Breaking the Sound Barrier

What is the Speed of Sound?


Because of the variables in measuring the speed of sound, we measure it at Mach 1. Mach 2 is twice the speed of sound, etc. The fastest any airplane has ever flown is Mach 6.7 -- a record set by the X-15, flown by Air Force Capt. Peter Knight on Oct. 3, 1967. The X-15 was built by North American.

Why "Sound Barrier"?
Before 1947, it was believed that the speed of sound created a physical barrier for aircraft and pilots. As airplanes approach the speed of sound, a shock wave forms and the aircraft encounters sharply increased drag, violent shaking, loss of lift, and loss of control. In attempting to break the barrier, several planes went out of control and crashed, injuring many pilots and killing some.
Eventually, the barrier proved to be mythical. Capt. Chuck Yeager, who punched through the barrier in the X-1, later wrote in his autobiography: "I thought I was seeing things! We were flying supersonic! And it was as smooth as a baby's bottom. Grandma could be up there sipping lemonade."

What is a "Sonic Boom"?
Sonic booms are created by air pressure. Much like a boat pushes up a wave as it travels through water, a vehicle pushes air molecules aside in such a way they are compressed to the point where shock waves are formed. The shock waves move outward and rearward in all directions and usually extend to the ground. As the shock cones spread across the landscape along the flightpath, they create a continuous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard on the ground as the sonic boom.

Fifty years ago, aircraft encountered serious turbulence from the accumulating shock wave. As the first to successfully ?punch through? the sound barrier, Col. Yeager was the first to report that smooth flight resumed ?on the other side.? Trying to break the sound barrier had already killed several pilots who lost control when they hit the shock wave. The 24-year-old Yeager encountered the same turbulence as other pilots, but tried something new -- he slammed the throttle forward and literally punched his way through the previously impenetrable barrier. On the other side, the flight returned to its routine smoothness.

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The Prandtl–Glauert Singularity, a.k.a. "vapor cone"

Many of you have ben asking how the vapor cone pictured in the sound barrier post forms.  This vapor cone is known as the Prandtl–Glauert Singularity or P.G. Singularity.  It is sometimes also referred to as a shock collar or shock egg.

This effect can be seen anytime air is forced to rapidly expand and contract, such as during supersonic flight, and even during bomb explosions.

Apollo 11

Operation Crossroads

F-18

The cause of the PGS is complicated & still being studied, but in general what occurs is that rapid air pressure changes in the wake of the aircraft traveling at or above the speed of sound cause a temperature drop.  If the tempertaure drops below the dew point, water vapor in the area condenses into water droplets.  So the cone-shaped shock wave is, in simple terms, a cloud that forms in the region of the shock wave, much like the cloud-in-a-jar that you saw in 7th grade.

Source: Wikipedia.org