Hermeus Corp. said last week that it has demonstrated turbojet to ramjet transition for its Chimera turbine-based combined cycle engine (TBCC)–a cross between a turbojet and ramjet engine–to allow reusable hypersonic planes to take off from regular runways.

“This is one of the most important technological feats to making operational hypersonic flight a reality,” Hermus said. “The cost and speed at which the Hermeus team achieved this milestone is notable. Hermeus designed, built, and tested Chimera in 21 months for $18 million.”

Hermus said that it tested Chimera at the Notre Dame Turbomachinery Laboratory, which supplies heated air to mimic high-Mach temperatures and pressures.

“At low speeds Chimera is in turbojet mode – just like any jet aircraft,” Hermeus said. “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. “

In August last year, the U.S. Air Force announced that it had awarded a $60 million contract to the Georgia-based Hermeus to speed the commercial development of hypersonic aircraft and propulsion systems (Defense Daily, Aug. 5, 2021).

The Air Force Life Cycle Management Center’s (AFLCMC) Presidential and Executive Airlift Directorate, the Air Force Research Laboratory, and venture capital funded the contract, part of a larger effort–the “Vector Initiative”–to spur high speed passenger travel and the possible advancement of Air Force technologies for senior leader transport; air mobility; intelligence, surveillance, and reconnaissance; and other missions.

Hermeus said it plans to build its first hypersonic drone, Quarterhorse, and begin flight testing it late next year. Hermeus is also developing Darkhorse, an uncrewed aircraft that can sustain hypersonic flight, and Halcyon, which is to be a Mach 5 commercial passenger aircraft.

“The contract establishes a number of objectives for Hermeus to meet within three years,” AFLCMC said last year, including increasing the understanding of enabling technology and mission capabilities for reusable hypersonic aircraft; scaling and flight testing a reusable hypersonic propulsion system; developing, building and testing three of Hermeus’ Quarterhorse concept aircraft; providing a payload integration guide for future hypersonic flight testing with Quarterhorse; and providing wargaming data for use in Air Force strategic analysis tools.

After the three-year contract with Hermeus, the Air Force plans to evaluate the company’s progress, the maturity of hypersonics, and the alignment of the Hermeus effort with service priorities.

In May, RTX Ventures, a capital investment group led by Raytheon Technologies [RTX], said that it made a strategic investment in Hermeus to develop commercial and military hypersonic aircraft (Defense Daily, May 12).

Ramjets, which function best between Mach 3 and Mach 5, mix compressed air with ignited fuel to create thrust.

“Hermeus’ TBCC engine is unique in the field of hypersonics,” the company said. “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.”

Hermeus has compared its Quarterhorse, which is to have a top speed of Mach 5 and a cruise altitude of 95,000 feet, to Lockheed Martin‘s [LMT] legendary SR-71 Blackbird, which first flew in 1964 and which had a top speed of more than Mach 3 and a cruise altittude of 85,000 feet.