A new test rig at Purdue is set up to fire rocket exhaust into the inlet of a hypersonic scramjet engine. This capability — a first of its kind at any university — is one of a series of platforms at the Maurice J. Zucrow Propulsion Laboratories that enable the rapid development, integration and operation of new propulsion concepts.

This capability was developed with funding from the Air Force Research Laboratory (AFRL). It represents the United States’ desire to rapidly produce new hypersonic engines that will outperform those of its adversaries.

"There’s a national need for these facilities," says Carson Slabaugh, associate professor of aeronautics and astronautics. "Our near peers are booked out for years in terms of work, and that’s a sign that we need more of these testing capabilities in this country. We can now support critical programs here, including priority efforts for national security."

Why scramjets?

Slabaugh says that in a conventional turbine engine or even a ramjet engine, the incoming air is slowed down to facilitate combustion. But the temperature and pressure of the air entering the combustor increases exponentially with the vehicle’s speed, which creates a challenge.

"There’s a national need for these facilities. We can now support critical programs [at Purdue.] including priority efforts for national security."

— Carson Slabaugh, Associate professor of Aeronautics & Astronautics

"At lower flight speeds, we’re decelerating the flow within the engine so we can actually have a flame inside of the combustion chamber. You can only do that deceleration process up to a certain point. At really high speeds, you just can’t decelerate the air enough without huge losses and, even if you could, it’s so hot that you can’t even add more heat with the fuel," he says.

Will Senior, a PhD student and graduate research assistant, aligns optics for a laser-diagnostic technique applied in a high-speed airbreathing combustion environment.

"At around Mach 5, the factors become so severe that you have to change your method of combustion. You have intake temperatures around 2,000 degrees Fahrenheit and pressures at a few hundred psi. That’s like the exhaust temperature of most engines."

In order for a vehicle to go hypersonic with an air-breathing engine, aerospace engineers turn to scramjets — engine designs where the air and fuel travels through the combustor at a supersonic rate. The main challenge in improving these engines is that there’s a very small window of time to complete the burn before the air and fuel has exited the engine.

That’s why the advanced tools available at Zucrow Labs are so important to further development. Slabaugh, a leader in the field of laser-based combustion diagnostics, helped develop Purdue’s competitive arsenal of laser sources and highspeed detection equipment. Special quartz-based windows replace a portion of the combustor shell, allowing researchers to directly observe the combustion happening inside an engine.

But before Slabaugh could test a scramjet, he needed something that would simulate its operating environment. Wind tunnel systems that supply air from a heat exchanger, Slabaugh says, top out at about 1,500 degrees Fahrenheit. This is roughly equivalent to Mach 4 flight conditions.

To test hypersonic engines, you need to feed it with a rocket.

A cost-effective alternative

Slabaugh says the most straightforward approach is to build a free jet tunnel, which is large enough to test an entire vehicle. Two of these exist in the U.S., one each belonging to NASA and the Air Force.

"In a free jet, you basically have a massive, massive rocket engine that produces high temperature, high pressure air that corresponds to whatever your Mach number is," Slabaugh says. "The crazy thing about that is the scale of these tunnels. They require huge infrastructure to operate, which can also make it prohibitively expensive to test novel, high-risk concepts."

In a direct-connect rig like the kind at Purdue, researchers test just the internal scramjet engine components by connecting the rocket, called a vitiating air heater, into the engine’s air inlet.

"Our direct-connect system allows us to replicate hypersonic conditions in a much smaller configuration, so we don’t have to run a facility that’s quite so large as a free jet. As a way to build confidence in your propulsion system design without having to run tests that are that expensive, we extract the isolator, combustor and expansion sections and we effectively replicate the flow that’s going in and the flow that’s going out," Slabaugh says.

The vitiator works like a rocket, by reacting hydrogen and oxygen and producing high-temperature water vapor. That rocket exhaust is supplemented with enough nitrogen and oxygen to simulate atmospheric air before it enters the scramjet combustor. A vacuum system, called an ejector, is attached downstream of the combustor to replicate the exhaust-side airflow at high altitude. Computational methods used during data analysis compensate for the effects of the moisture that the vitiator introduces to the system.

Engineering research at scale

This rig, like the other propulsion test stands at Zucrow, lives in an isolated test cells with 20-inchthick reinforced concrete walls and steel explosion-proof doors.

It’s also loud. When Slabaugh and his students first fired up this new rig in May 2022, the sound vibrations were literally rattling electronic component boards apart inside the datalogging computers. "We learned an important lesson about the resilience of our electronics that day and had to make a few adjustments," said Slabaugh.

The repaired computers have been relocated to the other side of the thick concrete walls of the test cells. They now live in the same room as the laser diagnostics equipment that serves as a cornerstone of Purdue’s combustion research program — a safe space from the harsh testing environment.

Purdue is the only academic institution to stand up this type of testing capability. Despite the challenges, Slabaugh is excited to have the scramjet test stand operating. "Our team always finds a way forward, because we are committed to working these fundamental, relevant problems at scale."

Any setbacks they face day to day are taken in stride.

"I come to work and shoot lasers at fire," he says with a smile. "I can’t complain."