Third Stage The second staging event

It was only when the S-II tanks had run dry and a signal had been sent to shut down its engines, that the Saturn's computer could begin the next stage of the ascent - the staging event that discarded the spent S-II and ignited the S-IVB for the first of its two burns. The same signal began Timebase 4 in the instrument unit to choreograph everything that had to occur.

Unlike the dual-plane separation between the first and second stages, this cut was made across a single plane at the top of the interstage that separated the S-II and S-IVB stages. This conical structure was actually manufactured as part of the S-IVB, although it was discarded with the S-II. Within a second of S-II cut-off, solid-fuel retro rockets mounted around the interstage ignited along with two ullage rockets at the base of the S-IVB. A pyrotechnic device then cut the two stages free. Engineers were less worried about the possibility of accidental contact because the S-IVB carried only a single centrally mounted engine and its extraction from the interstage occurred well above the atmosphere.

Crews generally found this staging event much less violent than the first, as Dave Scott opined after his Apollo 15 flight. ''The S-II to S-IVB staging was about a quarter to a fifth the force of the S-IC staging. It was again a positive kind of feeling, but it wasn't a violent crash like we felt on the S-IC.'' Eugene Cernan pointed out other differences: ''On the S-II [shut down], although it's sharp and a very hard hit, you don't unload the entire stack like you do when you're on the S-IC.'' However, Ed Mitchell had been so keyed up for the S-IC separation on Apollo 14 that he was unprepared for the jolt the S-II staging delivered. ''I thought the S-II cut-off was more dramatic than the S-IC. Maybe that's because I had been thinking about the SIC being the dramatic one and not thinking about the S-II.'' On his Apollo 10 flight, Cernan told Capcom Charlie Duke how a cloud of debris was produced on staging that moved with the stack. ''Charlie, lots of stuff out the window on staging. We're catching up and passing it now.''

The ullage motors continued to burn for about 8 seconds, helping to push propellant down the pipes and into the turbopump, during which time, the single J-2 engine brought itself up to full power. They were then jettisoned from the stage to avoid their dead weight being carried to orbit. The start sequence of this engine was identical to the J-2 engines used in the S-II, except that the fuel was allowed to flow through the engine walls for a longer period before ignition. The dead S-II, having reached at least 90 per cent of the speed required for orbit, managed to coast to a watery impact in the Atlantic Ocean, 4,500 kilometres from Kennedy Space Center. The S-IVB continued to push the spacecraft to orbit with a burn that typically lasted 140 seconds.

The crews felt an acceleration of only 0.5 g, which rose to 0.75 g as the burn progressed, until the Saturn's guidance system had sensed that the required speed had been achieved and shut down the engine. This was also the signal to start Timebase 5, which sequenced all the tasks required to settle the stage and its spacecraft payload in their orbital coast. In only 11% minutes, the Saturn V had accomplished its first major task by getting the spacecraft into orbit, completing one of the riskiest parts of the mission.

During a post-flight debriefing, Eugene Cernan summed up the Saturn V in layman's terms: ''I think the S-IC acted and performed like some big, old, rugged, shaky, big monster. It has to be noisy, has lots of vibration and smoothed out somewhat after max-Q, but still was a rumbling bird. The S-II was a Cadillac: quiet, less than 1 g flight most of the time until we built up our g-load prior to staging. It was quiet, smooth, had very little noise, or feeling of rumbling or anything else. The S-IVB: a light little chugger, is probably the best way I can describe it. It just sort of rumbled on, not anywhere near the extent of the S-IC, but just sort of continued to rumble on through the burn.''

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