Sonic Boom

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A Sonic Boom
Credit: Ensign John Gay, USS Constellation, US Navy

F/A-18 "Hornet" Passes Through Water Vapor Cloud at Speed of Sound
1st prize (of 42,000 entries) Science and Technology World Press Photo Contest 2000

Many people have heard a sonic boom, but few have seen one. When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, an unusual cloud might form. The origin of this cloud is still debated. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets. Above, an F/A-18 Hornet was photographed just as it broke the sound barrier. Large meteors and the space shuttle frequently produce audible sonic booms before they are slowed below sound speed by the Earth's atmosphere.

Here is the story behind it:

Through the viewfinder of his camera, Ensign John Gay could see the fighter plane drop from the sky heading toward the port side of the aircraft carrier Constellation.  At 1,000 feet, the pilot drops the F/A-18C Hornet to increase his speed to 750 mph, vapor flickering off the curved surface of the plane. In the precise moment a cloud in the shape of a farm-fresh egg forms around the Hornet 200 yards from the carrier, its engines rippling the Pacific Ocean just 75 feet below, Gay hears an explosion and snaps his camera shutter once.

"I clicked the same time I heard the boom, and I knew I had it," Gay said.  What he had was a technically meticulous depiction of the sound burrier being broken July 7, 1999, somewhere on the Pacific between Hawaii and Japan.

"At sea level a plane must exceed 741 mph to break the sound barrier, or the speed at which sound travels. The change in pressure as the plane outruns all the pressure and sound waves in front of it is heard on the ground as an explosion or sonic boom. The pressure change condenses the water in the air as the jet passes these waves. Altitude, wind speed, humidity, the shape and trajectory of the plane - all of these affect the breaking of this barrier. The slightest drag or atmospheric pull on the plane shatters the vapor oval like fireworks as the plane passes through," he said. "You see this vapor flicker around the plane that gets bigger and bigger. You get this loud boom, and it's instantaneous. The vapor cloud is there, and then it's not there. It's the coolest thing you have ever seen."

 

Copyright 2010 Tim Stouse
Last modified: December 10, 2010
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