Hermeus’ Quarterhorse jet breaks supersonic barrier for the first time

Atlanta-based aviation startup Hermeus has officially broken through the supersonic barrier with its Quarterhorse technology demonstrator, as it continues on its quest to field the world’s first fully reusable hypersonic aircraft.

As it stands, Hermeus holds the distinction of operating the world’s first privately developed, unmanned supersonic jet.

Hermeus’ Quarterhorse Mk 2.1 reached a reported speed of Mach 1.21 in just its third test flight this month. This means that this exotic technology demonstrator broke that Mach 1 barrier just less than one year (actually, 364 days) after the first-ever test flight of its own iterative predecessor, the Quarterhorse Mk 1.

“Our customers at the Department of War are paying close attention to how fast this program is moving,” AJ Piplica, CEO and Co-founder of Hermeus, said in a press release.

“This flight demonstrates a pace of execution that is extremely rare in modern aviation. Our country’s ability to deliver new asymmetric military capability at scale depends on teams that can solve hard technical challenges quickly. That’s exactly what we’re proving with each test flight we conduct and each new aircraft we build at Hermeus,” Piplica added.

Hypersonic flight, which is generally defined as flying at speeds in excess of Mach 5, has been an area of increasing focus for the U.S. and other militaries around the world in recent years.

In a modern-day arms race, the U.S., Russia, and China are all fielding hypersonic missiles, or weapons that travel at speeds higher than Mach 5 while retaining the ability to maneuver. But rather than pursuing similar single-use hypersonics, Hermeus has taken a novel approach to developing fully reusable hypersonic aircraft at relatively low cost, largely using existing technologies in creative new ways.

Promotional video showing Quarterhorse’s Mk 2.1 first flight in May 2025. (Hermeus)

Arguably, Quarterhorse is not a single program, but rather two separate ones.

The first program is the aircraft itself, which is being designed, built, and tested internally by Hermeus using a combination of traditional techniques and advanced manufacturing processes like 3D-printing titanium.

And the second program is the exotic turbine-based, combined-cycle propulsion system called “Chimera,” which is being devised to push this aircraft beyond Mach 5 and keep it there for an operationally relevant period of time.

This new propulsion system combines two different kinds of air-breathing jet engines into one.

The first engine is a Pratt & Whitney F100 turbofan that accelerates the aircraft from a dead stop all the way up to speeds above Mach 2. Then a ramjet engine kicks in. The aircraft needs to be moving at high supersonic speeds to for the ramjet to function, but once online, it can potentially propel the aircraft to speeds approaching Mach 6.

This first supersonic flight, however, occurred with only the F100 turbofan onboard, as Hermeus continues to take iterative steps toward its ultimate goal.

The firm already demonstrated Chimera’s ability to function and transition between turbojet and ramjet power (using a J85 turbojet rather than the larger F100) in a wind tunnel as far back as November 2022.

Feature Image: FAA agents inspect the Quarterhorse Mk 2.1. (Hermeus)

Read more from Sandboxx News

How a kamikaze drone that is just ‘good enough’ is changing warfare

Long-distance targeting is much harder than people think – and that’s bad news for China

The F-14 Tomcat fighter wasn’t the US Navy’s first choice

America’s LUCAS kamikaze drones will get a lot scarier thanks to Hivemind AI

A Green Beret remembers his favorite foreign weapons