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Boom Supersonic has finally made a crucial step toward developing a new supersonic passenger plane.

The aerospace startup said Tuesday that it had signed an agreement with three different companies to develop an engine for its planned Overture aircraft.

Dubbed “Symphony,” the engine that would power the aircraft still has a long way to go. The engine’s design is still to come, and Boom said it would turn to Florida Turbine Technologies to help develop it. FTT is one of three firms Boom has turned to for the engine, saying it has also tabbed GE Additive “for additive technology design consulting” and StandardAero for maintenance.

News of the agreement comes just more than three months after Rolls-Royce backed out of a collaboration with Boom on a conceptual study for the program, withdrawing itself from consideration to be the engine maker.

“Rolls-Royce has determined that the commercial aviation supersonic market is not currently a priority for us,” the company said at the time. Boom said in a statement that it would announce an engine supplier by year’s end.

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Since then, the status of Boom and the Overture program has been in doubt. The company unveiled a refreshed quad-jet design for the airframe during the Farnborough Airshow this summer, but critics were quick to point out that without an engine, any aircraft renderings were only conceptual at best. Some at the time cited comments that referred to Boom as the “Theranos of aviation,” referring to the fraudulent medical testing company founded by Elizabeth Holmes — a comparison that CEO Blake Scholl roundly rejected.

Boom aims to build and certify its Overture jet by the end of the decade, and says that it can make supersonic air travel economically and environmentally sustainable. The company has received orders from several airlines, including American Airlines and United Airlines in the U.S.

However, detractors have questioned several of Boom’s claims, pointing out that the Overture would suffer the same limitation as the Concorde: Due to the ultraloud sonic boom created when traveling faster than sound, the plane would be restricted to slower speeds near overpopulated areas. This would effectively mean that it could not go full speed over land. Because its expected range means a fuel stop would be necessary on flights across the Pacific (thus negating the time savings of supersonic flying), the aircraft’s advantages would be largely restricted to the transatlantic market.

Whether Boom will ultimately succeed remains unclear, but the agreement with an engine maker nevertheless represents a major step toward a first build and successful flight.

“Developing a supersonic engine specifically for Overture offers by far the best value proposition for our customers,” Scholl said in a statement. “Through the Symphony program, we can provide our customers with an economically and environmentally sustainable supersonic airplane—a combination unattainable with the current constraints of derivative engines and industry norms.”