A rotating detonation engine has flown in space, run for over four minutes on a test stand, and been bolted to the front of a gas turbine. None of them is yet in service. Where will rotating detonation engines fly first — and what is still in the way?
Three homes, three different deals
The same physics offers different bargains to different machines.
Rockets. A rotating detonation rocket engine (RDRE) needs no compressor and no turbine downstream, it already tolerates enormous pressures and heat fluxes, and its propellants (hydrogen, methane or kerosene with oxygen) have tiny cell sizes — so the annulus can be small. As Lesson 12 showed, the Isp benefit is modest; the attraction is a chamber only a few centimetres long, lighter and potentially cheaper than a conventional one, with the possibility of an aerospike nozzle wrapped naturally around the centre body. JAXA's 2021 space flight and NASA's long-duration tests have made rockets the front-runner.
Gas turbines. Here the prize is the largest: a few percentage points of cycle efficiency in power plants and aircraft engines, worth enormous amounts of fuel. But the turbine is the problem. It must swallow a swirling, pulsing exhaust at kilohertz frequencies without losing efficiency or shaking its blades apart, and the RDE must deliver true pressure gain after its injector losses — which, as Lesson 12 showed, is still not achieved.
Ramjets and high-speed propulsion. A ramjet already has no turbomachinery and gets its compression from flight speed. An RDE combustor can be shorter than a ramjet's, may operate over a wider speed range, and could improve efficiency at lower flight Mach numbers where ram compression is weak — exactly where Lesson 1's cycle analysis says detonation helps most. Engine companies and start-ups are testing RDE combustors for missiles and high-speed aircraft.
What still stands in the way
- Heat. The walls of an RDE see a detonation pass thousands of times a second, at high pressure, with very thin boundary layers. Heat fluxes are several times those of a conventional combustor of the same size. Short tests use the walls as heat sinks; long runs need regenerative or film cooling designed for unsteady loads — NASA's long-duration RDRE used an additively manufactured copper alloy for exactly this reason.
- Operability. The engine chooses its own wave count, can switch modes, can fall into counter-rotating or pulsing behaviour, and can fail to start or blow out if the fill layer drops below its critical height (Lesson 11). A production engine must start reliably and stay in a known mode across its whole operating range.
- Pressure gain. For air-breathing engines, the case rests on true pressure gain, measured honestly with the equivalent available pressure — still the field's central challenge (Lesson 12).
- Unsteadiness and noise. The exhaust pulses at kilohertz frequencies with intense acoustic emissions, which turbines, nozzles, structures and people all have to tolerate.
- Diagnostics and modelling. Waves at two kilometres per second in a channel a few millimetres wide are hard to see and hard to compute. Much of what we know comes from high-speed cameras looking down the annulus and from simulations that are still too coarse to capture the cellular structure in a full engine.
- Rockets (RDREs): compact chambers, tiny cells with oxygen, no turbine downstream — the front-runner, flown by JAXA in 2021 and run for minutes by NASA in 2023.
- Ramjets: no turbine, shorter combustors, most benefit at low ram compression.
- Gas turbines: the biggest prize, but need true pressure gain and a turbine that tolerates kilohertz pulsations.
- Open problems: heat loads, operability and mode control, pressure gain, unsteadiness and noise, diagnostics.
Next, the finale: design an RDE yourself, and see whether it runs.