{{short description|Condensation pattern in transonic flight}} [[File:FA-18 going transonic.JPG|thumb|An F/A-18F during transonic flight]] A '''vapor cone''' ('''shock collar''', or '''shock egg''', also known as a '''Mach diamond'''{{sfn|Campbell|1994|p=15|ref=NASA-SP-514}}) is a visible cloud of condensed water that can sometimes form around an object moving at high speed through moist air, such as 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 (static 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 experience supersonic flow while others "see" subsonic air — a flight regime called transonic flight.{{sfn|Campbell|1994|p=12|ref=NASA-SP-514}}
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.<ref name="wilk4">{{cite web |last=Wilkinson |first=Jeff |date=August 15, 2007 |title=Breaking the Sound Barrier (and Vapor Cones around Jets) |url=http://www.wilk4.com/misc/soundbreak.htm |url-status=live |archive-url=https://web.archive.org/web/20220724175431/http://www.wilk4.com/misc/soundbreak.htm |archive-date=2022-07-24 |accessdate=2012-10-31 |website=Wilk4 |publisher=}}</ref>
== Examples == 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.
== Gallery == <gallery widths="250" heights="220"> File:Ares1-X(28OCT2009).jpg|Ares I-X test rocket during launch October 28, 2009 File:FA-18 Hornet breaking sound barrier (7 July 1999).jpg|F/A-18A at transonic speed File:F-A-18C Hornet in transonic flight, with condensation cones.jpg|Complex set of condensation cones surrounds an {{nowrap|F/A-18C}} at transonic speed File:F-35 with double vapor cone, 2016 airshow RNAF, Leeuwarden.jpg|F-35 with a double vapor cone </gallery>
== See also == *Sonic boom *Sound barrier *Shock diamond *Shock wave *Contrail *Condensation cloud *Prandtl–Glauert singularity *Condensation
== References == {{reflist}}
==Further reading== *{{Cite book |last1=Campbell |first1=James F. |url=https://ntrs.nasa.gov/citations/19950024579 |title=Patterns in the sky: Natural Visualization of Aircraft Flow Fields |last2=Chambers |first2=Joseph R. |publisher=NASA Langley Research Center |year=1994 |location=Hampton, VA |pages=12–16 |language=en |chapter=3.2 High-Speed Flows |id=NASA-SP-514 |ref=NASA-SP-514}}
== External links == {{commons category|Aerodynamic condensation from shockwaves}} *{{YouTube|f0RNZnTmKzg|Vapor cone}} *{{YouTube|-mr9tam_c9g|F-18 Hornet High-speed (transonic) flyby}}
Category:Aircraft aerodynamics Category:Shock waves