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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, 2020 at 19:12

2 Answers 2

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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, 2020 at 3:04
  • $\begingroup$ @Tristan could be. There was a 'remove before flight' cover over the nozzle exit too. $\endgroup$ Nov 13, 2020 at 4:22
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In addition to Organic Marble's answer, I want to add some information specific for the F-1.

In "F-1 Engine Familiarisation training manual (R-3896-1)" by Rocketdyne (link to full PDF on archive.org, 17MB), the throat plug is mentioned in the engine checkout procedures (section 1-170 and 1-190):

1-170. SINGLE-ENGINE CHECKOUT.
1-171. Single- engine checkout is done after receiving inspection and after engine buildup tasks are completed. The engine is installed in the test stand, where the ignition monitor valve sense tube is disconnected, Thrust Chamber Throat Security Closure G4089 removed, and Thrust Chamber Throat Plug G3136 installed. All connections are made between the engine and Engine Checkout Console G3142; facility electrical, pneumatic, and hydraulic sources are applied to the console; and the console is prepared for operation. Electrical system function and timing tests, a turbopump torque test, pressure tests, valve timing tests, and leak and function tests a r e done in accordance with the detailed requirements in R-3896-11. Upon completion of engine checkout, the ignition monitor valve sense tube is connected, Thrust Chamber Throat Plug G3136 removed, and Thrust Chamber Throat Security Closure G4089 installed.

As Organic Marble mentions, the plug is used specifically for testing purposes - see also "F-1 Rocket Engine Operating Instructions", specifically the various thrust chamber leak tests (e.g. section 1.3.30) (link to 325 MB PDF on archive.org). For protection during transport, the Thrust Chamber Throat Security Closure is used, which inserts into the same threaded hole in the centre of the injector plate. This page on heroicrelics.org has an excellent illustrated description of the security closure.

Thrust chamber closures

Image from "F-1 Rocket Engine Illustrated Parts Breakdown", via heroicrelics.org. I think that item 33 is the "Thrust Chamber Throat Plug" and item 33A is the "Thrust Chamber Throat Security Closure", but I'm unable to locate the original document to confirm.

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