Avionics Design Architecture for Low-Cost CubeSat Missions and Lessons Learned from R5-S2 and R5-S4

The Realizing Rapid, Reduced-cost high-Risk Research (R5) project is primarily intended to develop and operate a capability to host low technology readiness level (TRL) payloads and demonstrations much faster and cheaper than the current state of the art. The primary stakeholder for this project is the Small Spacecraft Technology program within the Space Technology Mission Directorate at NASA with the project based out of Johnson Space Center. Unlike typical NASA satellites, R5's spacecrafts utilize many commercial off-the-shelf (COTS) components while designing custom hardware only when necessary. This paper explores the avionics subsystem design philosophy, the resulting design, and the lessons learned throughout the design, build, test, and operations of R5 Spacecraft 2 and R5 Spacecraft 4 (R5-S2 and R5-S4). The avionics subsystem has divided its design into three main functional groups: power, propulsion control, and payload interfaces. The power group contains the solar panels, batteries, and a battery management system. The propulsion control group contains the electronics needed to control thrusters, monitor propulsion systems, and operate reaction wheels. The payload interfaces group contains a variety of standard COTS interfaces to facilitate the integration of a broad range of low TRL prospective payloads (not requiring payloads to develop to a singular data interface). The avionics subsystem design supports late changes to mission goals and spacecraft configuration, reducing the need for redesign and testing time. Typical CubeSat architectures use the PC/104 specification, a modular framework in which avionics PCBs are stacked. As the R5 project utilizes many COTS components and interfaces with a wide variety of payloads, the PC/104 specification does not meet the needs of the project. Instead, the R5 avionics subsystem has developed an alternative framework that allows for flexible mechanical and electrical integration with this variety of components. Throughout the design, build, and test process of R5-S2 and R5-S4, the team learned many lessons that were then applied to future designs (spacecrafts 3 and 5 and beyond). This paper will highlight the lessons learned and how the subsystem design has evolved to reduce risk for future missions and enhance capability.