How do you go supersonic without a boom?
It involves reshaping the aircraft, especially the nose, to weaken the shock wave. You can never completely eliminate the shocks, but they might be reduced enough to become tolerable -- quieter and more like distant rolling thunder than a single sharp bang. NASA has tested a design that reduces boom strength by 40% on fighter jets, but I have my doubts that large supersonic airliners will ever be able to break the sound barrier over populated areas: a 50,000 pound fighter is a far cry from a 750,000 pound SST.
the initial question, Concorde's subsonic range was about 3,300 miles, if memory serves. GDB would know for sure. Being pure turbojets designed for supersonic cruise, the Olympuses were not great performers in subsonic cruise and were quite inefficient (loud and gas-guzzlers) at takeoff and landing.
Could it have gone supersonic twice during the same flight without burning too much fuel?
Probably not. The acceleration and climb from 30,000 feet and Mach .9 to 50,000 feet and Mach 2 consumed an enormous amount of fuel: remember that the afterburners were lit until Mach 1.7. I doubt Concorde would ever have had enough fuel onboard to do it twice in a normal flight.
Is it a possible engineering achievement?
Possible, yes. Economical, not yet. Off the top of my head, there are two significant design elements that will boost subsonic efficiency:
- Variable-bypass engines act like high-bypass turbofans at takeoff, subsonic cruise, and landing, reducing noise and fuel consumption, and then "change gears" to act like pure turbojets or low-bypass fans at supersonic cruise to provide the necessary exhaust velocity.
- Variable-sweep wings adjust the aerodynamic profile to fit subsonic and supersonic requirements more closely. Delta wings are fine at supersonic speeds, but create enormous drag at slow speeds and aren't wonderful at subsonic speeds. Folding the wings back into a delta or arrow configuration at supersonic speeds achieves the best of both worlds.
Both elements are heavy, expensive, and very complex, but could bring an SST within striking distance of some subsonic airliners for cruise efficiency over land.
Keynes is dead and we are living in his long run.