High-Agile Spacecraft with Flexible Appendages: Limits, Risks, and Mitigation

Modern Earth-observation missions increasingly rely on high-agility retargeting to maximise target acquisitions. At the same time, new-generation spacecraft are adopting larger, lightweight, and lightly damped appendages such as high-power solar arrays and deployable SAR antennas, which introduce low-frequency flexible dynamics and tighter slew-and-settle constraints. The combined trend raises a central question for mission design: how far can agility be increased without incurring residual vibrations that degrade pointing performance and structural margins?

This work investigates an agile spacecraft equipped with a Control Moment Gyroscope (CMG) attitude control system and a reduced-order flexible dynamics model capturing the dominant appendage modes. Using an Earth-observation retargeting scenario as a motivating use case, we show that increased agility can improve target acquisition by up to 500% compared with a conventional reaction-wheel cluster. At the same time, this benefit can be limited by residual vibrations that extend manoeuvre settling time, potentially eroding the net advantage of CMG agility and reinforcing the perception that high agility and large flexible appendages are incompatible on the same platform.

The first part of the study determines the natural-frequency regime in which residual vibrations become non-negligible following aggressive (bang-bang) slew manoeuvres. The second part evaluates practical mitigation strategies compatible with classical control architectures (e.g., PD), including jerk-limited reference trajectories and input-shaping techniques (e.g., ZVD). Sensitivity to modelling uncertainty and time delays is also assessed. Results indicate that CMG-actuated agile spacecraft can preserve rapid retargeting capability while satisfying pointing and structural requirements, demonstrating that high agility and flexible appendages can be successfully combined within a single satellite.