It’s sleek and shimmering, gliding through sun-tinted clouds. It’s moving at nearly four times the speed of sound – faster than the famous SR-71 Blackbird, which set the airspeed record some 50 years ago.
Its name is Quarterhorse, its mission is to be the world’s first high-Mach autonomous aircraft, and its creator is the U.S. startup company Hermeus.
But powering the futuristic aircraft and its ambitious goals is an engine familiar to aviation enthusiasts – the Pratt & Whitney F100.
Rather than build a new engine, Hermeus will use a modified F100, a propulsion system with 30 million flight hours and a track record aboard the F-15 Eagle and F-16 Falcon, which celebrates 50 years of flight in 2024.
Now, the engine is looking toward its next 50 years.
“Fifty years of life combined with the opportunity to remain in service for another 50 years is something that’s unique,” said Josh Goodman, senior director of the F100 program. “Pratt likes to do hard things, and utilizing the F100 in this solution gives us the opportunity to once again go solve hard problems.”
But first, Hermeus must prove its Chimera turbine-based combined cycle engine, which will use the F100 as its gas turbine, can achieve hypersonic speeds for Darkhorse, an uncrewed multi-mission military aircraft that would fly through contested environments at Mach 5+.
Then, it will build its third aircraft, Halcyon, a 20-passenger commercial aircraft that would make the New York-Paris flight with a yet-to-be-determined engine.
“People see it as ambitious, a little crazy,” said A.J. Piplica, Hermeus’ co-founder and CEO, “but also highly grounded in rational, realistic roadmaps to get there.”
The F100 will appear in the third prototype of Quarterhorse in 2025, where it will help the vehicle surpass Mach 2.5. Then, in the fourth prototype, it will help solve an especially hard problem called mode transition, which is when the propulsion system switches from turbo to ramjet during flight.
Here’s how they’ll do it.
The Chimera engine works a little like a car’s manual transmission system, but rather than changing gears to go faster, the engine changes propulsion systems.
For speeds up to about Mach 2, the Chimera engine will use a turbine propulsion system, where an engine creates thrust by taking in air, compressing it and igniting it. For anything faster, it switches to its ramjet system, which needs no compressor because the air around it is already moving so fast.
Something called a precooler – a clever way of getting more performance out of the turbine engine – will help the two systems work together. By lowering the temperature of the air it takes in, the precooler gives the turbine more headroom to work harder and get the aircraft up to higher speeds.
Hermeus has already completed two major milestones by proving mode transition and precooler technologies in separate lab-based tests.
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