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The Galileo Orbiter is a "dual-spin" spacecraft--a spinning section will allow instruments to measure fields and particles in space while a despun (nonspinning) section will provide a stable platform for the camera and other remote-sensing instruments. These two sections are mechanically connected by a device known as the spin-bearing assembly (SBA) and electrical signals are passed between the two sections via the SBA.
The spin-bearing assembly was functionally tested on the flight spacecraft in JPL's Spacecraft Assembly Facility. The purpose of the test was to check mechanical clearances between the spun and despun sections and to allow the spin-bearing assembly to dynamically operate and flow signals and power across the spinning slip ring and rotary transformer interfaces.
The Orbiter was raised into the air until it was hanging about 50 centimeters (20 inches) above the work stands. Then, after carefully separating the joint between the spinning and nonspinning sections of the spacecraft and checking clearances, commands were sent to the SBA to initiate the spinning motion. For this test, the spun section was held stationary while the despun section rotated at about 3 revolutions per minute.
In flight, the spacecraft will operate in several modes: inertial, cruise, and all-spin at rates from 3 to 10 rpm for stabilization. The entire spacecraft will use the higher spin rate in all-spin during Probe release, orbit insertion, and the perijove raise maneuver. During the normally quiet cruise mode and the high-performance inertial mode, only the spacecraft's spinning section will rotate.
The design of the SBA presented a unique challenge in transferring power and data between the two parts of the Orbiter. Since one part spins with respect to the other, cables could not be used since they would become wrapped around the joint that connects the two parts. The solution is twofold: power and low data rates will be transmitted via 48 slip rings, while high-rate data will be transmitted through 23 rotary transformers (see cutaway drawing).
"In the slip rings, a flexible brush contact rides on a large revolving ring that is electrically connected to the spinning side," explains Garry Burdick, assistant manager of JPL's Guidance and Control Section. The rotary transformers consist of pairs of coiled wires around a ferrite core. One coil is electrically connected to the spun section. Signals are transmitted without contact by means of the magnetic field generated by pulses of current in the coils."
The relative rotational position between the spun and despun sections is continuously measured and derived from an optical encoder within the SBA. The SBA is controlled by the attitude and articulation control subsystem (AACS) onboard the Orbiter.
"Galileo's configuration is unique in that the 400-Newton main engine, which is rigidly attached to the spinning section, is located within the envelope of the despun section," notes Frank Locatell, cognizant engineer for the configuration of the SBA and despun section. "Thus, the spun propellant lines and engine cables are routed through the center of the SBA. This is the first configuration of this kind."
A significant number of glitches and noise interfered with the electrical signals of two slip ring modules during life tests begun last year. Similar problems were found with the flight SBA this spring. Such interference could have been troublesome for the mission. After an intensive effort (headed by Joe Savino of JPL's Electronics and Control Systems Division) to determine the cause of the problem and appropriate action, the flight SBA was disassembled, inspected, analyzed for contamination, retrofitted with new slip-ring modules, wired, and reassembled. Since the voltage transients were most probably caused by chemical contamination accumulated during the building process, the SBA has been rebuilt with extremely clean components and internal redundancy has been reapportioned to provide more system immunity to this type of electrical noise. In addition, system modifications were made to assure that the spacecraft would be less sensitive to noise. The rebuilt SBA is being delivered to JPL's Spacecraft Assembly Facility in September 1985 following performance checks and functional tests.
At launch, the spun and despun sections will be rigidly held together. After launch, the sections will be separated and remain separated for the duration of the mission. The centerfold drawing describes the separation joints and mechanisms.
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