HST Two-Gyro Handbook
3.1 Gyroscope Overview1
HST has six rate-sensing gyroscopes. Under normal operating procedures, three of the six gyros must be functioning to provide sufficiently accurate pointing to achieve guide star acquisitions and science data collection. The gyroscopes aboard HST sense whenever the attitude of the observatory is changing, whether during large angle slews from one target to another or during small pointing changes as a result of subtle forces acting upon the observatory. Each gyroscope senses the motion about a single axis. The relative orientations of the gyro axes within HST are different so that the torques exerted on the gyroscopes by attitude changes affect each gyro differently. As a result, any combination of three gyros can be used to define a set of three orthogonal axes around which changes in the roll, pitch, and yaw of the observatory may be measured.
There are many different types of gyroscopes available, but only gas bearing gyros are capable of providing the combination of extremely low noise, excellent stability, and high sensitivity to motions that is required for HST observations. Each gyro has a wheel spinning at a constant rate of 19,200 rotations per minute on gas bearings. The wheel is mounted in a sealed cylinder, which floats in a thick fluid. Electricity is carried to the motor that spins this wheel by thin wires, or flex leads, approximately the width of a human hair. The wires are immersed in the fluid along with the wheel. Changes in the gyroscope rates induced by movement of HST are captured by onboard electronics. This information is then fed to Hubble's central computer where it is analyzed. The HST pointing is changed through the use of several reaction wheel assemblies. Each assembly contains spinning wheels, which when spun at varying rates, create the appropriate torques required for the desired movement.
The gyroscopes are packaged in pairs, in devices called rate sensing units (RSUs). Each RSU weighs approximately 24.3 pounds and is 12.8 x 10.5 x 8.9 inches in size. The individual gyroscopes weigh approximately 6 pounds and are 2.75 x 6.5 inches in size. Figure 3.1 shows an exploded view of one of the HST gyroscopes. Figure 3.2 shows a gyro after assembly.Figure 3.1: Exploded View of a Gas Bearing Gyroscope
Figure 3.2: Assembled
Figure 3.3: Schematic of the Hubble Space Telescope after Servicing Mission 3B
Major components are labelled, and definitions of the U1, -U2, -U3 (V1, V2, V3) spacecraft axes are indicated.
The HST gyroscopes are attached to the focal plane structure at the aft end of the observatory on the same side as the fixed-head star trackers (FHSTs). Figure 3.3 shows the HST field of view following SM3B in the standard HST coordinate system. The RSUs are accessed by opening the large cargo bay doors on the U3 side of the observatory.1Some of the information in this chapter was reproduced from the Hubble Space Telescope Gyroscopes Hubble Facts sheet available from the GSFC HST Program Office.
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