A Variable Speed Control Moment Gyro (VSCMG) is a novel actuator for
spacecraft attitude control and represents the future of current
momentum exchange devices. The proposed concept consists of a spacecraft
with a single VSCMG for attitude control: this particular actuator
design is proposed for the purpose of high manoeuvrability with a
minimal actuator configuration. The system under investigation is
under-actuated and the full-attitude cannot be controlled. Based on this
consideration, a reduced-attitude representation is proposed and
exploited to control the spacecraft for pointing the bore-sight. Here it
is developed, for the first time, a completely new, non-singular and
nonlinear control law able to re-point at discrete intervals or track a
pointing reference continuously. The asymptotic stability of the system,
in the different operative modes, is proved using Lyapunov stability
theory. The controls are demonstrated with numerical simulations and
then further validated with an application to a 12U CubeSat rendezvous
utilizing a single throttable electric thruster.