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One of the dramatic moments of Apollo 13 (in real life, not just the movie) was when they had to build scrubbers for the LM ("fit the square peg in a round hole").

Why did they have to do that? The LM was attached to the CM, and the airlock was open. Why couldn't the CM scrubber scrub from the CM rather than the LM?

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They needed to remove carbon dioxide from the air. As the astronauts exhaled, the carbon dioxide accumulated and if left unchecked, would reach a high enough partial pressure that they would no longer be able to eliminate carbon dioxide from their bodies and take in oxygen.

The Apollo spacecraft (both the CM and the LM) used lithium hydroxide canisters for this purpose. As they became saturated in the LM, they needed to be replaced to continue working. But there were no spares available for the LM, as it was only intended to support two astronauts for a few days. Thus they had to adapt CM canisters to work in the LM.

The CM scrubbers could not operate, as the entire spacecraft was shut down. It could not use electrical power from the SM, as all the fuel cell oxygen there had been lost. The only electrical power it had available was in the reentry batteries, and they needed to conserve that for actual reentry.

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  • $\begingroup$ So why couldn't they purify the air from the CM? $\endgroup$
    – shuttle
    Feb 8, 2018 at 4:43
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    $\begingroup$ The CM was shut down due to lack of power. The scrubbers there could not operate. $\endgroup$
    – Fred Larson
    Feb 8, 2018 at 4:43
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    $\begingroup$ Active. The would be no passive way of passing air through the scrubbers, it had to be blown by a fan. No convection in zero G, you know, and I don't think there would be any significant convective flow even on earth. $\endgroup$
    – Fred Larson
    Feb 8, 2018 at 4:46
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    $\begingroup$ The removal itself is passive. Exposing lithium hydroxide to carbon dioxide results in lithium carbonate and water, plus some heat. The reaction occurs at room temperature. In order for this reaction to occur, the LiOH needs to be exposed to CO2 -- and that means a fan is needed. $\endgroup$
    – David Hammen
    Feb 8, 2018 at 10:10
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    $\begingroup$ @FredLarson: And manually pumping the air through the scrubbers would probably produce more CO2 than would be removed by the scrubbers. $\endgroup$
    – Vikki
    Apr 25, 2019 at 0:03

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