Breaking the sound barrier by air, by land, and in free fall

On October 14, 1947, high above California's Antelope Valley, Charles "Chuck" Yeager became the fastest man alive. That day Yeager—an Appalachian farm boy-turned-fighter ace—flew an experimental rocket plane called the Bell X-1 through the sound barrier and into the history books. Fifty years and one day later (and only about 500 miles due north), another fighter pilot—RAF Wing Commander Andy Green—equaled Yeager's feat but on four wheels. Thrust SSC was the name of his ride, and it made Green the fastest man on Earth. It's a title he still holds.

But 65 years to the day after Yeager's supersonic flight, an Austrian skydiver named Felix Baumgartner got his own entry into the record books. Baumgartner rode a helium balloon from Roswell, New Mexico, (yes, that Roswell) 128,100 feet (39,045m) into the atmosphere and then stepped out of its gondola, breaking the sound barrier with nothing more than a pressure suit and the laws of gravity. Luckily for Green, Baumgartner became the fastest man en route to Earth.

All three of these historic supersonic firsts happened on (or about) October 14, but the pursuit of speed isn't some endeavor confined to a single day. The speed of sound—otherwise known as Mach 1 after an Austrian physicist—varies depending upon the medium through which that sound is passing. On a warm day at sea level, it's about 768mph, or 343.2m/sec if you prefer to think in SI units. Throughout the 17th century, scientists in England and France worked to calculate the speed of sound, getting ever closer before William Derham got there—or thereabouts—in 1709. Derham used a telescope, a pendulum, and his church tower in Upminster (now a far suburb of London) to arrive at the answer by observing the interval between seeing a rifle flash and hearing its crack. It would be more than 200 years before a human could attempt to travel that fast, however.

The Right Stuff

The first challenger would have to wait for World War II. The demands of aerial combat during the war meant fighter planes were pushed ever faster, but there appeared to be a limit to just how fast. Aerodynamic drag would build up the closer a propeller-driven plane got to the speed of sound, aka before things started to go wrong. A plane's control surfaces would lock up as the air began to flow faster and faster over them, overwhelming and resisting any inputs from the cockpit (this is called compressibility). More than a few unlucky pilots rode their uncontrollable vehicles into the ground as they got closer to Mach 1, usually in a steep dive. This led to the idea of the sound barrier—literally a speed beyond which a plane could not pass.

Towards the end of the war, the US National Advisory Committee for Aeronautics (NACA, the organization that eventually became NASA) wanted to explore powered flight at these transonic speeds. Wind tunnels, like one built at NACA's Langley Memorial Aeronautics Laboratory, proved useless. As the air was blown through at faster velocities, shockwaves would form that bounced off the tunnel walls and the models within. To properly understand flight at these speeds, manned aircraft would have to be flown by test pilots. In 1944, NACA, the Army, and the Navy agreed to such a research program to investigate high-speed flight.

As might be expected, a rivalry began between the Army and Navy, with each picking a different contractor to build their test planes. The Army went with the Bell Aircraft Company, the Navy with Douglas, but both were powered by Reaction Motors' rocket engines that burned ethyl alcohol and liquid oxygen. The Bell machine, first called the XS-1 but later renamed the X-1, would be carried aloft by another plane and released mid-air, saving weight that could then be devoted to making it go fast (as well as carrying several hundred pounds of instruments to gather data). The X-1 would be required to fly for at least two minutes to an altitude of 35,000 feet (10,688m) at speeds of up to 800mph (1,287km/h), and it would be capable of withstanding 18G (the actual planes would fly faster and much, much higher).

It was known that .50 caliber bullets traveled at supersonic speeds, so Bell built a plane that had roughly the same shape (plus a pair of wings and a tail, obviously). Bell was based in Buffalo, New York, and the X-1's flight tests were initially conducted in Florida, but the need for a long runway on which to land it led the Army to relocate the program to the dry lake beds of the Southern California desert. Its home would be Muroc Army Air Field, later renamed Edwards Air Force Base (the US Air Force was spun out of the Army in September 1947). Muroc Dry Lake is in Antelope Valley in the eastern Mojave Desert. Although only 100 miles from Los Angeles, it was the far side of the San Gabriel mountains, making it remote and difficult to reach by road. The Army used it as a bombing range during the war and decided it was also a perfect location for a secret test flight program.

Despite this remote location, the Army set about building the infrastructure needed for the X-1. Liquid oxygen and nitrogen tanks were constructed, as was a loading pit to mate the plane to its mothership, a Boeing B-29 Superfortress. Bell's test pilot, Jack Woolams, led the program at first, but he was tragically killed in a crash practicing for the National Air Races in 1946. He was replaced by Chalmers "Slick" Goodlin, who made 26 flights in the X-1; However, arguments between Bell and the Army over compensation for the test program meant he lost the chance to take the X-1 supersonic. Rather than paying Bell and Goodlin—who wanted $150,000 to break the sound barrier—in April 1947 the Army decided to use one of its own pilots. Enter Chuck Yeager.

Yeager was a gifted and natural pilot, and he loved flying the X-1, describing it as "the best damn airplane I ever flew." He made his first powered flight in the X-1—a plane he named Glamorous Glennis after his wife—on August 29, 1947, building up speed over the following weeks in a carefully controlled test program. At speeds above Mach 0.85, the X-1's sweet handling characteristics changed. Describing a flight on October 5, 1947, Yeager likened the experience of reaching Mach 0.86 to being as if "I was driving on bad shock absorbers over uneven paving stones." Things got worse the closer he got to Mach 1. At Mach 0.94, a shock wave formed on the plane's elevator, removing his pitch control (the ability to point the nose up or down).

Regardless, the team continued the test program, tweaking the horizontal stabilizer (that pair of wings on the X-1's tail). October 14, 1947 would be the year's ninth powered flight; the 50th for the X-1. Even then, things weren't entirely smooth. Yeager managed to fall off a horse the weekend before the flight, breaking a rib and injuring his shoulder, something he kept quiet from his superiors lest he be grounded.

On that fateful morning, Yeager and the X-1 were dropped from the B-29 mothership at 20,000 feet (6,096m). The B-29 wasn't actually going fast enough, and Yeager had to fight the plane to prevent a stall before firing all four chambers of the Reaction Motors' XLR-11 rocket. Leveling off at 42,000 feet (12,802m), he found the faster the plane went, the smoother things got. On the ground, NACA staff were tracking his progress when they heard his sonic boom. Yeager had done it, reaching a speed of Mach 1.07 (700mph or 1,127km/h at that altitude). The sound barrier had been broken.