After analysing comments in related question How cryogenic oxygen was heated up for CM cabin repressurization? It appears that LM had oxygen stored as either gas or supercritical fluid but not at cryogenic temperatures. According to wikipedia, PLSS could be topped up from LM oxygen tanks.

And the same source quotes 70 to 99 bar oxygen pressure in PLSS tank depending on a mission type. Logically, to make gaseous oxygen flow from LM tank to PLSS tank, LM tank has to have somewhat higher pressure than that. What was the merit to use gaseous oxygen in LM? Liquid would occupy less volume. Was it for the reason to not having the need to heat it up for PLSS's recharge? And, finally, what was the pressure and temperature in the LM oxygen tanks?


2 Answers 2


(Partial answer)

The LEM had ...

... four oxygen supplies : two, in the descent stage, provide oxygen during the descent and lunar- stay phases of the mission: two, in the ascent stage, during the ascent and rendezvous phases of the mission.

The caution and warning limit values can tell us the range of pressures expected to be nominal.

An absolute pressure transducer in each descent oxygen tank feed line generates an output proportional to tank pressure. The output is routed to the Instrumentation Subsystem (IS) ,where it is conditioned to provide a telemetry signal, a caution indication, and through the 02 /H20 QTY MON selector switch (panel 2) , a display on the 02 QUANTITY indicator. When descent tank 1 pressure drops 1 below 135 psia (approximately 5% of capacity), a signal is routed to the O2 QTY caution light.

This tells us the nominal pressure in the descent tanks was between 2700 and 135 psi (~187 to ~9 bar). Page 2.6-13 confirms the upper limit "Descent tank pressure, approximatey 2690 psia..."

A pressure transducer in the fill line of each ascent oxygen tank generates an output that is also conditioned in the IS to provide a telemetry signal, a caution indication, and a display on the 02 QUANTITY indicator. The quantity of oxygen remaining in the tanks is read on the indicator by setting the selector switch to ASC 1 or ASC 2, as applicable. The 02 QTY caution light goes on if, before staging, the pressure in either ascent oxygen tank is less than 684 psia (less-than-full condition ). After staging, the signal that causes this indication is inhibited; instead, the light goes on when pressure in ascent tank No. 1 is less than 100 psia.

This tells us the nominal pressure in the ascent tanks was between 684 and 100 psi (~47 to ~7 bar)

From LM Handbook Volume 1 page 2.6-7

Speculation about why liquid / cryogenic O2 wasn't used: Desire to reduce weight and complexity of the LEM.

Have not found temperature data yet.

  • 1
    $\begingroup$ Agree for less complexity, but "Desire to reduce weight" - don't know... You need much thicker tank wall for 187bar rather than for ~60bar. Ascent stage probably lower pressure because it doesn't refills PLSS and designed for much shorter mission phase $\endgroup$ Nov 18, 2019 at 13:56
  • 1
    $\begingroup$ @LeoS agreed, but then you have to carry something to heat up the LOX before the crew can breathe it. That weighs something. I don't know the tradeoffs, so I wrote "speculation" :) $\endgroup$ Nov 18, 2019 at 13:57

From the Apollo Document NASA TN D-6724


a large descent-stage gaseous oxygen tank (approximately 48-pound capacity at 3000 psi) and two ascent-stage gaseous oxygen tanks (approximately 2.4-pound capacity each at 900 psi).

A pressure regulator was used to reduce the high pressure of the descent stage tank to 900 psi, the ascent stage tanks were connected behind the regulator.

So the design pressure of the descent stage tank was 3000 psi and of the ascent stage tanks were 900 psi, in metric units 206.9 and 62 bar.


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.