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Looking at this elevation view of the Falcon 9, the nozzle exit of the core engine in the cluster is in a different plane with respect to the nozzle exits of the surrounding eight engines.

enter image description here

Why is this? Is there an advantage to this layout?

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  • $\begingroup$ A wild speculation: It is the one engine used during landing (attempts) and perhaps is offset from the other engines in order to gimbal? $\endgroup$
    – LocalFluff
    May 6, 2015 at 8:43
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    $\begingroup$ Could also be to prevent pogo oscillation. Good question! $\endgroup$
    – TildalWave
    May 6, 2015 at 9:01
  • $\begingroup$ Another factor could be that, IIRC, the centre engine is throttled less than the rest. It might have something to do with the stresses that creates or something to do with optimising the exhaust pressures. $\endgroup$ May 6, 2015 at 9:33
  • $\begingroup$ At the time the design was chosen, SpaceX believed this was an advantage. There are many factors that go into a decision like this, so it is not always a simple answer to why. $\endgroup$ May 9, 2015 at 2:44
  • $\begingroup$ @RossMillikan Could the legacy from the single engine Falcon 1 be a reason? The F9 as an F1+8 where the 8 of the engines only are used during launch and kind of plays the role of the strap-on boosters of the Atlas V or Soyuz, although instead of discarded they are integrated for potential reusability and have maximum commonality for economic purposes. $\endgroup$
    – LocalFluff
    May 9, 2015 at 9:32

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I think ForgeMonkey is onto something. I just ran across this article and thread, which includes speculation that the center engine gets some aerospike effect from the exhaust plumes of the surrounding engines, and maybe the vertical positioning of the nozzle is trying to optimize that.

Seems plausible, but I don't understand the fluid dynamics enough to say if that's really it.

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