<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Track:
Does the Concorde fly at 52,000' then to get out of the way of slower planes, or is it more efficient up there?</font>
Concorde doesn't fly at 52,000', it cruise climbs reaching nearly 60,000', depending on weight, prior to starting it's descent. At lower altitudes it step-climbs until climbing above other traffic. At that point ATC gives them a block altitude allowing them to cruise-climb slowly throughout the flight staying at the most efficient altitude for it's current weight.
A supersonic airplane's speed is limited by skin temperature. Higher altitudes have thinner air which produces lower skin temperatures.