In the transcript (1 and 2) of Apollo 11 lunar landing we can see ground control and crew talking about "T1" moment and "T2" moment:

  • 102:47:09 Duke: ...T1. Over. Eagle, you are Stay for T1.
  • 102:47:12 Armstrong: Roger. Understand, Stay for T1.
  • 102:47:15 Duke: Roger. And we see you venting the Ox(idizer).
  • ...
  • 102:51:45 Duke: Eagle, Houston. You are Stay for T2. Over.
  • 102:51:50 Duke: Correction, you're...
  • 102:51:52 Armstrong: Roger. Stay for T2. We thank you.
  • 102:51:54 Duke: Roger, sir.

I find very difficult to find official documentation explaining this 2 moments, so I tried collecting & comparing all the descent checklists in this page, based on images found here:

"T1" is explicitly declared at page 10 of Timeline Book:


It means that T1 = PDI+15min, and PDI is defined at page 8 as being 102:35:13:


Hence T1 = 102:50:13

Then things become tricky, because apparently T1 is also defined as TD+3 (top of page 10):


But page top also says that page itself covers period from TD+3 to T2... but apparently T2 is not explicitly declared: we only have a "no-stay" checklist at PDI + 19:30:


PDI+19:30 means 102:35:13 + 00:19:30, hence 102:54:43 .

Is there any confirmation that this is T2 time? I read about that T2 is instead the first time CSM comes back over the LM after one orbit.... but then I found also this on page 11:

1st REV

Is this "first CSM revolution"? It occurs at 104:42:05, i.e. around 2 hours after touchdown (102:47:11 planned, 102:45:40 actual), and some sources declare that T2 was planned 2 hours after touchdown... But probably this is instead T3?

Then what is this on page 11?


  • PDI+13 = 102:35:13 + 00:13:00 = 102:48:13
  • PDI+23 = 102:35:13 + 00:23:00 = 102:58:13

Aldrin in 1969 debriefing said "the Tig for this T-2 abort, which is PDI plus 23", explaining why ground control asked him to input 10254 "rather than something else" (which he typed wrong, he typed erroneously PDI+12).

I ended up with this preliminary table (from timeline, flight plan and CMP solo checklist), but I am really confused...

time table

  • 1
    $\begingroup$ I thought this would be defined in the Mission Rules, but if it is, I can't find it. $\endgroup$ Commented Feb 24, 2022 at 16:47
  • $\begingroup$ I have this vague recollection that it had something to do with whether the abort would be accomplished with the descent engine vs. the ascent engine. But I have no references to back that up, so it's probably completely wrong. $\endgroup$
    – Dave Tweed
    Commented Feb 27, 2022 at 16:49
  • $\begingroup$ The descent engine tanks are suppose to be depleted at landing $\endgroup$
    – jumpjack
    Commented Feb 27, 2022 at 21:51

1 Answer 1


As far as I can tell, T1 and T2 abort times were determined based on information calculated with the Real-Time Computer Complex (RTCC).

Nominally, according to Luminary Memo #66 (Cherry, February 1969), T1 abort would be at approximately TFI=980s (PDI+16:20) and T2 abort at approximately TFI=1380s (PDI+23:00), where TFI is time from PDI ignition. I believe, however, that these times vary slightly depending on specific orbits of the CSM and the position of the LM on the lunar surface.


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