THIRD STAGE The second staging event

It was only when the S-II’s tanks had run dry and a signal had been sent to shut down the engines, that the Saturn’s computer could begin the next part of the ascent – the staging event that discarded the spent S-1I 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 conical interstage that separated the S-II and S-IVB stages. Although it was discarded with the S-II, this structure was actually manufactured as part of the S-IVB. Within a second of S-II cut-off. solid-fuel retro rockets mounted around the interstage ignited, as did two ullage rockets at the base of the S-IVB. A pyrotechnic device then severed the tw o stages. 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.

Crew’s generally found this staging event much less violent than the first, as Dave Scott opined in test-pilot prose after his Apollo 15 flight. "The S-II to S-IVB staging w’as 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 [shutdown], 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 tvas 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 S-IC 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 at the base of the S-IVB continued to burn for about eight seconds and helped 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 .1-2 engines used in the S-II. except that the fuel was allowed to How through the engine walls for a longer period before ignition. The dead S-II coasted to a watery impact in the Atlantic Ocean. 4,500 kilometres from the launch site. With a mixture ratio of 5.0 to 1. the S-IVB continued to drive for orbit with a burn that typically lasted 140 seconds.

The crews experienced an acceleration of only 0.5 g. which rose to 0.75 g as the burn progressed. When the Saturn’s guidance system sensed that the required speed had been achieved at the necessary altitude, it shut down the engine. This was also the signal to initiate Timebase 5. which sequenced all the tasks required to settle the stage and its spacecraft payload in their orbital coast. In only 11 Vi minutes, the Saturn V had accomplished one of the riskiest parts of the mission by achieving orbit about 180 kilometres above Earth.

During a post-flight debriefing, Eugene Ccrnan summed up the Saturn V in layman’s terms: “I think the S-IC acted and performed like some big, old, rugged, shaky 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."