• Hypersonic flight (speeds faster than Mach 5) is the perceived future of human aviation.
  • In order to achieve that vision, engineers need to develop hybrid engines capable of handling subsonic, supersonic, and hypersonic speeds.
  • Aviation company Hermeus successfully demonstrated mode transition between turbojet and ramjet engines for the Air Force’s Quarterhorse hypersonic aircraft, a major hypersonic flight milestone.

Few planes ignite the awe-inspiring passions of aviation nerds quite like the SR-71 Blackbird. From its stealthy design to its secretive history, the Blackbird simply screams cool—and it also helps that it’s the fastest air-breathing (a.k.a. “not a rocket”) crewed aircraft in human history.

Capable of flying in excess of Mach 3.2 (around 2,200 mph), the SR-71 has been the pinnacle of aviation speed for nearly half a century, but the Atlanta-based aviation company Hermeus thinks it’s time for a little competition. The company’s vision of hypersonic flight (meaning speeds faster than Mach 5) isn’t one reserved for clandestine spy missions, however. That’s because Hermeus wants to bring hypersonic airliners to an airport near you.

In 2021, the Air Force awarded Hermeus a $60 million contract to develop three uncrewed concept aircraft, including the hypersonic “Quarterhorse.” Late last year, Hermeus passed a major milestone by successfully firing a turbojet-ramjet hybrid engine, known as “Chimera.”

The air-breathing monster behind the powerful SR-71 is a Pratt & Whitney J58 turbojet engine, which maxes out at speeds around Mach 3. In Hermeus’s hypersonic design, a ramjet, which can only operate at high speeds as it uses this air to pressurize air and fuel in the combustion chamber, kicks in and carries the theoretical Quarterhorse aircraft to Mach 5 and beyond. For any plane hoping to dethrone the Blackbird, its engine needs to somehow seamlessly transition between turbojet, ramjet, and back to turbojet—and Hermeus has already pulled off that delicate piece of aviation engineering.

“We just demonstrated a mode transition, which means we went from turbojet mode to ramjet mode,” Hermeus co-founder and CTO Glenn Case said in a recent video. “This is probably the most critical challenge in unlocking hypersonic flight.”

In order to test its Chimera engine, Hermeus needed to simulate the high-speed pressures of Mach 4 flight, so the company packed up its engine and shipped it to the Notre Dame Turbomachinery Lab, a high-Mach test facility. There, Hermeus demonstrated the Chimera’s ability to transition between engines by guiding supersonic air around the turbojet and into the ramjet. The hypersonic engine for Quarterhorse joins similar initiatives to unlock the era of hypersonic flight, including the Air Force’s Project Mayhem, which hopes to deliver a hypersonic bomber in the foreseeable future.

So how far off are these hypersonic planes from takeoff? If history is any indicator, Pratt & Whitney first tested the J58 in October 1958, and six years later, the SR-71 was streaking across the sky. However, the J58 was just essentially a souped-up version of an already-existing technology, whereas Hermeus, and other hypersonic companies, are designing a more complex hybrid platform. For the record, Hermeus says it’s on track to break the Blackbird’s “outright air-breathing aircraft speed record next year,” per New Atlas.

Who knows? Maybe by the end of the decade, human aviation will finally enter the era of hypersonic flight.

2023-01-25T15:07:33Z dg43tfdfdgfd