Hypersonic developer Hermeus has begun testing a proprietary pre-cooler engine technology that will be a critical feature of the company’s ultra-fast Quarterhorse aircraft.
Pre-cooling refers to the chilling of engine intake air before compression and combustion. Colder intake air allows standard turbojet jet engines to operate at higher speeds, with greater efficiency and reduced performance degradation.
Although Hermeus has not revealed design specifics, pre-cooling can be accomplished using a heat exchanger and a cryogenic fuel, such as liquid hydrogen.
Such a system is an integral part of the Hermeus approach to breaking the Mach 5 hypersonic barrier, which involves accelerating to low-Mach flight with the use of a standard jet engine, before transitioning to a separate ramjet propulsion system to reach hypersonic speeds.
Static testing of the Hermeus pre-cooler system took place at Edwards AFB, the company said on 14 May. The US Air Force is a financial backer of the California start-up.
Hermeus is currently developing its pre-cooling system for integration with the Pratt & Whitney F100 fighter jet engine, which will power its forthcoming Quarterhorse Mk 2 flight vehicle.
“Data collected from these tests inform how the F100 handles the Hermeus pre-cooler under a variety of loads and will lead into future testing with heated air, simulating high-Mach flight conditions,” the company says.
The Mk 2 aircraft, which is the third airframe in the Quarterhorse line, will be the company’s second airworthy design and the first to be capable of supersonic flight. Hermeus plans to fly Quarterhorse Mk 2 sometime in 2025.
The experimental design will feature a delta wing, a variable air intake and be “about the size of a [Lockheed Martin] F-16”, Hermeus says.
While the Quarterhorse Mk 2 is intended to reach speeds of roughly M2.5 – short of hypersonic flight – the design will incorporate the pre-cooler technology as part of the turbine-based combined cycle (TBCC) propulsion system that Hermeus hopes will power its next vehicle to M5 and beyond.
With its TBCC engine, which Hermeus calls Chimera, the company hopes to address one of the most vexing challenges of hypersonic flight: the transition from jet turbine to ramjet propulsion.
While high-speed ramjet engines are capable of reaching hypersonic speeds, the physics of current designs require airspeeds of around M3.5 to achieve compression significant enough to produce ignition and generate thrust.
Contemporary ramjet engines, and the related scramjet configuration, perform most efficiently at M4 and faster.
That means an air vehicle has to reach those speeds before the ramjet can even begin producing accelerative thrust – a problem for current turbojets. The fastest crewed aircraft ever flown – Lockheed’s iconic SR-71 Blackbird – reached M3.
Addressing the gap between conventional jet engine speeds and the minimum velocity needed for ramjet ignition is essential for reusable hypersonic flight. Several US companies are pursuing different strategies to that end.
Hermeus appears to be tackling the engine transition problem with the pre-cooler approach, which could allow the F100 to reach higher speeds.
By contrast, propulsion heavyweight GE Aerospace is taking another tac: developing a more efficient ramjet.
Using a technique called rotating detonation combustion to improve performance, the engine-maker thinks it can lower the airspeed needed to achieve combustion with a hypersonic-capable ramjet.
Meanwhile, California start-up Stratolaunch is pursuing a rocket-based solution for its Talon A air-launched vehicle, closer in style to the hypersonic munitions currently being developed by the USA, Russia and China.
That rocket engine is being developed by Colorado propulsion start-up Ursa Major.
