Description

The IMU unit was a key part of the entire Apollo guidance, navigation and control system developed by the MIT Instrumentation Laboratory. The full three axis inertial guidance unit contains gyros and accelerometers. Designed by MIT, this unit was manufactured by the A.C. Spark Plug Division of General Motors. A.C. Spark Plug also made the gyroscopes and Sperry manufactured the accelerometers. In David Hoag's report, "The History of Apollo On-Board Guidance, Navigation, and Control," (presented at the Internation Space Hall of Fame Dedication Conference, October 1976), he wrote: "The inertial measurement unit borrowed its technology heavily from the Polaris missile guidance experience at the Laboratory. John Miller assembled a Laboratory team and was supported by A.C. Spark Plug in the inertial system design. The mechanical design was undertaken by John Nugent, who had done that work for Plaris. In order to simplify the design considerably and to achieve more accuracy in the alignment to the stars, the inertial measurement unit was provided with only three degrees of freedom in its gimbals, although four gimbals would have permitted unlimited all-attitutde freedom. With the natural choices for aligning the system for flight, only some unusual attitudes of the spacecraft would put the gimbals into lock where the alignment would be lost. The resulting constraint in the design irritated the astronauts, although, in retrospect, they had no particular trouble with the attitude limitations during missions." "It was the stellar alignment of the inertial measurement unit which made this design significantly different from that of the Polaris system which was erected with gravity and gyrocompass action." This unit was built for testing and included is a very heavy base used to secure the spinning unit when in operation here on earth. The unit is marked with: Part no: 2018601, Serial no: 200-6, Contract no: NAS-9-153.