Weak systems thinking

The Block II redefinition had great import for the Apollo guidance computer (AGC) and hence for the human role in the missions. First to go was the idea that the astronauts might repair the system in flight. Shea found North American's principle of inflight maintenance ''another example of weak systems thinking.''45

All during this management shakeup, engineers at North American and the IL still struggled with the problems of in-flight repair as its implications became more and more cumbersome. For their part, the astronauts were none too happy about having to learn maintenance techniques—the hottest test pilots didn't want to be repairmen in space. One IL engineer recalled Alan Shepard responding to the prospect of in-flight repair with insightful sarcasm: ''Yeah, and we should all train to be brain surgeons so we can operate on each other.''46

To make such repair practicable, the capsule would have to carry an extra copy of every electronics module in the computer, which amounted to carrying an entire additional computer. By October 1963, that's exactly what the IL decided—to include two complete computers in the capsule and eliminate the possibility of in-flight repair. That option, however, either doubled the volume or halved the computing power that would be available, neither of which was acceptable.

Meanwhile, Mercury was flying, and in May 1963 a new data point came in. When Gordon Cooper orbited the earth during the last Mercury flight, his control system failed, forcing him to orient and fire the reentry by hand. While it was easy to hail the flight as a triumph of manual control, something had gone deeply wrong. Upon examination, engineers found that the urine collection system had failed and globules of urine had migrated into the electronics, which, when combined with moisture from Cooper's own exhalations, proved to be rapidly corrosive. North American began a study of the effects of humidity on Apollo electronics and recommended design changes to protect the circuits from moisture. The Apollo electronics must be sealed shut.47

This change ruled out in-flight repair. Under Shea's direction NASA removed the requirement from the North American and Grumman contracts for the spacecraft, instead requiring the companies to install redundant systems in the vehicles.48 Bell-comm showed the increase in reliability from two computers to be negligible or nonexistent, and the IL engineers agreed. Each Apollo spacecraft would have only one computer.

IL engineers would now make the single computer aboard the spacecraft ultra-reliable—an uncertain proposition given the poor record of most electronics. But what if it did fail? What would be the backup? Without a second computer, the astronauts might have to navigate home from the moon with a failed computer. Could they do it with paper charts?

The answer came from the ground. During the early 1960s, with experience gained from the Mercury and Gemini flights, advances in electronics and signal processing, and the advent of atomic frequency standards for very precise timing, NASA learned that it could track a spacecraft with great precision from ground-based antennas. A transponder on the spacecraft listened for radar interrogation signals from three huge antennas on earth (in Spain, Australia, and California) and echoed them back. Precise measurement of time delays and Doppler shifts, improved by averaging over time, allowed NASA to calculate positions in lunar orbit to within ten meters, and velocities to 0.5 meters per second. These numbers became so accurate that greatest uncertainty in the ground-based navigation fixes became the knowledge of the coordinates of the antennas on the surface of the earth, which could only be pinpointed to a few meters.49

So in the words of NASA engineer Cline Frasier, ''Why not free up the whole mission planning and everything, and just go ahead and do it from the ground as the primary mode?''50 Primary navigation from the ground would not only eliminate the problem of a backup, it would also significantly reduce the workload of the astronauts, who could turn their scarce attention to other things. The primary navigation system, then, became ground-based tracking, backed up by the onboard computer. The computer, of course, would still be primary when the spacecraft, on the far side of the moon, lacked access to earth-bound signals. Implicit, of course, in this decision was a demotion of the MIT effort—for most of the flight now the onboard navigation would be a backup system. Furthermore, it meant abandoning the original requirement of ''autonomous'' navigation—the IL engineering value of being able to navigate missiles and aircraft with no references to the outside world. Their approach seemed reasonable enough in wartime, but by 1964 the idea that the Soviets would try to jam navigation signals for peaceful moon shots began to seem far-fetched.

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