US engine maker GE Aerospace says it has achieved a significant breakthrough in efforts to develop a reusable engine capable of powering ultra-fast hypersonic flight.
GE Aerospace on 14 December revealed that scientists at its Global Research Center in Upstate New York successfully tested a dual-mode ramjet (DMRJ) engine subscale demonstrator that uses a novel technique called rotating detonation combustion (RDC).
The company says the design could enable high-speed, long-range flight with greater efficiency than what is available with current scramjet engines, for both expendable and reusable vehicles.
“As the aerospace sector sets its sight on the future of hypersonics, GE Aerospace is well positioned with the right capabilities, experience and scale to be a leader in driving new developments for our customers,” says Amy Gowder, chief executive of GE Aerospace’s defence unit.
The company believes its DMRJ prototype is first example anywhere of a hypersonic-capable engine that makes use of RDC.
The US Air Force describes the technique as a “more-efficient type of combustion, characterised by a closely coupled shock wave and reaction zone, where the fresh propellants are rapidly compressed, heated and burned”.
The subscale RDC test took place sometime in the last quarter of 2023, according to officials at the Global Research Center near Albany, New York.
SWITCHING TO SCRAMJET
Crucially, GE Aerospace believes the RDC hypersonic engine will address a critical obstacle to hypersonic flight: transitioning between conventional jet engines and ultra-fast DMRJs.
While Tom Cruise’s Pete “Maverick” Mitchell character made reaching Mach 10 look easy in a scene from latest cinematic entry to the Top Gun franchise, hitting the M5 threshold for hypersonic flight is significantly harder in real life.
The secretive Darkstar aircraft from Top Gun: Maverick used a jet engine to reach M3.5, at which point Maverick flips a few cockpit switches to cut off the craft’s conventional turbine powerplant, alter the shape of the air intakes and ignite the scramjet – or DMRJ – powering the fictitious Darkstar beyond M10. The aircraft then malfunctions and Maverick safely ejects.
In real life, transitioning between propulsion systems remains a problem.
The physics of conventional DMRJ engines require airspeeds of around M3.5 to achieve compression significant to produce ignition and generate thrust. Current scramjet engines perform most efficiently at M4 and faster.
The fastest crewed aircraft ever flown – Lockheed Martin’s iconic SR-71 Blackbird – reached M3.
Propulsion engineers must address the speed gap between roughly M3 and M3.5 before crewed hypersonic flight, in a reusable vehicle like the Blackbird, can become a reality.
“There is still a lot of invention that needs to take place,” says Craig Young, director of engineering and hypersonic propulsion at GE Edison Works, the company’s advanced technology development unit.
One option is to use rocket boosters, rather than jet turbines, to reach high sub-hypersonic speeds.
Examples of the rocket-powered approach include Stratolaunch’s uncrewed TalonA vehicle and hypersonic weapon systems like Lockheed’s Hypersonic Air-Breathing Weapon Concept and the Raytheon-Northrop Grumman Hypersonic Attack Cruise Missile.
Young says best-in-class hypersonic systems are currently achieving a transition from conventional propulsion to scramjet power somewhere between M3.2 and M3.7.
“We can do better than that,” he says.
BENDING THE CURVE
One might expect a company known for jet engines to approach the scramjet transition by building an even-better jet engine. But GE Aerospace is taking the opposite approach – and RDC is the key.
Rather than build a faster turbojet, scientists at the Global Research Center say a DMRJ engine with RDC will be able to ignite and produce thrust at slower speeds than conventional scramjets.
Young says the approach brings potential for “significant improvement” over existing systems.
While improvement means a lower airspeed threshold for scramjet ignition, it also takes the form of smaller boosters for rocket-powered systems, more compact DMRJs and more efficient combustion in high-Mach flight.
That makes the recent RDC engine milestone significant not just for the prospect of crewed hypersonic flight, but also expendable systems.
Joseph Vinciquerra, senior director of aerospace research at GE Aerospace, describes the company’s DMRJ concept as “platform agnostic” – meaning it could someday be used to power a missile, an aircraft or even a commercial vehicle bound for orbit.
While much development work remains incomplete, the company is pouring resources into solving the hypersonic problem. In just one segment of the effort, GE Aerospace has invested some $1.5 billion into scaling up the production of ceramic matrix composite materials, known as CMCs.
The high-strength, lightweight and heat-resistant components will support the push into the hypersonic realm, as well as existing products like the CFM International Leap turbofan engine.
GE Aerospace is also aggressively pursuing opportunities with the US government, with federal contracts now making up roughly 30% of its roughly $250 million in annual scientific spending.
“One of the things that we’ve really honed over the past five years is the ability to seek out problems that the government is trying to solve,” Vinciquerra says. The company is seeking government opportunities that align with “product priorities that we’re looking to advance within the enterprise”, he notes.
The DMRJ engine falls squarely within that sweet spot – and GE plans to move rapidly on advancing the concept.
The initial RDC test used a sub-scale demonstration article housed in the submarine-like basement of the Global Research Center. In a hardened test cell, seemingly innumerable pipes and lines direct fuel, compressed air and coolant into an engine assembly, while scrubbed exhaust is vented to the surface.
This facility allows researchers to explore and address challenges related to scramjet ignition, combustion stability, throttle control, excessive vibration, heat loads and material selection.
GE Aerospace intends to test a full-scale DMRJ engine with RDC by early 2025 or sooner.
Part of the company’s strategy for developing and expanding the technology was the acquisition earlier this year of New York-based propulsion-development company Innoveering. That firm successfully tested its own DMRJ in the first quarter of 2023, including transitioning between gas turbine and scramjet engines.
Mark Rettig, general manager of business development at Edison Works, calls the Innoveering acquisition “huge” for GE Aerospace, saying Innoveering’s DMRJ design formed the basis of GE Aerospace’s RDC-based prototype.
In practice, Rettig explains, mode transition would allow an aircraft to be launched under power of a gas turbine engine, transition to scramjet power to reach hypersonic speeds and perform a mission, then decelerate and transition back to turbine-powered flight for landing.
“That mode transition is critical to the efficiency of the vehicle,” Rettig says. “It’s critical to the vehicles being able to get the range and performance they need to execute their missions.”
Optimising efficiency and engine performance during those mode transitions will be key to proving the concept has realistic possibility, he notes.
While the recent RDC test featured only the DMRJ demonstrator engine, the full-scale test planned for early 2025 will include mode transition with a jet turbine engine.
If GE Aerospace proves that design’s feasibility, the prospect of crewed hypersonic flight becomes “an engineering problem”, as Young describes it, rather than a matter of invention.
Solving that engineering problem will fall largely to airframers like Lockheed, Northrop or Boeing. They will be challenged to devise ways of integrating two radically different propulsion systems with unique air-intake requirements.
The fictional Darkstar – designed by Lockheed in the Top Gun universe – uses variable air intakes, which mechanically adjust during the mode transition depicted on screen.
While that transition might have seemed pure fiction when the film was released in 2022, the recent propulsion breakthrough at GE Aerospace makes the concept less fanciful.
Still, hypersonic experts at the Global Research Center say one aspect of the Darkstar remains firmly within the realm of fiction: the prospect of surviving an ejection at M10.