5.3 Typical Requirements and Design Considerations

Within the scope of the spaceflight mission, the electrical power system must support the other subsystem designs and properly interface electrical and data connections. Like with any subsystem, the size, weight, and power are obvious requirements or design drivers. Requirements for spaceflight include:

• An orbit average and peak power on the power distribution (wire harnessing, motherboard, and daughterboard control)

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• A specific amount of power generated and/or stored (solar array and battery size)
• Voltage conversion and limits (circuit board design)
• Current regulation or limits (circuit board design)

Let’s think about sources for requirements generated from external constraints from all parts of the spacecraft lifecycle. Just like the structures and mechanisms system, the electrical power system design also needs to adhere to many considerations outside of the spaceflight mission.

During manufacturing and assembly, electrical engineers and technicians need to take proper care when handling electrostatic-sensitive components. Workbenches must be clear of paper and other debris, and sensitive components must be handled on grounded electrostatic discharge mats with the handler connected with a discharge wrist strap. A clean work surface prevents debris from contaminating the electronic board and potentially short-circuiting the exposed circuitry. This holds particularly true for any metal shards, which act as conductors. Paper and cardboard boxes can interfere with the path-to-ground of the grounded ESD mat if ESD-sensitive components are placed on top of the paper instead of the mat [Desco]. Another consideration is the growth of “whiskers” from tin or zinc-finished surfaces. These whiskers can grow up to 10 mm in length but are typically less than 1 mm and typically around 1 micrometer in diameter. The reason for their growth is unknown. To reduce the risk of tin whiskers, requirements could include the exclusion of pure tin-plated components and independent verification of the plating composition of the products [NASA]. Other practices include reflowing and alloying the tin plating, replating the whisker-prone area, conformal coating or foam encapsulation, or accepting the risk. Bonding requirements for the ISS may be found in Space Station Electrical Bonding Requirements [SSP 30245 Revision E]. The Artemis CubeSat Kit will include an ESD mat, does not use tin or zinc-plated components, and follows all bonding requirements.

During testing, external requirements from the launch provider commonly include battery safety and survivability through vibration tests. “All cells and batteries on the CubeSat shall adhere to the design and testing requirements for spacecraft flight onboard or near the ISS as derived from the NASA requirement document JSC 20793 Crewed Space Vehicle Battery Safety Requirements”. After vibration testing specified in the structural requirements, the electrical power system must be functionally tested to ensure safe operations. As the Artemis CubeSat Kit may be soft-stowed with astronauts through launch and handled by astronauts on the ISS for deployment, we have followed the evaluation, qualification, and acceptance testing of the kit batteries and avionics.

During transport and handling, requirements may include restrictions on batteries or pressurized vessels. Battery composition or the maximum amount of charge on the battery may be regulated during transportation. As an example for airplanes, lithium batteries must not exceed 100 Watt-hours per battery and may only be allowed in carry-on bags, not checked bags [TSA]. Small compressed gas cartridges (for spacecraft) are not permitted through TSA. The exceptions are if the gas cartridges are for personal medical use or if the cylinders are empty.

Artemis Kit Specific

From the time of delivery through on-orbit deployment, the CubeSat power system shall be at a power-off state, utilizing an RBF pin, which cuts all power to the satellite once it is inserted into the satellite. The power is inhibited through a minimum of three (3) independent inhibit switches actuated by physical deployment switches. “The CubeSat shall have, at a minimum, one deployment switch, which is actuated while integrated into the dispenser” [NR-NRCSD-S0004]. During in-orbit operations, “the CubeSat shall not operate any system (including RF transmitters, deployment mechanisms or otherwise energize the main power system) for a minimum of 30 minutes after deployment. Satellites shall have a timer (set to a minimum of 30 minutes and requiring appropriate fault tolerance) before satellite operation or deployment of appendages. CubeSats shall incorporate battery circuit protection for charging/discharging to avoid unbalanced cell conditions ” [NASA LSP-REQ-317.01]. The Artemis CubeSat Kit has an RBF pin, at least three independent inhibit switches, a 30-minute timer, and battery circuit protection. Outside of these external launch provider requirements (or really, constraints), the electrical power system’s primary responsibility is to enable the other spacecraft subsystems, particularly the payload. The EPS requirements are highly prone to iteration following the dynamic design process. Here are the initial requirements that drove the Artemis CubeSat kit.

Artemis Kit Specific