Backfire, don't forget that the SUBSONIC version of Boeing's Sonic Cruiser was supposed to be as economical as a 767. I'd imagine the supersonic version would burn more fuel but there are many who say it would actually be more efficient than the .98 mach version, being out of the transonic buffer zone. Airbus believes that, though they haven't as yet done serious R&D on it. With the right configuration and engines, speed AND relative fuel efficiency may not be mutually exclusive concepts. Boeing's SC
based SST concept would be a twin and would get to its' top speed, say Mach 1.2-1.4 WITHOUT afterburners, perhaps Concorde's main liability, since it was relatively efficient in cruise at Mach 2 with the burners off. Much work would need to be done but there's the promise here of a relatively efficient and environmentally friendly SST that might be pursued if the air travel market recovered sufficiently to demand it. Despite the obstacles, speed remains an enticing goal.