Jan 28th, 2019

Classic Photo of an F-18 Breaking the Sound Barrier

On July 7, 1999, Navy Ensign John Gay snapped this photo of an F-18 breaking the sound barrier.  The photo ranks among the classic aviation photos of all time.  This is a bit of his story.

The aircraft carrier Constellation was sailing the Pacific Ocean between Hawaii and Japan.  Ensign John Gay was assigned to the carrier.  He and his eight-man team were responsible for naval intelligence photos taken from the F-14 (also assigned to the carrier).

The F-18 Hornet was traveling about 75 feet above the water at roughly 750 mph.  When the ensign heard the sonic boom, he snapped the photo.  For more of his story see:  http://www.wilk4.com/misc/soundbreak.htm

F-18 Vapor Cone

What is the cone around the jet?  The cone is called a vapor cone.  It is sometimes referred to as a shock egg or a shock cone.  It is simply water condensation.  It is formed by the effects of the aircraft.  Part of the aircraft is travelling supersonic and part is traveling subsonic. 

The speed of sound varies depending on an aircraft’s altitude.  Near sea level, Mach 1 (the speed of sound) occurs at over 700mph.  However, due to the shape of the aircraft, parts of it may have supersonic airflow around the plane and other part can be subsonic.  This is called the transonic region.

The air density, air pressure and air temperature around the transonic aircraft can drop below the dew point.  As we know from wet grass in the early mornings of summer, below the dew point we get water condensation.  The condensed water is what causes the vapor cone.

The ensign went on to some short-lived notoriety.  The picture remains a classic.

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