RDE Ramjet hypothetical design
DISCLAIMER
This is just a conceptual design of a hypothetical hybrid engine, this whole project is just an expanded upon "shower thought" I had in 2024. I wanted to explore the idea of a propulsion system that could take a vehicle from a standing start to hypersonic speed without any moving parts. This is the original version and may include mistakes, inaccuracies, and oversights. I am not a propulsion engineer, and this is not a real design.
What it is
The goal was a hypothetical hybrid engine — a Rotating Detonation Engine (RDE) combined with a ramjet — meant to take a vehicle from a standing start to hypersonic speed without any moving parts.
Back in the days of the first flight of the X-15 or the A-12 — the predecessor of the SR-71 Blackbird — the race for the fastest manned aircraft was dominated by either rocket engines or modified turbojets. Go even further back, and the German V-1 flying bomb used a pulsejet engine, which was noisy, had very poor fuel efficiency, and produced low thrust at speed. The shutter valves in the combustion chamber were moving parts, and that was one of its mechanical limitations.
The basic concept of using pulse combustion for propulsion isn't inherently flawed, though. The V-1's Argus pulsejet used deflagration, not detonation, which is precisely the distinction that makes an RDE different. In recent years, a propulsion system known as the Rotating Detonation Engine (RDE) has been developed by NASA, the US Air Force Research Laboratory (AFRL), JAXA in Japan, Venus Aerospace, university research groups in the US, Poland, and elsewhere, and other organizations, which have found that it could potentially be more efficient than conventional rocket engines, thanks to the pressure gain in detonation-based combustion. My proposed design uses the RDE during the initial phase of flight — from a booster-assisted start up to approximately Mach 2.5, which is sufficient to transition into ramjet operation. This brings its own challenges, though: RDEs require already-compressed air to function, meaning some form of pre-compression is necessary, and cooling is a serious issue given the extreme temperatures generated during continuous detonations. Those challenges, and some possible ways around them, are what this project explores.
How a Rotating Detonation Engine works
RDEs are an evolution of the pulse detonation engine (PDE), which itself evolved from the earlier pulsejet engines used, for example, in the V-1. In a typical PDE, fuel and oxidizer are injected into the combustion chamber, mixed, and ignited by a spark plug. The resulting detonation creates a high-pressure wave that pushes exhaust out, producing thrust in discrete pulses.
An RDE works on a similar principle, but in a circular, annular (toroidal) combustion chamber. A continuous detonation wave travels around the ring, allowing for near-continuous thrust generation — combining the efficiency of detonation-based combustion with more stable operation than pulsed systems.
One of the biggest advantages of RDEs over conventional propulsion is pressure-gain combustion, which improves thermodynamic efficiency and specific impulse compared with conventional constant-pressure combustion — an efficiency gain rather than a dramatic thrust-density advantage. RDEs also have very few moving parts: unlike engines that rely on complex assemblies like turbines and compressors, an RDE operates with minimal mechanical components, which reduces weight and increases reliability under extreme conditions such as the dynamic pressure and temperature of hypersonic speeds.
My design
I started with the goal of an engine that could go from standstill to hypersonic and doesn't have any moving parts. That meant exploring several initial ideas, some of which were ultimately discarded due to technical limitations, before I realized RDEs are a genuinely viable option for aircraft. Below is a simple 3D concept.
The main problem with this design is that the RDE still needs a compressor to operate. One way around that is a short booster phase at the start — something like a rocket booster or JATO. At around Mach 0.5 the RDE would begin to take over, and with help from the compressor, bring the aircraft up to the minimum speed a steady ramjet needs to assume the role.
So the original goal — a standing start with no moving parts — is not met by this design. The booster and the compressor are both concessions I could not find a way around.
Open questions
This is just a starting point — there are several other ways an RDE and a ramjet could be integrated together, and plenty of room to keep developing the idea.