As reliable as a parachute

However innovative the architecture, design was not the major challenge of sending a computer to the moon. The difficulty would be making this computer robust, reliable, even bulletproof. Moreover, how did one measure such things when the machine could not be tested in its real environment for years to come? Early on, the IL group recognized that reliability would largely determine the success of their enterprise. If their computer failed in testing, it would never be allowed on the mission. If it failed in flight, it could cost the astronauts' lives.

NASA manager David Gilbert made the IL responsible for the AGC's reliability and gave them a set of policies. First on the list: ''Make maximum use of the man to simplify equipment.'' Design conservatively. Rigorously test and select parts. Require the contractors to test the components, test the final assembly. Carefully report and track each failure. Gather data about them, analyze the data, and review the design accordingly.12

Robert Chilton remembered that Hall and his group paid constant attention to reliability questions, though NASA wasn't prepared to give them a specification: ''[Hall asked:] How reliable is this? How reliable is that? How many failures per thousand hours, or how many hours between failures? They asked me how reliable has the computer got to be, this guy in charge of computers. How reliable does a computer have to be? I didn't know anything from reliability, I said, ''It has to be as reliable as a parachute.''13

But what did ''as reliable as a parachute'' mean? Chilton responded vaguely because reliability requirements for Apollo were notoriously imprecise across the board. Eventually, the project settled on .999 for safety and .99 for mission success. That is, a one in a hundred chance the mission would fail, and a one in a thousand chance the astronauts would not survive. Max Faget recalled that, using these numbers, ''one of the study contractors came to me and pointed out that wasn't very much different from the expected mortality from three 40-year old individuals on a two week mission if you took the standard actuary tables.''14 These were not useful criteria for design decisions.

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