How does the sound barrier work?

At ordinary speeds an aircraft announces itself. Pressure disturbances run ahead of it at the speed of sound, and the air begins to part before the nose and wing arrive, flowing smoothly around them. As the aircraft approaches the speed of sound, that warning has less and less time to work. Air arrives at the surfaces before it can adjust, and instead of easing aside it is compressed abruptly across a very thin region called a shock wave. Across a shock the pressure, temperature and density jump almost instantly while the flow slows. The first shocks appear not at Mach 1 but earlier, typically around Mach 0.8, because air accelerating over a curved wing top is already locally supersonic even when the aircraft as a whole is not.

Shock waves cost energy, and the aircraft pays for it as drag. Between roughly Mach 0.8 and Mach 1.2, the transonic range, drag can multiply several times over as shocks form, strengthen and move across the wings and fuselage. Behind a shock the boundary layer often separates from the surface, causing buffet and a loss of lift. The centre of lift moves rearward, pitching the nose down in a tendency called Mach tuck, and control surfaces sitting behind a shock can lose their effect because the air reaching them has already been disturbed. Propeller-driven aircraft of the Second World War met these effects in high-speed dives, and several were lost, which is where the idea of a solid barrier came from.

The barrier turned out to be a matter of thrust and shape rather than an absolute limit. Jet and rocket engines supplied thrust that kept increasing at high speed, unlike propellers whose tips went supersonic first. Thin wings and sharp noses delay and weaken the shocks; sweeping the wings back lowers the speed the air sees across the chord and pushes the drag rise to a higher Mach number. An all-moving tailplane, rather than a hinged elevator, keeps pitch control through the transonic range. The area rule, discovered in the 1950s, showed that smoothing the way an aircraft's total cross-section changes along its length cuts transonic drag sharply, which is why some supersonic designs have pinched fuselages. Once past Mach 1 the shocks settle into a stable pattern and the flight becomes calmer again, though the drag remains higher than in subsonic cruise.