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During Apollo 13, the Lunar Module had to be used as a lifeboat in order to save the astronauts. Then, it was only jettisoned from the Command Module shortly before Earth re-entry (even after the Service Module separation). (Source: Apollo 13)

My thinking is that it would have been wise for succeeding missions (14-17) to keep the LM docked for the way back to Earth, to have a lifeboat in case of any similar problems with the SM/CM.

Instead, the LM was jettisoned while still being in lunar orbit and commanded to impact on the moon surface to measure the seismic effects with sensors deployed while at the moon surface. (Source: Apollo 14)

If I had to weigh up the pros/cons of having a lifeboat to save the Astronauts in case of emergency against some seismic measurements, I would rather go with the lifeboat. However, for some reasons NASA seems to have decided differently. Am I missing anything? Does anyone have any insights or ideas and can think of any reason why they did it this way?

jakob.j
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    Ask yourself: What's the difference between a fresh, unused LM, and one where the consumables are all expended? – Organic Marble Jul 11 '21 at 16:45
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    Most of the resources the LM contained would have been used during the lunar excursion mission. The LM would have predominantly been dead weight which would have required more fuel to be transported to move both the CSM & the LM. – Fred Jul 11 '21 at 16:46
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    It has weight. That means fuel to move it, and that fuel needed to be brought all the way. So something else would have to go to make room for that. – Thorbjørn Ravn Andersen Jul 12 '21 at 10:47
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    It'd have made a welcome toilet – Innovine Jul 12 '21 at 14:26
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    Another thought is that the LMs 14-17 had all been on the Moon. What if, maybe, something snuck aboard?... – Oscar Bravo Jul 13 '21 at 10:01
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    "in case of any similar problems" in too imprecise. What kind of failure scenario are we talking? The fact that Apollo 13 escaped total loss is down to extreme luck IIRC. Had the event occurred a bit later, or had someone made a bad decision at the control center, there would not have been a chance for Apollo 13 to come back with life aboard. If you have a makeshift lifeboat and the command module becomes unusable for some reason, you are no better off: No re-entry for you. – David Tonhofer Jul 13 '21 at 15:05
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    Turns out that I was thinking too much "ship lifeboat" like and too less "spacecraft lifeboat" like, with brings the importance of energy reserves and resources with it. Thanks for all the great answers and comments! – jakob.j Jul 13 '21 at 17:55
  • @Innovine: After the Apollo 10 floating mystery turd, I doubt it would be welcome. – DrSheldon Jul 14 '21 at 02:17

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Without the descent stage, you have a far less capable lifeboat. Looking at Wikipedia, the ascent stage had two batteries with a total capacity of 592 Amp-Hours (some of which would have been used in the ascent), while the descent stage had four batteries with a total of 1660 Amp-Hours (raised to 2075 Amp-Hours for Apollo 15-17). So most of the power that kept Apollo 13 going would not be available in your scenario.

Looking at oxygen, this source the ascent stage carried 2.43 pounds of oxygen, and 48 pounds in the descent stage. For comparison, the service module carried 652 pounds of oxygen.

Given that some of this limited supply would have been used during the LM ascent and docking, there weren't enough supplies aboard the ascent stage to be useful.

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Eugene Styer
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    Also the A-13 astronauts used fuel from the LM to make periodic course corrections on the way back to Earth. The LM in a successful Moon landing would have used most of its fuel for that operation. – Oscar Lanzi Jul 14 '21 at 00:46
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The primary purpose of every Apollo Moon landing mission was to land on the Moon. Once the vehicle had passed safety checks and had made the lunar orbit insertion burn, the next step was to separate the Lunar Module from the Command and Service Module. If these steps did not happen the mission would have been a deemed loss of mission failure.

Aside: There's a huge difference between Loss Of Mission and Loss Of Life. Apollo 13 is the quintessential example of a Loss Of Mission failure that was not a Loss Of Life failure.

In the case of a successful separation, Moon landing, ascent, and docking, the job of the Lunar Module was finished upon docking with the Command and Service Module. There was no point in keeping the ascent stage attached, and many reasons to not keep it attached. The primary reason against keeping the ascent stage attached was that the ascent stage represented a huge mass penalty.

The next task for the Command and Service Module after ascent stage docking was to perform the burn that would take the CSM back to Earth. The Command and Service Module was sized to perform that maneuver with three humans and bunch of lunar rocks as payload. Keeping the ascent stage attached would have represented a huge mass penalty. This would have required a resizing of the Service Module.

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David Hammen
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    the burn that would take the CM back to Earth was done by the Service Module. This would have required a resizing of the Service Module. Separation of the CM from the SM was done just before reentry. – Uwe Jul 11 '21 at 18:59
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    Looking at Apollo by the Numbers, I think the "empty" LM ascent stage would have added about 15% to the CSM weight at return. So it might just have been possible within existing safety margins for fuel, had they needed to do it as a once-off - I can't find a clear statement of how much the CSM had left over. Agree it would have needed resizing if a planned thing, though! – Andrew is gone Jul 12 '21 at 09:06
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    @Andrew: Bay 1 had ballast so some fuel capacity existed by replacing it with tankage. Also, the LM had fuel remaining and it's not well-known but the ascent engine could be restarted so you had little more margin. And of course, if your fuel burn was higher than expected, you leave it. – Joshua Jul 12 '21 at 23:44
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Another factor here: The reason the lunar module existed in the first place. The Saturn V was nowhere near powerful enough to send a rocket to the moon and bring it back. It would have taken at least two Saturn V launches to lift something that heavy. This was being a big problem for NASA in mission planning--which was finally solved by the approach NASA used of not taking the return fuel down to the surface and not bringing the lander back.

To try to bring back the lander would have required a lot more fuel in the service module which would have put it way over the weight the Saturn V could lift. Besides, if you wanted more reserve capacity it would have been a lot more efficient to simply add it to the service module than take it down to the lunar surface and bring it back up.

Loren Pechtel
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    Another point to consider is that the CM/SM combo was expected to burn fuel to enter lunar orbit, and again to leave it. Skipping those parts of the mission meant that Apollo 13 craft were able to carry the LEM back to earth, but if the failure had happened after the lunar insertion burn, there wouldn't have been enough extra fuel to get the LEM out of orbit. – supercat Jul 12 '21 at 05:28
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    This demonstrates clearly how close to the limit of technology this was. They only barely could do it. – Thorbjørn Ravn Andersen Jul 12 '21 at 10:48
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    @ThorbjørnRavnAndersen Not really, they could have built a larger rocket or assembled something in Earth orbit using multiple launches of the rocket they had. It all comes down to how much money you want to spend which is a purely political decision. – Nobody Jul 12 '21 at 15:01
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    @nobody. A larger rocket? It still has to lift itself. It would be much more expensive. – Thorbjørn Ravn Andersen Jul 12 '21 at 16:24
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    @nobody also I don’t think you understand how much effort went into doing this due to the politically decided deadline… – Thorbjørn Ravn Andersen Jul 12 '21 at 16:26
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    @ThorbjørnRavnAndersen Because you used that same phrasing: "I don’t think you understand" the point regarding "It all comes down to how much money you want to spend which is a purely political decision" when you answer "It would be much more expensive." – Nobody Jul 12 '21 at 18:44
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    @nobody the budget was pretty big. https://www.planetary.org/space-policy/cost-of-apollo The hard limit was “before the end of the decade”. – Thorbjørn Ravn Andersen Jul 12 '21 at 19:41
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    @ThorbjørnRavnAndersen Well, that and "before the Russians". Without that competition, they might not have put quite so much resources into it. – Darrel Hoffman Jul 12 '21 at 20:33
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    @ThorbjørnRavnAndersen Yet no one starved or was worked to death, so they could have gone bigger with the budget. – Nobody Jul 13 '21 at 07:53
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    @Nobody Engineers in western countries tend to quit and look for better options if worked to death. – Christian Jul 13 '21 at 16:32