Apollo 12 was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969. Apollo 12 launched on schedule from Kennedy Space Center, under completely overcast rainy skies, encountering wind speeds of 174.6 mph during ascent, the highest of any Apollo mission. Lightning struck the Saturn V rocket 36.5 seconds after lift-off, triggered by the vehicle itself, discharging down to the Earth through the ionized exhaust plume. Protective circuits detected overloads and took all three fuel cells offline, along with much of the command service module or CSM instrumentation. A second strike at 52 seconds knocked out more equipment. However, the Saturn V continued to fly normally; the strikes had not affected the guidance system, which functions independently from the CSM. The loss of all three fuel cells put the CSM entirely on batteries, which were unable to maintain normal 75-ampere launch loads on the 28-volt DC bus. These power supply problems lit nearly every warning light on the control panel and caused much of the instrumentation to malfunction. Electrical manager John Aaron remembered the telemetry failure pattern from an earlier test when a power supply malfunctioned in the CSM signal conditioning electronics which converted raw signals from instrumentation to standard voltages for the spacecraft instrument displays and telemetry encoders. Aaron made a call which switched the SCE to a backup power supply. The switch was fairly obscure, but Lunar Module Pilot Alan Bean, flying in the right seat as the spacecraft systems engineer, remembered the SCE switch from a training incident a year earlier when the same failure had been simulated. Aaron's quick thinking and Bean's memory saved the mission. Bean put the fuel cells back on line, and with telemetry restored, the launch continued successfully. The lightning strikes had caused no serious permanent damage.
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