There are folks here who can correct me, but in the meantime, what I think I know is....
The DC-Dallas route, entirely over populated land, could not be flown at supersonic speeds (regulations, noise pollution, sonic booms), but Concorde could do it in high-subsonic cruise at around Mach 0.95, somewhat faster than the norm for regular subsonic transports.
I believe the DC-MIA route was flown mostly supersonically, by climbing subsonically at Mach 0.95 straight down the Potomac to the Atlantic at Norfolk, Va., and then, 20+ miles offshore, turning SW towards Miami and making the supersonic acceleration-climb out over the water. Remained offshore (dodging the coastal bulge of the Outer Banks) until about 250nm from Miami. where the descent/deceleration phase would slow it to subsonic speed before getting too close to the shoreline.
Once at ~28,000 feet at Mach .95 - and over the water - it only took a few moments, after turning on the reheat/afterburners, to punch through Mach 1, and maybe 20 minutes (depending on weight) to reach 51000 feet* and Mach 2.02 (air termperature permitting.) And maybe 20 minutes for the deceleration/descent to Mach 0.95 at ~34000 feet.
(*I believe the afterburners were switched off at Mach 1.7 - usually about 42000 feet? - at which point the dry thrust of the engines and fancy shockwave-pressurized nacelle design could maintain the IAS and (reduced rate) climb (and increase the Mach) all by themselves.)
Across the Pond, short "experience flights" from both Paris and London were made from time to time - get out over the Atlantic, light up the afterburners, and tool around at supersonic speeds for some part of an hour before returning to base.
I'm pretty sure subsonic flight was never really efficient at any speed. Concorde was dependent on Mach 1.7 or so (and high altitudes) to maintain the efficiency of nacelle thrust modulated by supersonic intake shockwaves, without very thirsty afterburners. I think that over the Atlantic, losing just one engine (25% of thrust) was enough to make it instantly a fuel emergency situation - you were going to come down into thicker air and fuel burn would skyrocket.
Last edited by pattern_is_full; 26th Jan 2023 at 03:55.