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What is the interstage used for, how big it is and is it deorbiting separately?

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On the Falcon 9, the interstage is attached to the top of the first stage and remains attached to the first stage.

It also contains the mechanisms for, and the grid fins themselves.

The nozzle of the second stage, sticks out from the bottom of the second stage, and is covered by the interstage.

The interstage provides a mount point for the second stage to attach.

Per Wikidepia:

The Falcon 9 interstage, which connects the upper and lower stage, is a carbon-fiber aluminium-core composite structure. Reusable separation collets and a pneumatic pusher system separates the stages. The original design stage separation system had twelve attachment points, which was reduced to just three in the v1.1 launcher.

On the Saturn V the first stage interstage was a ring that was discarded and reentered on it own as seen is this nice question. The second to third stage interstage remained attached.

On some boosters, (looking at you Soyuz) where the next stage actually starts firing before separation the interstage will be a mesh/grid that allows the startup exhaust out during second stage startup.

Specifically:

To accommodate the third stage, the core booster is crowned with a lattice structure, featuring 12 connectors and six locks, which are designed to be cut with pyrotechnics to separate the second and third stage in flight.

The lattice structure makes it possible to ignite the engine of the upper stage moments before the separation of the core stage below it. A special cone-shaped, titanium-covered deflector above the instrument section is designed to facilitate the flow of hot exhaust gas from the third-stage engine, when it begins firing.

Soyuz family of boosters

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  • $\begingroup$ Saturn V interstage space.stackexchange.com/q/26973/6944 $\endgroup$ – Organic Marble Oct 7 '20 at 1:13
  • $\begingroup$ re the apollo interstage, it was only the first interstage that separated from both stages, the second remained attached. $\endgroup$ – user20636 Oct 7 '20 at 10:08
  • $\begingroup$ This question comes close to answering -- but does not quite answer -- the "what is it for" question: like any interstage, it serves two primary purposes -- 1) provide the load path from the second stage structure to the first stage structure, and 2) provide protection for the second stage engine from aero loads. Plus the grid fin and mechanism stuff you mentioned. $\endgroup$ – Tristan Oct 7 '20 at 14:35
  • $\begingroup$ @Tristan Please use the Edit function and add it in. If I do not lke I will call you names and rant and rave, then revert it. So what could go wrong? :) $\endgroup$ – geoffc Oct 7 '20 at 19:14

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