A U.S. Navy Sailor Snapped a Picture of a Jet Fighter Breaking the Sound Barrier–Disputed!
Summary of eRumor:
This email features a picture of an F-18 jet fighter flying through a cloudy circle of what is said to be visible evidence of breaking the sound barrier. The story says the photo was taken by Ensign John Gay when the plane swished past the U.S. aircraft carrier Constellation at 750 miles per hour.
Ensign Gay is real and he did take the picture. There is controversy among people who have seen the picture, however, as to whether it is authentic. Some observers say that what Ensign Gay has photographed is not a breaking of the sound barrier, but a different phenomenon involving a combination of the aerodynamics of the airplane, the atmospheric pressure, and the temperature. Also, Ensign Gay says he took the picture at the moment he heard the sonic boom. Critics have said that is not likely since the plane was flying faster than sound, so any sonic boom would have been heard after the plane passed.
To get a professional perspective on this, TruthOrFiction.com turned to Professor James R. Frysinger of the Department of Physics and Astronomy at the University/College of Charleston in Charleston, South Carolina and who served in the United States Navy. He says such pictures can be authentic and explains why. He also describes why the person taking a picture of a supersonic jet would indeed hear a sonic boom even if the jet had exceeded the sound barrier long before arriving at the photographer’s location:
Dr. Frysinger wrote:
A conically shaped high pressure surface is formed by objects traveling faster than the speed of sound in a medium, e.g., a jet flying through the air.
This cone is caused by the sound source (jet) traveling faster than the sound waves that are produced by it can travel.
The result is an “envelope” of overlapping circles (“sound wave crests”), each with its center lying ahead of the last sound crest. That envelope is conically shaped with its point at the actual sound source. Lying just inside this conically shaped sound pressure wave (above ambient pressure) is a similar, conically shaped surface of below-ambient pressure air (“sound wave trough”).
This is a necessary result of the wave nature of sound waves. Any decent physics text will have a sketch of this. If the humidity level is high enough (e.g., just above sea surface on a warm Pacific Ocean afternoon), the humidity in the air may condense in that trough of low pressure and form a cloud, only to be reabsorbed by the air when pressure returns to normal.
Such clouds tend to form on the noses, nacelles, an leading egdes or tips of wings, canards, etc. These are not unlike contrails except that the water vapor is present in the air before the jet passes, as opposed to being the result of burning jet fuel.
The sonic “boom”
Pictures of these show the result of moisture condensing in the conically shaped wave trough just behind the shock wave. The sonic “boom” reported with these events is NOT caused at the time the jet’s speed rises past the speed of sound. It is in fact the continuous sound that the shock wave “crest” represents. A person farther down the path of the jet will hear the same “boom” but later than a person nearer the jet’s direction of approach. The speed of the sound crest is equal to the speed of the jet in the direction of travel and equal to the speed of sound perpendicular to the cone’s surface. The movie clip on my website at www.cofc.edu/~frysingj/fs14ss.mpg shows the jet approaching and at that moment it has not yet been heard by the observer. Many of the stills being passed around are single frames from similar video clips; the camera was rolling before the “boom” was heard.
A personal account
I was fortunate during my last tour of duty to be standing atop a building at the end of the main runway at NOB Norfolk, a large naval base during a show by the Navy Blue Angels. My height above ground was equivalent to being on the fourth floor, but I was on top of it and “in the elements”. The announcer let us know that a jet was coming in at a supersonic speed and would pass at that speed down the length of the runway and “very close to the deck”. I saw it coming and aimed my camera slightly downward to its altitude and then tripped the shutter on my tripod-mounted camera as it went by. Immediately, I was blown over backwards and my camera and its tripod landed on top of me. The sequence–all in a fraction of a second–was click, boom, thud. Having been alerted I didn’t need to “wait for the boom” to tell when to take the picture. And since the shock wave is NOT a momentary phenomenon, I could let my eye judge when he would be “dead ahead”. All this happened so fast that it seemed for a moment to be simultaneous, but that was an illusion. Yes, I got the picture, looking slightly down into the cockpit and with a clear view of the pilot. Humidity levels that day were too low for a contrail-type cloud to form as they did in the movie clip cited above.
The technical explanation
The geometry works out such that the sine of the apex half-angle (half the angle at the point of the cone) is equal to the ratio of the speed of sound in air of that temperature, pressure, and humidity to the jet’s speed. That speed ratio is called the Mach Number. So, sin(theta) = V/v or sin(theta) = 1/(Mach Number) where v is the speed of the jet, V is the speed of sound at that location, and theta is half the angle made at the point of the cone.
Notice that if the speed of the jet equals the speed of sound, the “cone” is actually a flat surface perpendicular to the direction oftravel. If the jet’s speed is less than the speed of sound, theequation is not valid; there IS no superposition of crests taking place(although there is a large pressure build up, similar to a bow wave ona ship).
Our thanks to Professor Frysinger