During EVA 2 of Apollo 17, Gene Cernan pointed out that having his PLSS on the "min cooling" setting was a little warm for him. That's probably because driving the lunar rover made his metabolic rate ~10% higher than Jack Schmitt's. Cernan even asked mission control if his cooling was working fine.

Why didn't he simply increase the cooling setting of his PLSS while driving? If the astronauts had to follow a strict cooling water rationing protocol (e.g. only use the "min cooling" setting while riding the LRV), where can I read more about it?

I understand that the astronauts had to ration cooling water (and other resources), but was there a protocol in place forcing them to use the "min cooling" setting on the while riding the LRV? (This particular EVA was the longest of all Apollo missions).

  • 1
    $\begingroup$ Cooling was done by evaporating water into the vacuum, increased cooling needed more water. If the lunar rover would have failed, walking back to the Lunar module would have increased the water consumption. There should be enough water to return by feet, enter the LM and connect the suits to the LM oxygen supply. $\endgroup$
    – Uwe
    Dec 30, 2022 at 21:04
  • $\begingroup$ @Uwe ... NASA should have equipped them with a parasol. The PLSS contained 2.7Kg of water, significantly more than the mass of an umbrella. I wonder how long a walk-about would need to be for the mass of saved water to exceed the mass of the parasol. Wouldn't need to worry about it blowing inside out. esquire.com/lifestyle/g3482/best-beach-umbrellas /;0) $\endgroup$
    – Woody
    Dec 30, 2022 at 22:13
  • $\begingroup$ Ha that's indeed food for thought! Going back to operating with the PLSS, were there any guidelines that the astronauts had to follow, especially for this particular EVA? (For instance, only use min cooling during LRV traverses?) I understand that they had to save water, but I haven't read anywhere what was the procedure to ration it during the EVA. $\endgroup$
    – olamarre
    Dec 31, 2022 at 1:30
  • $\begingroup$ @olamarre ... I'm sure there were extensive contingency plans for the EVA dealing with consumables (water, O2, battery) as well as back-ups (emergency O2) and back-ups to the backups (buddy breathing and sharing cooling capacity). What would the consumption rates be if one astronaut was injured and need to be carried? The combinations boggle my mind. I bet the route was planned fast-out, slow-back. $\endgroup$
    – Woody
    Dec 31, 2022 at 2:47
  • $\begingroup$ @Woody, the EVA contingency was quite simple: if the cooling system, the oxygen system, the LiOH system, the battery, the fans, or just about anything else failed, the astronaut would switch to the emergency oxygen supply. This was a pressure-driven, open-circuit system that would provide fresh air and cooling for 30 minutes -- enough time to get back to the LM and hooked up to the LM's suit supply system. $\endgroup$
    – Mark
    Dec 31, 2022 at 4:06

1 Answer 1


The PLSS doesn't have fine control over cooling. It has three settings: minimum (circulate water at 75-80 degrees F), intermediate (circulate water at 60-65 degrees F), and maximum (circulate water at 45-50 degrees F). Astronauts would keep the cooling system at "minimum" almost all the time, occasionally raising it to "intermediate" during periods of heavy exertion. During something as low-activity as driving the rover, it's likely that increasing the cooling would result in Cernan shivering from the cold.

  • $\begingroup$ Great reference! Do you know if the diverter valve control handle had "detents" and could only be placed in the 3 positions? It doesn't look like it from the drawing. i.imgur.com/25enbmx.png But perhaps anything not in "hot" or "cold" was "intermediate". $\endgroup$ Jan 4, 2023 at 22:39
  • 3
    $\begingroup$ The Apollo Lunar Surface Journal notes that the valve had detents and could theoretically be set between those detents. Given that the valve is being operated by touch using fairly clumsy gloves, I don't think any non-detent setting is practical. $\endgroup$
    – Mark
    Jan 4, 2023 at 22:50
  • $\begingroup$ Another great link! Thanks. $\endgroup$ Jan 5, 2023 at 0:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.