Two decades later, some start-up companies, including Boom and Spike Aerospace, are pushing ahead with new designs and plans.

Boom, which is working with Rolls-Royce, the British jet engine maker, said its plane would be more efficient than the Concorde; United estimates it will be 75 percent more efficient. Boom’s planes will not be as noisy as the Concorde because their engines will create a sonic boom only when flying over water “when there’s no one to hear it,” said Boom’s chief executive, Blake Scholl, who previously worked at Amazon and Groupon.

In recent years, many people have also grown increasingly concerned about air travel’s contribution to climate change. Supersonic jets are expected to use more fuel than regular jets per passenger per mile, according to experts.

Mr. Scholl said the engines on Boom’s planes would rely entirely on sustainable aviation fuel, which can be made from waste, plants and other organic matter. Experts say such fuel could reduce emissions, but its supply is limited, it is expensive and its use does not eliminate greenhouse gas emissions.

United said it was too early to know how much it would charge for the flights, which it would run out of its hubs in Newark and San Francisco to start. But another big question mark about the plane is how many people will be willing to spend the thousands of dollars that each ticket on a supersonic flight is likely to cost.

United has long focused on business travelers, including by adding flights to Israel, China and other destinations popular with executives and by offering more business class seats on its planes. Mr. Leskinen called the idea of supersonic travel a “really powerful tool for business.”

“You can have a business meeting and still be home to have dinner with your family,” he said.

But corporate and international travel is expected to rebound slowly from the pandemic, and some experts say it might not recover fully for years because companies have realized that they can be effective without as many in-person meetings.

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