A 50-foot-long tube installed last week in the new Gas Dynamics Imaging Lab in Aerospace Engineering will take experimental flight research at the University of Michigan deep into hypersonic territory. “Scramjet” technology, the focus of the research to be conducted in the lab, could underpin reusable space rockets, drones that could launch from the US and reach any location on the planet in under three hours, and perhaps even a 3-hour flight from New York to Tokyo.
Scramjets, or supersonic combustion ramjet engines, have been in the works for half a century, but even today they struggle to fly under their own power for more than a few minutes. Unlike traditional jet engines, combustion in a scramjet requires neither a compressor nor a spark. When flying at supersonic speeds, the air compresses and heats up as it is funneled through the engine, causing the fuel to spontaneously combust.
“It’s exciting because it’s an area where almost everything is unknown,” said James Driscoll, the Arthur B. Modine Professor of Aerospace Engineering. Even in fifty years, engineers around the world have only produced a handful of engines that can handle air rushing through them at supersonic speeds.
Candle in the wind
One of the major stumbling blocks is “engine unstart.” Just like it sounds, the jet is burning fuel and then the flame dies. The culprit, explained Driscoll, is the pattern of shockwaves that the high-speed air produces inside the engine. These waves can spill out the mouth of the engine, preventing fresh air from entering and continuing the combustion.
Another problem is maintaining stable combustion while air whips through the engine. It’s popularly compared to keeping a match lit in a hurricane, but Mirko Gamba, an assistant professor of aerospace engineering who is building the new lab, explained that the problem isn’t just the wind blowing out the flame – it’s the fuel blowing out of the engine. In a 1.5-meter-long engine, the fuel would leave a millisecond after it was injected if the engine’s design didn’t force it to stick around.
“Within that millisecond, you have to mix the air and fuel at a molecular level and burn it,” said Gamba. If the fuel isn’t thoroughly blended with the air, it won’t burn quickly enough.
To facilitate the mixing, engineers try to control how the air swirls, or forms vortices, inside the engine. “If you just use the natural way to mix the fuel and air so they can react, it would be impossible,” he said. “Vortices can enhance mixing.”
Driscoll has been exploring supersonic flight for nearly 40 years. His lab contains a wind tunnel that produces flows up to Mach 5, or five times the speed of sound, and can sustain them for as long as 30 minutes. These flows can be hot, up to 2,000 degrees Fahrenheit for simulating conditions inside an engine for combustion studies, or cold, mimicking the flow patterns of air entering the engine.
“A hypersonic vehicle starts at Mach 5. Mirko Gamba’s facility will go to Mach 13 flight conditions with temperatures that can be as high as 8,000 degrees Fahrenheit,” said Driscoll. “We only have a few facilities in the world that can simulate those high temperatures.”
The price of attaining those high speeds and temperatures is time – Gamba’s lab will have only a millisecond or so to conduct their experiments. Rather than pushing or pulling air through the tube, as occurs in a wind tunnel, their “expansion tube” will rely on pressure differences and shockwaves to create extreme speeds.
Firestorm in a bottle
The expansion tube is a long metal tube divided into three sections by metal seals. In the first, a lightweight gas such as helium is pumped up to a pressure tens or hundreds of times that of Earth’s atmosphere. At the designed pressure, the metal seal breaks. Then the gas rushes into the middle chamber, which contains the test gas – often air for scramjet studies.
When the high-pressure gas hits the low-pressure air, at a fraction of Earth’s atmospheric pressure, it sends a shockwave through the air. The shockwave raises the air’s temperature and pressure and pushes it toward the third section. When the shockwave reaches the second seal, it breaks through, sending another shockwave through the third section – a lightweight gas at very low pressure.
These shockwaves are each born with a twin, known as an expansion wave, traveling in the opposite direction. As the expansion wave moves back through the high-temperature air, it will turn some of the thermal energy into motion, whipping it to hypersonic speeds while retaining temperatures in the thousands of degrees Fahrenheit.
The heated, hypersonic gas rushes into the fourth and final section where, for example, a test model of a scramjet engine might be located. In this millisecond that conditions mimic the interior of a scramjet, Gamba’s team will inject fuel into the model and watch it mix with the air. Lasers will illuminate particles introduced to track the motion of the fluid, and from their movements, the team will map the streams and swirls of air. “It is kind of like observing the motion of leaves outside of your window as they are moved around by the wind,” said Gamba.
The chemicals in the air, such as oxygen or fuel molecules, or the products of combustion can also be traced. Different molecules absorb and emit different wavelengths of light, and Gamba’s team will use this identification technique to study where the unmixed fuel and air congregate – the fuel that is well mixed with oxygen reveals itself by combusting.
Predicting scramjet performance
Between Gamba’s lab and Driscoll’s, experimental supersonic flight research in Aerospace will be able to explore a wide range of conditions that could exist inside scramjet engines, and outside the vehicle. The data they collect about the behavior of the air and fuel then feeds computer simulations developed by aerospace professors Philip Roe, Iain Boyd and Kenneth Powell.
Used by NASA and Air Force researchers, these simulations can help with the development of scramjet designs that avoid engine unstart, disperse and burn the fuel quickly and don’t overheat.
“Together we make a strong team,” said Driscoll.