United Airlines said it would buy ultra-fast jets from Denver-based aerospace company Boom Supersonic, bringing back supersonic passenger travel which died out with the retirement of the Anglo-French Concorde in 2003.

Under the agreement, the airline will purchase 15 of Boom’s “Overture” aircraft once they meet United’s safety, operating and sustainability requirements, with an option for an additional 35 aircraft.

The announcement comes less than two weeks after Aerion’s plans to produce a supersonic business jet faltered due to a lack of funding for production.

Supersonics have come under criticism from environmentalists for burning more fuel per passenger than comparable subsonic planes.

United, which has pledged to reduce its greenhouse gas emissions 100% by 2050, said the aircraft would be optimized to use 100% sustainable aviation fuel (SAF) from its entry to service, projected to be in 2029.