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The first stage of Saturn V has 5 main F-1 engines, but only 4 of the outer engines can gimbal and the engine at the center was fixed (Source : https://youtube.com/clip/UgkxCWZFo19a9Af_N1bcg9XuMjkJLNRzPTzB).

I was thinking about this for a while about why 4 and not 2 gimballing engines?

I was thinking if redundancy was the problem, if that's the case, they could've gone with gimballing 2 of the outer engines which are diametrically opposite to each other. But, why 4?

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    $\begingroup$ ...a solution to what? $\endgroup$ Dec 29, 2022 at 12:06
  • $\begingroup$ Please edit to clarify exactly what you are asking. Does 4 gimbaling engines sound like a lot to you? $\endgroup$ Dec 29, 2022 at 12:50
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    $\begingroup$ @Woody differential throttle provides far less control over direction and none over roll. One outboard gimbaled engine can't independently control direction and roll. The minimal set for full control without additional vernier engines or other control mechanisms is a pair of gimbaled engines. You only get redundancy with the third engine, and lose it with one engine failure. Four gimbaled engines gives double redundancy (which, as Uwe noted, was needed on one mission). $\endgroup$ Dec 29, 2022 at 17:32
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    $\begingroup$ The dual engine failure was on a 2nd stage, but it did have the same TVC setup. @ChristopherJamesHuff your comment could be an answer. $\endgroup$ Dec 29, 2022 at 20:03
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    $\begingroup$ Since the F-1 wasn’t throttleable at all, differential throttling would have been an additional added complexity. $\endgroup$ Dec 29, 2022 at 20:21

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Assuming there are no vernier engines or other control mechanisms, two gimbaled engines are the minimum necessary for full attitude control, however only two gimbaled engines do not provide redundancy. One functioning gimbaled engine can't independently control direction and roll.

Failure tolerance thus requires at least a third engine to be gimbaled. Four gimbaled engines gives double redundancy, allowing the vehicle to tolerate two failures of the "outboard" engines and still have full attitude control, as well as providing symmetry that helps simplify the design and vehicle control.

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  • $\begingroup$ Furthermore, making all "outboard" engines gimballing simplifies construction in some way; since the center engine in the only ungimballed, and all other engines are gimballing. If only two engines were gimballing; you would have two gimballed "outboard" engines, and two ungimballed ones. The former case has probably a higher parts count, however, the latter case has more constructive variety. $\endgroup$
    – Dohn Joe
    Jan 2, 2023 at 10:10
  • $\begingroup$ @DohnJoe "providing symmetry that helps simplify the design" $\endgroup$ Jan 2, 2023 at 15:06
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    $\begingroup$ I did not see it mentioned but F-1 gimbaling (IIRC) is only in one axis not two. The gimbal axis is parallel to the line that joins opposite engines. This way neither engine of a pair can pivot toward the center engine. Gimbaling in only one axis greatly simplifies structural and plumbing complexity and heat control. (Opposite engines can gimbal in same direction for pitch or yaw; opposite directions for roll.) Note, too, that some launchers need to have a particular side toward ground for telemetry (antennae often not on all sides of the vehicle) or to maintain the guidance reference plane. $\endgroup$
    – Chris Ison
    Jan 2, 2023 at 18:27
  • $\begingroup$ @ChrisIson I originally was going to mention that, but couldn't find a good reference. Note that it still has double redundancy, except in the case of losing 2 opposed engines. However, losing an engine does reduce the amount of control: the Firefly Alpha has a similar configuration (without a center engine), and the first flight had an engine shut down soon after liftoff, then lost control when trying to go transsonic. $\endgroup$ Jan 2, 2023 at 18:44
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    $\begingroup$ @ChrisIson The F-1 gimbals along two axes ("F-1 ENGINE FAMILIARIZATION TRAINING MANUAL" R-3896-1, paragraph 1-21) $\endgroup$
    – Ludo
    Jan 2, 2023 at 19:33

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