ATC Watcher
it was possible - if the total route was short enough. And you had ~60000 kilos of fuel to waste.
Key points:
- Concorde's speed was directly related to altitude - going subsonic required descending to FL400 or below. With a corresponding decrease in Mach/true airspeed. Very poor fuel efficiency below Mach 1.7 - couldn't hold speed without afterburner/reheat. Don't forget how much Concorde's flight profile and range absolutely depended on turning the engine/nacelle system into a ramjet from Mach 1.7-2.02 to work "commercially" at all. The old "at Mach 2.02 cruise, 85% of the thrust came from the nacelle" idea.
- to get
back to supersonic flight required repeating the whole climb-and-accelerate profile with AB/reheat fuel flowing by the tonne, until re-acquiring Mach 1.7 at FL400±.
which leads to:
Did the BAH to SIN route involve slowing down over India?
Nope. It was far more efficient to simply maintain Mach 2.02 and
bypass India (and Sri Lanka) to the south. Circling all the way around them while maintaining supersonic speed and altitude used
less fuel than: descend - slow to subsonic - cross India on a direct route - climb and accelerate back to supersonic.
You can google up some maps of Concorde routes (e.g. Paris-Dakar, Dakar-Rio). Actual routes, not airline "schematics." And see that it was almost
always preferable and more efficient to get out over an ocean ASAP and get the ramjet effect going at Mach 1.7+, and then
stay out over water as long as possible. Even if it meant an indirect "dogleg(s)" route covering more miles. Except for some intentionally "transcontinental" routes like KHI-CCU, Perth-Sydney, Dulles-Dallas.