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I just read a comment by @RussellBorogove about the Apollo 11 abort options that the landing module had about 700 pounds of fuel left on touchdown. I assume that this is the intended behavior for any detached craft (you probably always plan in a little reserve that you do not want to use up), I wonder if it is generally possible to use that leftover fuel in the next stage:

In an Apollo-style mission, can you recover leftover fuel from the descent stage for the ascent stage and from the ascent stage for the command module? I.e. can you pump it from the earlier stages to the current stage? Did the Apollo missions do that or did they leave fuel on the moon (and whereever the ascent module went)?

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    $\begingroup$ While in case of a staging point as firmly determined as land/relaunch this isn't viable, in general case you can just delay stage separation until you exhaust the last drop from a lower module, and begin the next phase of flight with full tanks farther along the way. Say, don't drop the transfer stage while in lunar orbit, but initialize braking with it, then separate once already in suborbital trajectory. $\endgroup$
    – SF.
    Commented Mar 16, 2016 at 23:58
  • $\begingroup$ Oh, that's a good point. LM could theoretically have started ascent on the last of the descent stage fuel, and staged during ascent, but it would have substantially complicated the guidance program. I think that's a more interesting digression than crossfeed, I'll edit. $\endgroup$
    – Russell Borogove
    Commented Mar 17, 2016 at 0:05

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In general, no; rockets aren't equipped to pump propellant between stages. You load each stage with enough fuel to do its job, plus a safety margin, and if you discard leftover fuel you congratulate yourself for having built a system with reliable performance, and on future missions, you might load a little less propellant and/or take a little more payload!

In the specific case of the Apollo architecture, after the descent, there was known to be more than enough propellant in the LM ascent stage to do the ascent. If the ascent stage underperformed for some reason and launched into too low an orbit, the command/service module carried enough extra propellant to come and get it.

Building additional unused tankage capacity into the ascent stage, anticipating tapping some propellant from the descent stage, would have made the ascent stage bigger and heavier, and keeping the weight of the LM to a minimum was a major engineering challenge in the development of Apollo.

So in the end, the Apollo spacecraft either vented or abandoned propellant from the descent stage (on the moon), the ascent stage (after CSM/LM rendezvous), and the service module (before separation from the CM prior to reentry).

However, the LM ascent stage did have separate tankage for the small RCS thrusters, which used the same propellants as the ascent engine, and it was possible to pump between those tanks! According to Apollo By The Numbers, Apollo 9, 10, 11, and 12 transferred some ascent engine fuel (~20-45kg) to the RCS tanks, and 16 transferred some from the RCS tanks to the ascent tanks. The ascent engine could fire only once, and RCS was used to complete the rendezvous maneuvering with the CSM after ascent, so it makes sense that some unused fuel might be more useful in the RCS tanks at that point, but 130-190kg of propellants remained unused in the ascent tanks on all the landing missions.

As SF. reminds me, the propellant left in the descent stage could have been used by simply reigniting the descent stage when it was time to leave and beginning the ascent with that, then staging onto the ascent stage when descent fuel ran out. This would be a similar flight plan to the LM's abort-from-descent option. All it would accomplish would be saving more fuel to discard later in the mission.

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The Apollo Lunar Module had no fuel transfer capabilities, nor did the astronauts have any equipment to transfer it. Therefore, any left-over fuel was vented, in the case of both stages. (venting is mentioned multiple places in the mission transcripts, for example page 319 of the linked PDF).

A concept for using fuel from the ascent stage to power a smaller "life boat" was considered during the program, but never implemented. (see Lunar Escape System.)

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