Related on TestTube:
What is Mach Speed
How Did a Human Survive Breaking the Sound Barrier?
In the 1880s, Austrian physicist Ernst Mach was studying the supersonic flow of gases when he was the first person to successfully photograph a bullet traveling faster than the speed of sound. What his images showed was a bullet with a shock wave in front of it and another one trailing behind it, clear proof of the phenomenon of compressibility. This discovery eventually led to the measurement that bears his name: Mach speed. Mach is followed by a number, which is the ratio of the speed of an object traveling through a gas to the speed of sound in that gas. So for example, Mach 6 (which is how fast the X-15 rocket flies), is six times the speed of sound.
We sometimes may forget, but air is made up of molecules, and like any other substance, air molecules can compress and expand. When they compress, they form shock waves. These shock waves make flying faster than the speed of sound--or "breaking the sound barrier"--a challenge. In the 1940s, pilots were coming up against this challenge as new aeronautic technologies were making planes fly faster until they could reach the speed of sound. Shock waves vibrate the air molecules they're made of, and they vibrate whatever is compressing them. In the case of an airplane whose speed approaches the speed of sound, the shock waves can be strong enough to rip it apart.
Engineers in the 1940s realized that if they were ever going to be able to fly faster than the sound barrier, they would have to build an aircraft that was both solid and fast enough to break through this "wall". The Bell Aircraft Corporation, working with the US Army Air Force, first devised the solution: taking inspiration from that supersonic bullet Ernst Mach first photographed, engineers designed the X-1 aircraft to break through that wall of compressed air in the sky and fly faster than the speed of sound.
But supersonic flight presented another challenge: the sonic boom. The sound an airplane makes when it smashes through the sound barrier continues as long as the plane travels at supersonic speed, not unlike the wake of a boat; this manifests as a loud bang to observers on the ground and follows in the wake of the aircraft. This is why the world's only supersonic passenger plane, the Concorde (which flew from 1976 to 2003) was only permitted to fly over oceans.
NASA Centers Team Up to Tackle Sonic Boom (nasa.gov)
"Since the Concorde's final landing at London's Heathrow Airport nearly a decade ago, commercial supersonic air travel has been as elusive as a piece of lost luggage. However, this hasn't stopped NASA from continuing the quest to develop solutions that will help get supersonic passenger travel off the ground once more."