An international team of engineers that includes combustion experts from the University of Sydney's School of Aerospace, Mechanical and Mechatronic Engineering are working on a revolutionary new rocket engine that can not only breath air while in the atmosphere, but can burn incoming air at supersonic speeds. If it can be made operational, the Rotating Detonation Engine (RDE) promises to make space launches cheaper and more efficient.

It's said that those who ignore the past are doomed to repeat it. It's also true that ignoring the past means overlooking old ideas that could be dusted off, given a new twist, and turned into something new and exciting.

A case in point is the rotating detonation engine, which is a potentially major advance in rocketry based on a very old technology. During the Second World War, one of the Nazis' secret weapons was the Vergeltungswaffe 1 (Vengeance Weapon 1), better known as the V1 flying bomb. Almost 10,000 of these primitive cruise missiles were launched against England in 1944 and '45, and were the direct ancestors of modern-day versions like the Tomahawk and Storm Shadow.

Key to the V1 was the pulse engine that propelled it. Put simply, this was a jet engine that consisted of a round box with an exhaust pipe at one end and spring-mounted slats on the front face. In operation, air would flow through the slats, mix with fuel, and ignite. This formed a pulse detonation that pushed the slats shut for a second and pushed the missile forward.

It was a very simple engine that was cheap and easy to build, but it was also very inefficient because the pulse detonations were intermittent and also pushed a large fraction of the unburned fuel out the exhaust.

RDE is a more advanced version of this engine, but instead of a combustion chamber, the fuel and air are injected into an open, circular channel and ignited. The combustion pulse flows around the channel and becomes self-sustaining as it generates waves that continually cycle as the hot gases are pushed out through the open end of the channel.

One of the advantages of the RDE is that it can act as both a jet and a rocket. In the lower atmosphere, air can be drawn into the vehicle and fed into the channel, but in the vacuum of space, this can be substituted with liquid oxygen or some other oxidizer.

The other plus is that, unlike other air-breathing engines, the RDE can sustain combustion at supersonic speeds. Most other supersonic or hypersonic engines have to slow down the air to subsonic speeds before it can be fed into the combustion chamber, but in an RDE, the air can come in at a much faster speed, allowing for a simpler design.

Currently, there are no functioning RDEs, but the US Navy, NASA, Aerojet Rocketdyne, Russia's NPO Energomash company, and partnerships like the University of Sydney and defense contractor DefendTex are all working on theoretical models that will be needed to build actual engines. Sydney's University's Clean Combustion Group along with DefendTEx and other international partners are working under the International Responsive Access to Space project funded by the Australian government grant to develop an RDE for the country's space industry.

Led by Associate Professor Matthew Cleary, the Sydney team is looking at how combustion works in the RDE, with special emphasis on computational fluid dynamics simulations to determine the engine's efficiency. The hope is that the RDE will allow for smaller, lighter, and less expensive rockets that could launch larger payloads into orbit and might power Australia's first sovereign space launch capability.

"Since the project kicked off we have worked with our collaborators to develop new computational methods to investigate supersonic combustion, which is a process known as detonation," says Cleary. "Our preliminary findings from simulations of a model rotating detonation engine have led to some interesting findings about the stability of detonations in an annular channel, in particular with regard to the importance of designing the combustor geometry such that the detonation is stable and rocket thrust can be sustained continuously. This information is being fed to our collaborators who are now starting work on ground testing an engine."

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