Smart guidance

With much of the atmosphere behind it and the second stage working smoothly, the Saturn changed the way it guided itself to orbit. So far, it had not compensated for the distance that the wind and other forces had pushed it from its ideal flight path. Nor had it tried to correct for any under- or over-performance of the engines. Instead, during the tilt sequence, the instrument unit had merely kept track of where it was at any moment. The new guidance regime was given a typically NASA-ese name of the iterative guidance mode. While in force, equations in the Saturn's computer plotted the most efficient flight path from 'wherever the vehicle was' to 'the point in space where it wanted to go' - in this case, insertion into a parking orbit around the Earth. To anthropomorphise the situation, what the computer was thinking was, ''OK, I know where the wind and such has pushed me to. What do I need to do to reach the position and speed that I have to get to?'' As the S-II powered ahead, the computer monitored the vehicle's progress and sent steering commands to the four gimballed outer engines of the stage as necessary to achieve the desired result. This steering was maintained for the rest of the S-II burn, then suspended and the stack's attitude held steady, which allowed S-II staging to occur and the third stage to ignite and begin to power the vehicle to orbit. With the S-IVB thrust established, the steering recommenced.

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