What Breaks When You Break the Sound Barrier? A Wall Made of Pressure

An object moving through air pushes it aside, and the information that something is coming travels ahead as pressure waves at the speed of sound, allowing the air to begin moving out of the way. As the object approaches that speed, the waves have progressively less head start and pile up closer together in front of it, so the air is compressed abruptly rather than gradually, forming a shock. At the shock, pressure, temperature and density change almost instantaneously across a very thin region, which alters the forces on the object substantially. The speed of sound itself is not a constant, varying with temperature and therefore with altitude, which is why aircraft speeds in this regime are quoted as a ratio to the local speed rather than in absolute terms.

Aircraft approaching the speed encountered several effects at once:

• •A sharp rise in drag, which early engines could not overcome
• •Loss of lift as shocks formed on the wing and disturbed the airflow behind them
• •Severe buffeting from unsteady separated flow
• •Control surfaces becoming ineffective or reversing in effect, since the shock sits ahead of the hinge
• •The centre of lift shifting backwards, forcing the nose down
• •Several aircraft lost in dives during the 1940s, which produced the idea of an impassable barrier

The solutions were aerodynamic and structural rather than a matter of brute power. Thin wings with sharp leading edges delay shock formation and reduce its severity. Sweeping the wing backwards means the airflow component crossing it is slower than the aircraft's speed, which postpones the problem, a principle worked out in Germany in the 1930s and applied widely after the war. All-moving tailplanes restored pitch control where conventional hinged elevators had failed. Area ruling, which shapes the fuselage so the total cross-section of the aircraft changes smoothly along its length, substantially reduces transonic drag and is why many aircraft of the period have a waisted fuselage. Powered controls replaced direct linkages, since the forces involved exceeded what a pilot could apply.

The first confirmed supersonic flight by an aircraft took place in October 1947, when a rocket-powered research machine dropped from a bomber exceeded the speed in level flight over California, and the details are instructive about how the problem was actually solved. The aircraft was shaped deliberately like a bullet, since bullets were known to be stable above that speed. It had very thin wings. Crucially it had an all-moving tailplane, added because conventional elevators had proved useless in the transonic range, and that feature is generally credited as the decisive one. The programme was one of a series of research aircraft built purely to gather data in regimes nobody understood, flown at considerable risk, and the flight itself was kept secret for months. The pilot flew it with two broken ribs after a riding accident, which he concealed to avoid being grounded.

The noise heard on the ground is continuously produced rather than made at the moment of transition, which is the most common misunderstanding. A supersonic aircraft trails a cone of shock waves behind it, and that cone sweeps across the ground as the aircraft passes, so anybody within the affected corridor hears the pressure change as a sharp double crack while the aircraft continues normally. It is not a single event at the moment of exceeding the speed and it does not stop afterwards. The overpressure is enough to startle people and occasionally to break glass, which is why supersonic flight over land is restricted in many countries, a restriction that was central to the commercial failure of supersonic passenger transport and that current low-boom aircraft programmes are trying to work around.

Pressure waves that normally warn the air to move aside pile up as the speed of sound is approached, forming a shock across which pressure and temperature change almost instantly. Drag rose, lift fell, buffeting set in and controls reversed, which is what made the barrier real as an engineering problem. Thin swept wings, all-moving tailplanes and area ruling solved it.