In Their Own Words: Don Eyles on Gyroscopes
"The Gyros in the Apollo Inertial Measurement Unit
A gyroscope wants to keep its orientation because the momentum of its spinning wheel resists the tilting of the axle. The orientation it “wants” to keep has nothing to do with the surface of the Earth or any other local phenomenon. It is fixed to inertial space. Point the spin axis of a perfect gyroscope at a particular star in the sky and it will forever point to that star. As a matter of fact stars do very slowly shift—but that perfect gyroscope that you could hold in your hand might as well be bolted to the cast iron framework of the universe itself.
The gyroscopes in an IMU like the one on Apollo took advantage of another characteristic of gyroscopes, that if you disturb one by rotating it around an axis perpendicular to the spin axis, it will try to rotate around the third axis. The output force can be turned into an electrical signal proportional to the angular rate of the disturbance. In other words, a gyroscope can provide a direct readout of angular rate.
The function of the gyroscopes in the Apollo IMU was to maintain the alignment of the inertial platform. The wheel in each gyroscope, consisting of a beryllium hub and a steel rim, was about 4 cm in diameter, weighed 84 grams, and spun at 24,000 rpm. By itself such a gyro was too feeble to hold its orientation against a serious disturbance, but these gyros were not effectors but sensors — sensors of sublime sensitivity — linked to motors in each of the gimbals. When the gyros sensed a disturbance the motors would react. As a result the inertial platform never moved. Hence its second name: the stable member.
So it was that deep inside an Apollo spacecraft, surrounded by electronic assemblies, suspended inside a spherical case, was an object that never budged from its fixed relationship to the stars, no matter how the spacecraft maneuvered. That was the equivalent of a compass. A compass does not tell you where you are on the ocean, it tells you a single angle — and that is enough. The inertial sensor goes beyond that. It gives you three angles that together define your orientation in three dimensions, and it works anywhere — even in space away from Earth’s magnetic field.
Add optics for sighting on stars, and a table of star vectors, and you can align the inertial platform to any orientation you like — and correct for the drift of gyroscopes that, after all, are not quite perfect. Add precise information on the motion of nearby bodies like the Earth and Moon, and you can use the same optics to determine your position and velocity from scratch by sighting on horizons and landmarks. And if you add a pilot, engines, and a radar, you can attempt complex maneuvers like landing on another planet.
But you must also add a digital computer — to collect data from the IMU and other sensors, from ground transmissions, from the switches and controls that the crew operate; to run the software that navigates, guides and controls; to display information to the crew and send telemetry data to the ground; and to command the engines, maneuvering jets, antennas, and the other effectors that animate the spacecraft. Most of all, the computer will write the plot, apply the higher-level logic that pulls the strings to accomplish the mission. The IMU has the star role, spare like Garbo; the guidance computer plays Svengali."
Sunburst and Luminary, an Apollo Memoir, pages 41-42 © Don Eyles