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Did the astronauts aboard Apollo 11's Lunar Module have any stated or imposed limitation or restriction on the amount of fuel they could use while in lunar orbit and performing landing maneuvers? I am asking this question because of following possibilities which I could imagine:

  1. They could have ran out of fuel (because of excessive maneuvering during descent) from descent stage before actual touchdown and consequently touch down at higher speed.

  2. Similarly running out of fuel while ascend, and eventually unable to dock with CSM.

Were there any instructions or schedules for fuel tank swapping or consuming from a specific tank?

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  • $\begingroup$ I've made some small edits for grammar, can you double check that I haven't accidentally modified the meaning of your question? Please feel free to edit further. $\endgroup$
    – uhoh
    Dec 26, 2018 at 5:45

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The Apollo LM had three independent propellant supplies: tankage in the descent stage usable by the descent engine, tankage in the ascent stage for the ascent engine, and in the ascent stage for the reaction control system (RCS) thrusters.

Prior to the initiation of descent and landing, only the RCS would be used, and very little of it.

During the descent, fuel quantity was carefully monitored. There was a low-level threshold (5% of total tankage) which would illuminate a fuel quantity light in the LM cabin; once that light came on, mission control started a 90-second countdown to a "bingo" call, meaning a fuel-critical state. At the "bingo" call, the mission rules specified that the LM had to touch down in the next 20 seconds or abort. This gave the final decision to the mission commander in the LM; if they were 50 feet from the surface and coming down smoothly at "bingo" they could go ahead and land. The abort mode in this case would involve going to full thrust on the descent engine (if there was significant fuel left), then staging and firing the ascent engine to return to a stable orbit. Touchdown at higher speed wasn't likely; if they couldn't land safely they'd abort.

Apollo 11 and 14 came closest to depleting the descent stage fuel; for 11, at touchdown there was 18 seconds left to "bingo", and probably 45-60 seconds of fuel actually remaining -- 770 lbs of fuel. The other landings had 1100+ lbs of fuel at touchdown.

I don't know if there were mission rules regarding the RCS fuel during descent; the gimbaled engine on the descent stage allowed most of the descent phase to be executed with very little use of the RCS.

The ascent trajectory was also computer-controlled, so there was relatively little variation in fuel usage until orbital insertion. Once in orbit, only the RCS would be used to maneuver the LM to rendezvous. It was possible to transfer remaining ascent fuel to the RCS tanks, and if, somehow, the RCS was completely depleted, the Command/Service Module (CSM) had its own RCS and could take over the active role in the rendezvous, and reach the LM.

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  • $\begingroup$ @ Russell Borogove. Thanks a lot. Very interesting information. $\endgroup$
    – Niranjan
    Jan 2, 2019 at 6:42
  • $\begingroup$ @Niranjan If you feel the question is sufficiently answered, you can click the check mark to accept it. $\endgroup$
    – Russell Borogove
    Jan 2, 2019 at 13:50
  • $\begingroup$ Any clue about fuel burn rate for descent stage at various thrust percentage? I can only find data for ascent stage engine: hq.nasa.gov/alsj/LM09_Main_Propulsion_ppMP1-22.pdf $\endgroup$
    – jumpjack
    Jan 11, 2022 at 12:07
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    $\begingroup$ @jumpjack Per the answer to another question here, at 100% throttle the descent engine's specific impulse (thrust per mass of fuel expended) is 305 seconds, and at 30% throttle it's 298 seconds -- that is, it changes very little with throttle on the LM, so fuel burn rate is approximately proportional to thrust within a couple of percent. $\endgroup$
    – Russell Borogove
    Jan 13, 2022 at 22:18

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