How exactly do you “break” the sound barrier?
By: Boom Supersonic
Tags: Aerospace, Technology, Aviation, Air Travel, Engineering

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.