A New Kind of Rocket that’s Lightweight and Easier to Construct: a Rotating Detonating Engine. Unfortunately, it’s Also Completely Unpredictable

In the current era of space exploration, the name of the game is “cost-effective.” By reducing the costs associated with individual launches, space agencies and private aerospace companies (aka. NewSpace) are ensuring that access to space is greater. And when it comes to the cost of launches, the single-greatest expense is that of propellant. To put it simply, breaking free to Earth’s gravity takes a lot of rocket fuel!

To address this, researchers at the University of Washington recently developed a mathematical model that describes the workings of a new launch mechanism: the rotating detonation engine (RDE). This lightweight design offers greater fuel-efficiency and is less complicated to construct. However, it comes with the rather large trade-off of being too unpredictable to be put into service right now.

The study that describes their research (“Mode-locked rotating detonation waves: Experiments and a model equation“) recently appeared in the journal Physical Review E. The research team was led by James Koch, a UW doctoral student in aeronautics and astronautics, and included Mitsuru Kurosaka and Carl Knowlen, both UW professors of Aeronautics & Astronautics; and J. Nathan Kutz, a UW professor of applied mathematics.

In a conventional rocket engine, propellant is burned in an ignition chamber and then channeled out of the back through nozzles to generate thrust. In an RDE, things work differently, as Koch explained in a UW News release:

“A rotating detonation engine takes a different approach to how it combusts propellant. 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 sets the RDE apart from conventional engines, which require a lot of machinery to direct and control the combustion reaction so that it can be turned into acceleration. But in an RDE, the shock wave generated by the ignitions creates thrust naturally and without the need for additional engine parts.

However, as Koch indicates, the rotating detonation engine field is still in its infancy and engineers are still not certain what they are capable of. Hence why he and his colleagues decided to test the concept, which consisted of recasting the available data and looking at pattern formations. First, they developed an experimental RDE (shown below) that allowed them to control different parameters (like the size of the gap between cylinders).

They then recorded the combustion processes (which took only 0.5 seconds to complete each time) with a high-speed camera. The camera recorded every ignition at a rate of 240,000 frames per second, allowing the team to watch the reactions unfold in slow-motion. As Koch explained, he and his colleagues found that the engine actually performed well.

“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 continue 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.

Next, the researchers developed a mathematical model to mimic what they observed with their experiment. This model, the first of its kind, allowed the team to determine for the first time whether an RDE would be stable. And while this model is not yet ready for other engineers to use, it could allow other research teams to assess how well specific RDEs will perform.

As noted, the engine design does have a downside, which is its unpredictable nature. On the one hand, the process of combustion-driven shocks naturally leads to the compression of the shocks by the combustion chamber, resulting in thrust. On the other, once started, the detonations are violent and uncontrolled – something that is completely unacceptable when it comes to rockets.

But as Koch explained, this research was a success in that it tested this engine design and quantitatively measured its behavior. This is a good first step and could help pave the way towards the actual development and realization of RDEs.

“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,” said Koch. “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.”

Koch and his colleague’s research was made possible thanks to funding provided by the U.S. Air Force Office of Scientific Research and the Office of Naval Research. While it is too soon to say, the implications of this research could be far-reaching, resulting in rocket engines that are easier to produce and more cost-effective. All that is needed is to ensure that the engine design itself is safe and reliable.

Further Reading: UW News