CHAPTER 9 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL 170 a commandable (i.e., set by the SPARTAN flight controller) velocity to rotate the array at a constant rate irrespective of the relative motion of the sun. Rate mode does not automatically track the sun, therefore reducing power generation. It is only used in special circumstances, such as verifying the rotational capability of the BGA or obtaining imagery of all sides of the SAW. The directed position mode uses a commandable position to fix the BGA at a specific angle. Once in position, a motor will hold the BGA at the command position by attempting to correct for any external forces (i.e., thruster plume impingement). In cases where visiting vehicles are docking, it is desirable to park the BGAs at a fixed position to minimize plume impingement but at the expense of generating power. However, in safety-critical situations (i.e., an EVA crew member working near the arrays), one of the anti-rotation latches can be used to mechanically inhibit BGA rotation. In manual free mode, the motor is disabled, thereby allowing free rotation of the gimbal assembly. This mode might be used following a BGA failure by allowing an EVA crew member to manually position the SAW for better power generation for repair. Solar Array Constraints The USOS solar arrays can be rotated into any position required to point at the sun—barring any shadowing from other hardware— through the combined use of the SARJs and BGAs. After the ISS was completely designed, many constraints were applied to solar array positioning to protect the solar array hardware from damage. Thruster Plume Impingement— Structural Loads and Array Erosion Thrusters that control attitude on the ISS and visiting vehicles work by combusting fuel and oxidizer. The combustion products exit the thruster nozzle at a high speed, thus imparting a force on the vehicle in the opposite direction. The departing combustion products are known as a thruster plume. Thruster plume impacts, or impingement, on the USOS solar arrays, can have two effects—both negative. First, the thruster plume can impart a force on the solar array structure and cause bending and/or torsional loads. If these loads are too high, the solar array structure can be damaged. Second, if combustion products come in contact with solar cells, they can chemically or abrasively degrade the cells. This degradation would reduce the capability of the solar arrays to produce power. To combat the negative effects when the thrusters are firing, the USOS solar arrays are stopped and positioned facing edge on to, and as far as possible from, the thrusters. This reduces the forces imparted on the solar arrays and their exposure to combustion products. Many thruster plumes need to be avoided, especially when visiting vehicles arrive to or depart from the ISS, which creates a narrow range of acceptable locations for the USOS solar arrays. The less flexibility on solar array positions, the more power constrained the ISS, requiring load powerdowns to stay in energy balance. If the forces involved are high enough, the BGAs or SARJs (or both) may be mechanically locked into position to prevent inadvertent, perhaps plume-induced, rotation into a position that would risk damage to the solar array. Longeron Shadowing When the USOS solar arrays were deployed, some of the components combined to form four longerons that run the length the mast of each SAW. See Figure 4. When a solar array is tracking the sun, each of the four longerons are exposed to sunlight. However, if the solar array is not tracking the sun (e.g., parked for thruster constraints) or if other equipment blocks the sun (i.e., during high beta), it is possible these longerons can become shadowed. Whether a longeron is in sunlight or shadow will change the temperature of the longeron and can cause it to expand (lengthen) or contract (shorten). If the longerons of a single array mast are unevenly shadowed (e.g., three longerons fully in the sun while one longeron is fully in shadow), this expansion and contraction can cause uneven tension and compression loads on the longerons. Analysis has shown that uneven shadowing for as few as 20 minutes can cause enough thermal loading differences to damage the mast. When the constraints to prevent longeron shadowing and thruster plume impingement are combined, it can be very difficult to develop a solar array plan that protects solar array hardware while still producing enough power to meet the ISS needs. These constraints came about mainly due to design changes from early space station concepts. The biggest design change that magnified these solar array constraints was the addition of Russia as an international partner. The USOS solar arrays were originally designed for the Space Station Freedom. When Russia joined