Massive as the Saturn V seemed to an onlooker, it would not naturally go in a straight line. On the contrary, lacking a guidance system it would have been as unpredictable as a child’s skittering balloon. The job of the guidance system was to ensure that the line of thrust of the launch vehicle was aligned with the center of mass. To that end, the guidance system checked the vehicle’s position, attitude, velocity, propellant levels, and a few dozen other variables every two seconds, and then sent messages to the four outboard F-ls (the center engine was fixed).
