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Regarding this tweet from the man himself. I take it to mean abort modes in some form or other for the entire mission? Perhaps just after LM descent? What are the 18 failure modes, and (if there's any way to speculate), which 10 would Collins be most likely to have been apprehensive about?

Linked content:

enter image description here

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  • $\begingroup$ Did you see the responses at your link? At least some would be literal "rescues" involving reducing the CM orbit to try to meet an LM that didn't get to the orbit it was supposed to. $\endgroup$ – Chris Stratton Oct 9 at 18:25
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The 18 rescue cases are detailed in the CMP Solo Book and they are all for the undocked phase of the mission after the LM undocks from the CSM in lunar orbit. In most cases the LM can not do or can't fully do the burns required to get back to the CSM in an emergency, so the CSM has to come and rescue it.

Here is the page with the list of rescue cases: Apollo 11 Rescue Cases

Source: Apollo 11 CMP Solo Book

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  • $\begingroup$ Thank you! Could you provide more information on what distinguishes these cases? $\endgroup$ – Anton Hengst Oct 10 at 0:17
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    $\begingroup$ @AntonHengst most of them are varying cases of "the LM got to stage X in the descent before it had to abort", with many of them subdivided according to whether a certain burn didn't happen at all, the partial burn was small, or the partial burn was larger. Each of those cases would have the LM in a different-shaped orbit. Other cases are the LM making an unscheduled emergency liftoff after landing, or lifting off without a working primary guidance computer. $\endgroup$ – hobbs Oct 10 at 5:05
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indy91 posted an image copy of a page from Michael Collin's CMP Solo Book. The text contents of that image are

APOLLO 11 FLIGHT DATA FILE CMP SOLO BOOK
RESCUE INDEX
CASE 1 - PARTIAL DOI ( <25 FPS) (CSM ACTIVE)
CASE 2 - PARTIAL DOI (>25 FPS) (CSM ACTIVE)
CASE 3 - NO PDI1, +12 (LM ACTIVE)
CASE 4 - <60 NO PDI1, +12 (CSM ACTIVE)
CASE 5 - ≥60 NO PDI1, +12 (CSM ACTIVE)
CASE 6 - PDI1, <10 VARIABLE INSERTION (LM ACTIVE)
CASE 7 - PDI1, +12 (10-12.5 MINUTES) (CSM ACTIVE)
CASE 8 - PDI1, +14:12 (12.5-15 MINUTES) (CSM ACTIVE)
CASE 9 - PDI1, +21:24 PREFERRED LIFT-OFF(T2) (CSM ACTIVE)
CASE 10 - NO PDI2, +12 (LM ACTIVE)
CASE 11 - <40 NO PDI2 +12 (CSM ACTIVE)
CASE 12 - 40 - 90 NO PDI2, +12 (CSM ACTIVE)
CASE 13 - ≥90 NO PDI2, +12 (CSM ACTIVE)
CASE 14 - PDI2 <14:30 VARIABLE INSERTION (LM ACTIVE)
CASE 15 - PDI2 +19:22 PREFERRED LIFT-OFF (T2) (CSM ACTIVE)
CASE 16 - CONTINGENCY INSERTION ORBITS (CSM ACTIVE)
CASE 17 - MANUAL INSERTION (CSM ACTIVE)
CASE 18 - ANYTIME LIFT-OFF (CSM ACTIVE)

These eighteen cases can be distinguished by the contingency that required a rescue mission to be performed, and which vehicle is the active1 vehicle in the rendezvous. Regarding which would have made Michael Collins apprehensive, I suspect that included those cases in which

  • The Lunar Module and/or the Command/Service Module had a chance of crashing into the Moon,
  • Orbital eccentricity made the Clohessy-Wiltshire approximations used for proximity operations invalid,
  • The computer on the LM failed, requiring the LM crew to perform the liftoff or orbit insertion manually, or
  • One or both of the astronauts on the LM had a serious medical issue.

I'm surprised Michael Collins didn't say all of them made him apprehensive, but then again I am nowhere close to having the nerves of steel required of an astronaut.


1Modern nomenclature uses "chaser" instead of "active", and "target" instead of "passive".
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