The evolution of space missions toward large, low-mass platforms has made the interaction between the attitude control system and structural dynamics—commonly referred to as Control-Structure Interaction (CSI)—a critical issue during the design phase. Although control law synthesis typically relies on rigid-body models, performance validation based solely on such assumptions is insufficient to capture potential instability risks and pointing degradation arising from structural flexibility. The first contribution of this work focuses on the development and validation of a nonlinear, fully parametric multi-body dynamic model capable of accurately representing the coupling between a rigid central hub and flexible appendages. Open-loop validation confirms the physical consistency of the proposed model, which proves to be a versatile and reliable tool for the preliminary analysis of different mission configurations.