Expert: Paul Soderman - 9/19/2006

Question
When jet fighters are pouring on the thrust, the exhaust flame from the engine is conical with rings of fire evenly spaced in the cone.  What causes these rings of flame?  You can see an example with an F-18 here:  http://www.pbase.com/mark_yuan/image/67058795

Thanks!

Answer
Mark
A jet operating at design speed will not create shock waves.  But when the pilot pours on the thrust, as you say, an off-design condition is created and the jet exhaust will generate shock waves.  You are looking at shock cells, which are a series of shock waves - actually a series of compression and expansion waves.  These cells cause the water vapor in the exhaust to condense at certain places (probably the expansion regions since low pressure causes a sudden temperature change and condensation); the condensed water droplets become visible.  I don't think you see flames in the photo, but if there was uncombusted fuel in the exhaust, it might show flames in the same shock cells.
Paul