131 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 torques. Although CMGs provide excellent fine attitude control, they have a capability limited by physics as the CMGs gimbal. Generally, their momentum axes are pointed in different directions (Figure 15). As the CMGs provide torque and absorb momentum, the spin axes of the flywheels begin to align. When the spin axes of the four-CMG system line up, the system loses control and is referred to as saturated (Figure 16). The CMG system will become saturated relatively quickly for any significant torques (e.g., a small air vent overboard will saturate the CMGs within a few minutes) and they are normally incapable of performing an attitude maneuver of more than about 1 degree, unless the CMG maneuver is specially designed. Torque Producing a Torquewell CMGs Saturatedwheels CMGs Producing a Torque Figure 15. CMGs CMG system with spin axes separated and able to react to external forces. Torque CMGs Producing a Torque CMGs Saturated CMGs Producing a Torque Figure 16. CMG system with spin axes aligned. This CMG system is saturated and the need to be repositioned (this will require thruster firings, also known as desaturations). The number of CMGs (four) was determined by how much momentum would be required to maintain this fine control during normal day- to-day operations in momentum management (explained below). The ISS simply maintains its attitude during these periods as the crew lives and performs research in between events such as visiting vehicle arrivals and reboosts. The basic capability was to maintain momentum management control without firing thrusters to support microgravity research and conserve propellant over long periods of time (~30 days). Three CMGs were required to meet this minimum level of capability. A fourth CMG was added to introduce redundancy, so operations could continue uninterrupted in the event of a failure. In comparison, a thruster provides a translational force that acts as a torque when applied over a distance between the thruster itself and the center of mass of the space station. Thrusters are significantly more powerful sources of force and torque, and they control the attitude of the ISS more coarsely than the CMGs. Thrusters are used to perform large attitude maneuvers such as those required to reposition the vehicle attitude for dockings. Thrusters are also used to help control vehicle attitude when the CMGs become saturated. In a process called desaturation, the CMG gimbals are moved out of alignment while the thrusters fire to absorb the torque generated. Desaturation is an automatic software function on the ISS, where the USOS GNC MDMs compute a “desaturation request” (i.e., essentially a vector with desired momentum correction) and hand it off to the Russian Segment Terminal Computers. While the GNC MDM gimbals the four-CMG system to a lower momentum state, the Russian Segment Terminal Computer computes and fires thrusters on the Service Module and/or Progress vehicles to react to the CMG desaturation event and maintain the attitude of the ISS. In some cases, the CMG system may not be able to maintain attitude control for difficult attitude control situations such as an overboard vent, a problem in the GNC MDM or its software, or the loss of multiple CMGs due to an electrical failure. In these cases, software in the C&C MDM will automatically hand over attitude control to the Russian Segment thrusters or, in limited scenarios, the crew may perform the handover manually in response to a warning message. Although thrusters are powerful devices, they have disadvantages. Most obviously, they use propellant that needs to be replenished, and that must be launched from Earth. Several tons of propellant must be launched to the ISS, annually, using Russian cargo vehicles. Another major but less-obvious concern is the health of the ISS solar arrays. The ISS solar arrays are lightweight and were built to be deployed and unfurled on orbit. Because of this, the arrays and the structure that supports them are quite fragile. Imagine a large version