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What would have happened if the third stage of Saturn V had exploded during TLI? Could the CM of Apollo 11 have safely returned to earth?

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    $\begingroup$ I think it's better to clarify what you mean by "exploded". E.g. one situation is that CM and LM are intact and can be safely detached (so it's like SIVb haven't completed the burn) $\endgroup$ – OON Jun 21 at 12:41
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If the failure of the S-IVB third stage didn't significantly damage the Command/Service Module, and the CSM was able to separate from the S-IVB, then this is a surprisingly benign failure mode and the crew would be likely to return to Earth safely.

Firstly, note that at all times during the TLI burn, the spacecraft trajectory is an elliptical orbit with perigee close to Earth, and apogee rising toward the moon's altitude over the course of the burn.

If the failure happens early in the TLI, then the period of the orbit is a matter of hours, and a slight adjustment of course will put the CSM on a reentry trajectory; this can be done with the small RCS thrusters even if the main SPS engine of the CSM is damaged.

If the failure is later in the burn, the resulting orbit would longer, up to 6 days or so. In this case, the SPS engine can be used to slow the spacecraft, significantly shortening the orbit. The CSM has a great deal of maneuvering capability, so even if the TLI was 90% done, the CSM could cancel all the speed gained during the TLI burn!

Interestingly, the preferred option if the TLI failed for any reason was to attempt an alternate mission plan, i.e. conduct operations in low Earth orbit rather than return immediately. It's even conceivable, if the TLI had progressed far enough, that the CSM could have continued with a lunar flyby or even a lunar orbit mission. If the S-IVB exploded for unknown reasons, however, and it was unclear if there was damage to the CSM, I can imagine them cutting the mission short.

Apollo Experience Report: Systems and Flight Procedures Development talks about TLI aborts a little bit. I believe there wasn't an automated abort system for TLI (since the spacecraft was on a "safe" trajectory and there was plenty of time to decide what to do about any problem).

TLI abort was indicated if the attitude error reached 45º (!) or if attitude rates exceeded 10º per second -- which would probably be the case if the S-IVB were to explode out from under the CSM.

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  • $\begingroup$ @ Russell. Excellent answer. you have covered even those doubts which I did not mention. Thank you so much. Looking forward to more such help in future. $\endgroup$ – Niranjan Jun 22 at 4:06
  • $\begingroup$ Suggestion: explain why the CSM had so much delta-V? $\endgroup$ – John Dallman Jun 22 at 10:46
  • $\begingroup$ @JohnDallman: I think that's worthy of being a separate question, if it hasn't been asked already. I haven't found such a question yet. I think this one comes closest. $\endgroup$ – Fred Larson Jun 22 at 14:12
  • $\begingroup$ The CSM was originally designed as a multipurpose spacecraft, with the capability to lift off from the moon and return to earth on its own. On the Apollo landing missions, it needed to brake both itself and the fully loaded lunar module into lunar orbit, then leave orbit and return to earth. I don’t know off the top of my head how much propellant was loaded and used for a nominal landing mission, but Apollo By The Numbers would have the answers. The CSM didn’t need anywhere near that much delta-v for LEO missions, and the tankage was reduced for the Skylab crew missions and ASTP. $\endgroup$ – Russell Borogove Jun 22 at 15:45
  • $\begingroup$ @FredLarson It’s been discussed in passing in several answers but I don’t think it’s been asked as an independent question; I’ll expand my comment here into an answer if someone asks it. $\endgroup$ – Russell Borogove Jun 22 at 15:46
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Almost certainly, assuming the CSM wasn't damaged. During Apollo 13, they considered (but chose not) to use "direct abort", which would have used the SM engine to return without going around the moon. A failure during TLI would have been much closer to earth, and most likely not travelling as fast.

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