The speed of sound is somewhere around 340 meters per second (roughly 760 miles per hour). As something gets close to that speed, it starts to experience increased aerodynamic drag, which makes accelerating significantly harder—this is the sound barrier. So, who was the first human to exceed it? Let’s find out.
What Is The Sound Barrier?
The sound barrier is a bit of a misnomer. It’s not a set line that’s impossible to cross. Instead, anything that moves in the range of speeds close to it (transonic speeds) will experience increased aerodynamic drag. The speed of sound is measured in Mach numbers. Mach 1 corresponds to the local speed of sound (as we see in a second, speed of sound changes); Mach 0.5 is half the speed of sound, Mach 2 is double it, and so on. Anything less than Mach 0.8 is safely subsonic, and anything over Mach 1.3 is supersonic. The range from Mach 0.8 to Mach 1.3 is transonic, and it’s where the drag effects of the sound barrier occurs.
The local speed of sound depends on the temperature of the air and its composition. At 68 ºF in dry air, it’s 343 meters per second. Go higher, and the air gets cooler and the speed of sound actually gets slower. At 11,000 meters (the cruising altitude of commercial jets), the air is around -70 ºF, so the speed of sound is about 295 meters per second (or 660 miles per hour). This means that the transonic range varies, too, although it always occurs at the same ratio to the local speed of sound—about Mach 0.8 to Mach 1.3.
What about Humans Breaking the Sound Barrier?
Now, let’s talk about humans getting to transonic speed. Human-powered objects have been doing it for millennia—the crack of a bullwhip is the tip creating a small sonic boom as it crosses the sound barrier—but the first people to get close were World War II pilots.
In dives, propeller aircraft, such as the Mitsubishi Zero, Supermarine Spitfire, and Lockheed P-38 Lightning, could all get close enough to the transonic range to experience problems. It led to plenty of crashes where the increased aerodynamic pressure on the control surfaces made it impossible for the pilot to pull out of the dive, although aircraft manufacturers mostly solved the issues with later models.
During WWII, some pilots claimed they’d broken the sound barrier in a dive, although their reports aren’t considered very credible. Airspeed indicators aren’t accurate in the transonic range, and the planes they were using generally started to experience serious problems above Mach 0.85. For example, every flight over 0.84 the P-51 Mustang flew caused vibration damage to the aircraft. A Spitfire taken to Mach 0.92 was forced to land after the engine was damaged by over-revving. It’s possible that some fighter pilot broke Mach 1.0 in a propellor plane a during dive, however, it’s just that they died doing it.
The first credible person who may have broken the sound barrier was Lothar Sieber, a test pilot in the Luftwaffe. He was flying a Bachem Ba 349 Natter, an experimental vertical takeoff rocket-powered plane—in other words, he was sitting on top of a missile. The flight lasted just 55 seconds, and he flew almost 9 miles before crashing into the terrain and dying. While the flight speed wasn’t tracked, it’s the first instance of someone piloting an aircraft undeniably capable of exceeding Mach 1. Unmanned missiles, like the German V-2, were by then hitting Mach 4.
During the war, the British and U.S. militaries researched high-speed supersonic planes, but it wasn’t until after the war that their efforts paid off. Bell Aircraft company developed the Bell X-1 based on the British Miles M.52. It was in this plane that the first human broke the sound barrier in level flight.
Okay, So Who Really Broke The Sound Barrier?
On the 14th of October, 1947, Air Force Captain Charles “Chuck” Yeager climbed into a Bell X-1. Rather than taking off from a runway, the rocket-powered plane was drop-launched from the bomb bay of B-29 bomber. It was flight number 50 for the experimental aircraft, and for the first time it achieved its aim—supersonic flight. Yeager reached a top speed of Mach 1.06 (361 meters per second or 807 miles per hour), becoming the first person in history confirmed to break the sound barrier.
Alas, that “confirmed” still needs a little explaining. Two other unconfirmed contenders exist for the first person to break the sound barrier. Yeager himself believes he achieved it in the Bell X-1 in flight number 49, when he reached a recorded top speed of Mach 0.997. (The speed indicator wasn’t accurate to that level of decimal places.) Also, George Welch claims to have broken the sound barrier on the 1st of October, 1947, and again on the 14th, 30 minutes before Yeager did it, in an XP-86 Sabre. The Sabre would later officially break the sound barrier, and witness reports and instrument readings suggest Welch achieved supersonic speeds. However, because the flight speeds weren’t officially observed, Yeager gets the nod.
Two other sound barrier breaking records are of note:
- On the 15th of October, 1997, Andy Green broke the sound barrier and set the land speed record in a vehicle that met the Fédération Internationale de l’Automobile’s definition of a car, when he reached 763 miles per hour in Black Rock Desert, Nevada.
- On the 14th of October, 2012, Felix Baumgartner became the first human to break the sound barrier unpropelled when he set the world record for the highest sky dive, jumping from 24.26 miles and reaching a top speed of 833.9 miles per hour—Mach 1.26.