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A vapor cone, shock collar, or shock egg is a visible cloud of condensation which can sometimes form around an aircraft. The phenomenon is frequently encountered at high-subsonic Mach numbers, but can also occur in lower-speed conditions, such as high-g maneuvers by fighter aircraft. The cloud is caused by extremely low pressure in an "expansion region" of the airflow around the aircraft, usually in relatively humid atmospheric conditions. A common misconception is that this effect is the aircraft "breaking the sound barrier."
Examples
These condensation clouds were frequently seen during Space Shuttle launches around 25 to 33 seconds after launch when the vehicle is traveling at transonic speeds. Similar effects are also visible in archival footage of some nuclear tests. Scientists observing the Operation Crossroads nuclear tests in 1946 named the transitory cloud a "Wilson cloud" for its superficial similarity to the Wilson cloud chamber effect. The effect is also noticeable in modern super-high-bypass turbofan jet engines when operating at takeoff power, due to the low pressure and transonic fan blades in the engine inlet.
| Ares I-X Test Rocket during launch October 28, 2009
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