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I just tuned in to the live "broadcast" for the Crew Demo-2 and it looks like the astronauts sat down over an hour and a half ago, and there's still more than an hour to go.

Why do (did) they have to sit there for so long before launch? To calm down? To test all of the controls and run through various drills one last time? Something else?

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All of the above, probably.

Here are just a few of the things that must happen before liftoff.

35 minutes before liftoff, propellant loading starts on the Falcon 9 first and second stage. At this time, the only thing that can save the astronauts is the launch escape system. Which means, the launch escape system needs to be armed before that, then it needs to be checked and verified. Before arming the LES, the astronauts need to be safely secured in their seats with visors down and life-support umbilicals connected. Before that, they need to perform functionality checks and leak checks on their suits. Obviously, the hatch can only be closed after the pad crew helps them do that, but must be closed before the LES is armed. After the hatch is closed, but before the pad crew leaves, they must perform leak checks on the capsule. Only after that can the pad crew leave, and only after the pad crew has left can the LES be armed.

All of this takes time, especially since everything is done very slowly, very carefully, and always quadruple and quintuple checked.

Just as example: I didn't watch the beginning of the webcast, but I did watch the end, and when the pad crew opened the hatch, there was basically one person working very slowly and carefully on the panel, and two people whose sole job, from the looks of it, was to simply watch them and compare every move they are making with a checklist on their pads.

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