EPD - Ulysses
Power & Thermal Subsystem
Power Dumpers and Power Regulation
The RTG provides 202W to power the spacecraft (this figure was last updated on 26 July 2005). At any given time, most of this power is used to run Ulysses' engineering subsystems and science instruments. Any power that is not immediately needed is routed to twelve Internal Power Dumpers (IPDs), which are resistances distributed inside the spacecraft thermal enclosure - this regulates power as well as providing general heating to the fuel lines, fuel tank, and electrical units which need a stable thermal environment. You can see the locations of the IPDs by viewing the upper and lower spacecraft platform diagrams. The amount of power dumped in the IPDs is regulated to maintain the spacecraft Main Bus Voltage within a 2% tolerance of 28V under all operational conditions. The Main Bus control curcuitry consists of three error amplifiers each of which feeds redundant Majority Voting Logic units.
Because Ulysses trajectory takes the spacecraft to varying distances from the Sun, solar heating input on the spacecraft can be between 45W per square metre at aphelion to about 1300W per square metre at perihelion. During the hottest times of the mission, the power dumped to the IPDs can cause excessive temperatures inside the spacecraft thermal enclosure, therefore External Power Dumpers (EPDs) are mounted outside the spacecraft to radiate excessive electrical power not needed for heating. The determination of how much power to dump externally is not made by the power subsystem but made on the ground by the Flight Control Team at JPL. EPDs can be switched by telecommand in combinations of 10, 20, 40, 50, and 100W.