How far is the sound barrier?

The phrase suggests a wall somewhere in the sky, but the sound barrier is a speed: the point at which an aircraft's airspeed equals the local speed of sound, Mach 1. That speed depends on air temperature. In the standard atmosphere it is 340 m/s, or 1,225 km/h (661 kn), at sea level and 295 m/s, or 1,062 km/h (573 kn), at 11 km where the air is at −56.5 °C. So the barrier is 'closer' at altitude, which is one reason high-speed record flights and airliner cruise both happen high up. There is no distance to it, only a margin: an Airbus A350 or Boeing 787 cruising at Mach 0.85 is about 160 km/h short of it, and will never go faster because its wing is not designed to.

The expression dates from the late 1930s and 1940s. As fighters dived toward Mach 0.8, air over their wings went locally supersonic, shock waves formed, controls froze or reversed and drag rose so sharply that some aircraft were lost. Early wind tunnels choked at these speeds and gave no useful data, so the transonic region looked like a wall. In fact the drag rise is a hump, not a barrier, and had been crossed by artillery shells and bullets for a century. The task was to build an aircraft strong enough, with enough thrust and with controls that still worked, to push through the hump.

On 14 October 1947 Chuck Yeager flew the rocket-powered Bell X-1, dropped from a B-29 over the Mojave Desert, to Mach 1.06 at about 13,000 m (43,000 ft). The X-1 had a fuselage shaped like a .50-calibre bullet, thin straight wings and an all-moving tailplane, the feature that solved the control problem and was later copied onto the F-86 and every supersonic aircraft since. Within a decade fighters such as the F-100 exceeded Mach 1 in level flight, and by 1976 Concorde carried passengers at Mach 2.

Inside a supersonic aircraft there is no bump at Mach 1; the Mach meter simply passes it, with a small trim change as the centre of lift moves aft. The dramatic part happens outside: the shock waves trail to the ground as a sonic boom, and in humid air a condensation cloud can flash around the airframe as the pressure drop behind the shocks chills the air. That cloud, often photographed on aircraft at airshows, appears at high subsonic speed as well and is not itself proof that the barrier has been crossed.