Reliability or Repair The Apollo Computer

If I wanted to write a philosophical novel about Apollo and say where did this technological capability come from? And what was it? You'd have to go back to these two things: you developed a group of men and an approach, a systems approach if you will, that let you undertake high-speed flying, you developed a systems approach that lets you undertake risky missions and be able to call them ethical.

That system plus the digital computer that's the Apollo mission. —George Rathert, NASA Control Engineer

While ''mini'' computers were beginning to come on the scene in the 1960s, the word was still relative, and a small computer was still the size of a phone booth. Specialized machines, like the seven-function unit that ran rendezvous calculations for Gemini, or the ballistic solvers in the Polaris missile, could be made reasonably compact, but the IL engineers envisioned a ''general-purpose'' computer, one that could be reprog-rammed at will to do any possible task with the data at hand.

Such a machine afforded great possibilities. New, specialized functions could be added at any time, merely by changing the programs, allowing late-stage design fixes, mission changes, or improved calculations. The computer could take over routine tasks from the astronauts, improve the accuracy of their flying, and help them manage the spacecraft's systems. Imagine punching in a series of numbers, then sitting back while the computer automatically oriented the spacecraft and fired the engine, in just the proper direction for just the right amount of time.

But would it work? Digital computers in the early 1960s seemed impossibly complex and broke down on a regular basis. Impressive, highly mathematical analyses, not to mention practical experience, showed that any such machine would fail frequently. How could NASA ensure that an Apollo computer would run for an entire two-week moon flight? Mercury and Gemini were banking on redundancy, backup systems, and manual takeover. What alternatives would the computer offer? A new technology, the integrated circuit, seemed to promise great increases in capability in very small packages. But the little chip was a black box, seemingly immune to human understanding and repair—and nobody had ever tried it in such a life-critical application.

From 1962 to 1965, NASA and IL engineers struggled through these problems as they developed the hardware for the Apollo Guidance Computer (AGC). At odds were differing engineering cultures, differing technical philosophies, and differing visions of the astronauts' roles. The design decisions, while considering subtle, technical details, also stemmed from judgments about human performance. Astronauts had been hailed as the ultimate backup systems. Could they take over if the computer failed? Could they help make the computers more reliable? How could the IL convince NASA and the astronauts to trust their lives to the machine?

It has become fashionable to denigrate the computers of the past with phrases like ''we flew to the moon with less computing power than I have on my wristwatch,'' or ''can you believe the entire Apollo program fit into a mere 36 k of memory?'' Simply focusing on memory size, or the computer's speed, however, misses the important engineering accomplishments of the Apollo computer. For who among us would risk our lives on our desktop computers, with all their speed, accuracy, and memory, and rely on their working flawlessly for two straight weeks? The space shuttle flies with five redundant computers. Any fully digital airliner has a minimum of three. Apollo had only one. It never failed in flight.

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