Hermeus and the Mach five startup building a hypersonic airplane in Atlanta

Hermeus, an Atlanta-based aerospace startup founded in 2018, is developing a reusable hypersonic aircraft capable of Mach 5 — roughly 3,800 miles per hour. Their approach centers on a turbine-based combined cycle (TBCC) engine that merges a conventional turbine and a ramjet into a single propulsion unit, potentially enabling New York-to-London flights in 90 minutes. If successful, Hermeus would solve one of aerospace engineering’s longest-standing challenges: building a reusable aircraft that takes off from a normal runway and accelerates to hypersonic speeds.

Who Is Behind Hermeus?

The founding team includes A.J. Piplica, Skyler Shuford, Glenn Case, and Mike Smayda, engineers with backgrounds at SpaceX and Generation Orbit. They started with a pointed question: why has nobody built a reusable hypersonic aircraft that can take off from a conventional runway?

The answer, for the past six decades, has been propulsion.

Why Is Hypersonic Propulsion So Difficult?

A conventional turbine engine works well from standstill up to roughly Mach 2 to 2.5. Beyond that, compressed incoming air generates so much heat that turbine materials hit their thermal limits and efficiency collapses.

A ramjet solves the high-speed problem — it compresses air purely through forward velocity with no moving parts, making it efficient at high Mach numbers. But a ramjet produces zero thrust at zero airspeed. It typically needs speeds above Mach 2 before it becomes useful.

Historically, reaching hypersonic speeds required either a rocket boost (as the X-15 used) or a separate carrier vehicle — both expensive and logistically impractical for routine operations.

How Does the Hermeus TBCC Engine Work?

Hermeus’s solution is their Chimera engine, a turbine-based combined cycle (TBCC) system that shares a common flowpath between a turbine and a ramjet.

The sequence works like this:

Low speed (takeoff through transonic): The turbine operates conventionally — compressing air with spinning blades, mixing fuel, igniting, and producing thrust.
Mid-speed transition (around Mach 2): As the aircraft accelerates past the turbine’s efficient range, the turbine spools down and the ramjet takes over using the same inlet and exhaust path.
High speed (Mach 2+ to Mach 5): The ramjet compresses incoming air purely through vehicle speed, powering the aircraft to hypersonic velocity.

One engine, two modes, covering the full speed range from runway standstill to Mach 5 and beyond.

The concept has existed in research papers since the 1960s. The SR-71 Blackbird’s Pratt & Whitney J58 engines performed a loosely similar turbojet-to-partial-ramjet transition. But no one has built a production-ready, reusable TBCC engine that transitions cleanly across the full speed range. That is what Hermeus is attempting.

What Hardware Has Hermeus Actually Built?

This is not a slideware company. In 2020, Hermeus ran a full-scale engine test of their Chimera combined cycle demonstrator. They integrated a General Electric J85 turbojet — a small engine originally designed for the T-38 Talon trainer — into a ramjet flowpath. It ran. It produced thrust.

The J85 is a surrogate, not the final engine. Hermeus used it because it was available, well-characterized, and affordable enough for rapid iteration. The production Chimera will use a different turbine core. The point was to validate the combined cycle architecture, inlet design, transition mechanism, and thermal management — and they did.

What Is the Hermeus Development Roadmap?

Hermeus has three planned aircraft:

Quarterhorse is an uncrewed autonomous demonstrator roughly the size of a large fighter jet. Its job is to prove the full vehicle can accelerate through the turbine-to-ramjet transition in actual flight, manage aerothermal loads at Mach 5+, and do it repeatedly. It takes off from a conventional runway in turbine mode, transitions to ramjet past Mach 2, reaches hypersonic speeds, then flies back and lands conventionally. No parachutes, no ocean recovery.

Darkhorse is a larger uncrewed platform designed for operational military and commercial missions — a bridge between demonstrator and passenger aircraft.

Halcyon is the passenger aircraft. Hermeus envisions it as a Mach 5+ airliner carrying approximately 20 passengers on transoceanic routes. New York to London in 90 minutes. Los Angeles to Tokyo in roughly two hours.

What Are the Biggest Engineering Challenges?

Several significant obstacles remain:

Materials. At Mach 5, aircraft skin experiences extreme aerodynamic heating. Leading edges of wings and nose can exceed 2,000 degrees Fahrenheit — beyond what aluminum or standard titanium alloys can handle. Solutions include carbon-carbon composites, ceramic matrix composites, and actively cooled structures that route fuel or coolant through channels in the skin. The SR-71 famously leaked fuel on the ground because its titanium panels were designed with expansion gaps that only sealed at high temperature. Hermeus faces similar challenges at even higher temperatures.

Mode transition. Switching from turbine to ramjet in flight is mechanically and aerodynamically complex. Inlet geometry may need to change, and the airflow path shifts fundamentally. A failed transition can cause thrust interruptions, pressure instabilities, or an unstart — where the shockwave inside the inlet gets pushed forward, causing near-instantaneous thrust loss. An unstart at Mach 4 is catastrophic.

Sonic boom. At Mach 5, the sonic boom is enormous — far worse than Concorde’s, which killed overland supersonic routes. NASA’s X-59 Quesst program is working on boom mitigation at Mach 1.5, but solving it at Mach 5 is exponentially harder. Without mitigation, Halcyon would be restricted to overwater routes, significantly limiting the addressable market.

Fuel consumption. Ramjet specific fuel consumption at Mach 5 is extraordinarily high. The fuel fraction — the percentage of total aircraft weight dedicated to fuel — will be very large, directly limiting passenger and payload capacity.

Economics. Concorde failed commercially not because the aircraft didn’t work, but because per-seat operating costs were so high that only a tiny fraction of travelers could afford tickets. Hermeus argues that modern materials, manufacturing techniques, and TBCC reusability change the math. That claim remains unproven.

Who Is Funding Hermeus?

Hermeus has attracted serious institutional backing. The United States Air Force awarded contracts through the Presidential and Executive Airlift Directorate — the office responsible for Air Force One and senior leader transport. The Air Force wants a hypersonic transport capable of moving the President or senior officials across the globe in under two hours, a real operational requirement with real funding. Hermeus has also received investment from major venture capital firms and strategic defense investors.

How Does Hermeus Compare to Other Hypersonic Companies?

The hypersonic industry is growing:

Reaction Engines (UK) is developing the SABRE engine, a precooled combined cycle concept that chills incoming air before it reaches the turbine, extending turbine operability to much higher Mach numbers.
Venus Aerospace (Houston) is pursuing a Mach 9 passenger transport concept using a rotating detonation engine.
Destinus (Europe) is working on hydrogen-powered hypersonic cargo aircraft.

Hermeus is arguably the furthest along in demonstrated hardware for a runway-to-hypersonic combined cycle approach. They have built hardware, fired engines, and have an active flight test program.

Why Does Hypersonic Technology Matter for All of Aviation?

The engineering challenges Hermeus is solving — thermal management, propulsion integration, advanced materials, autonomous flight control at extreme speeds — have downstream effects across the industry.

Composite materials developed for hypersonic skins become lighter, stronger structures for general aviation airframes. Autonomous flight control algorithms built for Quarterhorse’s uncrewed missions inform next-generation autopilot systems. Manufacturing techniques that make reusable hypersonic engines viable reduce costs for conventional turbine engines.

Aviation has always followed this pattern. Pressurization technology from World War II high-altitude bombers gave us pressurized airliners a decade later. Fly-by-wire systems built for military fighters produced the Airbus sidestick. GPS satellites launched for military precision gave general aviation pilots panel-mount navigation.

What Is the Realistic Timeline?

Hermeus has discussed Halcyon entering service in the early 2030s, though aerospace startup timelines frequently slip. The engineering is exceptionally difficult, the economics are unproven, and certification for passenger-carrying hypersonic flight is uncharted regulatory territory.

But the engine works. The hardware is real. The Air Force is invested. And the propulsion transition problem — turbine to ramjet in a single flowpath — is one of the great unsolved challenges in aerospace engineering. If Hermeus cracks it cleanly and repeatably, that achievement alone reshapes what is possible for the next century of flight.

Key Takeaways

Hermeus is building a TBCC engine that merges a turbine and ramjet into one unit, enabling conventional runway takeoff through Mach 5+ flight.
The Chimera engine has been tested at full scale, validating the combined cycle architecture with real hardware — not just simulations.
Major challenges remain in materials science, mode transition reliability, sonic boom mitigation, fuel efficiency, and operating economics.
The U.S. Air Force is funding Hermeus through the Presidential Airlift Directorate, providing both strategic validation and real defense dollars.
Hypersonic technology developed here will trickle down to general aviation through advanced materials, manufacturing techniques, and autonomous flight systems.