Despite the diversity of possible electrical power sources (see Table 9.1 and Figure 9.6), the vast majority of spacecraft operate on a solar array/battery combination. This is particularly true for Earth-orbiting spacecraft, which we shall focus on here. Figure 9.9 is a simplified diagram showing how this typical arrangement works. The main feature is the connection of the spacecraft's electrical loads across the solar array (via points 6 and 7), so that the loads are supplied by the array while the spacecraft is in sunlight. However, there is a complication. We know from the above discussion about solar arrays that if a specified area of array is presented to the Sun, it will produce a specified amount of electrical power. But we also know that the spacecraft loads vary; for example, payload instruments will be switched on and off at various times, and subsystem elements such as reaction wheels, or the communications equipment, will run when they are required. So, on the one hand, the solar array output is constant, but then on the other, the
electrical loads it needs to supply are continuously varying. To address this mismatch, a solar array regulator is fixed across the solar array (at points 1 and 2). This can be a simple device that takes the excess power not required by the spacecraft loads and dissipates it externally as heat—a bit like an electric bar fire. At the other end of the scale, it can be a sophisticated device, controlled by the onboard computer, which switches patches of solar array area in and out to ensure that the array output matches the loads at any particular time.
While all this is going on, the battery system is also connected across the solar array (via points 3 and 4), so that it can be charged up while the spacecraft is in sunlight; it becomes just another element of the spacecraft's electrical loads. Then, when the spacecraft enters Earth's shadow on each orbit, the electricity for the payload and subsystems can be supplied by the stored energy in the batteries (through points 4 and 5). This process of charging and discharging the batteries needs to be carefully controlled, to ensure that the batteries last long enough to do their job throughout the entire spacecraft mission. The main factors that govern the lifetime of the batteries are the total number of charge/discharge operations needed during the spacecraft's mission, and the amount of stored energy that is taken out of the battery on each such operation. For a typical Earth-orbiting spacecraft, this charge/discharge operation usually occurs once per orbit, and so for a spacecraft in a LEO there are typically about 5000 charge/discharge cycles per year. The charge/discharge controllers (points 3 and 5) are essential components to make sure the batteries do not die prematurely. If you think again about the battery in a car, the battery charge and discharge process is not usually controlled in any way; in fact the driver takes this role. Unlike the battery system in a spacecraft, which can last for 10 or 15 years as a consequence of careful control, the battery in a car can expire at the most inopportune moment before its mission (the car's lifetime) is completed!
This section has discussed how the power subsystem achieves the vital role of keeping all the electrical spacecraft systems alive. We now turn to the basics behind spacecraft communications.
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