What if…?

After only 90 minutes in space, the Apollo stack on top of the S-IVB had made one revolution of Earth and was coasting over United States territory where it had near­continuous communication from Hawaii to the Atlantic. The most important task during this 20-minute opportunity was for Capcom to read up to the spacecraft three huge lists of numbers, called PADs (short for pre-advisory data)} Each list gave pertinent details of an engine bum relevant to the next few hours. One of these, the TL1 PAD, would almost definitely be used by the crew. The other two were for bums they hoped they would never have to make, because they were contingencies to abort the mission.

Throughout every mission, NASA implemented strategies that would attempt to

1 See Chapter 8 for a fuller explanation of the PAD, including a worked example.

make any reasonable technical failure snrvivable. Sometimes this was by providing redundant systems. Another strategy was to lay down procedures so that the crew and the flight controllers already knew what to do at any point in the mission, should a problem make a return advisable. The strength of these approaches was dramatically and successfully vindicated on Apollo IS when they overcame a failure that was both crippling to the spacecraft and completely unforeseen.

One proeedure was based on the premise that the crew might lose communica­tions with Earth while on their way to the Moon, or in lunar orbit. In case this happened, mission control always ensured that while they could still talk with the erew, they would keep them updated with enough data to enable them to return home as safely and as quickly as was appropriate. This was the function of the two contingency PADs read up while passing over Hawaii. They gave the crew all the information they would need if. for any reason, they had to return home soon after TLI without the help of mission control.

The first was calculated on the basis of an ignition time 90 minutes after TLI and was therefore called the ‘TLI – 90′ PAD. It would have required the CSM to burn its main engine for more than five minutes against their momentum away from Earth. For the early Apollo flights, the second was known as the "TLI — 4 hours’ PAD, but this became a TLI – 5 hour’ burn on Apollo 11. For the remainder of the missions, it was calculated on a time relative to lift-off, usually eight hours after lift­off. Once the crew had the two abort PADs copied down onto paper forms, they could concentrate on the Lranslunar injection PAD and the preparations for the burn itself.