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   Winchell Chung ⚛🚀 (nyrath@spacey.space)'s status on Sunday, 12-Jan-2025 00:48:41 JST Winchell Chung ⚛🚀

   Mastodon spaceflight mass-mind, please come to my aid. My google-fu is failing me.

   In the Apollo program, Saturn V’s S-II (second stage) employed four ullage motors situated on the aft interstage skirt. Additionally, the third S-IVB stage incorporated an Auxiliary Propulsion System designed for ullage purposes. These settled the fuel so the engines did not injest vapor and shred.

   Question: why didn't the ullage motors need ullage motors?

   • Embed this notice
     Thomas Sturm (tsturm@famichiki.jp)'s status on Sunday, 12-Jan-2025 00:48:40 JST Thomas Sturm

     in reply to

     @nyrath Actually found this: The ullage motors for the second and third stage were solid motors.

     https://apollo11space.com/what-is-the-ullage-rockets-on-saturn-v/

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     Thomas Sturm (tsturm@famichiki.jp)'s status on Sunday, 12-Jan-2025 00:48:40 JST Thomas Sturm

     in reply to

     @nyrath To clarify: Only in the diagram are the ullage motors described as "solid".

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     Thomas Sturm (tsturm@famichiki.jp)'s status on Sunday, 12-Jan-2025 00:48:40 JST Thomas Sturm

     in reply to

     @nyrath I'm seeing there that the third stage of the Saturn V also had a auxiliary hydrazine propulsion system. Wonder what that was used for. Never heard of that.

   • Embed this notice
     Charlie Stross (cstross@wandering.shop)'s status on Sunday, 12-Jan-2025 00:48:40 JST Charlie Stross

     in reply to

     • Thomas Sturm

     @tsturm @nyrath Ullage, maybe? It had to re-light in flight for translunar injection. (Checks wikipedia) Yup, they were ullage motors, also used for attitude control during coast phase.

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     Thomas Sturm (tsturm@famichiki.jp)'s status on Sunday, 12-Jan-2025 00:59:49 JST Thomas Sturm

     in reply to

     • Charlie Stross

     @cstross @nyrath Somehow I had no idea that the third stage have attitude control. And apparently it had both solid motors and the hydrazine engine for ullage. Options!

   • Embed this notice
     Charlie Stross (cstross@wandering.shop)'s status on Sunday, 12-Jan-2025 00:59:49 JST Charlie Stross

     in reply to

     • Thomas Sturm

     @tsturm @nyrath Remember the third stage had to fly a stable enough profile for the CSM to dock with the LM in flight? (If it started tumbling or rolling when the CSM detached, that would be Bad for "you are not going to the moon today" values of Bad.)