As we recall from Table 7.1 in Chapter 7, the main function of the power subsystem is to provide a source of electrical power to support payload and subsystem operation. This job is critical to the overall health of the spacecraft. Just about every type of spacecraft payload and all the subsystems, with the exception of the structure and possibly the thermal control, need a reliable source of electrical power to operate. A failure, or brief interruption, of the power subsystem function can have catastrophic consequences for the spacecraft mission.
In the same way that the spacecraft has primary and secondary propulsion (see the previous section), the spacecraft also has primary and secondary power systems. The primary power system is the main source of electrical energy; for example, for Earth-orbiting satellites this is often the conversion of sunlight into electricity using a solar panel (or array). As you have read through the previous chapters in this book, you've seen numerous pictures of spacecraft, and the majority of them are equipped with solar arrays. The secondary power system comprises electrical storage devices. In the vast majority of spacecraft, this implies the use of battery technology. However, there are other possibilities, although they are rarely used. For example, a fly wheel can be installed as an alternative electrical storage device. While the spacecraft is in sunlight, solar panel power can be supplied to a torque motor to spin a large wheel. When the spacecraft is in darkness, and the primary power source no longer works, the rotational energy in the wheel can be extracted and converted back into electricity.
A spacecraft is very much like an automobile inasmuch as there is a primary power source combined with a secondary power system. In a car the primary system is the generator that provides electrical power all the time that the engine is turning. The secondary power system is the battery, which allows the car's systems to operate even when the engine is not running. Despite the terminology, the secondary system is very important, in that it provides a means of starting the car and bringing the primary power source on line!
The most common mode of operation for the supply of electrical power on board Earth-orbiting spacecraft involves a solar array/battery combination. While the spacecraft is on the sunny side of the Earth, the solar array operates to produce power for the vehicle's payload and subsystems. At the same time, an extra amount of power is produced by the solar array to charge the battery system. Then, when the spacecraft enters the Earth's shadow on each orbit revolution, the solar array ceases to function and the vehicle's systems are powered by discharging the stored energy in the batteries.
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