economic obstacles and one fatal Concorde crash prevented them from being used to their full commercial potential. Design principles Supersonic flight brings with it substantial technical challenges, as the aerodynamics of supersonic flight are dramatically different from those of subsonic flight (i.e., flight at speeds slower than that of sound). In particular, aerodynamic drag rises sharply as the aircraft passes the transonic regime, requiring much greater engine power and more streamlined airframes. Wings To keep drag low, wingspan must be limited, which also reduces the aerodynamic efficiency when flying slowly. Since a supersonic aircraft must take off and land at a relatively slow speed, its aerodynamic design must be a compromise between the requirements for both ends of the speed range. One approach to resolving this compromise is the use of a variable-geometry wing, commonly known as the "swing-wing," which spreads wide for low-speed flight and then sweeps sharply, usually backwards, for supersonic flight. However, swinging affects the longitudinal trim of the aircraft and the swinging mechanism adds weight and cost. Use of a delta wing, such as those used on the Aerospatiale-BAC Concorde generates a vortex which energises the flow on the upper surface of the wing at high speeds and attack angles, delaying flow separation, and giving the aircraft a very high stall angle. It also solves the issue of fluid compressibility at transonic and supersonic speeds. However, it is, of course, inefficient at lower speeds due to the requirement of a high angle of attack, and therefore need the use of flaps. Heating Another problem is the heat generated by friction as the air flows over the aircraft. Most subsonic designs use aluminium alloys such as Duralumin, which are cheap and easy to work but lose their strength quickly at high temperatures. This limits maximum speed to around Mach 2.2. Most supersonic aircraft, including many military fighter aircraft, are designed to spend most of their flight at subsonic speeds, and only to exceed the speed of sound for short periods such as when intercepting an enemy aircraft. A smaller number, such as the Lockheed SR-71 Blackbird reconnaissance aircraft and the Concorde supersonic airliner, have been designed to cruise continuously at speeds above the speed of sound, and with these designs the problems of supersonic flight are more severe. Engines Some early supersonic aircraft, including the first, relied on rocket power to provide the necessary thrust, although rockets