Internal Details
The attitude of the vehicle was measured relative to a coordinate system that was fixed just prior to launch with the X coordinate vertical, the Z coordinate in the direction of the pitch maneuver (down range, roughly East), and the Y coordinate perpendicular to the other two, cross range, roughly South. At the heart of the ST-124 was a platform that was held in a fixed orientation; hence the name "stabilized platform". It is connected by three gimbals that allowed the vehicle to roll, pitch and yaw but the stable platform to be held fixed in space. It was being translated, of course, but did not tilt during flight.
The platform is stabilized by three gyros mounted on it. One measured any rotations about the X axis, one about the Y, and one about the Z axis. They generated signals that were shaped in feedback circuits and sent back to torquers on the inner, middle and outer gimbals that exactly countered the rotations, nulling the gyro outputs and keeping the platform stable.
The inner gimbal also carries three accelerometers, two pendulums, and a pair of prisms. The accelerometers measured vehicle acceleration along the X, Y, and Z axes. Their outputs were used by the LVDC to measure actual vehicle motion, for the purpose of navigation. The pendulums were used to set the X axis exactly vertical, and the prisms were used to align the Y and Z axes, just before launch. The prisms reflected infrared beams sent into the ST-124 by a theodolite stationed 700 feet away from the launch pad. Commands from the theodolite were transmitted via cables to the vehicle, to torquers in the ST-124 to orient the stable platform toward the correct azimuth.
The gyros, accelerometers and pendulums contain almost frictionless nitrogen gas bearings. These required very precise machining and very small gaps between the bearing surfaces. Dimensions were held to tolerances of 20 microinches (0.5 µm), and the gap filled by the nitrogen is about 600-800 microinches. Nitrogen entered the gyros at about 15 psi and was vented to space via a pressure regulator in the bottom of the ST-124 that opened at 13 psi. The large silver sphere to the left of the ST-124 held the supply of nitrogen for the bearings.
The ST-124 includes many components made of anodized beryllium. This material was chosen for its stiffness, light weight, machinability and stability. The case of the ST-124 is a short cylinder, 7.5 inches high and 21 inches in diameter, made of beryllium. The ends of the cylinder are closed by two approximately hemispherical aluminum covers. The gimbals and several parts of the gyros and accelerometers are also made of beryllium.
In contrast to beryllium, which is lightweight, the rotors of the gyros are made of a very dense, strong material, Elkonite. This is a sintered form of tungsten, with about 10 percent copper, to make it machinable.
Heat generated by torquers and other electrical equipment inside the ST-124 was carried away by cooling coils built into the aluminum covers. A mixture of methanol and water at 15 degrees C was circulated through the coils. The internal temperature of the ST-124 stabilized at about 42 degrees C.
Read more about this topic: ST-124-M3 Inertial Platform
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