Engineers at the University of Washington are working on a new type of rocket engine that holds the promise of being lighter, more efficient, and simpler to make than conventional liquid-fuel rockets. Called a Rotational Detonation Engine (RDE), one of the biggest hurdles to making it practical is to develop mathematical models that can describe how the very unpredictable engine design works in order to make it more stable.

An RDE is a rocket engine that is similar to the pulse jet engines that powered the infamous German V1 cruise missile of the Second World War, which used a simple combustion chamber with an exhaust pipe at one end and spring-mounted slats on the front face. In operation, air would come in through the slats, mix with fuel, which was then detonated, producing a pulse of thrust. An RDE takes this idea one step further.

"A rotating detonation engine takes a different approach to how it combusts propellant," says James Koch, a UW doctoral student in aeronautics and astronautics. “It’s made of concentric cylinders. Propellant flows in the gap between the cylinders, and, after ignition, the rapid heat release forms a shock wave, a strong pulse of gas with significantly higher pressure and temperature that is moving faster than the speed of sound.

"This combustion process is literally a detonation — an explosion — but behind this initial start-up phase, we see a number of stable combustion pulses form that continues to consume available propellant. This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust."

Put simply, what is happening is that when the shockwave is established, it becomes self-sustaining without the need for the complex support systems that conventional rocket engines need to control how the propellants mix and burn. This means an RDE can be much simpler from a mechanical point of view.

The tricky bit is that the detonations that form and maintain the shockwave are extremely complex and unpredictable. To gain a better understanding of what is going on, the Washington team built an experimental RDE to study how it operated under different parameters during a series of half-second experiments, which were recorded by a high-speed camera at 240,000 frames per second.

The data returned from this a setup helped the team produce a mathematical model to determine if the engine was stable as configured. However, there is still a long way to go before the overall performance of the RDE can be determined an improved upon.

“My goal here was solely to reproduce the behavior of the pulses we saw — to make sure that the model output is similar to our experimental results,” Koch says. “I have identified the dominant physics and how they interplay. Now I can take what I’ve done here and make it quantitative. From there we can talk about how to make a better engine.”

The research was published in Physical Review E.

The video below shows the test engine forming a shockwave.

RDE test footage

Source: University of Washington