To skip or not to skip

If the entry plan required the spacecraft to skip out of the atmosphere for a period, perhaps to extend the flight path as happened on Apollo 11, the computer moved on to P65 which controlled the ascending part of the skip-out trajectory. P66 then controlled their attitude during the period the spacecraft was outside the atmosphere. On the second re-entry, control was passed to P67. In most cases however, re-entry did not include a skip-out phase so P64 handed directly to P67.

In the case of Apollo 11, as they approached Earth on their 3-day coast, the weather in the prime recovery area looked increasingly poor so the decision was taken to maintain their Earthbound trajectory, but alter the re-entry flight path to include a skip-out, thereby extending their flight through the atmosphere from 2,200 kilometres to nearly 2,800 kilometres. ''I wasn't very happy with that,'' said Collins at his debrief, ''because the great majority of our practice and simulator work had been done on a 1,187 [2,200-km] target point. The few times we fooled around with long-range targets, the computer's performance and the ground's parameters seemed to be in disagreement. So, when they said 1,500 miles [2,800 kilometres], both Neil and I thought, 'Oh God, we're going to end up having a big argument about whether the computer is computer is Go or No-Go for a 1,500-mile entry.' Plus 1,500 miles is not nearly as compatible. It doesn't look quite the same on the EMS trace. If you had to take over, you'd be hard-pressed to come anywhere near the ship. For these reasons, I wasn't too happy about going 1,500 miles, but I cannot quarrel with the decision. The system is built that way and, if the weather is bad in the recovery area, I think it's probably advantageous to go 1,500 miles than to come down through a thunderstorm.''

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