How do Control Moment Gyros (CMGs) differ from reaction wheels?

CMGs generate torque by changing the direction of a spinning wheel's angular momentum, while reaction wheels change the magnitude. CMG operation: Wheel spins at constant high speed, Gimbal rotates wheel axis, Torque = omega_wheel x omega_gimbal (cross product), and Torque perpendicular to both spin axis and gimbal rate. Advantages over reaction wheels: Much higher torque per unit mass (100x typical), Better for large, agile spacecraft (ISS, agile imaging). Disadvantages: More complex (gimbals, singularity avoidance), Momentum capacity limited by gimbal angles, and Singularity states where torque cannot be produced in certain directions. Configurations: Single-gimbal CMGs (simpler, singularity issues), Double-gimbal CMGs (singularity-free but heavier). Used on ISS (4 CMGs, 4760 Nms each), Hubble, and agile satellites. Steering laws avoid singularities while meeting torque demands.