Shock Waves: Why Jet Makes 'Vapor Cone'

Pilots in the years after World War II used to think of the sound barrier as an invisible wall in the sky -- something they would feel but not see as they broke through it.

There is no actual sound barrier. But as the Navy picture here shows, there is often a beautiful visual phenomenon as a plane gets close to the speed of sound. Here's an F-22 Raptor fighter jet as it passes through "high transonic" speeds, probably going more than 700 miles per hour.

Why the spectacular spray of vapor around the plane? Engineers believe droplets of water form from moisture in the air around the plane as it speeds by, creating a drop in air pressure around it.

The phenomenon is called a vapor cone or a shock collar, and, if you want to get really technical, a Prandtl–Glauert singularity.

"Since the plane is thick, it has to push the air ahead of it out of the way," said Peter Coen, principal investigator of the Supersonic Fundamental Aeronautics Project at NASA's Langley Research Center in Virginia. "The shock waves the plane creates, they're actually fairly strong. Some of the air moving around the plane is going at supersonic speed, even if the plane isn't.

"And if there's humid, moist air around it, you get what you see in the picture."

The picture above was taken last week over the Gulf of Alaska, during a Navy exercise called Northern Edge 2009. But the same thing can happen to planes anywhere in the world, and is often seen around a rocket (see the picture of Apollo 11, below) as it races toward orbit in the moments after launch. The air around it, having to race around the body of the rocket or aircraft, actually goes faster than the ship itself does.

One last little fact: When a plane or rocket creates a vapor cone, it is not actually going faster than sound, only 90 to 96 percent as fast. Once it reaches supersonic speed, the speeds are too great for any mere water droplet to keep up.

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