Tinstaafl
Can't fly subsonic below FL450?
Well, we always tried to on landing, but I think I know what you mean!
That's its equivalent of a low speed buffet boundary at that level?
Yes, in effect. Using the more traditional 1.3V
S as a minimum speed in cruise would not be particularly relevant or useful on Concorde, and in fact V
S is difficult to determine.
Instead, Concorde uses a speed called V
LA (Lowest Authorised Speed) as her minimum speed for any stage of flight, and at FL410 her V
LA is 300 kts IAS, which means that she must be at or above M1.0 to fly above FL410.
In practice, passing FL450 in a climb she would be nearer M1.8, with an IAS of 530 kts, some 230 kts above her minimum speed, not a bad margin!
Regards
Bellerophon