The OLYMPUS solar arrays are regulated using the sequential switching shunt technique as described above. In this case the regulating equipment is divided between a mode control unit and independent power dump modules. The power bus voltage is regulated to be 50 volts + 1% at the central distribution point. Battery discharge regulators meter out stored energy from two batteries in order to maintain the power bus at its regulated voltage during eclipse. Both nickel-hydro- gen and nickel-cadmium batteries will be used in this initial technology development mission. Battery charge regulators are used to recharge the batteries from the power bus. Charge management is provided by a battery management unit. Power is routed from the central distribution point to the communications payloads which are comprised of a direct broadcast television transmitter, business services and various high frequency transponders at 20/30 GHz. 3.13 Power System Requirements for Deep Space Missions ESA has flown a limited number of deep space missions to date. However, the few spacecraft which have been developed for these missions have performed quite well, resulting in highly successful missions. The 1986 GIOTTO mission to investigate Halley's comet was an outstanding success for ESA. The spacecraft approached to within 500 kilometers of the cometary nucleus and returned closeup pictures of it to Earth. ESA's current deep space efforts include the solar polar explorer ULYSSES, and the conceptual definition of the TITAN PROBE for the American CASSINI mission. 3.13.1 Cassini TII'AN PROBE The TITAN PROBE power system is based on an Li-SO2 primary battery. This provides power via a regulated 28 volt bus at a maximum of 300 watts for 3.5 hours. The probe will be brought to Titan by the proposed American CASSINI spacecraft. The probe will then enter Titan's atmosphere at high velocity, where aerodynamic braking followed by parachute deployment will be used to carry it through the predominant atmospheric phase of its scientific mission. It is necessary to differentiate between the major mission categories discussed above, i. e. LEO missions, GEO missions, and interplanetary missions, in order to provide a systematic description of solar array requirements. Most requirements have their main impact at some specific level - systems level, array level, assembly level, or component level. 3.2.1 Solar Array System Requirements Absolute power requirements have a large impact on the array system, as emphasized in section 3.1. Both rigid and deployable arrays are now commonly used at low and intermediate power levels. At high power levels flexible blanket arrays become advantageous. The appropriate technology as a function of power to mass
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