Gimbal Reliability

The IL group faced a similar showdown with Grumman, this time over the issue of gimbal reliability. An inertial guidance system can have any number of gimbals, or gyroscopes, depending on what the system needs to do. The gimbals keep the acceler-ometers fixed in inertial space so their frame of reference is not affected by changes in the orientation of the spacecraft. Three or four gimbals would be required to track a full navigational solution.69

When Hoag looked at the problem, he found that all of the thrusting that the command module would be doing, and hence all of the accelerations the inertial system would have to measure, were limited to a single plane. Because of this constraint, the inertial system could be designed with three, rather than four gimbals. The drawback was ''gimbal lock,'' a subtle condition where the solution hit a mathematical singularity and the platform could not produce useful measurements for a given alignment. Certain orientations and maneuvers would just not be allowed with a three-gimbal system (fighter planes, e.g., required four gimbals to accommodate their extreme maneuvers).

Hoag realized the system simply did not have to fly about in all directions—it only had to fly to the moon and back. Put another way, the spacecraft needed to fly around the moon but not over it. So Hoag designed in three gimbals, which, incidentally, had been the number in Polaris. ''We were motivated by simplicity, low-weight, reliability,'' Hoag explained, ''and adding the fourth gimbal fights each one of these aspects.'' There would be just a small cone of attitudes, very far off normal, that the spacecraft could not assume.70 More trade-offs.

Strict adherence to a set of procedures could avoid gimbal lock, and the computer could monitor itself and warn when gimbal lock approached. But the astronauts and Grumman were accustomed to systems from aircraft that had four gimbals, including Gemini, and nervous about the prospect of ''forbidden attitudes'' and the danger of gimbal lock. The issue hinged on the knotty problem of reliability—what was the likelihood that a gyro would fail, and hence require the redundancy of four instead of three? The trouble was, reliability is notoriously difficult to predict, and conflicts arose over how to interpret the sparse data.

The IL provided its own estimates of reliability, based on experience with Polaris, showing that three gimbals would be reliable enough. But Grumman too developed estimates, which NASA found ''highly pessimistic.'' Grumman extrapolated reliability numbers from earlier missile programs and used them to argue for a redundant, four-gimbal platform. ''It was a very dramatic forecast of how bad the reliability would be,'' Hoag recalled, ''and we were incensed.''

In January 1964, NASA manager Joe Shea brought the engineers from the various parties together, ''to find the truth and punish the guilty.'' He gathered the men for a tense standoff. As they entered the room, Shea announced that the meeting was being tape recorded. ''I intend to force a black-and-white conclusion to this meeting. . . . Someone, Grumman or MIT, will have to leave this meeting admitting he was wrong.'' Grumman presented their case, noting its data came from Minneapolis-Honeywell and the Titan missile program.

To counter, Hoag presented the IL case, meticulously exposing errors in the Grumman data, showing how the reliability predictions improved as he corrected the errors. ''I'll never forget the day,'' Grumman's Gavin recalled. ''What happened, of course, was that Davey [Hoag] had a bunch of facts that we did not have. He wound up showing what they could do was perhaps not quite as good as what they had advertised, but it was a darn sight better than what we were saying they could probably do.''

Grumman fell hard. In Kelly's words, ''MIT had blown Grumman's analysis out of the water.'' Grumman engineers recalled the incident with bitter remorse, and felt their reputations permanently damaged. ''For years afterward we endured strained relations with MIT.''71

Ironically, the three-gimbal platform came to be one of IL's regrets, an institutional victory but a loss for the project. Returning from the lunar surface, when Neil Armstrong was docking the LM to the command module, ''I flew it right into gimbal lock.'' Gimbal lock loomed as a constant fear on Apollo 13. ''We felt we were being unnecessarily restricted in the maneuvers we could perform,'' Michael Collins wrote.72 Hoag wished he had done it differently, because the conflict and recriminations it caused were not worth the reliability improvements that his design provided.73

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