In the SpaceX Inspiration4 Launch broadcast at about T-01:31:05 (91 minutes before launch) the term "L minus five" is used. While "T-minus" is a standard phrase in use since the early days of spaceflight, I've never heard "L minus" or "L plus" used before.

Question: How old is the use of "L minus" to denote the number of days before launch?

So the procedure for picking the launch window is actually a little bit different.

We reserved a couple of days -- a couple of twenty four hour days for the mission, but had to wait until L minus five, or five days before the first launch day that was selected to narrow it down to twelve hours. And then after that, we waited until L minus three to select the five hour window that we have today.

And then we have four opportunities in each of those launch windows about an hour apart, to lift off.

So it's a very unique case; so we really just knew just three days ago what the T zero in the launch window would be.

SpaceX Inspiration 4 Launch broadcast at about T-01:31:05 (91 minutes before launch) where the term "L minus five" is used

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    $\begingroup$ It was standard in the shuttle program. L minus was the wall clock time before launch, T minus was the countdown time before launch. So L minus was T minus plus the built in holds. $\endgroup$ Sep 15, 2021 at 23:42
  • $\begingroup$ It's still used. I remember hearing it on several launches recently. Might have been Antares and Arianespace. We normally don't hear it on SpaceX because the do load-and-go, and thus don't have the possibility for holds. $\endgroup$ Sep 16, 2021 at 18:34


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