General Electric Aerospace: New architecture with Rotating Detonation Combustion

On December 14, 2023, General Electric Aerospace announced a breakthrough in jet engine design with the development and test of a new architecture called the Hypersonic Dual-Mode Ramjet that will employ Rotating Detonation Combustion. This technology is extremely efficient for hypersonic aircraft that will be able to perform with a longer flight range and a speed exceeding Mach 5. It is planned to demonstrate a full-scale version of this system next year.

*Explanatory image of the propulsion architectures. Credits: SANDBOXX*

In the field of supersonic and hypersonic propulsion, there are many concepts to take into account, and on which General Electric Aerospace has always been working. These include the in-depth study of high-temperature tolerant materials (superalloys and ceramic matrix composites), silicon carbide employed for the high-temperature electronic field, thermal management, additive technologies, and high-speed aerodynamics.

Dual-Mode Ramjet

In 2022, General Electric Aerospace acquired Innoveering LLC, a company specializing in hypersonic propulsion. This collaboration led to the development in just 12 months of the new dual-mode RamJet technology using the rotary detonation combustion principle.

Generally speaking, Ramjets and scramjets do not have the compressor (or moving parts), therefore cannot operate while stationary, and are inefficient at low speeds. However, these technologies perform well beyond Mach 3, taking advantage of the shock waves created through the (intentional) design of the air intake to compress it. This greatly increases the pressure before it is mixed with the fuel to be ignited for propulsion.

The performances achievable by the ramjet/scramjet in terms of specific impulse (employing hydrogen fuel) are as follows shown in the figure.

Achievable performance of ramjets, scramjets, turbojets, and rockets. Credits: Falempin, F. - Ramjet and Dual Mode Operation — *Achievable performance of ramjets, scramjets, turbojets, and rockets. Credits: Falempin, F. – Ramjet and Dual Mode Operation*

A dual-mode ramjet/scramjet, such as the one used by General Electric Aerospace in their bench tests, is the design of a single engine that can operate as a ramjet at low speeds and a scramjet at higher speeds. This is done by modifying the inlet geometry to create the appropriate shock-wave system for each flight regime.

When the subsonic jet engine tech (turbojet or turbofan) is combined with one of these supersonic speed jet engines (ramjet or scramjet), is it possible to obtain the so-called turbine-based combined cycle (TBCC) engine. This would allow an aircraft to take off using the low-speed engine, then switch to the high-speed one for operational flight, and then switch back to the low-speed engine for landing.

GE states that their dual-mode Ramjet architecture with rotary detonation combustion could reach higher speeds than other hypersonic engine designs, while also offering a significant increase in fuel economy and therefore range.

Rotating Detonation Combustion Principle

A rotary detonation engine (RDE) is an innovative propulsion system that surpasses traditional jet engines in efficiency thanks to a particular ignition mode.

*3D model and CFD simulation of the combustion chamber of a Rotating Detonation Rocket Engine. Credits: NASA*

To better understand the concept, we can consider it to be an extension of Pulse Detonation Engines (PDEs), which are, in turn, an extension of Pulsejets:

In pulse jet engines, as in almost all combustion engines, the way the air/fuel mixture burns is known as deflagration (which refers to the ignition and subsonic combustion of the air/fuel mixture).

*Explanatory scheme of the difference between Detonation and Deflagration phenomena. Credits: Detonation phenomena of conserved explosives*

In pulsed detonation engines (PDE), the ignition of the air/fuel mixture occurs in pulses like a pulsed jet. Still, instead of using deflagration, it uses detonation (the mixture burns at supersonic speeds) allowing them to push vehicles at higher speeds. Detonation releases more energy than deflagration, and detonation engines are also more efficient, producing more thrust with less fuel. While rockets and common jets use the Brayton cycle in which the heat is applied at constant pressure with the working fluid expanding in the combustion chamber (volume increase), detonation engines use the Humphrey cycle, adding the heat at constant volume. This is represented by the vertical steps 2-3 in the following picture, which makes us understand that the area (which represents the additional available work) is increased, compared to the Brayton cycle.

*Humphrey and Bryton Cycles. Credits: Pulse Detonation Propulsion System Paper*

A rotary detonation engine (RDE) elevates the PDE concept by innovatively redirecting the detonation wave within the engine’s circular channel, rather than propelling it rearward as in traditional designs. Fuel and oxidants are added to the channel through tiny holes, which are then struck and ignited by rapidly circulating detonation waves. The result is an engine that produces continuous thrust.