GE Aerospace says it successfully demonstrated an advanced jet propulsion concept that involves a dual-mode ramjet design utilizing rotating detonation combustion. This could offer a pathway to the development of new aircraft and missiles capable of flying efficiently at high supersonic and even hypersonic speeds across long distances.
A press release that GE Aerospace put out today offers new details about what it says “is believed to be a world-first hypersonic dual-mode ramjet (DMRJ) rig test with rotating detonation combustion (RDC) in a supersonic flow stream.” Hypersonic speed is defined as anything above Mach 5. Amy Gowder, President and CEO of the Defense & Systems division of GE Aerospace, previously disclosed this project, but offered more limited information, at this year’s Paris Air Show in June.
A rendering of a rotating detonation engine design. USAF/AFRL via Aviation Week
“A typical air-breathing DMRJ propulsion system can only begin operating when the vehicle achieves supersonic speeds of greater than Mach 3,” the press release explains. “GE Aerospace engineers are working on a rotating detonation-enabled dual mode ramjet that is capable of operating at lower Mach numbers, enabling the flight vehicle to operate more efficiently and achieve longer range.”
“RDC [rotating detonation combustion] enables higher thrust generation more efficiently, at an overall smaller engine size and weight, by combusting the fuel through detonation waves instead of a standard combustion system that powers traditional jet engines today,” the press release adds.
To elaborate, in most traditional gas turbines, including turbofan and turbojet engines, air is fed in from an inlet and compressed, and then is mixed with fuel and burned via deflagration (where combustion occurs at a subsonic rate) in a combustion chamber. This process creates the continuous flow of hot, high-pressure air needed to make the whole system run.
A rotating detonation engine (which involves combustion that happens at a supersonic rate) instead “starts with one cylinder inside another larger one, with a gap between them and some small holes or slits through which a detonation fuel mix can be pushed,” according to a past article on the general concept from New Atlas. “Some form of ignition creates a detonation in that annular gap, which creates gases that are pushed out one end of the ring-shaped channel to produce thrust in the opposite direction. It also creates a shockwave that propagates around the channel at around five times the speed of sound, and that shockwave can be used to ignite more detonations in a self-sustaining, rotating pattern if fuel is added in the right spots at the right times.”
The video below offers a more detailed walkthrough of the rotating detonation engine concept.
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In principle, rotating detonation requires less fuel to produce the same level of power/thrust as combustion via deflagration. The resulting sustained shockwave builds its own pressure, as well, leading to even greater fuel efficiency. Pressure is steadily lost during deflagration.
In addition, rotating detonation typically requires far fewer moving parts than are needed in traditional gas turbines. In theory, this should all allow for rotating detonation engine designs that are significantly smaller, lighter, and less complex than existing types with similar very high power/thrust output.
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“GE engineers are now testing the transition mode at high-supersonic speeds as thrust transitions from the RDE-equipped turbine and the dual-mode ramjet/scramjet,” GE Aerospace’s Gowder said in Paris earlier this year, according to Aviation Week.
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A combined ramjet and rotating detonation concept could be an especially big deal for future missiles, like the ones DARPA’s Gambit project is envisioning, and possibly high-speed air vehicles for reconnaissance use. This propulsion arrangement could allow for greater efficiency and lighter (and potentially smaller) airframes, which in turn allow for greater performance — especially in terms of range — and/or payload capacity. If rotating detonation combustion can reduce the minimum speed required to get the ramjet working, this would reduce the amount of initial boost such a system would need at the outset, too. This would mean a smaller overall package. All of this opens doors to new levels of operational flexibility.
This new engine concept could also potentially become one component of what is known as a turbine-based combined cycle (TBCC) engine arrangement, of which much talk over the years about in recent years. Most TBCC design concepts revolve around combinations of advanced ramjets or scramjets for use at high speeds and traditional turbojet engines that work better a low speeds.
A graphical depiction of a notional turbine-based combined cycle engine arrangement. Lockheed Martin
A practical TBCC concept of any kind has long been a holy grail technology when it comes to designing very high-speed aircraft. A propulsion system that allows for this kind of high and low-speed flexibility would mean an aircraft could take off from and land on any suitable existing runway, but also be capable of sustained high-supersonic or even hypersonic speeds in the middle portion of a flight.
