Breaking the sound barrier: Chuck Yeager and the Bell X-1

An orange aircraft, not even ten meters long, drops from the belly of a four-engine bomber, fires its four rocket chambers one after another and shoots into the sky over the California desert. A few minutes later something has happened that many engineers had considered extremely risky: a human being has flown faster than sound. The story of breaking the sound barrier with the Bell X-1 is about courage and improvisation, about deliberate research – and about why a new chapter of aviation began on October 14, 1947, one whose traces you can find today at every military airfield and in many museums.

The sound barrier in brief

On October 14, 1947, Captain Charles “Chuck” Yeager of the US Air Force flew the Bell X-1 over the Mojave Desert at about Mach 1.06 – according to the Smithsonian Institution, the first flight of an aircraft faster than sound. The X-1 was a pure research aircraft with rocket power, built by Bell Aircraft for a joint program of the US Army Air Forces and the NACA, the predecessor organization of NASA. It was carried aloft by a Boeing B-29 and released there. The flight stayed secret at first and was not officially confirmed until June 1948. The aircraft, named “Glamorous Glennis,” now hangs in the National Air and Space Museum in Washington.

Why sound became a “wall”

In World War II, the fastest fighters ran into an invisible limit. In a steep dive they approached the speed of sound, and then something eerie happened: the aircraft began to shake, the nose pitched down, and the controls barely responded. Engineers spoke of compressibility – at these speeds the air can no longer simply be pushed aside but is compressed. Shock waves form that make the airflow separate from wings and control surfaces and make drag jump suddenly.

For pilots this was deadly dangerous. As early as 1941 a pilot died on a test flight when a Lockheed P-38 could not be pulled out of a fast dive. In Britain, the de Havilland DH 108 broke apart in the air at about Mach 0.9 on September 27, 1946; test pilot Geoffrey de Havilland Jr. died. Accidents like these fed the idea of a “sound barrier” beyond which no aircraft would stay controllable. The British aerodynamicist W. F. Hilton later claimed to have coined the term as early as 1935 – whether he really was the first is disputed.

The speed of sound itself is not a fixed number. It depends mainly on the air temperature. At 20 degrees on the ground it is about 343 meters per second, or roughly 1,234 km/h. At high altitude, where it is far below freezing, it drops noticeably. That is why pilots give their speed in this regime as a Mach number: Mach 1 is exactly the local speed of sound, no matter how high you fly.

A bullet with wings

At the end of 1944, the NACA and the US Army Air Forces began a joint research program for an aircraft meant to push specifically into the region around Mach 1. On March 16, 1945, Bell Aircraft of Buffalo, New York, received the contract for three examples. The designation was initially XS-1, for “Experimental, Supersonic,” and later became simply X-1.

Bell went with rocket power. The jet engines of the time were too weak and lost too much power at high altitude. The engine came from Reaction Motors and had four combustion chambers fed with alcohol and liquid oxygen. Each chamber could be switched on individually; together they delivered about 6,000 lbf of thrust, roughly 27 kN. The propellant lasted only a few minutes, though.

For the fuselage, the designers took as their model an object known to fly stably at supersonic speed: a .50-caliber bullet of the kind used in heavy machine guns. This produced the characteristic pointed fuselage with short, straight, thin wings. The X-1 was built from high-strength aluminum; the propellant tanks were made of steel.

A crucial detail was the adjustable horizontal stabilizer. Instead of moving only the elevator, the angle of the entire stabilizer could be changed. That is exactly what later saved the record flight, because the normal elevator barely worked anymore around Mach 0.9.

Specification	Bell X-1 No. 1 (“Glamorous Glennis”)
Manufacturer	Bell Aircraft Corporation, Buffalo (New York)
Serial number	46-062
Length	30 ft 9 in (about 9.4 m)
Wingspan	28 ft (about 8.5 m)
Height	10 ft 8.5 in (about 3.3 m)
Weight according to the Smithsonian	6,130 lb (about 2,780 kg)
Powerplant	Reaction Motors rocket engine, four chambers, about 6,000 lbf thrust
Crew	1 pilot
Launch	Drop from a Boeing B-29

From glide flight to rocket flight

The first flights still took place without power. On January 19, 1946, Bell test pilot Jack Woolams released over Pinecastle, Florida, for the first glide flight. Woolams was killed in August 1946 on a practice flight for an air race, after which Chalmers “Slick” Goodlin took over the testing. On December 9, 1946, the first rocket-powered flight followed.

The tests later moved to the Mojave Desert in California, to the area around the dry salt lake Muroc Dry Lake – the site of today’s Edwards Air Force Base. The huge, flat lakebed offered room for emergency landings in any direction. For the actual push to the speed of sound, the Air Force took over the program itself and chose a young fighter pilot.

Chuck Yeager: the pilot

Charles Elwood Yeager was born on February 13, 1923 in Myra, West Virginia, and grew up in Hamlin. He had no college education. In September 1941 he joined the US Army Air Forces as an enlisted soldier, first worked as an aircraft mechanic and was then trained as a pilot. In the war he flew the North American P-51 Mustang from England. On his eighth mission in March 1944 he was shot down over France, escaped to Spain with the help of the French Resistance and returned to combat. He is credited with 11.5 aerial victories, including five on a single day in October 1944 and one against one of the new German jet fighters, the Messerschmitt Me 262.

After the war Yeager became a test pilot. In favor of the X-1 was that he was considered an exceptionally calm, precise flyer – and that he was willing to take the risk without demanding a special bonus for it.

October 14, 1947

Two nights before the decisive flight, Yeager fell from his horse on a ride and broke two ribs. He kept the injury quiet because he feared being taken off the program. The problem: the X-1’s entry hatch had to be latched from the inside with the right arm, and that was impossible with the pain. His colleague and engineer Jack Ridley simply sawed off a broom handle with which Yeager could close the hatch with his left hand.

The procedure was the same as on the previous flights: the B-29 climbed to altitude with the X-1 in its bomb bay, Yeager climbed down a ladder into the cramped cabin, and then the X-1 was released. He fired the rocket chambers, kept climbing and accelerated. At about Mach 0.94 the familiar problem returned – the elevator barely worked. Yeager flew using the adjustment of the horizontal stabilizer and so kept control. Then the X-1 passed the speed of sound. The Smithsonian Institution gives Mach 1.06 at about 43,000 feet (roughly 13,000 m), which corresponds to about 1,130 km/h; other sources give slightly different values.

What stood out afterward was mostly what did not happen: beyond Mach 1 the X-1 flew more smoothly than in the region just below. The “wall” was mainly a zone of violent turbulence and control problems, not an impenetrable obstacle.

The record stays secret

The Air Force classified the success as secret. It leaked anyway: the trade magazine Aviation Week reported on December 20, 1947, and the Los Angeles Times shortly after. Air Force Secretary Stuart Symington did not officially confirm the supersonic flight until June 10, 1948. Yeager, Bell founder Lawrence D. Bell and NACA researcher John Stack shared the Collier Trophy, one of the most prestigious awards in American aviation.

Was Yeager really the first?

To this day a counterclaim comes up: North American test pilot George Welch is said to have gone faster than sound in a dive in the XP-86 prototype as early as October 1, 1947. This claim is considered possible but is not officially confirmed, because the flights were not properly measured. Officially, the XP-86 – the forerunner of the F-86 Sabre – did not exceed Mach 1 until April 26, 1948. And unlike a dive, Yeager’s flight was a controlled, measured push in level flight and climb.

Work it out: how fast is Mach 1?

How many km/h Mach 1 means depends on the air temperature and therefore on altitude. The calculator shows you which Mach number a given speed produces at different altitudes.

What came after the X-1

With October 14, 1947, the question of whether an aircraft can fly supersonic under control was answered. Testing continued: on March 26, 1948, Yeager reached Mach 1.45 in “Glamorous Glennis,” and on January 5, 1949, he even managed a takeoff from the ground, straight from the lakebed at Muroc. In all, according to the Smithsonian, X-1 No. 1 flew 78 times – 19 manufacturer test flights and 59 Air Force test flights – before the tests ended in mid-1950.

The program produced further developed versions:

Version	What set it apart	Fate
X-1A	larger, more propellant; Yeager reached Mach 2.44 on December 12, 1953 and then fell into a dangerous spin	lost in an explosion in 1955
X-1B	heat research and reaction control jets, whose technology later went into the X-15	National Museum of the US Air Force, Dayton
X-1D	lost in a propellant explosion before its first powered flight in 1951	not preserved
X-1E	conversion of the second X-1 with a very thin wing and turbopump	in front of the NASA Armstrong Flight Research Center, Edwards AFB

The X-1A flight showed that the next limit was not sound but stability at very high Mach numbers. After the record, Yeager tumbled down over 15,000 meters before he could recover the aircraft. Experiences like these shaped the design of later supersonic aircraft.

The X-1 was the start of the long line of American X-planes. The data gained went into fighter jets such as the Lockheed F-104 Starfighter, which flew with the Luftwaffe and Navy in Germany, and into high-speed reconnaissance aircraft such as the Lockheed SR-71 Blackbird. Things also moved on in civil aviation: Concorde (more on the type in the profile of the Aérospatiale/BAC Concorde) carried passengers across the Atlantic at twice the speed of sound from 1976.

Where you can see the X-1 today

For spotters who collect aviation history, the X-1 is one of the big targets. The surviving aircraft are all in the USA:

• Bell X-1 No. 1 “Glamorous Glennis” (46-062): On August 26, 1950, the Air Force handed the aircraft over to the Smithsonian Institution. It belongs to the National Air and Space Museum in Washington and is displayed in the Boeing Milestones of Flight hall – the same hall as other milestones of aviation. Because the museum has undergone extensive renovation in recent years, it is worth checking the current opening hours and gallery information before you visit.
• X-1B: It stands in the National Museum of the US Air Force in Dayton, Ohio, where the history of the X-planes is also extensively documented.
• X-1E: It is displayed in front of the main building of the NASA Armstrong Flight Research Center at Edwards Air Force Base. Because it stands on a military site, access is not comparable to a normal museum visit.

If you are in Washington anyway, plan a second day for the Steven F. Udvar-Hazy Center, the large annex of the Air and Space Museum at Dulles Airport. Among other things, an SR-71 and a Concorde are on display there. From the spot at the Udvar-Hazy Center you can also watch the traffic at Washington Dulles Airport.

Supersonic today: what spotters notice of it

For everyday life at the fence, supersonic flight has become almost invisible. Civil aircraft have not flown supersonic on a regular schedule since the end of Concorde in 2003. Fighter jets such as the Eurofighter Typhoon are supersonic-capable, but fly much slower near an airfield. Supersonic flights take place in designated training airspace and mostly at high altitude – or on quick reaction alert scrambles, whose boom then occasionally makes the news.

Even so, the legacy of the X-1 is worth a look, especially on military aircraft:

• Recognize fuselage shapes: Pointed noses, thin wings and swept or delta wings reveal that a jet is built for high Mach numbers. How to tell the types apart in the sky is shown in the guide Recognizing fighter jets in Europe.
• Read the serials: The X-1 carried the Army Air Forces serial number 46-062 – the year of procurement is in the first digits. This system lives on to this day, as the article on US military serials explains.
• Photograph Eurofighters: Where to catch the Luftwaffe’s supersonic jets best in Germany is covered in the post Spotting Eurofighters in Germany.
• Understand condensation: The vapor cones that sometimes appear around an aircraft on fast passes are caused by pressure and temperature changes in humid air – not a reliable sign of supersonic speed. More on that in the article on contrails.

Chuck Yeager after the record

Yeager stayed loyal to flight testing for a long time. In 1962 he became the first commandant of the Air Force’s Aerospace Research Pilot School, where later astronauts were also trained. In December 1963 he survived, with severe burns, an ejection from a rocket-boosted NF-104 Starfighter that had gone into a flat spin. Later he led combat units, among other places in Germany, was promoted to brigadier general in 1969 and retired in 1975.

He became popular with a wide audience through the book and film The Right Stuff (1983), in which Sam Shepard played him. For the 50th anniversary of his record, Yeager flew supersonic once more in 1997 in an F-15D, and in 2012, at 89, he sat in an F-15 again for the 65th anniversary. He died on December 7, 2020 at the age of 97.

Conclusion

A good 44 years after the Wright brothers’ first powered flight, the Bell X-1 proved that the sound barrier is not a limit but a difficult transition region. What made it possible was a rocket aircraft designed single-mindedly for research, an adjustable horizontal stabilizer – and a pilot who took off despite broken ribs and with a broom handle in his hand. The findings from Muroc are in every supersonic jet you see today at a military airfield or an airshow.