Out-of-plane stability analysis of collinear spinning three-craft Coulomb formations

• Collinear Coulomb formations display marginally stable in-plane motion.
• The out-of-plane motion is unstable and diverges from the nominal solution.
• The Coulomb formation degrades as the out-of-plane errors grow.
• For small out-of-plane errors, the Coulomb formation is maintained.
• Forcing out-of-plane errors to near zero preserves the formation indefinitely.

The results of a stability analysis focusing on the out-of-plane motion of collinear three-craft Coulomb formations with set charges are discussed. Such a formation is assumed to be spinning in deep space without relevant gravitational forces present. Assuming in-plane motion only with circular relative trajectories and initial position and velocity perturbations confined to the orbital plane, the previous work analytically proves marginal stability in the linear sense and numerically shows marginal stability in the short term. In this paper, the equations of motion are presented in the cylindrical coordinate frame in order to analyze the out-of-plane motion in more detail. The out-of-plane motion is shown to decouple to first order from the marginally stable in-plane motion. A simple control law is developed and applied, which directly controls the out-of-plane motion only within specified deadbands. For a wide variety of out-of-plane perturbations, the control law succeeds in preserving the in-plane variant shape despite some out-of-plane motion. A trend between the settling time and deadband, which defines the largest out-of-plane errors allowed before the controller is turned on, is determined, which illustrates how large the deadband may be before the in-plane motion is affected.