Aircraft

Turbojet to hypersonic ramjet: Hybrid Chimera engine shows its range

Turbojet to hypersonic ramjet: Hybrid Chimera engine shows its range
In order to take off from standstill on a runway, then accelerate through to hypersonic speeds, a hybrid engine is required
In order to take off from standstill on a runway, then accelerate through to hypersonic speeds, a hybrid engine is required
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Hermeus has gone for a Pratt & Whitney F100 turbofan out of the F-16 for the turbine portion of the Chimera II engine that'll power its larger Darkhorse aircraft
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Hermeus has gone for a Pratt & Whitney F100 turbofan out of the F-16 for the turbine portion of the Chimera II engine that'll power its larger Darkhorse aircraft
The Halcyon: Hermeus eventually wants to create a Mach 5+ hypersonic airliner
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The Halcyon: Hermeus eventually wants to create a Mach 5+ hypersonic airliner
In order to take off from standstill on a runway, then accelerate through to hypersonic speeds, a hybrid engine is required
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In order to take off from standstill on a runway, then accelerate through to hypersonic speeds, a hybrid engine is required
The turbojet cuts out and the ramjet kicks in once the aircraft is moving faster than Mach 3
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The turbojet cuts out and the ramjet kicks in once the aircraft is moving faster than Mach 3
The Quarterhorse's stated goals are to break Mach 4, validate the Chimera engine, and break the 50-year-old airspeed record still held by the SR-71 Blackbird
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The Quarterhorse's stated goals are to break Mach 4, validate the Chimera engine, and break the 50-year-old airspeed record still held by the SR-71 Blackbird
Darkhorse is intended to be an uncrewed aerial system capable of sustained flight at Mach 5 – something that's never been done yet
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Darkhorse is intended to be an uncrewed aerial system capable of sustained flight at Mach 5 – something that's never been done yet
The turbojet will get you from zero to Mach 3, after which the ramjet kicks in for the real party starts
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The turbojet will get you from zero to Mach 3, after which the ramjet kicks in and the real party starts
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Atlanta company Hermeus has its sights set on Mach 4 next year, to beat the Mach 3.3 benchmark set by the SR-71 Blackbird way back in 1976. Watch its Chimera hybrid engine nailing the switch between low-speed turbojet mode and hypersonic ramjet mode.

Under US Air Force funding, the company is charging forth on its Quarterhorse project, a nuggety, dart-shaped, remotely-piloted UAV designed to sprint faster than any aircraft before it.

With funding only awarded back in 2021, the pace of development on this one is pretty wild, and Hermeus claims it's still on track to break the outright air-breathing aircraft speed record next year.

To get there, it needs an engine similar to what the Blackbird ran; a hybrid propulsion unit that can operate in two stages. When it's time for maximum thrust at hypersonic speeds, you need a ramjet. But ramjets rely on airflow to compress and heat air ready for combustion; they can't start you off on a runway, they produce basically zero thrust below Mach 0.5, and they don't become more efficient than a regular turbojet below somewhere around Mach 3.

The turbojet will get you from zero to Mach 3, after which the ramjet kicks in for the real party starts
The turbojet will get you from zero to Mach 3, after which the ramjet kicks in and the real party starts

Thus, the hybrid. Hermeus has designed and built an engine it calls the Chimera that runs a ramjet and a slower-speed turbojet in the same tube. The turbojet is functional even at a standstill, so it's capable of getting Quarterhorse down a runway and into the air, where it'll accelerate to Mach 3 before shutting off. The supersonic intake air will then be routed around the turbojet into the ramjet, which will compress it, ignite it, and blast the aircraft to its top speed.

The team took the Chimera engine to the Notre Dame Turbomachinery Lab last year, where it was mounted to a bench in a high-Mach test facility specially modified to deliver inlet air at speeds high enough to simulate Mach 4 flight. The engineers ran the jet in bursts, typically late at night to mitigate the insane electricity costs involved. The test campaign lasted a full five months before the Hermeus team was able to announce it had achieved a full and stable transition between the turbojet and ramjet stages in November.

“The Notre Dame facility allowed us to create conditions similar to what we’ll see in flight,” said Hermeus co-founder and chief technology officer Glenn Case in a press release. “Completing this testing on the ground significantly de-risks our Quarterhorse flight test campaign which will begin late [this] year.”

The Quarterhorse's stated goals are to break Mach 4, validate the Chimera engine, and break the 50-year-old airspeed record still held by the SR-71 Blackbird
The Quarterhorse's stated goals are to break Mach 4, validate the Chimera engine, and break the 50-year-old airspeed record still held by the SR-71 Blackbird

NDTL director Joshua Cameron said, “We are pleased that Hermeus chose NDTL as their partner for this exciting test program.” Cameron, who is also a research assistant professor in Notre Dame’s Department of Aerospace and Mechanical Engineering, added, “I am also so proud of my team for developing the facility and executing a successful test campaign on a very aggressive schedule.”

Beyond Quarterhorse, Hermeus is planning a larger Darkhorse unmanned platform, which will run an enormous Pratt & Whitney F100 engine out of an F16 as its turbojet stage. This machine is targeted at delivering sustained hypersonic flight at Mach 5, a military capability no country has yet in its arsenal.

Hermeus has gone for a Pratt & Whitney F100 turbofan out of the F-16 for the turbine portion of the Chimera II engine that'll power its larger Darkhorse aircraft
Hermeus has gone for a Pratt & Whitney F100 turbofan out of the F-16 for the turbine portion of the Chimera II engine that'll power its larger Darkhorse aircraft

But the end goal for Hermeus is not military. Its ultimate plan at this stage is to build a civilian, commercial, Mach 5 hypersonic airliner with a titanium-alloy body and a range around 4,600 miles (7,400 km), capable of ferrying up to 20 passengers across 125 different trans-oceanic routes. This "Halcyon" plane would blast you from Paris to New York in 90 minutes at a monster 90,000 ft altitude, higher than the legendary Blackbird was allowed to cruise.

Five times faster than any commercial aircraft on the market today, Halcyon frankly looks like a bit of a pipe dream at this stage, given that it's an exponentially harder thing to build, test and certify than a supersonic jet – and even those are proving critically difficult, with Florida company Aerion shutting down in 2021 despite more than US$10 billion in advance orders for its 50-passenger Mach 1.4 jet. Not to mention, it's a huge leap from developing military UAVs to getting a hypersonic airliner certified for commercial flight and into serial production, let alone cleared to fly at those speeds. Don't hold your breath on this one.

Still, Hermeus has made some pretty wild progress since it was founded in 2018. It's a young company, with a preternaturally young-looking team, working on some groundbreaking gear. They're shooting for the moon, who knows where they'll land? Check out the engine bench test in the video below, it's as awesome as you'd expect.

Hypersonic Engine Test Milestone

Source: Hermeus

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12 comments
12 comments
paul314
20 passengers? Even if it can run 2-3x as many flights a day as a conventional aircraft because of the speed, those are going to be some incredibly pricey tickets. Either for the super-rich to impress one another, or highly-prized corporate minions. This seems really cool but not that useful for anything but military applications.
Ornery Johnson
The diagrams don't show it, but I wonder how the engine blocks the air going to the turbofan during the transition to ramjet? Probably some type of elongated cone that snaps into place in front of the turbofan intake.
paul314
@Ornery Johnson at those kinds of speeds you don't necessarily have to block the intake. Some kind of cone or other shape(s) to create a stationary shockwave in the right place can divert the airflow around the turbofan. Very much in line with what the SR71 did.
Robt
If it works, this is going to cost a fortune to certify, but the split between civil and military use might just make it economically feasible. I certainly hope so; these guys seem to know what they’re doing
darkcook
As a mechanical engineer with 25-years experience, it's encouraging to see such a sharp and energetic group of young engineers tackling new challenges. Makes me happy. Interesting concept that I hope to continue seeing encouraging developments from.
Nelson Hyde Chick
Imagine the ingredible amount of greenhouse gases this thing is going to spew-out?! Why do we need this? So we can kill people faster? Or maybe so rich people can not take as long to fly to Davos for the yearly conference?
P51d007
If/when they do break the SR-71 record, LOL, just pull an SR-71 out of mothballs to break it, or "officially" release the flight data of the SR-72. ;)
ljaques
I wonder if they can grab a Wagyu burger for me in Tokyo and...
The test fire of Quarterhorse a year ago (YT vid) was interesting. She's a big, pretty, loud bird!
Smokey_Bear
Awesome. People should not still be moved in slow metal tubes. Sub-sonic should be reserved for freight, humans should be hypersonic. We have things to do, and nobody likes a long flight.
Ranscapture
@Smokey_Bear I actually like the 14hr flights across the pacific because it’s the only time of waking hours where I can’t be working or cooking or cleaning. I can sit there and watch movies, eat, play games, and read.
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