How a Human First Broke the Sound Barrier

Before October 14, 1947, the general belief was that humans were simply not capable of traveling faster than approximately 343.2 meters per second, or Mach 1. Hence the term, born sometime a decade earlier: "the sound barrier."

A bullet or a cannon ball? Sure. But, it was thought, the transonic drag would cause severe instability to an airplane, and probably rip it and its pilot to shreds.

The experimental aircraft piloted on that day by 24-year-old Air Force test pilot Chuck Yeager, however, was designed specifically to test this theory. The  Bell X-1—nicknamed Glamorous Glennis, after Yeager's wife—had four rocket engines, thin, upswept wings, and a nose modeled after a .50 caliber bullet. Whereas fighter aircraft of the time were built to withstand 12 times the force of gravity, this was built to absorb 18 Gs.

It wasn't, however, designed for control.

"Basically the X-1 was a pure rocket," Yeager told NOVA in 1997. "It burned liquid oxygen and a mixture of five parts alcohol to one part water. You know, we'd been fooling around with jets. Jet engines didn't have the thrust to push the airplane into the region of the speed of sound or beyond."

He explained that the X-1 could only hold enough rocket fuel for two and a half minutes of flight. Thus, it had to be taken into the air by a  B-29 Superfortress and dropped into flight, at which point the rockets could fire and then burn out, and the plane would glide to safety on the desert floor.

But as with most experimental planes, safety was iffy. Col. Albert Boyd, in charge of the US Air Force's test pilots, originally sought a pilot who had no dependents. "I said, 'Yeah, I'm married and I, I've got a little boy, and I, I think that makes me more careful,'" Yeager said. "And that worked out. He said, 'Well, OK, be careful.'"

"I gave no thought to the outcome of whether the airplane would blow up or something would happen to me," Yeager said. "It wasn't my job to think about that. It was my job to do the flying."

Yeager's flight as depicted in "The Right Stuff" (1983)

Two nights before the flight, however, Yeager fell from a horse and broke two of his ribs. He was so worried about being removed from the mission that he forewent a doctor and instead visited a veterinarian in a nearby town for treatment. He kept the accident from his superiors and only told his wife and his friend and fellow project pilot Jack Ridley.

On the morning of the flight, Yeager was in such pain that he was unable to close the X-1's hatch by himself. So Ridley  left a broken-off broom handle in the cockpit, which allowed Yeager to seal the hatch.

Yeager and his plane were dropped from the Superfortress at 20,000 feet. At first, he stalled, but gained enough speed to get lift, and then switched on his rockets in sequence, accelerating to 0.88 Mach. The plane started shaking; the controls locked up momentarily. Yeager used the horizontal stabilizer, deactivated two of his rockets, and regained control. He fired the two rockets again, and, at over 42,000 feet, catapulted himself to a speed of 700 miles per hour, or 1,127 kpm. The needle on his Machmeter pushed past "1" and off the scale.

For about 20 seconds, Yeager enjoyed smooth supersonic flight at what was recorded as Mach 1.06. "There was no buffet, no jolt, no shock," he wrote later. "Above all, no brick wall to smash into. I was alive." He wanted to tell someone, anyone, that he'd just pierced the sound barrier. But to maintain secrecy, transmissions were restricted.

"Hey Ridley!" he recalled saying into the radio. "Make another note. There's something wrong with this Machmeter. It's gone completely screwy!" Ridley understood. "If it is, we'll fix it," he replied. "'But personally, I think you're seeing things."

To observers on the ground, the loud "ba-boom" sound that the X-1 made—a sonic boom—was the sound of the plane catching up with the sound waves it was emitting. Imagine these sound waves "piling up" in front of the aircraft as it accelerated; once it outran them, the resulting change in pressure was heard by those listening on the ground as an explosion, echoing across the California desert. (The intense build-up of pressures at this speed also creates clouds of water vapor around the wings.)

A newsreel from 1948

The news of Yeager's flight was kept secret until June, 1948. But once word got out, the race was on to find a new breed of men ready to travel by rocket, a mission dramatized by Tom Wolfe in his 1979 book  The Right Stuff. Yeager (portrayed by Sam Shepard in the 1983 film version of the book, and who had a cameo role as a bartender named Fred) would not join the group of test pilots who would become NASA's first astronauts, but he would break all sorts of other aviation records.

Meanwhile, speculation mounted that the barrier had already been busted by a manned flight earlier—by Air Force pilot George Welch during a high-speed dive, or by a test pilot in Nazi Germany. But Yeager's sonic boom was thought by the few people within earshot to be the first with a human attached to it.

Many speed demons would follow "the world's first supersonic man." The racing pilot Jackie Cochran was the first woman to break the sound barrier on May 18, 1953, in a Canadair Sabre, with Yeager as her wingman. As supersonic flight became more widely understood, the notion of "the sound barrier" continued to fade. Swept wings, aerodynamic design, and engines of ever-increasing performance would mean that many combat aircraft of the 1950s could routinely break the sound barrier in level flight, though not without some difficulty.

Supersonic flight is now relatively straight-forward for modern fighter jets, but attempts to bring Mach 1 to commercial flight have not been as successful. The Concorde, introduced in the 1970s, would be retired in 2003. It's also thought that a China Airlines 747 may have broken the sound barrier in an unplanned descent from 41,000 ft to 9,500 ft after an "in-flight upset" in 1985.

Land vehicles can also surpass Mach 1. On October 15, 1997—50 years and one day after Yeager's first supersonic flight—a rocket car called the ThrustSSC, for "Super Sonic Car," was the first, officially.

And in 2012, 65 years to the day after Yeager's record-setting flight, Felix Baumgartner would be the first human to break the sound barrier with his own body in his famous 840 mph Redbull-sponsored freefall. (Though Joe Kittinger may have beaten him to that title decades earlier.)

That same day, in Nevada, an 89-year-old Yeager, now an Air Force General, climbed into the backseat of an F-15 Eagle—nicknamed Glamorous Glennis III—and passed Mach 1 at more than 30,000 feet over California's Mojave Desert. Yeager reportedly flew the plane during take-off and landing. It was his last official Air Force flight.

Afterwards, when reporters asked what he made of Baumgartner's feat, he said he hadn't heard about it. A young girl asked him if he was scared. Yeager joked, "Yeah, I was scared to death."