Vapor cone

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A F/A-18F during transonic flight

A vapo(u)r cone, also known as shock collar or shock egg, is a visible cloud of condensed water which can sometimes form around an object moving at high speed through moist air, for example an aircraft flying at transonic speeds. When the localized air pressure around the object drops, so does the air temperature. If the temperature drops below the saturation temperature a cloud forms.

In the case of aircraft, the cloud is caused by expansion fans decreasing the air pressure, density and temperature below the dew point. Then pressure, density and temperature suddenly increase across the stern shock wave associated with a return to subsonic flow behind the aircraft. Since the local Mach number is not uniform over the aircraft, parts of the aircraft may be supersonic while others remain subsonic — a flight regime called transonic flight.

In addition to making the shock waves themselves visible, water condensation can also occur in the trough between two crests of the shock waves produced by the passing of the object. However, this effect does not necessarily coincide with the acceleration of an aircraft through the speed of sound or Mach 1.[1]


These condensation clouds can often be seen appearing around space-bound rockets as they accelerate through the atmosphere. For example, they were frequently seen during Space Shuttle launches, about 25 to 33 seconds after launch, when the vehicle was traveling at transonic speeds. Similar effects were 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.[2]

During acceleration towards Mach 1, the aerodynamic and mechanical aircraft sound is confined to a sound cone. At Mach 1, the angle of the cone margins is 45 degrees to the line of flight; the cone’s apex is the aircraft nose.

In 1883, Osborne Reynolds (Ref.) showed that noise interfered with laminar flow, precipitating turbulence at much lower flow rates. As Mach 1 nears, the Doppler Effect increasingly concentrates aircraft sound intensity, with associated extreme increase in ultrasonic noise frequency. The intense high frequency noise of air in the anterior sound cone prevents laminar flow, with much higher resistance to penetration by the leading edges (the “sound barrier” effect), leading to compression bands (“shock wave” effect).

The ultra-high ultrasound noise in the anterior sound cone “atomizes” raindrops into a cloud of fine water droplets that conforms to its shape (conical anteriorly). The high intensity anterior sound cone ultrasound field will heat the conical cloud disc.

As Mach 1 is being exceeded, the sound barrier is breached as the aircraft slides into silent, low resistance, uncompressed laminar air.


See also[edit]


  1. ^ Wilkinson, Jeff. "Wilk4: Breaking the Sound Barrier (and Vapor Cones around Jets)". Retrieved 2012-10-31.
  2. ^ "Operation Crossroads: Fact Sheet". Retrieved 2012-10-31.

External links[edit]