How do aircraft break the speed of sound?
Supersonic aerodynamics are much more complex than subsonic aerodynamics for a variety of reasons, the foremost being breaking through the transonic envelope (around Mach 0.85-1.2). This is because to pass through this speed range supersonic jets require several times greater thrust to counteract the extreme drag, a factor that raises two key issues: shockwaves and heat.
Shockwaves come from the passage of air (with positive, negative or normal pressures) around the fuselage, with each part of the aircraft affecting its progress. As such, while air is bent around the thin fuselage with minimal effect, as it reaches the wings – a huge change in the cross-sectional area of the jet – it causes shockwaves along the plane’s body. The resulting waves formed at these points bleed away a considerable amount of energy, and create a very powerful form of drag called wave drag.
To mitigate this, any supersonic jet design must allow for a smooth-as-possible change in cross-sectional areas, with the wings fluidly curving out from the fuselage. Heat is the other big concern. Sustained supersonic flight – as a by-product of the drag it generates – causes all of its materials to experience rapid and prolonged heat, with individual parts sometimes reaching in excess of 300 degrees Celsius (572 degrees Fahrenheit).
As such, conventional subsonic materials like duraluminium (or dural) are infeasible for a supersonic jet, as they experience plastic deformation at high temperatures. To counter this, harder, heat-resistant materials such as titanium and stainless steel are called for. However, in many cases these can push up the overall weight of the aircraft, so reaching a workable compromise between heat resistance and weight is the key.