If the airline can follow through with its plan, these would be the first commercial supersonic flights since the grounding of the Concorde jet in 2003. The economics of that fuel-guzzling supersonic jet and restrictions on flying it over land at speeds that would produce a sonic boom doomed the Concorde. Airlines had to try to fill the plane with passengers willing to pay a substantial premium over a first-class ticket on traditional aircraft. United is buying the supersonic jet