The thing about the sound barrier in the 1940s was that everyone in aviation knew that it was theoretically possible to travel faster than sound. High-powered bullets did it all the time.
But whether or not you could do it in an airplane — well, that was a whole different question.
In the early 1940s, the U.S. Army Air Force (the forerunner of the U.S. Air Force) and the National Advisory Committee for Aeronautics (the forerunner of NASA) asked Bell Aviation to build an experimental aircraft to explore the aerodynamics of flight near the speed of sound. It would be called the X-1 and the company’s engineers took their inspiration from a known success.
“They took the bullet shape of an M-2 Browning machine gun bullet, a .50 caliber bullet, and that became the fuselage shape of the X-1,” Bob van der Linden tells Inverse. He is the National Air and Space Museum Curator of Special Purpose Aircraft.
“They knew it was stable, because a high-powered bullet is accurate, which means we can fly straight,” he says.
After 50 test flights — and some key modifications — that’s exactly what Air Force Pilot Chuck Yeager did on October 14, 1947. He took the X-1 right up to the sound barrier and flew straight on through.
“The X-1 apparently flew beautifully,” van der Linden says. “It sailed right through the alleged barrier.”
The purpose of Bell X-1
When World War II ended and the Cold War began, the U.S. Military sought any advantage it could in an effort to surpass the military capabilities of the Soviet Union.
This included finding ways to make aircraft fly faster, according to van der Linden.
World War II fighter aircraft with piston engines had become about as powerful as such engines could allow, routinely edging up to, if not quite reaching, 500 miles per hour in level flight. But when reaching higher speeds in prolonged dives, pilots would encounter something termed “compressibility” — shockwaves built up in front of the aircraft, eventually freezing the control surfaces. Pilots would lose control and crash.
“He thought maybe it wasn’t possible to go supersonic.”
There was something about getting close to the speed of sound — which varies with air density but is around 761 miles per hour at sea level — that was causing big problems. (The speed at which sound travels is the sound barrier; when an aircraft exceeds this speed and experiences increased drag, that’s breaking the sound barrier.)
The U.S. Army Air Force and the National Advisory Committee for Aeronautics (NACA) knew you could theoretically design objects to fly at supersonic speeds: German V-2 ballistic missiles were known to fly at the speed of sound.
But American aerospace engineers had a big and important gap in their data on “transonic” flight — that region between Mach 0.8 and Mach 1.2 — van der Linden says. Transonic aircraft reach speeds very near the speed of sound, greater than 250 mph but less than 760 with about a Mach 1.
X-1 was designed to explore that space — and ultimately fly beyond it, becoming a supersonic craft.
Who is Chuck Yeager?
Chuck Yeager enlisted in the U.S. Army Air Corps in 1941 after graduating high school. He flew combat missions over France and Germany from 1943 to 1945. He shot down 13 German aircraft, five in one day, and was then shot down himself over occupied France in 1944 — but eluded capture.
After the war, Yeager became a test pilot, displaying a knack for excellent airmanship and flight analysis that proved essential for the X-1 program.
“Even though he only had a high school education, he had this innate understanding of things mechanical,” van der Linden says, adding the test pilot could fly an aircraft precisely the way the engineers needed him to. “Then when he landed he, like most other fine flight test pilots, was able to explain it to the engineers and they understood one another.”
Van der Linden points out that Yeager was not unique among test pilots, and that other test pilot might have flown the X-1 just as well.
“But it took a person of his skills and composure to be able to do it,” he says of Yeager. “He was a fine test pilot.”
So fine a pilot, van der Linden says, that Yeager was considered the top pilot in the Air Force at the time — which is why he was tapped for the X-1 mission.
“It was an Air Force program,” van der Linden says, “so it was natural to give it to him.”
Yeager would go on to serve in combat roles during the Korean and Vietnam wars and retire from the Air Force in 1975 with the rank of Brigadier General. He died in 2020 at the age of 97.
The first plane to exceed the speed of sound
It was a remarkably conservative design in a way, van der Linden says. To the proven bullet-shaped fuselage were attached two thin, but immensely strong, straight wings, as swept wings were still an unproven concept at the time.
But the X-1’s propulsion system would be different from the propeller-driven aircraft that dominated before and during the war or the air-breathing jet engines first deployed by the German Luftwaffe on their Messerschmitt Me 262 in the final days of World War II.
The nascent jet engine technology available wasn’t considered powerful enough for the task of breaching the sound barrier, van der Linden says, so the X-1 used the four-chambered XLR-11 rocket engine, which burned a mix of water and alcohol with liquid oxygen.
Because of the choice of rocket propulsion, the X-1 would be carried to around 25,000 feet above the desert around Edwards Air Force Base in the bomb bay of a B-29 Superfortress, and then dropped before igniting its engines and making a speed run.
“It went on through.”
“The X-1 was designed to take off from the ground,” van der Linden says, “But they didn’t want to do that because they could consume most of their fuel to get up to altitude.”
The control surfaces of the X-1 were also different than most other aircraft of the era, a design essential for eventually breaking the sound barrier.
“They had what's called a ‘full flying tail’ on it, which was very clever,” he says.
This meant the entire body of horizontal fins on the tail of the X-1 could pivot to act as elevators, control surfaces that move an aircraft up and down. Most other aircraft at the time had elevator flaps on the tail fins, which became a problem for planes as they got closer to the speed of sound and entered that “compressibility” zone with mounting shockwaves.
“The center of pressure would move backward,” van der Linden says. “When it got to the tail surfaces where the elevator was, it would blank out the elevator.” This kept the flap from moving on its hinge, and the pilot from controlling the airplane.
Even with the full flying tail, there were problems. In the test flights preceding October 14, Yeager was having difficulty controlling the X-1 as it neared Mach 0.997. “He thought maybe it wasn’t possible to go supersonic,” van der Linden explains.
The NACA however, made a simple fix: They changed the screw jack on the elevators, allowing the fins to respond more quickly to a pilot’s input.
On October 14, Yeager was able to adjust the controls by just a fraction of a degree “to get control back so that the center of pressure was no longer right on the hinge point,” van der Linden says before snapping his fingers.
“It went on through.”
The legacy of breaking the sound barrier
Yeager took the X-1 up to Mach 1.02 for about 20 seconds that October day and would later say the only real barrier had been in human knowledge. There’s a great lesson there, says van der Linden.
“At the time, this looked like it was an unsolvable problem,” he says. Everyone rose to the challenge and solved it anyway.
“It may seem impossible, but keep plugging, because it may be possible,” van der Linden says.
Unlike the triumphs of the early astronauts, there was no public celebration over the toppling of the sound barrier. The public didn’t hear anything about Yeager’s flight until the news was leaked to the press a few months later.
“It came out in the Aviation Week and the Air Force was very upset about that,” van der Linden says. “It kind of showed our cards to the opposition.”
The U.S. and Soviet Union were quick to their respective drawing boards, with the first U.S. Fighter jet capable of level supersonic flight, the F-100 Super Sabre, making its debut in 1954.
The Soviets followed suit with the MiG-19 in 1955 and supersonic flight has been a crucial part of military airpower ever since.
Cold War interceptors such as the American F-14 and the Soviet MiG 25 could fly in excess of Mach 2 in response to threats to their national airspace from strategic bomber aircraft.
Meanwhile, the U.S. spy plane, the SR-71, could cruise at an unbelievable March 3.2, according to van der Linden. It saw service beginning in 1966 — just 19 years after Yeager’s tentative first steps beyond Mach 1.
“They would take off from Southern California to fly up the coast and they're doing Mach 3 and they would make a right-hand turn,” he says. “By the time they get out of the right-hand turn, they're over Montana.”
Yeager and the X-1 not only showed the impossible was possible, but they also allowed the extension of the impossible to military aircraft capabilities undreamt of at the time of his flight.
The thing is, now that we have them, those capabilities are seldom used.
Why did supersonic air travel end?
Van der Linden was on the last transatlantic flight of the supersonic airliner Concorde in 2003.
First entering commercial use in 1976, the Concorde could cruise at Mach 2.04 and fly from Paris to New York in three and a half hours compared with the nearly five hours it took a subsonic Boeing 747 on a good day.
“My brother in law, who at that time was an F-18 pilot Hornet pilot in the Navy, asked me how long had I gone supersonic,” van der Linden says. “I said, ‘Oh, about two, two and a half hours.’”
“He goes, 'yeah, I thought so — you have more supersonic time than I do,’” van der Linden remembers.
Most fighter pilots rarely go supersonic because there’s little need without an emergency, he explains. But most importantly, there’s the hard reality of supersonic flight: It’s incredibly wasteful when it comes to fuel. “It requires a prodigious amount of fuel to go past Mach 1,” he says.
“It's expensive because of physics.”
This is due to drag — the resistance of the air against the forward motion of the aircraft increases dramatically as it approaches the sound barrier. “The drag does go down as you go past that, but it never gets back down to the level of transonic flight,” van der Linden says.
To go faster requires more power, more power requires more fuel and more fuel equals greater expense, which is what, combined with the limited flight paths due to loud sonic booms, eventually spelled the end of the Concorde. Supersonic airliners proved too expensive for commercial air travel, even with average round-trip tickets selling for $12,000, van der Linden says. That’s likely the way things will stay for commercial jets.
“No matter what you do in terms of breakthroughs in engine technology, airframe, whatever, it's always going to be more expensive to fly supersonic,” he says. “Because whatever breakthrough you might be able to develop in materials and whatnot for a [super sonic transport], you can apply that to a conventional airplane.”
Cost hinders the military too. Modern jets can go supersonic when they need it and they usually don’t need it — so they don’t.
This hardly means the legacy of Yeager and the X-1 is unimportant or an engineering cul-du-sac. No one knew what it would take to fly at supersonic speeds before they tried.
“They needed to find out and they found out it's expensive and it's expensive because of physics,” van der Linden says.
The future of experimental X-planes
It’s that spirit of inquiry — that willingness to risk the unknown to learn what is possible and push the boundaries of our engineering and imaginations — which is the true legacy of Yeager and the X-1.
This is not just in some philosophical or allegorical sense. The experimental X-plane program is still alive and active to this day, with recent examples following in Yeager’s footsteps.
- In 2004, NASA’s uncrewed X-43A hypersonic demonstrator flew two missions, reaching Mach 6.8 in the first and Mach 9.6 in the second — world records for an aircraft with air-breathing jet engines (as opposed to rocket engines which power orbital spacecraft to Mach 25).
The term hypersonic refers to flights faster than Mach 5, or five times the speed of sound, where aerodynamics change as dramatically as they do between subsonic and supersonic flight. This poses immense engineering challenges.
- NASA’s X-59 Quiet SuperSonic Technology, or QueSST, meanwhile, is being constructed to test whether it’s possible to reduce the sound of the sonic boom generated by supersonic flight, with test flights scheduled to begin in 2022.
Generated by the shockwaves caused by supersonic flight, a sonic boom can sound like an explosion. Along with fuel costs, this sound contributed to the Concorde’s demise, van der Linden says.
“These experimental aircraft of all different ways, shapes, and forms have enabled us to learn so much about flight,” he says, and “It started with the X-1.”