An associate professor of mechanical and aeronautical engineering at the University of Virginia, Christopher Goyne is the director of the Aerospace Research Laboratory at the University of Virginia. Goyne spoke with Air & Space senior associate editor Diane Tedeschi in August.
Air & Space: What drew you to a career in aerospace?
Goyne: My father was a pilot, and when I was a little boy, he used to take us flying. Sometimes we’d fly to a weekend vacation and go camping. He also introduced me to model aircraft, so I think that got me interested in the field. At university, I did mechanical engineering for undergrad and then specialized in aerospace engineering in graduate school. That was in Australia, where I grew up.
What is a scramjet?
A scramjet is a type of air-breathing supersonic jet engine. Unlike a jet engine that you might have on a Boeing or Airbus commercial aircraft, a scramjet doesn’t have any rotating parts. If the aircraft is already moving fast enough, and you have an inlet where you scoop the air in, you can increase internal temperature and pressure via compression inside the inlet. Then in the combustion chamber, when you mix the fuel in, you can generate a sustained flame, creating hypersonic propulsion.
I can see the appeal of an engine with no rotating parts.
[A scramjet] is operating in a very harsh environment. When you start flying at Mach numbers above five, the airframe encounters intense friction with the air, which leads to a lot of heat transfer to the engine. You also have the intake air flowing very quickly through the engine, so you need to add the fuel at exactly the right point and hope that it burns completely instead of spilling out the back of the engine and not generating thrust. So although a scramjet is a simple engine in principle, it’s difficult to operate [in the real world].
Tell me about a particularly satisfying moment that happened during your research.
At the University of Virginia, we have a high-temperature, high-speed wind tunnel where we can create the conditions a scramjet would experience when it’s flying at Mach 5. There’s an electrical heater that can heat the air inside the wind tunnel to almost 1,800 degrees Fahrenheit. And often, the experiment itself might be years in the planning: You start out with a concept for a scramjet, and then you build it and install it in the wind tunnel. One of the best parts of my job is to see when a new engine design actually holds a flame. When the scramjet is operating efficiently, you can see the fire coming out the exhaust system. At that point, the flow velocity through the engine is about one kilometer per second, and the noise inside the wind tunnel is about the same noise as a 747 on takeoff. It’s an exhilarating feeling when you see firsthand the results of your research.
Is there a timeline for when we might see scramjet vehicles go from testbeds to becoming operational?
The near-term application is in a defense—for high-speed cruise missiles. It would be a single-use hypersonic vehicle. The mid-term application would be in high-speed transportation aircraft: potentially a high-speed business jet or a high-speed military reconnaissance jet. And then the longer-term application is in getting to space, where you could use a scramjet to accelerate over some of the trajectory while you’re still low in the atmosphere and then use a rocket to boost you up into orbit. The Department of Defense is actively doing research on scramjets, and they’ve advertised that they’re trying to develop an operational system as soon as they can. So I think it’s possible they could have something operational in the next three to five years.
A scramjet-powered passenger aircraft is probably decades away?
Well, there are companies around the world that are working on the concept, and there are private companies in the U.S. that are developing the concepts. They’re trying to do something within 10 years.
As far as crewed hypersonic travel to Mars, would it be accurate to say that the technology exists and that the more pressing problem is the cost of such a mission?
People interested in exploring Mars have done a good job showing that we can put rovers on the planet—reliably. But when you send robots to Mars, you don’t need to send food. You don’t need to send water. And you don’t need to bring them back to Earth. If you talk about sending people, then you’re going to need a life-support system. You’re going to have to carry oxygen, water, food, and fuel. So the spacecraft starts to get bigger. We don’t have experience sending large spacecraft to Mars and getting them to safely enter the atmosphere and land like we’ve done with the rovers [such as Spirit, Opportunity, and Curiosity]. The technological challenge right now is how do we get a lot of mass landed on Mars so we can support a human presence and then return [the astronauts] to Earth. That’s the challenge, and that relates to why NASA and SpaceX are developing these larger rockets that can launch more mass into orbit.