How odd that two of the most significant technological achievements of our lifetimes both devolved in part from the arcane science of ballistics: not only Mach’s work on supersonics but the development of computers in the 1940s to create precise artillery firing charts for gun-laying.
Nonetheless, Mach wouldn’t be pleased that he’s universally remembered as a result of that virtually inconsequential experimental dalliance rather than for his work as a psychophysiologist, his criticism of classical physics and mechanics, or his contributions to Einstein’s theory of general relativity. Who remembers Mach’s band? (Not a ragtime group but a phenomenon that relates the physiological effect of spatially distributed light stimuli to visual perception.) Anybody you know been quoting the Mach principle? (Einstein’s term for Mach’s claim that the inertia of an isolated body can have no meaning.) Who gets any crossword puzzle mileage out of Mach angle? (The actual object of his supersonics research—the angle between a shock wave and the direction of motion of the object creating the wave.
No, its “Mach number” that will go down through the ages as the legacy of this stubborn, brilliant, and multi-talented Czech scientist-philosopher.
But don’t feel sorry for him. Save your laments for Jacob Ackeret. Remember him? He was the director of the Institute of Aerodynamics at the Swiss Federal Institute of Technology, and in 1929 he suggested the term “Mach number” for the ratio of the speed of an object to the speed of sound in the medium within which the object is traveling. If he’d had a good PR guy, we might today be quoting Ackeret numbers, discussing Ackeret-2 Concordes and determining critical Ackeret.
The Sonic Boom
They don’t call them “booms” for nothing. If you’re imagining rolling thunder, think instead of a cherry bomb in a trash can. Given the right combination of atmospheric conditions, altitude, and airplane—particularly its weight, shape, and maneuvering configuration—the sudden arrival of a shock wave can sound like trains colliding. Oddly enough, one parameter that matters little is speed: Above a certain Mach number, a sonic boom can actually weaken as the airplane goes faster.
The first human-made sonic booms (supersonic bullets and artillery shells aren’t heavy enough to create the phenomenon) were trailed by German V-2 rockets reentering the atmosphere, rattling English windows far below as the missiles headed London-ward to do far worse.
In 1949 sonic booms were still rare enough that the San Francisco sheriff’s department dashed about trying to find the source of mysterious “explosions” reported by worried suburban residents. Turns out that the NACA’s nearby Ames Laboratory had just acquired an early F-86 Sabrejet, the first production aircraft that could go supersonic, and two NACA test pilots were routinely doing just that.
The British soon discovered that sonic booms were so spectacular you could build entire airshow routines around them. The Brits had experimented with using booms as weapons, maneuvering an aircraft to “throw” the thunder at structures. Concluding that breaking the enemy’s dishes really wouldn’t change the course of the battle, Royal Air Force pilots nonetheless perfected the technique of pitching the sound at the crowd during the then annual Farnborough Show, and it got so the size of your boom was a greater measure of pilothood than wristwatch complexity.