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I wonder if there is any rule for how closely you can put engine nozzles to each other. Are there any vibration, heat, etc. constraints when you are assembling two or more engine bells together?

Saturn I had four central engines very close, with the external gimballing engines more distant. I know that the outboard ones were placed there, among other things, to allow better pitch, yaw, and roll authority. But are there any limits for clustering engine nozzles?

Saturn I engine bay. The engine nozzles are highlighted, and you can compare the distance between each nozzle

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Obviously there are differences between implementations that will make a general rule very hard to define.

For example, on the Space Shuttle, the 3 SSME engines were close together, and the two SRB's offset nearby. But when they looked at 4 or 6 RS-68A engines between the two SRB's for one of the many SLS/Ares iterations they found the heat load was too high.

Thus every engine will have different requirements, different capabilities, different tolerances.

As a contrast the 5 F-1 engines on the Saturn V were clustered pretty close together. The 9 Merlin 1C on the Falcon 1.0 in the tic tac toe pattern, then the change to the Merlin 1D on the Falcon 1.1 in the Octaweb pattern.

Thus I would argue that generally you can cluster them as close as they can handle. But the parameters there depend on the engines specifications. Or to put it more simply "It depends". Odd how often that is the answer.

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    $\begingroup$ In the case of Saturn I engines this is probably the reason for attaching the central flame shield. $\endgroup$ – Junior Miranda Mar 25 '15 at 14:04

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