What is the speed of a supersonic aircraft?

"Supersonic" describes any flight above Mach 1, and since the speed of sound varies with temperature, the threshold in km/h depends on altitude: 1,225 km/h near the ground and 1,062 km/h above 11,000 m, where nearly all supersonic aircraft fly. From there the range is enormous. Engineers distinguish the supersonic regime proper, up to Mach 5, from the hypersonic regime beyond it, where aerodynamic heating and air chemistry change the rules. No manned air-breathing aircraft has clearly exceeded Mach 3.3; the rocket-powered X-15 reached Mach 6.7 in 1967, but it flew like a winged missile and landed by gliding.

Concorde, the only supersonic passenger aircraft with a long commercial career (1976-2003), cruised at Mach 2.02-2.04 at between 15,000 and 18,000 m: about 2,180 km/h, which cut the London-New York crossing to under three and a half hours. Its limit was set by nose temperature, which at that speed exceeded 120°C and stretched the fuselage by several centimeters. The Soviet Tupolev Tu-144, somewhat faster on paper (Mach 2.15), barely entered passenger service. Current supersonic business jet projects aim for more modest figures, around Mach 1.4-1.7, to cut fuel burn and sonic boom.

Most modern fighters have a structural or intake limit between Mach 1.8 and Mach 2.5: the F-16 and the Eurofighter are around Mach 2; the F-15 and the MiG-31 reach Mach 2.5 and 2.8 respectively. In practice they fly supersonic only briefly, because afterburner use sends fuel consumption soaring; the F-22 and the Eurofighter can sustain a "supercruise" around Mach 1.5 without it. The extreme was set by the Lockheed SR-71 Blackbird, which in 1976 recorded 3,529.6 km/h over a timed course, Mach 3.3, and routinely cruised at Mach 3.2 above 24,000 m thanks to engines that at that speed functioned largely as ramjets and a titanium structure.

Each speed bracket demands different solutions. Up to Mach 2, a simple intake and swept or delta wings are enough; beyond that a variable-geometry intake is needed to slow the air to subsonic speed before the compressor, and at Mach 3 aluminum loses strength from heat, forcing the use of titanium or steel. That is why each additional Mach costs far more than the last, and except for reconnaissance and interception, designers settle for Mach 2.