CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 130 an electrically driven inner gimbal, which in turn is mounted completely inside an outer gimbal. Because of the inner/outer gimbal design, the spin axis of the flywheel can be oriented at any position within three- dimensional space. A CMG (Figure 14) is a device that produces torque (a rotational, twisting force). A torque is generated on the space station by electrically driving the inner and outer gimbals and pushing or “gimbaling” the spinning wheel. Compared to the size of the ISS, the torque is surprisingly small, usually 10 to 30 N-m (7 to 22 ft-lbs) depending on the velocity at which the gimbals are being driven by their motors. For a comparison, if a person could stand at the end of the space station truss, he or she could impart the same level of torque by simply pushing. Figure 14. A CMG prior to launch. The large box mounted on the left houses the power supplies and small computer that controls the CMG. The CMG measures approximately 130 x 130 x 130 cm (51 x 51 x 51 in.) and weighs 272 kg (600 lbs). Since the environment around the ISS consists of external forces that are much less than those encountered on Earth, the relatively low torque output of the CMGs is sufficient for all attitude control—except when large maneuvers need to be performed quickly. The ISS uses four CMGs, mounted on the Z1 truss segment, that work together in tandem under the command of the USOS GNC MDM flight computer. The MCS uses the CMGs to generate torque and correct the attitude when small external rotational forces act to push the space station out of its flight attitude. Applied over time, that torque is stored as momentum in the CMG system (momentum=torque multiplied by time). Since the gimbals of the CMGs can constantly be in motion and are powered electrically, they can provide a constant, fine attitude control that counteracts the small aerodynamic and gravity gradient