Although I have seen many perfectly symmetric nozzles, I have also seen designs such as this (Virgin Galactic) which is only symmetrical about one axis:
Why is this?
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Sorry I mean the Falcon yes. – Jonny Sep 16 '14 at 15:39
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According to comments on this thread, the shape of the nozzle on SpaceShip Two is to provide "free pitch-up" during the short powered flight period. That nozzle shape would direct thrust force downward slightly, under the craft's center of gravity, causing it to slowly rotate nose-upward. Presumably modifying the nozzle shape was easier than tilting the whole engine, and the nozzle can be customized for different flight profiles (i.e. more or less fuel carried on a particular flight).
In this video, you can see the pitch-up maneuver during the powered burn from roughly 0:30 to 1:00. Ordinary aircraft achieve pitch-up by using control surfaces (elevators/elevons), but using these creates drag. Directing the rocket motor thrust instead gives a pitching force, with less drag.
Russell Borogove
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7Just a note that the Falcon 9 v1.0 used 3x3 engine layout on the photo and that isn't the configuration they're flying now on Falcon 9 v1.1 that is using a petal layout with 1 central engine and remaining 8 in a circle around it. Nozzles still seem axially symmetric to me tho, so this doesn't change anything for the answer. – TildalWave Sep 18 '14 at 11:47
