Austrian daredevil Felix Baumgartner is set to break Joe Kittinger’s high altitude jump record today by sky diving from 120,000 feet.

But that’s not all. He’s going to break the sound barrier on his way down, and without the benefit of an aerodynamic casing like an aircraft fuselage. It’s fitting that Baumgartner’s supersonic jump comes very close to the 65th anniversary of the day Chuck Yeager became the first man to fly faster than the speed of sound — that is, faster than Mach 1. But the conditions of both men’s supersonic achievements are very different.


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The sound barrier, as it’s called, isn’t really a barrier. It’s the term that describes the building up air in front of and around something as it approaches the speed of sound.

As an aircraft approaches the speed of sound, air can’t move out of the way fast enough so it piles up in front of the wing increasing drag. But as the air traveling over the top of the wing reaches the speed of sound, it forms shock waves that move back and forth, disrupting airflow and causing the aircraft to buffet and shake.

The speed required to fly faster than Mach 1 varies with altitude because it’s dependent on air pressure and temperature. To exceed the speed of sound at 0 feet altitude, you’d have to drive a rocket car 761 miles per hour.

At 120,000 feet, Mach 1 is about 710 miles per hour. But at 43,000 feet, Mach 1 is equivalent to 660 miles per hour, and that’s how fast Chuck Yeager flew the X-1 when he became the first man to exceed Mach 1 in level flight on October 14, 1947.

The intense shaking and buffeting pilots experienced at high speeds caused many aircraft to fail. In the post-World War II aviation years, figuring out how to fly safely at speeds approaching and eventually in excess of Mach 1 became a serious problem in need of a solution. Unfortunately, wind tunnels didn’t help engineers solve the problems. The air interacted with the tunnel walls and gave engineers corrupt data.

The only way to gather real usable data on this transonic flight regime was to build an aircraft to fly through the sound barrier. This need gave rise to the experimental aircraft program — the X-planes — and the long list began with the US Air Force designed and Bell Aircraft built X-1.

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The X-1 was shaped like a .55 caliber bullet, an object known to travel faster than sound after a level shot from a gun. It stood to reason that the same shape in an aircraft would be a pilot’s ally in trying to break the sound barrier. The X-1 was air launched from underneath a B-29 bomber so it could conserve all its 8,000 pounds of fuel for the short powered flight, and it was painted bright orange so observers on the ground could see it.

The morning of Tuesday, October the 14, 1947, Chuck Yeager was in the cockpit of the B-29. Bob Cardenas was at the controls, and Yeager’s close friend and long time sidekick Jack Ridley was on board as well. Through the open bomb bay doors was the X-1 with its cockpit open to the wind. As the B-52 neared launch altitude of 20,000 feet, Yeager climbed down the ladder into the small orange aircraft and closed the hatch with a sawed off broom handle Ridley had left on the seat; he’d been thrown from a horse the night before and his broken rib made it impossible to shut the hatch unaided.

The flight plan that morning dictated that Yeager reach a top speed of Mach 0.97, which he was happy to do with a broken rib. But he was sick of these cautious flights, sneaking up on the sound barrier one one hundredth of a Mach number at a time. He was confident in his aircraft, he’d flown it eight times, and he knew it could withstand three times the stresses of flight that he could.

He also knew that every flight was dangerous, and that every flight he didn’t push the aircraft to Mach 1 was just an accident waiting to happen. He decided, sitting in the X-1 that morning, that barring some mishap on this flight he would push through the sound barrier next time he flew with Air Force permission or not.

Cardenas released the X-1. Yeager got his nose pitched down, starting a shallow dive to pick up speed. As he lit the X-1’s four rocket chambers, he pitched his nose back up and accelerated easily to Mach 0.88. He shut down two of his rocket chambers and let momentum carry him higher into thinner air where he reached Mach 0.92. He turned his third rocket chamber back on and accelerated to Mach 0.96, noticing as he did that the flight seemed to get smoother the faster he flew. He re-lit his last rocket chamber and watched as the needle on his Machmeter fluctuate and tipped right off the scale. Yeager couldn’t be sure he wasn’t seeing things.

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After all the anxiety pilots and engineers shared about breaking the sound barrier, Yeager was there. And it was as smooth as anything. He’d expected something, some bumping or shaking, to tell him he’d gone supersonic but he got nothing. Inside the X-1, breaking the sound barrier was less like crashing through a wall and more like easing through Jello.

Regardless of the lack of physical sensations, the flight opened a new chapter in the history of aviation. We’ll have to wait and see if Baumgartner’s supersonic jump will do the same for skydivers or daredevils around the world.

Image: Felix Baumgartner of Austria sits in his capsule and prepares for the pre-breathing procedure during the preparations for the final manned flight of the Red Bull Stratos mission in Roswell, New Mexico, on October 6, 2012. Credit: Jörg Mitter/Red Bull Content Pool