5.4 Design Process and Drivers

The main design driver in sizing the electrical power system is the power and energy consumption of the payload and other subsystems. This section will provide the best guidance as to how best to size the power generation’s initial capability in producing power. We start with an initial power budget is given by the New SMAD’s Table 10-9:

With the remaining 60 W, we can start to find subsystem component solutions that consume around the power allocated to them on average. The keyword here is average, where the average is taken over a single orbit or a set of multiple orbit completions. A more detailed power budget and profile are discussed in the analysis section but let’s discuss the intuition around variation in power consumption that must be taken into account when calculating averages:

• The attitude control subsystem has a high % allocation for average power consumption because many payloads require active pointing during their science operational phase. The attitude control subsystem must be active for other mission operational phases, like sun pointing to charge the batteries or radio pointing when passing overground stations. You’ll also notice that the minimum power consumption for an attitude control system is 0 W, lower than the other subsystems because the attitude control system can be held at standby with no power consumption.
• The communication subsystem has a rather high minimum power consumption but a small % allocation of average power consumption. The communications subsystem is power-hungry when it is on but does not remain on constantly. When sizing the communications subsystem, think about how much data must be transmitted, how long the transmitter must communicate with ground stations, and how frequently the spacecraft encounters a pass over a ground station.
• The command and data handling system is nearly always on unless an anomaly disrupts the system’s operations. If the onboard computer only has one power mode and uniformly consumes a steady amount of power, this uniform power consumption is the average consumption number. Some on-board computers have low and high power modes, toggling between low activity and high activity depending on the operational mode. A high power mode is potentially necessary during science operations and for on-board processing prior to downlinking data. A low power mode may be utilized if the spacecraft needs to conserve energy during a long transit. The average power consumption is a weighted average of the power levels during each mission phase and the length of time of each mission phase.
• The thermal system’s power allocation primarily includes active thermal control components, like heaters. Many thermal control solutions are passive and do not consume power, like radiators or surface coatings. For active components, the thermal control system does not need to be active through the whole mission and may be held at standby (0 W minimum power consumption). The heaters may not be needed during sun pointing, as a heat source is already present. Heaters may be necessary during an eclipse in which the satellite passes behind a planetary body and the sun is occluded by the planetary body.
• The power system must support all the other subsystems and commonly, an amount of power is necessary to sustain electronic drivers or other daughterboards at an overhead cost during standby. The supporting electronics from all the subsystems accumulate a significant amount of power consumption that varies with the mission phase, like all the other subsystems. This power consumption also includes line losses and other inefficiencies in power conversion or transfer. As the power subsystem is so pervasive in its involvement with all the subsystems, the electrical power system has the second-highest average power allocation, behind the payload.
• The structures and mechanism subsystem typically do not need power, outside of single events like deployment.

At the initial design, SMAD recommends a margin of 25% due to the design’s immaturity so SMAD advises a power generation solution that produces 125 W. As the initial design progresses, the payload and spacecraft system will grow in power consumption. The Aerospace Corporation conducted a historical study of the power budget growth through the design phases relative to the ultimate flight system [NASA & Aerospace]. They found the historical average of the instrument and spacecraft power growth is closer to 40%, which is better advice for margin. Although the results of this study are for traditionally larger spacecraft, this study is based on real data analysis. We should look for a power generation solution that produces 140 W.

The basic procedure to design the electrical power system is listed below. A more detailed procedure is broken out in section 5.10 Electrical Power System Design Tools

1. Define the power consumption and electrical characteristics of the spacecraft bus components
2. Define the necessary power generation and energy storage required to fulfill the power consumption requirements
3. Select the power generation and energy storage methods
4. Analyze the system’s power budget and profile from the beginning of the mission to the end of the mission to ensure the selected components are sufficient to supply power
5. Design a power conversion, management, and distribution subsystem to interface the power sources and power consumers
6. Procure and fabricate components
7. Conduct tests on isolated components
8. Conduct tests on integrated components

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