Hypersonic aircraft developer Hermeus says it has demonstrated turbojet to ramjet transition within its engine, Chimera, coming one step closer to an operational hypersonic flight.
Chimera is a turbine-based combined cycle engine (TBCC) – which basically means it’s a hybrid between a turbojet and a ramjet. 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.
Hermeus notes it designed, built, and tested Chimera in 21 months for $18 million.
“This achievement is a major technical milestone for Hermeus,” said CEO AJ Piplica. “But more than that, it’s a proof point that demonstrates how our small team can rapidly design, build, and test hardware with budgets significantly smaller than industry peers.”
With the successful mode transition demonstration, the Hermeus team now expects to manufacture the first Quarterhorse aircraft to begin flight testing in late 2023.
The engine testing took place at the Notre Dame Turbomachinery Laboratory which provides heated air to simulate high-Mach temperatures and pressures.
“The Notre Dame facility allowed us to create conditions similar to what we’ll see in flight,” said CTO Glenn Case. “Completing this testing on the ground significantly de-risks our Quarterhorse flight test campaign which will begin late next year.”
At low speeds Chimera is in turbojet mode – just like any jet aircraft. But as the temperature and the speed of the incoming air increase, turbojets hit their performance limit. This happens at around Mach 2.
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.
A ramjet is a simple propulsion system which “rams” the incoming high-pressure air to create compression. Fuel is mixed with this compressed air and ignited for thrust. Ramjets are optimal between Mach 3 and Mach 5.
Hermeus explains that its TBCC engine is unique in the field of hypersonics. Most hypersonic platforms are powered by a rocket engine. But this approach makes reusability much harder and inherently more dangerous for passenger flight.
By making a full-range air-breathing hypersonic engine that does not require a rocket to accelerate, Hermeus is setting the stage for operational hypersonic flight – meaning aircraft that can be rapidly re-used.
An additional benefit of this engine design is that it accommodates existing transportation infrastructure. Hermeus aircraft are designed to be operational at traditional airports.
