How exactly do you “break” the sound barrier?

Is the sound barrier an invisible wall? (Spoiler alert: No!)

It wasn’t long ago that people believed the sound barrier was a physical barrier, a real yet invisible wall.

Until Chuck Yeager broke the sound barrier on October 14, 1947, it was a commonly-held belief that exceeding the speed of sound — breaking the sound barrier — would destroy an aircraft.

Where did these ideas originate? Here’s a quick primer on the sound barrier.

What exactly is the sound barrier?

Today, we know that the sound barrier is the sudden increase in aerodynamic drag that happens when an object approaches the speed of sound — also known as Mach 1. It’s not a physical or solid barrier.

The moment an aircraft’s speed exceeds the speed of sound, it is said to have broken the sound barrier.
At what speed do you break the sound barrier?

The speed at which you break the sound barrier depends on many conditions, including weather and altitude. It’s approximately 770 mph or 1,239 kmh at sea level.

Why did people believe the sound barrier was a physical wall?

During World War II, pilots reported aircraft tearing apart and instruments freezing when they dove during combat — possibly at the moment they approached the speed of sound. It was described as hitting an invisible wall. [Click here to read more on this topic in the Washington Post archives.]

In the 1940s, the proper design techniques and aerodynamic details for a successful supersonic aircraft were unknown. Aircraft that are not specifically designed to fly supersonically — those having little or no wing sweep and that have thick wings with blunt leading edges — exhibit a sharp rise in aircraft drag as they approach the speed of sound. This increase comes from shockwaves forming in the accelerated flow over a wing, even though the aircraft itself is not yet exceeding the speed of sound. These shock waves cause pressure fields on the wing (and the rest of the aircraft) and can lead to significant flow separation behind the shock waves. Both of these phenomena can create significant aircraft drag. This shock formation and increase in drag is very sudden and large, and tends to be a “barrier” to any further acceleration of the aircraft. At the time, no aircraft had successfully overcome this drag rise, so some predicted that it might not be possible.

Did anything else break the sound barrier prior to 1947?

While bullets and cannonballs had exceeded the speed of sound for years, conventional wisdom held that humans could not exceed it. Further, there was skepticism that aircraft propulsion systems could ever propel an aircraft to the speed regimes in the same way that a projectile achieves this speed by being shot from a gun.

Did drag cause structural failures in WWII aircraft when approaching the speed of sound?

Increase in drag itself is not likely the cause of the structural failures, as drag forces on an aircraft typically do not critically affect the structure. There are two other failure modes that likely caused the destruction of aircraft trying to break the sound barrier in this timeframe. The first is aircraft flutter. Flutter is an unstable coupling of the aerodynamics of the aircraft and the natural vibration modes of the aircraft structure. Flutter is very sensitive to speed, and can be further exaggerated by the effects of shock waves forming on the wings and control surfaces. Flutter can occur almost instantaneously once a certain critical speed is reached, and in a split second the vibrations on the aircraft will exceed the strength of the aircraft — and the structure will catastrophically fail.

The second possible cause is changes to aircraft stability, which can over stress the aircraft to the point of failure. The presence of shock waves can change how the plane responds to gusts or control inputs, and sometimes this can result in an unstable response that leads to full aircraft failure.

Due to the sudden, extreme, and catastrophic nature of these aircraft accidents, and because the pilots rarely survived, very little was learned from each accident that could then be applied to future aircraft designs or modifications. These extreme accidents also fueled the myth that a “sound barrier” existed that no aircraft would ever successfully exceed.

How was the sound barrier broken?

U.S. Air Force Captain Chuck Yeager, officially broke the sound barrier on October 14, 1947 in the Bell X-1 rocket plane. Yeager passed Mach 1 following a drop from a B-29 airplane, proving that an aircraft with passengers could break the sound barrier without injury or harm. The flight took place over Muroc Air Force Base, now known as Edwards Air Force Base, in the California desert. Following this milestone, research continued, and by 1959, the X-15 rocket plane had traveled five times faster than the speed of sound.

The Bell X-1 rocket plane was the first plane in recorded history to break the sound barrier.
What causes a sonic boom?

Pressure waves, aka sound waves, propagate at the speed of sound. When an aircraft is moving faster than the speed of sound (breaking the sound barrier), the pressure waves do not propagate in front of the aircraft, but rather create a wave — similar to the wake of a boat — that follows along with the aircraft. A sonic boom is that sound wave passing by the observer.

Can you see a sonic boom?

This is the moment photographers dream of capturing with one click. But technically, you can’t see a sonic boom without very specialized imaging technology, such as Schlieren imaging, which resolves different densities in air or fluid. After more than a decade of research, NASA successfully captured supersonic shock waves for the first time this year. Click here to check out their images.

With specialized equipment, you might capture a “vapor cone” — the condensation that appears behind an aircraft as it approaches Mach 1. Also known as “shock collars” or “shock eggs,” you’re more likely to see these majestic cloud formations in humid conditions, especially over water. (Unfortunately, you can’t capture a vapor cone with your smartphone.)

And sometimes, if the conditions are right, you can see the sound waves propagating outward from a rocket launch.

Why was breaking the sound barrier such a huge achievement?

Breaking the sound barrier proved that the human body could move without injury at the speed of sound, taking us closer to the possibility of space flight.

What’s a real-life example of the speed of sound?

A great example is thunder, which is the sound caused by lightning. Both occur at exactly the same time, but you see a lightning flash before you hear its thunder because light travels much faster than the speed of sound. It takes the sound of thunder roughly 5 seconds to travel a mile or 3 seconds to travel a kilometer.

According to the National Weather Service, “If you count the number of seconds between the flash of lightning and the sound of thunder, and then divide by 5, you’ll get the distance in miles to the lightning: 5 seconds = 1 mile, 15 seconds = 3 miles, 0 seconds = very close.” Bear in mind that you should be in a safe place while counting — don’t wait to take cover.

Try applying this example the next time you see a fireworks display, especially if you’re watching from a distance.

Do we drag a sonic boom everywhere we go?

No, but we do create sound waves. All sounds are vibrations. Sound is a pressure wave, and we create these waves every time we breathe, move, speak and sing. We even make sound waves in our sleep (some more than others). Our waves are faster than you might think: the speed of sound in air is about 768 mph (1,234 kmph) under normal conditions.

Breaking the Sound Barrier in Chuck Yeager’s words:
Chuck Yeager, the first pilot to break the sound barrier

“Leveling off at 42,000 feet, I had thirty percent of my fuel, so I turned on rocket chamber three and immediately reached .96 Mach. I noticed that the faster I got, the smoother the ride. Suddenly the Mach needle began to fluctuate. It went up to .965 Mach — then tipped right off the scale … We were flying supersonic. And it was as smooth as a baby’s bottom; Grandma could be sitting up there sipping lemonade.” — Chuck Yeager (Source: Yeager: An Autobiography. ed. Bantam, 1986)


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