129 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 n Gravity gradient forces. Gravitational force acts on an object as a proportion of square of the distance from the Earth. Parts of the space station that are nearer to the center of the Earth are attracted more than ones that are farther away. While on Earth, and for most satellites, the difference would be considered miniscule the size of the ISS causes relatively significant gravity gradient torques in certain flight attitudes. Again, the attitude control system needs to constantly counter these torques to stay in attitude control. n Other minor forces, including solar radiation pressure (literally, pressure from light). At any instant in time, these forces are absolutely miniscule—e.g., the drag from the rarified atmosphere in low-Earth orbit is 100 times less than the force on the human hand when holding a sheet of paper. Over time, however, even this miniscule- but-constant aerodynamic force will cause the ISS orbit to slowly drop, usually on the order of 25 to 50 m (82 to 164 ft) per day, which drives the need for occasional reboosts. These external forces will also act to try to push the ISS out of attitude and cause it to tumble aerodynamic and gravity gradient forces, in particular, are not evenly distributed. This drives the need for attitude control devices. For the ISS, these devices are CMGs assisted by occasional thruster firings. Electrically powered CMGs are used for gently counteracting environmental forces most of the time, whereas propellant-consuming thrusters are used for maneuvering and desaturating the CMG system when required (discussed below). Controlling Attitude in Space—Control Moment Gyroscopes and Thrusters Two general categories of activities require attitude control. One is regular day-to-day operations where the ISS is maintaining a LVLH attitude and a stable platform for other vehicle systems and payloads. During these operations, only small adjustments are needed to be applied by the control system to counter the tiny forces introduced by aerodynamic drag and gravity gradient forces. The other category is special operations, where control of the ISS attitude may require rapid rotational maneuvering of the ISS attitude, use of stronger methods of attitude control during rendezvous operations of visiting vehicles, reboost operations, or recovery from an unplanned loss of attitude control. Two primary methods are employed to control the attitude of the ISS for these operations (Table 1). CMGs on the USOS MCS system perform fine attitude control using only electricity readily available from the Electrical Power System, and typically are fully in attitude control during day-to-day operations. Thrusters on the Russian MCS can be called upon to augment or take over attitude control from the CMGs during special operations. Although the thrusters offer more power, it comes at a cost of consuming propellant (which must be resupplied from Earth), increased operational complexity, and the potential to interfere with payloads dependent on a microgravity environment. Table 1. Comparison of CMGs and Thruster Control Type of Control CMGs Thrusters Advantages • Use only electricity. • Can hold attitude tightly. • No potential to damage solar arrays. • Do not interfere with microgravity payloads. • More powerful, can perform larger maneuvers. • More robust (can maintain attitude control when CMGs would be overwhelmed). Disadvantages • Limited power. • May require augmentation by thrusters for larger maneuvers or tighter control. • Require resupply of propellant from Earth. • Can require special positioning of solar arrays to avoid damage. • Spent propellant can cause contamination, especially for windows. • Firing may interfere with microgravity payloads. The effectors of the USOS MCS system consist of four CMGs. The CMGs each consist of a 98 kg (216 lbs) steel flywheel, which is spun by an electric motor at a constant rate of 6600 revolutions per minute. The flywheel sits on two mechanical, lubricated spin bearings that are electrically driven to keep the CMG running at full speed. The flywheel and spin bearings are mounted on