Ever since I was a kid watching launches in real time, I've always wondered how and why certain pre-launch events are scheduled as they are. Obviously things like propellant loadings have to be done on a certain sequence well in advance of launch, with time allowed for chilldown and such, but what about the timing of events closer to launch?
I'm thinking of the swingarm retractions, the switch to internal power, and the switch to internal guidance (which I assume means "guidance release"). The CM access arm was retracted several minutes before launch, one of the S-IC arms retracted some seconds before launch, but most didn't retract until after first motion. That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.
It just occurred to me that if you did retract the arms before liftoff and then had to hold, how would you empty the propellant tanks for a scrub? Maybe that's the reason -- once the arms retract, you're committed so you just have to make sure they work.
But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?
The switch to battery power occurs at T-50 seconds. Here the tradeoff is fairly obvious: you want to transfer as late as possible to conserve them, especially if you then have a hold, but at the same time you want to see them perform under load long enough to see that they're in good shape, so you can hold if they're not.
I assume "internal guidance" or "guidance release" refers to the moment that the inertial platform in the IU is physically uncaged and allowed to do its thing. I believe it's also the moment at which the launch REFSMMAT (the space-fixed coordinate system used by the platforms in the IU and CSM) has its X axis pointed straight up from the launch site. I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?
I can imagine that because there was so much manual prelaunch monitoring of the Saturn V (remember those shots of rows and rows of engineers at consoles in the KSC firing room) enough time had to be allowed after each event for the person monitoring it to call a hold if necessary. I suspect that with computer monitoring of modern launch vehicles, a lot of these events can be scheduled much more tightly with respect to liftoff.
