A pulse detonation engine spends most of its life waiting: for the tube to refill, for the products to leave, for the valve to cycle. The detonation itself lasts half a millisecond. What if the detonation never had to stop?
Bend the tube into a ring
Take the PDE's tube and bend it until its ends meet. Now a detonation launched into it never reaches an end: it runs round and round. Of course, it would consume all the mixture in one lap — unless fresh mixture keeps being fed in behind it.
So feed it. Make the ring an AnnulusThe ring-shaped gap between two coaxial cylinders that forms an RDE’s combustion chamber, typically 5–20 mm wide. — the gap between two coaxial cylinders, typically 5 to 20 millimetres wide — and inject fuel and oxidizer through holes in its closed end. The mixture flows axially into the chamber. The detonation runs around the annulus, burning the layer of fresh mixture that has accumulated since it last passed; behind it, the injectors start refilling; and the burnt gas, turned by the wave, expands axially out of the open end. That is a Rotating detonation engineAn engine in which one or more detonation waves run continuously around an annular chamber, consuming fresh mixture injected through the bottom..
Three things that a PDE needs, an RDE does not:
- Valves. The wave itself is the valve. As it passes over an injector, its pressure briefly exceeds the plenum's and the flow stops; about ten microseconds later (roughly 4% of a lap, in the course's solver), the pressure has fallen and the injector reopens.
- Re-initiation. The detonation is started once and then sustains itself as long as it finds fresh mixture ahead.
- Waiting. Filling and detonation happen at the same time, in different places around the ring. The engine is continuously busy.
From outside, the engine is steady: mass flows in steadily and out steadily. Only an observer riding the chamber wall sees the violent pulsing — and the wave as seen from the wall is steady too, in its own rotating frame.
A short history
The idea is older than it looks. In 1959, Bogdan Voitsekhovskii at the Lavrentyev Institute of Hydrodynamics in Novosibirsk observed a detonation spinning continuously in a disc-shaped chamber fed with acetylene and oxygen. In the 1960s, Nicholls and colleagues at Michigan tried, with limited success, to build rocket chambers on the same principle. For decades the Novosibirsk group, led by Fedor Bykovskii, kept experimenting and established most of the empirical rules this course uses.
The field exploded in the 2000s and 2010s, driven by Piotr Wolański's group in Warsaw, the US Naval Research Laboratory and Air Force Research Laboratory, and groups in Japan, China, France and elsewhere. In July 2021 a JAXA sounding rocket, S-520-31, fired an RDE in space for the first time. In 2023 NASA's Marshall Space Flight Center ran a copper-alloy rotating detonation rocket engine for 251 seconds at thousands of pounds of thrust.
How fast does it spin?
A wave of speed in an annulus of mean diameter completes a lap in seconds. With waves running, a pressure sensor on the wall sees
- An RDE is an annulus fed through its closed end; one or more detonations run around it continuously, burning the fresh layer injected since they last passed.
- The wave is its own valve, it is lit once, and filling and burning happen simultaneously in different places.
- Pioneered by Voitsekhovskii (1959) and Bykovskii; first flown in space by JAXA in 2021; run for minutes by NASA in 2023.
- Wave frequency: — kilohertz for laboratory engines.
Next: freeze the wave and look closely at the flow around it.