transitions to full supersonic combustor airflow at speeds in excess of Mach 5.

Preventing the engine from starting during this transition is critical.

The incoming wind interacts with the inlet walls in the form of a series of shock waves known as a “shock train”. Traditionally, the leading edge of these waves, which can be destructive to aircraft integrity, have been controlled by pressure sensors. The machine can be modified, for example, by moving the position of the shock assembly.

But where the leading edge of the shock train is located can change rapidly if flight disturbances change the dynamics in the air. The shock system can pressurize the inlet and thus create conditions for start-up.

So, “If you’re scanning at the speed of sound, but the engine processes are moving faster than the speed of sound, you don’t have a very long response time,” Goyne said.

He and his collaborators wondered whether it might be possible to predict pending takeoff by observing the properties of an engine’s flame.

Sensing the flame spectrum

The team decided to use an optical emission spectroscopy sensor for the feedback needed to control the leading edge of the shock train.

The optical sensor is no longer limited to information gathered on the engine walls, such as pressure sensors, but can identify subtle changes both inside the engine and in the flow path. The tool analyzes the amount of light emitted by the source—in this case, the reacting gases in an overcurrent combustor—as well as other factors such as flame location and spectral content.

“The light emitted by a flame in an engine is caused by the relaxation of molecular species that are excited during combustion processes,” explained Elkowitz, one of the Ph.D. “Different species emit light at different energies or colors and offer new information about the engine’s condition that is not captured by pressure sensors.”

The team’s wind tunnel demonstration showed that engine control can be both predictive and adaptive, seamlessly transitioning between scramjet and thrust function.

In fact, the wind tunnel test was the first demonstration that adaptive control of these types of dual-function motors could be achieved using optical sensors.

“We were very excited to demonstrate the role optical sensors can play in driving 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 environments.”

Building towards the future

Although much more work needs to be done, optical sensors may be part of a future that Goyne believes will be realized in his lifetime: traveling to space and back like an airplane.

Dual-mode scramjets would still require some kind of boost to get the plane to at least Mach 4. But there would be the added safety of not relying solely on rocket technology, which requires carrying highly flammable fuel along with large amounts of chemicals. oxidizer to burn fuel.

This reduced weight would allow more room for passengers and payload.

Such an all-in-one aircraft, which would glide back to Earth like the space shuttles of old, could even provide an ideal combination of cost-effectiveness, 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, it has yet to capture aircraft-like operations. Our findings could potentially build on Hyper-X’s storied history and make its access to space safer than current rockets.” technology based on