The countdown for Apollo 12 started on 8 November, with a view to lifting off at the opening of the launch window on 14 November. It began with the clock at T-98 hours, and a ’pre-count’ in which the launch vehicle and spacecraft activities were undertaken independently. The ’terminal count’, with coordinated activities, began at 9:00 p.m. EST on 12 November with the clock at T-28 hours. Two holds were planned, the first at T-9 hours for 9 hours 22 minutes and the second at T-3 hours 30 minutes for 1 hour. The work progressed smoothly until technicians tried to load liquid hydrogen fuel cell reactant into the service module and discovered that tank no. 2 was not chilling down.1 When the hydrogen flow was halted, the level in the tank declined rapidly. Upon peering in through an inspection panel, a technician observed frost on the exterior of the tank. A flaw in the outer shell had ruined the vacuum insulation. This issue was new to the manufacturer’s field team at the Cape. After consulting with the Manned Spacecraft Center, John Williams, the Spacecraft Operations Director, decided to replace the faulty tank with one from CSM-109, which was on-hand at the Cape for Apollo 13. An unscheduled hold was initiated at T-17 hours.2 After an access panel had been removed, the tank was cryogenically and electrically isolated from the hydrogen subsystem shelf, and the new tank fitted. Once this was done, the cryogenics were successfully loaded. To preclude delaying the launch, the scheduled hold at T-9 hours was reduced by 6 hours to compensate. Meanwhile, at T-15 hours the cask of plutonium which was to power the scientific station that the astronauts were to deploy on the lunar surface was affixed to the descent stage of the lunar module, and at T-10 hours the crawler began to withdraw the mobile service structure from the pad.