U.S.-based air carrier United Airlines announced Thursday that it had reached an agreement with a Denver-based startup company for 15 supersonic aircraft the company said would fly 1.7 times the speed of sound using sustainable fuel and be 100% carbon neutral.In a press release, the airline said its deal with aerospace company Boom Supersonic has an option to buy 35 more planes once the company develops its first aircraft. The airline said this was the world’s first purchase agreement for net-zero carbon supersonic aircraft. Financial terms of the agreement were not disclosed.On its website, Boom Supersonic said its Overture aircraft would be capable of flying 1.7 times the speed of sound — about 2,092 kilometers per hour — more than twice the speed of conventional airliners. The company said the plane would carry a maximum of 88 passengers, cruise at an altitude of about 18,288 meters and fly on 100% sustainable, carbon-neutral fuel. It hopes to deliver the planes to United by 2029.The Overture is still in development. On its website, Boom Supersonic said it hoped to build and test fly the first plane by next year.Should the plane get off the ground as planned, it will be the first supersonic airliner to fly since the Concorde was retired in 2003. The luxury plane was introduced in 1976 and flew passengers across the Atlantic via Air France and British Airways.But because of expense, noise and environmental issues, the planes never really caught on. In 2000, an Air France Concorde crashed into a hotel shortly after takeoff from Paris, killing everyone on board and four people on the ground.

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