Commemorating the miraculous recovery of Apollo 13 this week, we take a look at how NASA kept the giant Saturn 5 booster precisely in the center lane on the way to the Moon. In this information packed seven-minute video, the Primal Space channel explains how the inertial guidance platform in the Saturn 5 booster knew where it was and where it needed to go. The Apollo Command Module, of course, had its own guidance system and so did the Lunar Module. The booster’s guidance system lived in the Instrument Unit, a 21-foot diameter ring mounted to the top of the third stage and immediately under the truncated cone that housed the lunar module.

You’ll recall from your Apollo history that the third stage got the stack into orbit, but just as critical was its burn to put the spacecraft into translunar injection for the trip to the Moon. Once that was done, the crew turned the Command Module around and extracted the Lunar Module from its garage. The third stage–the S-IVB–still had a role to play. Like other S-IVB’s in the program, the booster was aimed at the Moon so its impact could be used for seismic experiments. That’s explained in the Apollo 13 Booster Impact Experiment. So those expensively developed inertial units ended their days in a splatter of debris on the lunar surface. What fun it would be to walk the site and pick up the pieces.

It wasn’t fun for Jim Lovell, however. When informed that telemetry showed the S-IVB had impacted the lunar service as planned, some 78 hours into the mission, Lovell said, “Well, at least something worked on this flight.” Lovell, Fred Haise and Jack Swigert still had another 64 hours in a dark, freezing Command Module before splashing down in the warm South Pacific Ocean on April 17th.

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