To minimize the re-entry mass of the Apollo command module, essentially all of the mission supply of command module consumables was stored in the service module. Was there a similar arrangement between the ascent and descent stages of the lunar module? In other words, to minimize lunar liftoff mass, were the majority of lunar module crew consumables (oxygen and water) stored in the descent stage? How much reserve was carried in the ascent stage (how much endurance was available for rendezvous attempts)?


3 Answers 3


Was there a similar arrangement between the ascent and descent stages of the lunar module? In other words, to minimize lunar liftoff mass, were the majority of lunar module crew consumables (oxygen and water) stored in the descent stage?

To some degree, yes, consumables were split across the descent and ascent stages with the majority kept in the descent stage.

From the specifications, we can see, for example:


  • Ascent stage: 85 lbs
  • Descent stage: 333 lbs


  • Ascent stage: 592 Amp-hours
  • Descent stage: 1660 (early) or 2075 (A15-A17) Amp-hours

From Apollo Experience Report - Lunar Module Environmental Control Subsystem:


  • Ascent stage: 4.8 lbs
  • Descent stage: 48 lbs

In addition to the O2 capacity, the lithium-hydroxide scrubber cartridges which removed carbon dioxide from the cabin air, as we know from Apollo 13, were also a limited consumable resource in the LM. The scrubbers do not return free oxygen to the air, so oxygen consumption is still a limit.

How much reserve was carried in the ascent stage (how much endurance was available for rendezvous attempts)?

Unfortunately, I have not found a simple answer to the "what was the endurance of the ascent stage" question.

@Uwe tells me a person uses 0.35-1.1 g/min of oxygen depending on activity levels; that works out to a 16-48 hour supply of oxygen for two crew if the ascent O2 tanks are full at liftoff.

For the CO2 scrubber cartridges, we can make a rough estimate using times from Apollo 13 and Apollo 12.

On A13, the crew of three were on the LM's life support starting at around 59 hours into the mission; they had to install an incompatible cartridge from the CM using an improvised adapter at around 90 hours, so the LM carts were expended after 31 hours: about 93 crew-hours worth of LM scrubbers. For the normal 2-person LM crew that would be ~46 hours.

On A12, two crew were on the LM's life support starting at about 105 hours, and the ascent stage left the moon at about 142 hours. Subtracting 8 hours of EVA (when the crew would be on PLSS rather than on the cabin air), that works out to 29 hours, so the ascent stage should be able to provide fresh air for roughly 17 hours after liftoff - more limiting than the O2 capacity.

A scenario where the ascent stage's endurance would be severely tested is unlikely. None of the Apollo flights had any difficulty with the ascent and rendezvous portion of the mission. If the ascent stage didn't reach a safe orbit, it would crash to the surface of the moon within 10 minutes. Once safely in orbit, either the CSM or the ascent stage could perform the rendezvous maneuvers: 100% redundancy of what were already, individually, reliable spacecraft. Even if the CSM and ascent stage couldn't hard-dock, the last resort would be to EVA back to the CSM cabin.

  • $\begingroup$ ... only around 10% ? ? $\endgroup$
    – amI
    Commented Nov 10, 2018 at 19:39
  • $\begingroup$ @aml Yes: days on the surface, 4 hours ascent-to-rendezvous. $\endgroup$ Commented Nov 10, 2018 at 19:49
  • $\begingroup$ CO2 scrubbers only removed the exhaled carbon dioxide from the cabin atmosphere. But to hold the pressure at nominal value, the consumed oxygen should be replaced, about 0.8 litre (at 1 bar or 2.4 litre at 0.33 bar) per astronaut at rest and minute. $\endgroup$
    – Uwe
    Commented Nov 10, 2018 at 19:49
  • $\begingroup$ @Uwe I thought the scrubbers freed the oxygen back. Do you know what the consumption rate is in mass units rather than volume? $\endgroup$ Commented Nov 10, 2018 at 19:51
  • 2
    $\begingroup$ @Uwe - Apollo used Lithium Hydroxide scrubbers, which made Lithium Carbonate and gaseous H2O -- no free O2. $\endgroup$
    – amI
    Commented Nov 10, 2018 at 20:10

The service module and descent stage did share the same design philosophy that any mass not needed to safely return the crew and scientific results would be left behind. However, there are both similarities and differences between the two modules.

  • Both the service module and the decent stage have a main engine, plus tanks with the fuel and oxidizer for these engines. However, the partner modules differ whether they also have a main engine/fuel/oxidizer; the ascent stage does, whereas the command module does not.
  • The service module has a reaction control system (RCS) for attitude control and low amounts of thrust; the descent stage has no RCS. In the partner modules, the ascent stage has a complete RCS, whereas the command module has a limited number of RCS thrusters.
  • Electrical power in the service module comes from three fuel cells. This is supplanted by 3 rechargable batteries in the command module for high current demands, reentry, and post-landing; 2 non-rechargable batteries in the CM for pyrotechnics; and after Apollo 13, one auxiliary battery in the SM. The descent stage has 4 batteries, and the ascent stage has 2 batteries.
  • The service module has oxygen tanks (which infamously blew in Apollo 13) for cabin oxygen and the fuel cells, plus hydrogen tanks for the fuel cells. The descent module has only an oxygen tank.
  • Water was produced by the fuel cells in the service module, and stored in tanks in the command module. In the descent stage, one or (Apollo 15-17) two tanks were filled with water prior to launch.
  • Neither the service module nor the descent stage carried crew, food, waste, personal items, instrumentation, or controls.
  • The service module had heat radiators; the lunar module used heat exchangers and sublimators to remove excess heat.
  • The service module had both high-gain (dish) and low-gain (scimitar) radio antennas. No antennas were mounted on the descent stage.
  • The descent stage and ascent stage each had radar. Neither the command module nor the service module had radar.
  • The descent stage was the only module with landing gear.
  • Prior to Apollo 15, only the descent stage had a bay to store scientific equipment. Starting with Apollo 15, it was enlarged to hold the lunar rover, and a bay was added to the service module for the service instrument module (SIM).

Update: The OP clarified the question for reserve oxygen (see section below). This section was my answer for reserve propellant.

As far as reserve propellant, the best information that I've found is the level for the "fuel warning" light:

... and a descent propellant quantity low­ level warning light. The low-level sensors provide a discrete signal to cause the warning light to go on when the propellant level in any tank is down to 9.4 inches (equivalent to 5.6% propellant remain­ing). When this warning light goes on, the quantity of propellant remaining is sufficient for only 2 minutes of engine burn at hover thrust (approx­imately 25%).

I am skeptical that the service module and descent stage had comparable levels of reserve fuel/oxidizer. The sizes of the tanks were already established well before the unmanned Apollo test flights. However, the amount of propellants actually used depend on the vehicle masses (which were changing throughout the Apollo program) and the flight parameters (which varied between flights). So the amount of propellants left over will vary a lot, and it's doubtful that they would be comparable between these two modules.

It's like asking how much "reserve" is in your car's gas tank. The tank has a fixed capacity, and the amount you actually use can vary trip to trip.

The missions did carry extra oxygen. Like propellants, the tanks were designed for a certain capacity, and the actual use varied from mission to mission. Here is the CSM usage for Apollo 15: 58.4% of the oxygen was planned to be used, and 59.0% was actually consumed.

Apollo 15 CSM cryogenics

and the lunar module usage for Apollo 15: 47.5% of the descent stage oxygen was planned for use, and 55.7% was actually consumed.

Apollo 15 LEM cryogenics

  • $\begingroup$ The lunar module had heat exchangers and sublimators to remove excess heat by evaporating water. The service module had heat radiators as well as evaporators. $\endgroup$
    – Uwe
    Commented Nov 10, 2018 at 21:42
  • $\begingroup$ @Uwe: I'll take your word on that, even though I can't confirm it. The PLSS for the lunar spacesuits did use a sublimator plate for cooling, but that's not the descent stage. $\endgroup$
    – DrSheldon
    Commented Nov 10, 2018 at 22:10
  • $\begingroup$ @DrSheldon Great answer, just a bit of confusion around "reserve". I was asking in regard to life support reserve, not propellant reserve, although it would be interesting to know how much propellant the ascent stage would have had remaining at rendezvous. $\endgroup$
    – Anthony X
    Commented Nov 10, 2018 at 23:18
  • $\begingroup$ @AnthonyX -- Orloff's Apollo By The Numbers has the info on propellant reserve after ascent. The figures are 284 pounds and up for the landed missions, which is an enormous amount of fuel for the ascent stage. The ascent engine couldn't be restarted, but propellant could be transferred between the ascent engine tanks and the RCS tanks, and the RCS thrusters were nearly as efficient as the ascent engine. Call it 400-500 m/s of ∆v on top of the already substantial RCS propellant allowance. $\endgroup$ Commented Nov 10, 2018 at 23:37

There were tanks for fuel, oxidator, water, helium (for pressurization) and oxygen in both the ascent and descent stage.

Also batteries in both stages, descent: four (Apollo 9-14) or five (Apollo 15-17) 28–32 V, 415 A·h silver-zinc batteries; 135 lb (61 kg) each, ascent: two 28–32 volt, 296 ampere-hour silver-zinc batteries; 125 lb (57 kg) each.

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The rendezvous radar needed for the return to the CSM only in the ascent stage, the landing radar only in the descent stage.


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