Boom Supersonic, the Colorado startup at Centennial Airport that’s been developing a vision of a new supersonic airliner, is parting ways with Rolls-Royce, the jet engine manufacturer that it had listed as a partner on the project.

Rolls-Royce had built the Olympus engines that powered the Anglo-French Concorde, the supersonic jetliner that resembled Boom’s sleek, 4-engine design. The Concorde left service in 2003.

“After careful consideration, Rolls-Royce has determined that the commercial aviation supersonic market is not currently a priority for us and, therefore, will not pursue further work on the program at this time,” the company said in a statement last week, noting that its contract with Boom was complete.

“The engine is the linchpin,” said Jon Ostrower, editor-in-chief of the aviation journal The Air Current, mirroring some doubtful remarks from commentators on Boom’s prospects to stay on schedule following the separation.

Boom, which has announced it will fly the first Overture by 2026, had achieved wide acclaim for its design – a little smaller and slower than Concorde, but slated to cross from New York to London in three-and-a-half hours, carrying 65 to 80 passengers.

Last month, American Airlines announced it was making a nonrefundable deposit on up to 20 Overtures, and United Airlines had previously committed to 15 supersonics. Defense aviation contractor Northrup Grumman had joined in a partnership for “special mission” variants of Overture.

Boom continually stresses Overture’s environmental features, saying it would fly 100% carbon neutral by burning sustainably produced aviation fuel rather than conventional, petroleum-based jet fuel.

Those and other claims have drawn doubts from some experts, but the question of obtaining a suitable engine is seen as a daunting hurdle for the aircraft.

In an interview last month with The Denver Gazette, Boom CEO Blake Scholl stressed that a selling point of Overture is that it relies on off-the-shelf technology that’s been proven on current subsonic airliners. “All the technology we need has been tested and used on other aircraft,” he said.

But aviation experts say a power plant that can push even a smaller airliner past the speed of sound requires a leap beyond current engines.

Boom released a statement timed with Rolls’ message. “We are appreciative of Rolls-Royce’s work over the last few years, but it became clear that Rolls’ proposed engine design and legacy business model is not the best option for Overture’s future airline operators or passengers.

“Later this year,” it continued, “we will announce our selected engine partner and our transformational approach for reliable, cost-effective and sustainable supersonic flight.” Boom offered no additional clarification despite inquiries from The Denver Gazette.

There are three other major engine manufacturers, including GE Aerospace, and CFM, a consortium owned by GE and French aerospace giant Safran. GE had been designing an engine for Nevada-based Aerion Supersonic before Aerion recently shut down operations. Safran Landing Systems is listed as a partner on the Boom Overture project.

Boom, with 250 employees and contractors, says it will test a “Baby Boom” mini-supersonic in California before year’s end and is planning a 70,000-square-foot testbed for a full-scale Overture aircraft for a site near Centennial Airport.