Apollo: The Race To The Moon

by Catherine Bly Cox

A fourth gimbal would have taken care of the problem—no matter what crazy position the spacecraft got itself into, the platform would have remained steady as long as the guidance system remained powered up. Gemini had used a four-gimbal system. But a fourth gimbal would have been heavy. It would have added substantially to the bulk of the system, because it would have had to fit around the other three. And the Instrumentation Lab at M.I.T., which designed the system, was comfortable with three gimbals—that’s what the Lab had used for Polaris. Three gimbals were enough for any maneuver the spacecraft might be required to make, as long as everyone paid attention to what was happening. After a long battle with the astronauts (Jim McDivitt threatened not to fly in a spacecraft without a fourth gimbal), a three-gimbal system was used.