Expert: Mark Janus - 7/19/2011

Question
I have seen in movies that while take off fighter jets(like F-18) contract their end of the booster exhaust so as to increase the thrust.So why not always keep the booster's end contracted so that we can always get the maximum output with same engine power consumed??

Answer
While changing the area ratio of the nozzle does improve nozzle performance under differing conditions, basically this is most important for when the afterburner is on...  (afterburners use a tremendous amount of fuel....thus are only used when necessary)...  A jet exhaust (with afterburner) is a supersonic flow and hence area changes have a great (and interesting) effect on the flow velocity....  fairly complicated to get into here...

Hope this helps,
mj