The Air Force wants air-breathing engines for hypersonic missiles

The U.S. Air Force is asking the defense industry to build air-breathing engines for hypersonic weapons.

The Air Force’s Request for Information (RFI) asks manufacturers whether they could produce up to 72 engines per year.

The RFI makes clear the challenge of designing an air-breathing engine to achieve hypersonic speed, which is defined as faster than Mach 5. Hypersonic weapons can range from fast anti-ship missiles traveling at Mach 8, to boost-glide weapons that use a rocket booster to loft a glider to the edge of the atmosphere, where they glide down at speeds of Mach 20-plus.

“Boost-cruise hypersonic missiles must endure stagnation temperatures of at least 2,000 degrees Fahrenheit for the entire mission and all sources for cooling must come from the fuel or auxiliary coolants, which are heat soaked during the trajectory,” the RFI explained.

“Furthermore, because these systems travel five to eight times faster than conventional systems, engine designs must be specially designed to ingest and combust air with the fuel at hypersonic velocities while maintaining consistent performance; all components of the engine must reliably survive the environment and operate with high precision in order to perform the mission,” the request adds.

For engine manufacturers, simply building these propulsion systems will be challenging. It will require “specialized equipment, materials, tooling, and designs in order to build novel inlet and combustor geometries, advanced fuel injection systems, high-performing fuels, effective thermal management systems, and durable engine structures such as nozzle throats, exit cones, and other supporting components,” according to the Air Force.

Breathing life into hypersonic missiles

hypersonic missile test — Members of the AGM-183A Air-launched Rapid Response Weapon Instrumented Measurement Vehicle 2 test team make final preparations prior to a captive-carry test flight of the prototype hypersonic weapon at Edwards Air Force Base, California, Aug. 8. (Air Force photo by Kyle Brasier)

Despite their complexity, air-breathing engines offer some significant advantages over conventional rocket engines or aircraft turbojet engines. A scramjet, for example, takes advantage of the missile’s high speed to compress air — at supersonic velocities — into the engine. This avoids the need to carry the oxidizers of a traditional rocket engine, or the heavy fans and other moving parts of a turbojet. For hypersonic vehicles that travel anywhere from Mach 5 to Mach 24, light weight and mechanical simplicity are highly desirable features.

“Air-breathing engines allow weapons to achieve longer ranges and increased payloads to the target,” the Air Force’s RFI noted. “These engine systems include ramjets, scramjets, combined-cycle engines, air-augmented rockets, and rotating detonation engines.”

The search for air-breathing engines comes after the Air Force just conducted its third successful test of a hypersonic missile. The Hypersonic Air-breathing Weapon Concept (HAWC) is an air-launched missile that uses a conventional rocket engine to accelerate the missile to a speed fast enough for the scramjet to kick in.

With Russia already having used hypersonic missiles in Ukraine – though apparently with disappointing results – and China developing them as well, the U.S. is in a race to catch up. But without a manufacturing base that can reliably produce air-breathing engines, America can’t win the contest.

Michael Peck is a contributing writer for Sandboxx and Forbes. He can be found on Twitter and LinkedIn.

Feature Image: An artist’s rendering of a hypersonic boost-glide vehicle (left) and a scramjet-powered cruise missile (right).