Spacecraft attitude control using an Adaptive Singularityfree Control Moment Gyroscope (ASCMG) cluster design for internal actuation is presented. A complete dynamics model is derived using the principles of variational mechanics, relaxing some common assumptions made in prior literature on control moment gyroscopes. These assumptions include perfect axisymmetry of the rotor and gimbal structures, and perfect alignment of the centers of mass of the gimbal and the rotor. The resulting dynamics display complex nonlinear coupling between the internal degrees of freedom associated with the CMG and the spacecraft base body's rotational degrees of freedom in the absence of these assumptions. This dynamics model is further generalized to include the effects of multiple CMGs placed in the spacecraft bus, and sufficient conditions for non-singular CMG cluster configurations are obtained. General ideas on control of the angular momentum of the spacecraft using changes in the momentum variables of a finite number of CMGs, are provided. A control scheme using a finite number of CMGs in the absence of external torques and when the total angular momentum of the spacecraft is zero, is presented. The dynamics model of the spacecraft with a finite number of CMGs is then simplified under the assumption that the rotor is axisymmetric, in which case it is shown that singularities are avoided. As an example, the case of three CMGs with axisymmetric rotors, placed in a tetrahedron configuration inside the spacecraft, is considered. The control scheme is then numerically implemented using a geometric variational integrator and the results confirm the singularity-free property and high control authority of the ASCMG cluster. Moreover, as rotor misalignments are addressed in the dynamics model, the ASCMG cluster can adapt to them without requiring hardware changes.