The race to field reusable hypersonic aircraft got a whole lot hotter last month, with GE Aerospace announcing a breakthrough in high-speed jet engine design that could potentially allow conventional aircraft to fly at speeds that exceed Mach 10.

According to GE’s press release, the company recently demonstrated “what is believed to be a world-first hypersonic dual-mode ramjet (DMRJ) rig test with rotating detonation combustion (RDC) in a supersonic flow stream.” This means leveraging rotating detonation combustion – a far more efficient means of power production – within a dual-mode ramjet (also sometimes called a dual-mode scramjet). Ramjets and scramjets are air-breathing jet engines that don’t function well at low speeds, but can power an aircraft or weapon from around Mach 3 up to Mach 5 and well beyond.

This would be a significant development toward producing dual-mode ramjet/scramjets with far greater range, but could represent an even bigger breakthrough if combined with a similarly Rotating Detonation Combustion-equipped turbofan engine in what’s commonly called a turbine-based combined cycle (TBCC) propulsion system. And those wheels are already turning. GE first acknowledged that it was working with the Defense Advanced Research Projects Agency (DARPA) and the Air Force Research Lab (AFRL) on an RDC-equipped TBCC engine in June of this year.

“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 Military Engines CEO Amy Gowder told Aviation Week.  

This TBCC engine would combine four different types of air-breathing jet engine technologies into a single combined system that would allow an aircraft to take off and land under conventional turbofan power, while also achieving hypersonic speeds under scramjet power during sustained flight – a concept that has been proposed by at least three other firms before.

But GE’s design is the first to incorporate Rotating Detonation Combustion, which could make such an engine far more compact and efficient than previous efforts, potentially resulting in a much more practical approach to hypersonic flight. 

A working TBCC engine has long been seen as the Holy Grail for reusable hypersonic aircraft, as the exotic propulsion systems powering today’s hypersonic weapons can’t function at low enough speeds needed to land aircraft, making them single-use. 

HOW GE’S HYPERSONIC ENGINE COMPARES TO OTHER DESIGNS

Last November, Atlanta-based aviation firm Hermeus demonstrated their turbine-based combined cycle Chimera engine could successfully transition from turbojet to ramjet power in a high-speed wind tunnel. About a month later, Virginia-based Leidos secured a $334 million contract from the Air Force Research Lab to continue development on an even more capable turbofan-to-scramjet design that would function similarly but could potentially achieve much higher speeds. And most secretive of all, Lockheed Martin announced success in ground-testing a similar turbofan-to-dual-mode-scramjet design with Aerojet Rocketdyne back in 2017 as part of the now highly secretive SR-72 program. 

But while each of these designs is quite promising, this new announcement out of GE Aerospace may represent the most promising combined cycle hypersonic propulsion system revealed to date, as it incorporates a similar turbofan-to-dual mode scramjet as the SR-72 and Mayhem efforts, but adds yet another exotic propulsion system into the mix in the form of a rotating detonation engine. 

This addition could help to overcome the biggest technical hurdle Lockheed Martin’s SR-72 program was known to face: bridging the gap between speeds attainable under turbofan power (which begins to drop in efficiency above Mach 2) and scramjet power (which functions less efficiently at speeds below Mach 4).

Based on GE’s claims, their Hypersonic Dual-Mode Ramjet with Rotating Detonation Combustion engine could not only achieve similar or even greater speeds as other hypersonic engine designs, but could offer a significant boost in fuel economy — and as a result, range — in what may prove to be an overall smaller and lighter package. 

This program has been underway for only about a year, according to the GE press release, but has matured rapidly thanks, in part, to GE’s acquisition of hypersonic-focused Innoveering LLC, last year. Inoveering brought a great deal of experience developing high-speed inlet designs, which play a vital role in the function of any jet engine. In a dual-mode scramjet, the inlet geometry must not only be precise but often also needs to be adjustable to manage the placement of shockwaves in the airstream at different flight speeds (more on that later).

GE now believes they will be able to demonstrate this exotic new engine system as soon as next year. 

WHAT IS A ROTATING DETONATION ENGINE?

In its simplest form, a rotating detonation engine (RDE) is a propulsion system that offers greater efficiency than traditional air-breathing jet engines thanks to a more effective means of ignition. 

In some ways, a Rotating Detonation Engine is an extension of Pulse Detonation Engines (PDEs), which are, in themselves, an extension of Pulsejets. While this lineage may seem confusing, walking through each of them as developmental steps may make it easier to wrap our heads around how RDEs work. 

Pulsejet engines work by mixing air and fuel within a combustion chamber and then igniting the mixture to fire out of a nozzle in rapid pulses, rather than under consistent combustion as you might find in other jet engines. In pulsejet engines, as in nearly all combustion engines, the way in which the air/fuel mixture burns is known as deflagration. In simple terms, deflagation means heating a substance until it burns away rapidly, but at subsonic speeds.

A pulse detonation engine works similarly, but instead of leveraging deflagration, it uses detonation. At a fundamental level, detonation is a lot like it sounds: an explosion. In contrast to deflagration, in detonation, the mixture burns at supersonic speeds. 

“You can get more efficiency by cramming all that reaction during a really short time in space. There’s, at least from a thermodynamic standpoint, potential for much higher efficiencies in engines that burn through detonation rather than deflagration,” Dr. Chris Combs, a Dee Howard endowed professor of hypersonic and aerospace engineering for the University of Texas San Antonio, told Sandboxx News.

When the air and fuel are mixed in a Pulse Detonation Engine, they’re ignited, creating deflagration like in any other combustion engine, but within the longer exhaust tube leading out of the engine, a powerful pressure wave compresses any unburnt fuel ahead of the ignition, heating it above ignition temperature in what is known as the deflagration-to-detonation transition (DDT). In other words, rather than burning through the fuel rapidly, the engine explodes it, producing more thrust from the same amount of fuel.

In pulse-detonation engines (PDEs), the air/fuel mixture ignition still occurs in pulses like a pulsejet, but the more powerful detonation method allows them to propel vehicles to higher speeds, believed to be around Mach 5. Because detonation releases more energy than deflagration, detonation engines are also more efficient, producing more thrust with less fuel.

A rotating detonation engine takes that PDE concept to the next level. Rather than having the detonation wave travel out the back of the aircraft as propulsion, it travels around a circular channel within the engine itself. Fuel and oxidizers are added to the channel through small holes, which are then struck and ignited by the rapidly circling detonation waves. According to Dr. Combs, it’s not uncommon for a rotating detonation engine to have three to five of these detonation waves circling the chamber at once.  

The result is an engine that produces continuous thrust, rather than thrust in pulses like a PDE, while also offering the improved efficiency of detonation combustion, rather than deflagration, as you’d find in a conventional jet engine. 

This sort of engine design alone can provide a great deal of power in a fuel-efficient package, which is why DARPA (the Defense Advanced Research Projects Agency), is actively developing an RDE to power its Gambit Missile. 

But GE Aerospace has found a way to incorporate this Rotation Detonation Combustion (RDC) process into its dual-mode ramjet propulsion system, and it claims to be working toward incorporating it into its turbofan technology as well. This may just solve two of the most pressing challenges facing hypersonic aircraft propulsion today: first, managing the transition from turbofan to scramjet power; and second, reducing the weight associated with such a turbine-based combined-cycle (TBCC) engine. 

But before addressing that, let’s briefly run through what a TBCC engine is.

HOW DO TURBINE-BASED COMBINED CYCLE ENGINES WORK?

Firms like Hermeus and Leidos are working to field a hypersonic aircraft that incorporates two different kinds of air-breathing jet engines into one: low-speed engines like turbojets and turbofans, and high-speed engines like ramjets and scramjets. This combination speaks to the innate challenges of managing airflow at speeds ranging from a dead stop, all the way up into the high-supersonic regime.

Turbojets and turbofans, the types of air-breathing jet engines that power many of today’s tactical aircraft, use a compressor fan near the inlet to suck air into the engine and compress it, before mixing that compressed air with fuel and igniting it for propulsion. As a result, these kinds of engines can work from a dead stop to accelerate an aircraft down a runway for takeoff and can continue to accelerate it up to around Mach 2 very effectively. Yet, they begin to see a drop-off in efficiency as they approach Mach 3. At that speed, the compressor at the front of the engine that’s meant to suck in and compress air instead becomes a hindrance to the air flowing in at the speed of the aircraft’s forward travel, limiting the engine’s efficiency.