It depends. For Minuteman I, the missile needed to be physically rotated in the silo to be aligned with the target. Then the "Targeting Van" connected to the missile, downloaded the new targeting data to the disk, and checked that the guidance system was aligned. As for the targeting data, it was generated by a mainframe that determined the right trajectory and produced the optimized navigation polynomials that the targeting algorithm used. It was something like 740 words of data per target so only two targets could fit in the computer.
Minuteman III used a smarter targeting algorithm that only needed 70 words of data per target, so the missile could support something like 8 targets at once, selected by a knob on the launch console. (The launch officers didn't know what the targets were; they were just told to use target #3 for example.) The targeting data was read off punched tape for Minuteman II and a magnetic tape cartridge for Minuteman III.
It was organized primarily via wargame scenarios, such that one target group comprised targets for a given scenario.
Simplified launch orders via the football, etc.
One group scenario might have been silo coordinates for an offensive first-strike. One group city coordinates for launch on warning strategic counter-strike, etc.
Each missile got a target from each scenario list programmed into a 'memory slot' with some overlap.
The organization/ optimization is mind-boggling.
But few understand that this is WHY the wargames and strikes had to be pre-planned ahead of time. It wasn't political hubris, but a technical requirement due to memory allocation.
> It wasn't political hubris, but a technical requirement due to memory allocation.
I don’t understand why it would be “political hubris”.
Proper targeting is hard work. You need to map your enemy territory to make optimal choices. Not just in a geographical “what are the coordinates” sense, but also in a “what are the important nodes to get the coordinates for” sense. At the same time your enemy doesn’t want to be mapped and resists your efforts.
Doing this properly takes time. On the order of months. But once you are under attack you don’t have that time. So you have to select your targets ahead of time.
It is not because the missiles have limited memory. If they would have needed more memory they would have added more memory. It is because the President doesn’t have time once under attack to name each enemy railway depot one by one and decide which ones are important, and which ones are better left unharmed. Instead what they have is a menu of options. Something like option 1 destroy all red military ports, military airports and military bases; option 2 destroy major military installations plus main industrial centers; option 3 destroy main population centers.
Thinking that the memory allocation is why it is the way it is is super tech centered and quite frankly putting the cart before the horse.
Minuteman III used a smarter targeting algorithm that only needed 70 words of data per target, so the missile could support something like 8 targets at once, selected by a knob on the launch console. (The launch officers didn't know what the targets were; they were just told to use target #3 for example.) The targeting data was read off punched tape for Minuteman II and a magnetic tape cartridge for Minuteman III.