by the gimbal motion. The primary practical benefit of the VSCMG when compared to the conventional CMG is an additional degree of freedom—afforded by the available rotor torque—which can be exploited for continuous CMG singularity avoidance and VSCMG cluster reorientation. Research has shown that the rotor torques required for these two purposes are very small and within the capability of conventional CMG rotor motors. Thus, the practical benefits of VSCMGs are readily available using conventional CMGs with alterations to CMG cluster steering and CMG rotor motor control laws. The VSCMG also can be used as a mechanical battery to store electric energy as kinetic energy of the flywheels. Spacecraft body If a spacecraft has rotating parts these can be utilized or controlled as CMGs. Potential problems Singularities At least three single-axis CMGs are necessary for control of spacecraft attitude. However, no matter how many CMGs a spacecraft uses, gimbal motion can lead to relative orientations that produce no usable output torque along certain directions. These orientations are known as singularities and are related to the kinematics of robotic systems that encounter limits on the end-effector velocities due to certain joint alignments. Avoiding these singularities is naturally of great interest, and several techniques have been proposed. David Bailey and others have argued (in patents and in academic publications) that merely avoiding the "divide by zero" error that is associated with these singularities is sufficient. Two more recent patents summarize competing approaches. See also Gimbal lock. Saturation A cluster of CMGs can become saturated, in the sense that it is holding a maximum amount of angular momentum in a particular direction and can hold no more. As an example, suppose a spacecraft equipped with two or more dual-gimbal CMGs experiences a transient unwanted torque, perhaps caused by reaction from venting waste gas, tending to make it roll clockwise about its forward axis and thus increase its angular momentum along that axis. Then the CMG control program will command the gimbal motors of the CMGs to slant the rotors' spin axes gradually more and more forward, so that the angular momentum vectors of the rotors point more nearly along the forward axis. While this gradual change in rotor spin direction is in progress, the rotors will be creating gyroscopic torques whose resultant is anticlockwise about the forward axis, holding the spacecraft steady against the unwanted waste gas torque. When the transient torque ends,