We all love the Apollo 13 story, but the famous lithium hydroxide CO2 scrubber of that mission has made it hard for me to find information about its regular use.

What happened to the old canisters after they were used up? Were they manually replaced? Were they soggy & wet & have to be wrapped up or otherwise prevented from leaking? Were they tossed behind the seats with the poop bags?


2 Answers 2


The canisters were manually replaced. Here is the appropriate checklist section.

enter image description here

From CSM Operations Checklist Section 10 Systems Management

Used canisters were stowed in the container that the unused canister was removed from.

The CO2-odor absorber filter change sequence involves numerically identified filters and alphabetically identified filter stowage containers. Although the filters are replaced in numerical sequence, the stowage containers are not opened in alphabetical sequence relative to filter replacement. Odd-numbered filters will always be installed in suit circuit canister A (upper) and even-numbered filters installed in canister B (lower). After the proper filter stowage container is located by the crewman, the correct filter is obtained and the filter change accomplished. The used filter is then stowed in the container from which the unused replacement came. Where two filters are stowed one above the other, the used filter will always be placed below the remaining unused filter. This provides a more readily accessible unused filter at the next 12-hour replacement period. Filter replacement data, such as filter number and mission time, is recorded by the crew in the flight log.

From Apollo Operations Handbook page 2.7-10 (emphasis mine)

  1. What happened to the old canisters after they were used up? Were they manually replaced? Were they tossed behind the seats?

@OrganicMarble's answer covers this with reliable sources quoted.

I only wish to add (after going through Apollo-7 to Apollo-11 flight journals), the exact mentioned in communication compartment numbers with (square type Command Module) LiOH canisters:

  • Apollo-7: A-2 (fresh canisters)
  • Apollo-8: A-3 (fresh canisters)
  • Apollo-9: A-5 (fresh canisters); B-5 (used canisters)
  • Apollo-10: A-3 (used canisters)
  • Apollo-11: B-6 (used canisters, to be later discarded with LM jettison to free up the space for moon soil samples)

Below is the graphical description of the compartments borrowed from Apollo-13 Flight Journal: enter image description here

The two types of cylindrical LiOH canisters in LM module, as well as cylindrical LiOH canister in PLSS were meant to be discarded either by jettisoning onto the lunar surface or with the ascent stage (being installed in the Environment Control System).

  1. Were they soggy & wet & have to be wrapped up or otherwise prevented from leaking? 

This NASA document (although related to Shuttle CO2 scrubbing systems) and this CO2 scrubber Wikipedia article (describing a generic LiOH CO2 scrubber) suggest that chemical reaction between solid LiOH and gaseous CO2 produces gaseous H2O as by-product:

2LiOH(s) + CO2(g) → Li2CO3(s) + H2O(g)

The reaction is exothermic, so perhaps the generated water vapour wasn't readily condensing within the LiOH bed itself. There were other systems to remove moisture from the circulated air, so the moisture from the CO2 scrubbing process would just add an additional load on those systems.

I believe that the assumption of the canisters being dry can be supported by the following quote from Apollo-7 crew when they were trying to troubleshoot the case of water leaking from the oxygen hose:

079:29:19 Cunningham: Roger. We also just discovered water coming out of our blue hoses, at least the one in the center couch...

080:15:40 Cunningham: Roger. And we checked our lithium hydroxide canisters. They are dry

Having said that, there was an issue with CO2 removal in LM after lift-off from the Moon, during rendezvous with CSM Apollo-11 Flight Journal Day6/Part2, when Mission Control suggested it might have been due to some water channeling inside the canister:

126:01:15 Evans: Eagle, Houston. Could you verify you switched lithium hydroxide canisters? Over.

126:01:24 Aldrin: That's affirmative. We started getting an erratic indication on primary [LiOH canister], so we switched to secondary [LiOH canister], and it was again erratic, and I thought it might have been a sensor. But it's settled down now, and we're on the secondary. Over.

126:48:47 Evans: Roger. In the event of the possibility that we may have had some water channeling in those hydroxide canisters, we recommend you stay in the cabin mode from now on. Over.

The above quote may be interpreted in the way that potential water in the canisters were treated as a source of malfunction rather than being a normal operation.

Emphasis added

  • 3
    $\begingroup$ Fantastic answer! If I could give two accepted, I would. Thank you for answering my second question in such thorough detail. $\endgroup$ Jan 27, 2020 at 5:46
  • $\begingroup$ Think you're right about the water. On shuttle the condensing heat exchanger which removed water vapor from the air loop was placed just after the LiOH canisters. nasa.gov/centers/johnson/pdf/383445main_eclss_21002.pdf page 3-2 $\endgroup$ Jan 27, 2020 at 12:22
  • 2
    $\begingroup$ @OrganicMarble The CO2 absorbition with LiOH works better with humid air than with dry air, therefore the condensing heat exchanger should not be placed before the LiOH canisters within the air loop. $\endgroup$
    – Uwe
    Jan 27, 2020 at 13:12

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