How do supersonic planes work?

Below the speed of sound, air ahead of an aircraft is warned of its approach by pressure waves and flows smoothly around it. Above Mach 1 the aircraft outruns its own pressure signals, and the air is deflected abruptly at shock waves attached to the nose, the intakes, the wing leading edges and the tail. Each shock costs energy, which appears as wave drag. A supersonic design begins by making those shocks as weak and as few as possible: a sharp nose, thin wings with sharp leading edges, and sweep or a delta planform so the leading edges sit inside the nose Mach cone. The fuselage is kept slender and its cross-sectional area varies smoothly from nose to tail, following the area rule that Richard Whitcomb formalised in the 1950s.

A jet engine's compressor cannot accept supersonic flow. Every supersonic aircraft therefore has an intake that decelerates the air to roughly Mach 0.5 before it reaches the fan or compressor face, and does so through a controlled series of oblique shocks rather than one violent normal shock. On Concorde this was done with hydraulically moved ramps inside rectangular intakes, which at Mach 2 delivered a large share of the net thrust simply by compressing the incoming air; the SR-71 used a movable centre-body spike in each nacelle. Getting the intake wrong causes an unstart, a violent loss of thrust as the shock pattern pops out of the duct.

Thrust itself comes from turbojets or low-bypass turbofans, usually with an afterburner that burns extra fuel in the exhaust for the push through the transonic drag rise. A few aircraft, such as Concorde and the F-22, can then cruise supersonically without reheat, which is far more fuel-efficient.

At Mach 2 the air compressed against the skin heats it to well over 100 °C; Concorde's nose reached about 127 °C and the airframe stretched by around 20 cm in cruise. Aluminium alloys tolerate roughly this limit; faster aircraft such as the SR-71 used titanium. Speed also moves the centre of lift rearward, which would pitch the nose down. Concorde solved this by pumping fuel between forward and aft tanks to shift the centre of gravity with it, rather than by carrying the drag of a permanent trim deflection. Control surfaces lose authority as shocks form over them, so supersonic aircraft use all-moving tailplanes or elevons that move the whole surface.

The shock waves trail behind the aircraft to the ground as a sonic boom, which is why civil supersonic flight over land has been banned in most countries since the 1970s. NASA's X-59 is testing whether a carefully shaped airframe can turn the boom into a soft thump; if it can, the rules may follow.