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This cutaway drawing of the F-1 injector plate has a "throat plug insert" annotated. What is a throat plug? What is it used for?

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    $\begingroup$ A throat plug may be a 'remove before flight' plug to prevent damage or pollution of the combustion chamber of the F-1 engine during transport or launch preparation. The plug insert at the injector plate is used to fix the plug using a thread cut into the plate. $\endgroup$ – Uwe Nov 12 '20 at 19:12
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A "throat plug" is a cover to prevent contamination from entering the engine's combustion chamber from the direction of the nozzle. It can also serve as a "stopper" for leak testing.

I could not find a picture of an F-1 throat plug, but here are some diagrams of the Space Shuttle Main Engine's throat plugs.

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This diagram shows the upper throat plug in use during a leak test of the combustion volumes of the engine. (The lower throat plug was used to leak check the combustion chamber / nozzle interface.)

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There was also an 'environmental protection' throat plug, provided for contamination protection post-landing.

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Source: SSME Pocket Data Book

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    $\begingroup$ I seem to recall reading that the F-1 injector design was classified SECRET at the time as well (can't seem to find a source now, so I can't assert correctness of this statement), so the throat plug would have had a security aspect as well for when the engine was moved. $\endgroup$ – Tristan Nov 13 '20 at 3:04
  • $\begingroup$ @Tristan could be. There was a 'remove before flight' cover over the nozzle exit too. $\endgroup$ – Organic Marble Nov 13 '20 at 4:22

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