CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 132 of a model made of tissue paper and toothpicks. Thrusters generate exhaust, which can flex and fatigue the arrays or slowly build up on the panels, thereby decreasing electrical generation. When thrusters are fired, the ISS solar arrays often need to be parked in particular positions to avoid being damaged by the thrusters, which usually reduces the amount of available power. Finally, the firings from the thrusters are not conducive to a microgravity environment for many payloads. Because of this, CMGs are in attitude control 99% of the time, with control being handed to thrusters for special events only, or in contingency cases such as loss of CMG attitude control or unplanned CMG saturation. Having both CMGs and thrusters available and working is critical to maintaining attitude control. Without thrusters, large maneuvers could not be performed, the ISS stack could not recover from a loss of attitude control event, and the ISS stack could not be put into position to dock or capture a rendezvousing vehicle. Without CMGs available to hold attitude between these events, the thrusters on the ISS would exhaust their fuel supply in a few months. A minimum of two CMGs are required to perform attitude control however, three CMGs are generally required to safely perform all attitude control functions. Individual CMGs can be replaced by a spacewalking astronaut with the assistance of the Space Station Remote Manipulator System (SSRMS) robotic arm. Two spare CMGs are carried externally on the ISS to replace failed gyros. Microgravity Probably the most well-known environment the ISS provides is one that is unique to space—an environment in which objects are weightless. Researchers can remove the variable of gravitational influence within their investigations. For example, on Earth, combustion is driven by convection, where warm air rises and cold air sinks. Crystal structures grown in the weightlessness of space can often be grown larger and more pure. Extremely sensitive experiments such as crystal growth experiments or those involving liquid flow may be negatively impacted by firing of the ISS thrusters, or even by movement of the ISS crew. For these reasons, more sensitive experiments may be run in racks that have vibration isolation, are usually planned when the USOS CMGs are in attitude control and thruster firings are not planned, and may be conducted at night when the crew is sleeping. US Segment Attitude Control The control system of the US Segment attempts to control three different variables: the attitude (how many degrees out of the desired attitude is the ISS located) attitude rate (how fast is the ISS rotating) and momentum (how close are the CMGs to saturation and therefore losing attitude control without resorting to thrusters). How much each of these variables, or controller states, are weighted by the attitude control software depends on how the software is configured. For example, some software controllers are designed to hold attitude and attitude rate (i.e., how quickly the attitude is changing, in degrees per second) tightly, but at the expense of allowing momentum to build in the CMGs and therefore requiring thruster firings. This type of controller is used for dockings but is unsuitable for attitude control of more than a few hours because it uses propellant. A non-propulsive controller is desirable for most attitude control—i.e., over 99% of the time. These controllers take advantage of the environment in which the ISS flies. External torques, and the associated momentum gain in the CMG system, often balance out over the course of an orbit. For example, as the solar arrays rotate, they can generate a torque in one direction at one part of an orbit, and then a corresponding torque in the other direction in another part of the orbit. The torques are conservative (i.e., they add up to zero) over a full orbit, with the CMGs providing the mechanism to store momentum on one side of the orbit by gimballing one direction, and then disperse the momentum on the other side of the orbit by gimballing the opposite direction. The CMGs also absorb small, unbalanced (on average) torques in orbit. Over the course of many hours, these unbalanced torques