This turboramjet engine could power the fastest aircraft in the world

A five-year-old startup has developed one of the most powerful aircraft engines.
Rupendra Brahambhatt
Hermeus Chimera
Hermeus Chimera


Atlanta-based startup Hermeus has developed a hybrid engine that can switch from low-speed turbojet mode to hypersonic ramjet mode during its flight. The company aims to develop the fastest-ever aircraft using its hybrid Chimera engine.

With a Mach 3.3 (2,532 miles per hour), Lockheed SR-31 Blackbird is currently the fastest jet in the world. To beat the Blackbird, Hermeus is developing Chimera engine-powered Quarterhorse aircraft that is likely to fly at Mach 4 (3,069 mph).   

Hermus conducted a simulated hypersonic flight test on November 17, 2022, at the Notre Dame Turbomachinery Laboratory in Indiana. The Chimera engine successfully transitioned from turbojet mode to ramjet mode during the test. 

The company's official news release noted, “Hermeus has demonstrated turbojet to ramjet transition within its engine, Chimera. This is one of the most important technological feats to making operational hypersonic flight a reality. The ability to switch between these two modes allows Hermeus’ first aircraft, Quarterhorse, to take off from a regular runway and then accelerate up to high-Mach speeds.” 

Difference between turbojet and ramjet

Ramjet engines are more fuel efficient and powerful than turbojet engines. For instance, a turbojet engine can go up to only Mach 3, but a ramjet engine has the potential to make an aircraft fly at Mach 6 (4,603 mph). While turbojet engines require a compressor to suck in air and compress the same for combustion. 

A ramjet engine doesn’t need a compressor because its high-speed motion is enough to ram and compress the air. However, these engines only operate at speeds above 300 mph and deliver outstanding performance at and above Mach 3. So although they are ideal for powering missiles, they can not be used in a crewed aircraft.

On the other hand, a turbojet engine will work efficiently at low and even at zero speed. These engines have become the go-to choice for powering commercial airliners and many army aircraft.  

Making of Chimera, a turboramjet

This turboramjet engine could power the fastest aircraft in the world
Turbojet and ramjet modes.

Chimera is a hybrid engine known as a turbine-based combined cycle engine (TBCC). Firstly, when the Quarterhorse aircraft is on a runaway, the TBCC engine functions like a turbojet, enabling the aircraft to take off, accelerate, and reach high speeds. As the speed and temperature of air coming towards Quarterhorse increases, the Chimera engine begins transitioning from a turbojet into a ramjet.

This process begins around Mach 2 speed and completes at Mach 3 when the engine starts drawing in air at supersonic speeds. This air is fast enough to achieve combustion without a compressor. This is where the turbojet engine becomes inactive, and the ramjet engine gets into the action, boosting the aircraft's speed and allowing it to fly at its highest speed.    

“Chimera has a pre-cooler that reduces the temperature of the air coming into the turbojet. This allows Hermeus to squeeze out a bit more performance from the turbojet before transitioning to ramjet. At around Mach 3, Chimera begins to bypass the incoming air around the turbojet and the ramjet takes over completely,” mentioned in the official news release. 

Chimera is the future of hypersonic flight

The Chimera engine is special because it could make hypersonic passenger flights a reality. Until now, hypersonic engines have only been used to power space rockets and missiles as they couldn’t be reused or employed in airplanes that require a runway to take off. With the "air-breathing" Chimera engine, this could change.

Hermeus claims their hypersonic engine doesn’t need a rocket to accelerate. It can be used in passenger flight operations where the aircraft is reused. Plus, it can allow a hypersonic aircraft to take off from a regular airport runway. 

The company received funding in 2021 only, and the team is excited about what it has achieved only in the past few years with the Chimera engine. They have also secured contracts from NASA and US Air Force.  

Apart from the Quarterhorse project, Hermeus is working on technologies to improve high-speed uncrewed flight applications. Hopefully, we’d soon be able to witness Quarterhorse’s hypersonic performance in the open sky.

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