Ramjets and scramjets, on the other hand, have no compressor (or often any moving parts at all), and as such, can’t function at a stop and are very inefficient at lower speeds. But at speeds above Mach 3, these engines come alive – using the immense pressure of inflowing air and, importantly, the shockwaves created through intentional inlet design to compress the air before it is mixed with fuel to be ignited for propulsion.

“You’ve got an inlet, and you have a shock that hits in that Inlet, and then you design these geometries so you get a shock system that bounces around inside,” Dr. Combs explained to Sandboxx News. “As the air goes through each one of those shocks, the pressure goes up, and then by the time you get to your combuster, you have this very high-pressure gas that you’re ready to light.”

Ramjets like the one used by Hermeus, are an older and more proven technology that slows inflowing air down to subsonic speeds for more manageable ignition. As a result, ramjets can often function at lower speeds (sometimes as low as Mach 2), using either a blocking body or intentional inlet geometry to create shockwaves that sufficiently slow the inflowing air. This slower airflow does make ramjets easier to ignite but also limits their maximum achievable speeds. It’s generally believed that Ramjets will top out at speeds of around Mach 6. 

More modern scramjets, on the other hand, allow for supersonic airflow throughout the engine, though the geometric design of the inlet is still very intentional.

“You’ve got a shock system, but it never actually shocks all the way down to subsonic speed,” Dr. Combs told Sandboxx News. 

This faster-flowing air makes ignition a significant engineering challenge that’s often compared to trying to light a match in a hurricane. But once you overcome this challenge, scramjets are generally believed to be capable of achieving speeds of Mach 10 or higher.

A dual-mode ramjet (also called dual-mode scramjet), like the one GE Aerospace recently used in its bench testing, is effectively a single air-breathing engine that can function as both a ramjet and a scramjet depending on the speed of the air flowing into it.

How exactly dual-mode ramjets manage this transition depends on the specific engine, but as Dr. Combs explained, both ramjets and scramjets function using “basically the same concept,” making it possible to design an engine that can function as a ramjet at lower speeds and a scramjet at higher speeds by changing the geometry of the inlet to create the appropriate “shock system” (series of shockwaves) for each flight regime. 

When you combine one of the lower-speed jet engines (turbojet or turbofan) with one of these higher-speed ones (ramjet or scramjet), you get what’s commonly called a turbine-based combined cycle (TBCC) engine. Such an engine would allow an aircraft to take off using the lower-speed engine, transition to the high-speed engine for operational flight, and then transition back to the low-speed engine for landing. 

Today, we’re aware of at least three American-based efforts to develop and field such an engine. Hermeus’s Chimera is a turbojet/ramjet design, though the firm has plans to field a larger and more capable turbofan/ramjet soon. Leidos has been more tight-lipped about their efforts tied to the Air Force Research Lab’s Mayhem program, though it’s understood the effort aims to field a more advanced turbofan/scramjet system. 

And then of course, Lockheed Martin claimed to have success with their own turbofan/dual-mode scramjet engine, designed in coordination with Aerojet Rocketdyne, back in 2017. Interestingly enough, this design was said to be experimenting with both Pratt & Whitney F100 and GE F110 turbofans as its turbine basis, though progress on this effort has since been shrouded in secrecy since the program went dark at the onset of the modern hypersonic arms race. 

GE Aerospace has admittedly not been among the big-name firms in the race to field hypersonic aircraft to date, but their ability to incorporate far more efficient rotating detonation combustion into these engine designs could allow them to not only catch up, but rapidly take the lead in this field. 

MANAGING THE TRANSITION

As Lockheed Martin’s Hypersonics Program Manager and the director of the SR-72 program, Brad Leland, explained in 2013, the transition from turbofan to scramjet power is a significant challenge because of the gap in the speed ranges each engine can function efficiently. 

“The turbine, which works well up to Mach 2, and the scramjet (supersonic combustion ramjet) work well at Mach 4 and above. By making those work together down at Mach 3 – below Mach 3 – that’s really the key,” Leland said at the time.

GE’s press release echoes this sentiment, highlighting that their dual-mode ramjet could normally only function efficiently at speeds above Mach 3, but by incorporating rotating detonation combustion (RDC) — effectively a ring of perpetual detonation circling the combustion chamber — it should be able to operate at lower speeds, helping to bridge the gap between turbofan and dual-mode ramjet power. 

But GE isn’t stopping at adding RDC to their dual-mode ramjet. They also intend to incorporate this technology into their high-powered turbofan engines, which could allow these lower-speed engines in their combined cycle design to achieve higher speeds more efficiently, which also helps to bridge the gap between efficient speeds for the turbofan and dual-mode ramjet. 

This turbofan design could also have significant implications for non-hypersonic tactical aircraft as well. In 2020, GE filed a patent for a turbofan engine equipped with a rotating detonation afterburner, which would offer a significant increase in airflow pressure, speed, and temperature ducted through bypass tubes directly into the engine’s afterburner. The design included a variety of heat-management measures intended to reduce the temperature of that airflow, ranging from inlet precoolers to porous flow path liner walls that would expose the airflow to cooling fluids.

Such an RDC afterburner could certainly help increase the speeds a turbofan engine can achieve, further bridging the velocity gap between turbofan and dual-mode scramjet, but importantly, could also apply to non-TBCC engines. An RDC afterburner used on conventional turbofan engines powering today’s tactical fighters could allow for higher top speeds and greater fuel efficiency — both of which are aims for 6th generation fighters in active development.

And the use of RDC in both the turbofan and the dual-mode ramjet could also have another significant benefit…

REDUCING WEIGHT

One of the biggest challenges to fielding an effective TBCC engine is weight. Weight is always a concern in aircraft design, but the issue becomes even more pronounced when your aircraft is forced to carry more than one form of engine. When flying under turbojet or turbofan power, the ramjet/scramjet is nothing but dead weight and vice versa. But engines that use rotating detonation combustion can help to offset this weight issue.

“RDC 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,” GE says in their write-up about their new engine. 

This could also allow for a shorter overall engine length, Dr. Combs explained, which is another important consideration for aircraft design, as larger internal space requirements lead to larger overall aircraft designs — which means more material and as such, even more weight.

And perhaps most importantly, more efficient engines reduce fuel requirements or extend ranges for the same amount of fuel carried. This would be essential for any Intelligence, Surveillance, and Reconnaissance (ISR) or precision-strike platform, which would need to cover great distances.

The SR-71’s unrefueled range of 2,500 miles required an internal load of 80,000 pounds of fuel, for example. While it’s difficult to predict fuel economy for GE’s experimental new engine design in an aircraft, it stands to reason that it would be far more efficient but would aim for significantly higher speeds, making the balance of weight to range just as essential as it is in supersonic designs. 

While Dr. Combs is not involved with GE’s RDC Dual-Mode Ramjet or TBCC engine programs, he posits that such a design could potentially propel an aircraft beyond Mach 10 and potentially even as high as Mach 12. 

“If these work as they’re supposed to and as the thermodynamics of all this type of stuff says that they should, then yeah, I think that there could be practical applications for this technology,” Dr. Combs said when we asked if this could produce aircraft with sufficient fuel range to be of use to the U.S. military. 

GE JUST BECAME A HEAVYWEIGHT IN THE HYPERSONIC AIRCRAFT RACE

According to GE, they managed to come this far in just 12 months, and now expect to be able to conduct a full demonstration of their DMRJ (dual mode ramjet) with RDC (Rotation Detonation Combustion) as soon as next year… Which, upon their release, isn’t all that far out at all. 

Thus far, GE has not offered any timeline regarding a TBCC engine that would combine their RDC dual-mode ramjet with a similarly RDC-equipped turbofan, though we know for certain that it’s something they’re exploring. If and when such an engine manifests, however, there will still be the challenge of producing an airframe around it that can withstand the friction, heat, and pressure of hypersonic flight.