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At the time of launch what was the air pressure in the Apollo LM, and the spacecraft-LEM adapter section (SLA)? Was the air vented from the SLA as the vehicle entered orbit?

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    $\begingroup$ I can't find any authoritative reference, but I would guess that the SLA had to be open to ambient pressure at all times, thus venting from sea level pressure to vacuum as the vehicle flew to orbit. On Apollo 6, one of the SLA panels failed because of increasing pressure inside the honeycomb-sandwich structure of the panel. As the SLA is made up of multiple separating panels, and has a number of access ports, it would be hard to pressure-seal it if you wanted to, and I don't think you'd want a lot of overpressure there. $\endgroup$ Commented Aug 8, 2018 at 15:44
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    $\begingroup$ I couldn't find a reference either but I am also sure there were vents. Next time I drive by the Saturn V at JSC I'll stop in and take a look at the SLA to see I if I can see them. Although I'm not 100% sure it is a real SLA. $\endgroup$ Commented Aug 8, 2018 at 18:13
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    $\begingroup$ @OrganicMarble The Saturn V at Marshall probably has a real SLA. Do you ever go there? You'd probably need a telescope to check for the vents: that Saturn V is upright! $\endgroup$ Commented Aug 9, 2018 at 0:58
  • $\begingroup$ @TomSpilker I have been to the visitor center there once. It's pretty awesome. My highlights were seeing the Shuttle-Centaur and the MPTA boat-tail, sitting beside each other outside. $\endgroup$ Commented Aug 9, 2018 at 1:27
  • $\begingroup$ The Field Guide to American Spacecraft says it's a real SLA, SLA-22. The page is a tad out of date though, the booster is displayed indoors now. I'll try and stop by tomorrow. americanspacecraft.com/pages/booster/sv-jsc.html $\endgroup$ Commented Aug 9, 2018 at 1:33

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I was able to answer this by taking some pictures of the Saturn V displayed at Johnson Space Center. Despite my skepticism, it is a real Spacecraft Launch Adapter (SLA), just as the Field Guide to American Spacecraft states.

We can see what are clearly a number of vents inside the SLA. (the crossed brace is not flight hardware, it merely supports the aft end of the SLA for display).

enter image description here

A closeup of one of the vents

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The outside of the vents on this SLA are covered with metal disks because it was displayed outside for many years. But knowing what to look for, we can see the vents on a stacked Saturn on the pad.

enter image description here


As far as the Lunar Module (LEM) goes, we know that operationally it had a ~ 5 psi pure oxygen atmosphere.
  • (original statement) It's inconceivable that the light weight LEM structure could have withstood 10 psi crush loads, so the LEM must have been pressurized to 14.7 psi on the pad with pure O2, which then bled down through the positive pressure relief valves during ascent.

  • (corrected) In fact the LEM cabin was pressurized not with pure O2, but with a sea-level type mix of 20% O2 / 80% N2. However, it did indeed blow down through the relief valves.

...the LM cabin at launch was instead charged with 20 percent oxygen-80 percent nitrogen, which was bled down to 5 psia in space, with pure oxygen makeup. During a lunar mission the LM cabin was vented to permit opening the front hatch. Upon the first repressurization the cabin would contain 5 psia pure oxygen.

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  • $\begingroup$ I know that some of the ports on the SLA were for loading propellants to the service module, though: heroicrelics.org/space-ctr/csm-sla/dsc49568.jpg.html -- I still haven't seen anything that's unmistakably an open pressure vent. $\endgroup$ Commented Aug 10, 2018 at 20:01
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    $\begingroup$ Those round circles on either side of the hinge on the side away from the tower certainly were not used to load anything, and match up with the vents in the picture I took. $\endgroup$ Commented Aug 10, 2018 at 20:06
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    $\begingroup$ Very nice detective work, must be handy to have a space center nearby ;-) $\endgroup$
    – uhoh
    Commented Sep 10, 2018 at 0:56
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    $\begingroup$ @uhoh Somewhat surprisingly, Rocket Park where the Saturn and other rockets are displayed is open to all for free - no pricey Space Center Houston tickets or NASA badge required. Just roll up to the main gate and tell the guard you're going to Rocket Park. Stop in when you're in Houston next. $\endgroup$ Commented Sep 10, 2018 at 19:41
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    $\begingroup$ The transcript from Apollo 17 suggests that's the case, while it only talks about pressurising the tunnel, it uses the whole of the CM re-pressurisation package to do so. $\endgroup$
    – JCRM
    Commented Aug 10, 2023 at 13:13

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