When watching (once again) a large History Channel documentary series about Gemini and Apollo, in some launch scenes with the original audio, the person doing the countdown usually says "all systems are internal". What does that mean? That the crew inside the rocket are "now" in full control of all its systems?


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This can refer to a number of things, the specific source (mission and point in time of the countdown) would be helpful.

It can refer to all systems running on internal power sources rather than external sources. In addition, it can also refer to additional systems that were using external sources, like the guidance system.

During the launch preparation/countdown, systems were switched from external to internal power to see whether that works correctly, then back to external power again to not waste the internal capacity. For Apollo 11, this test happened roughly half an hour before lift-off.

Shortly before take-off, all systems then need to switch to internal power as the external connections are cut. For Apollo 11, if I'm reading that correctly, there were two points: at T-15min the "spacecraft" (CSM?) was switched to internal power and 50 seconds before lift-off the remaining systems were switched to internal power.

There is another thing they were referring to as going "internal": the inertial guidance system, which was locked while on the pad but needed to be "released" shortly before lift-off. For Apollo 11, that Guidance Reference Release was done about 16 seconds before lift-off.

  • $\begingroup$ Very interesting link, thanks. I found this: "T minus 15 seconds, guidance is internal. The phrase 'guidance is internal' refers to the moment at T minus 17 seconds that the Saturn V's guidance system is no longer held in an orientation with respect to Earth. It is now inertial and holds an orientation with respect to the stars....This moment is more formally called Guidance Reference Release or GRR. GRR begins a 16-second period that inhibits the first of the Saturn V's control timebases." $\endgroup$ Aug 22, 2020 at 17:46

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