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I watched the launches of SpaceX Crew Demo 2 and Crew 1. I observed that there were no planned holds during the last hour of the countdown.

The Space Shuttle had planned holds at T-20 minutes and T-9 minutes. cite: NASA Countdown 101

Just as the title says, why does SpaceX NOT have these planned holds?

Note for confused commenters and answerers: A planned hold does not affect the launch time. It is already planned into the countdown!

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    $\begingroup$ Two words: Cryogenic Fuels. $\endgroup$ – GdD Dec 16 '20 at 16:21
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    $\begingroup$ Maybe you should be asking why the shuttle had planned holds rather than why SpaceX don't. $\endgroup$ – GdD Dec 16 '20 at 16:31
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    $\begingroup$ @GdD four words: Shuttle had cryogenic fuels $\endgroup$ – Organic Marble Dec 16 '20 at 17:37
  • $\begingroup$ What is the purpose of having a countdown during a rocket launch? is really relevant to this question and might help an answer. Also according to wikipedia "Launches of the Space Shuttle to the International Space Station did not allow for holds to be extended due to the launch window which was limited to no more than 10 minutes" $\endgroup$ – mgarey Dec 16 '20 at 18:23
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    $\begingroup$ On the last launch, SpaceX pushed the launch by about 90 minutes about 60 minutes before the originally scheduled launch. How is being able to rapidly reschedule functionally different from a planned hold? And if it is, what would be the advantage of having a planned hold as opposed to planing the hold time into the countdown in the first place? $\endgroup$ – Jörg W Mittag Dec 16 '20 at 21:09
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Two issues:

  1. The launch window for ISS rendezvous orbits are instantaneous. So how does a planned hold help? You cannot miss the launch window, regardless, else you wait another day.

  2. The Cryo fluids as @GdD commented, are sub-cooled, and in the warm Florida sun they warm up. They are cooled below the usual temperature, since they become denser when colder. The performance and trajectory assume the use of the sub-cooled propellants and thus if they warm up too much, they cannot launch.

So how would a planned hold help? They are on a ticking clock as things warm up regardless.

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    $\begingroup$ Shuttle rndz flights had planned holds. They don't affect the launch time because they are, urm, planned. I think you might want to examine your assumptions about what exactly a planned hold is. $\endgroup$ – Organic Marble Dec 16 '20 at 17:34

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