The International Space Station: Operating an Outpost in the New Frontier Page 169 (203 of 438)

169 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL CHAPTER 9 UTA Inboard Ring Outboard Ring Figure 9. Photo of a SARJ. The SARJ consists of two rings, with a utility transfer assembly (UTA) on the axel and two drive/lock assemblies (not shown). The rings consist of teeth (see Chapter 18) that move like a bike chain on the gear sprocket. The drive/lock assembly is used to either turn the gear teeth or lock the ring in position. The UTA allows commands and telemetry as well as electricity to pass back and forth using a roller ball assembly, shown in Figure 10. A utility transfer assembly (UTA) is located at the center of each SARJ and provides the path for power and data transfer. The UTA has a roll ring structure, which consists of multiple stationary metal plates surrounded by rotating metal rings. Flexible metal rollers, called “flexures,” between each plate and ring maintain a continuous conducting path to pass electrical power or computer signals between the stationary plate and rotating ring. One plate-roller-ring set is required for each power or data connection that must pass through the rotating joint. These roll rings allow for 360º continuous rotation with seamless power and data conduction. See Figure 10. Photo credit: McDonnel Douglas Image s07-17091 Stationary Plate Connection with Cable Idlers Stationary Plate Flexures Rotating Ring Connection Rotating Ring Cable Passthroughs Figure 10. Example roll ring. The gold stationary plate stays fixed while the outer section rotates. Part of a cable (black) is shown in one of the passthroughs on the inner stationary ring (the rotating ring connector is empty in this photo). Power or data is conducted through the metal flexures between the stationary and rotating rings to transfer electricity or data. Photo: Courtesy of Robert C. Dempsey Beta Gimbal Assemblies The BGAs rotate individual SAWs to provide beta angle array pointing capability (i.e., when the ISS is in the nominal +XVV attitude). Normally, each BGA will rotate approximately ±4º a day to compensate for the changing solar beta angle as the Earth orbits the sun. The BGAs provide the structural load path connecting the SAWs to the ISS truss structure while providing 360° rotational capabilities. They include roll rings for data and power transmission (similar to the SARJ UTA), a motor for SAW positioning, and two redundant anti-rotation latches for locking the BGA in specific positions. Software operates the BGA using multiple “modes.” The autotrack mode—the nominal mode—uses data from the Guidance, Navigation, and Control MDM on the relative positions of the ISS and the sun to calculate the angle that the BGAs should be moved to track the sun. The rate mode uses

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169 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL CHAPTER 9 UTA Inboard Ring Outboard Ring Figure 9. Photo of a SARJ. The SARJ consists of two rings, with a utility transfer assembly (UTA) on the axel and two drive/lock assemblies (not shown). The rings consist of teeth (see Chapter 18) that move like a bike chain on the gear sprocket. The drive/lock assembly is used to either turn the gear teeth or lock the ring in position. The UTA allows commands and telemetry as well as electricity to pass back and forth using a roller ball assembly, shown in Figure 10. A utility transfer assembly (UTA) is located at the center of each SARJ and provides the path for power and data transfer. The UTA has a roll ring structure, which consists of multiple stationary metal plates surrounded by rotating metal rings. Flexible metal rollers, called “flexures,” between each plate and ring maintain a continuous conducting path to pass electrical power or computer signals between the stationary plate and rotating ring. One plate-roller-ring set is required for each power or data connection that must pass through the rotating joint. These roll rings allow for 360º continuous rotation with seamless power and data conduction. See Figure 10. Photo credit: McDonnel Douglas Image s07-17091 Stationary Plate Connection with Cable Idlers Stationary Plate Flexures Rotating Ring Connection Rotating Ring Cable Passthroughs Figure 10. Example roll ring. The gold stationary plate stays fixed while the outer section rotates. Part of a cable (black) is shown in one of the passthroughs on the inner stationary ring (the rotating ring connector is empty in this photo). Power or data is conducted through the metal flexures between the stationary and rotating rings to transfer electricity or data. Photo: Courtesy of Robert C. Dempsey Beta Gimbal Assemblies The BGAs rotate individual SAWs to provide beta angle array pointing capability (i.e., when the ISS is in the nominal +XVV attitude). Normally, each BGA will rotate approximately ±4º a day to compensate for the changing solar beta angle as the Earth orbits the sun. The BGAs provide the structural load path connecting the SAWs to the ISS truss structure while providing 360° rotational capabilities. They include roll rings for data and power transmission (similar to the SARJ UTA), a motor for SAW positioning, and two redundant anti-rotation latches for locking the BGA in specific positions. Software operates the BGA using multiple “modes.” The autotrack mode—the nominal mode—uses data from the Guidance, Navigation, and Control MDM on the relative positions of the ISS and the sun to calculate the angle that the BGAs should be moved to track the sun. The rate mode uses

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CHAPTER 9 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL 168 Power Buses Power buses provide the physical mounting structure for Remote Power Control Modules (RPCMs) and provide access to command and data interfaces, power input and output connections, and cooling. They do not have any active components (i.e., no moving switches or gears) and only provide structural, thermal, power, and data support for RPCMs. The number of RPCMs in each power bus varies depending on power requirements in that particular location of the vehicle. Power buses were not designed to be replaceable and can be found both internally and externally on the ISS. Remote Power Control Modules The RPCMs are the interface between the EPS and all non-EPS equipment on board the ISS. Because RPCMs are the most numerous EPS devices on the ISS, and due to their direct interface to downstream loads, the crew and ground have the most interaction with these items. The distribution of secondary power to downstream loads can be controlled by opening and closing RPCs within the RPCMs. Protection of the EPS against downstream faults is accomplished by opening RPCs when too much current draw is detected. The RPCMs come in six different configurations, each with a different number of RPCs with different current ratings and current- limiting capabilities. The type of RPCM used in any particular location depends upon the downstream load requirements. All RPCMs have the same housing and the same standard interface connectors, and can be located either externally or internally to the ISS. Pointing Systems To maximize the power generated by the solar arrays, the USOS EPS was designed with multiple articulating joints to allow the solar arrays to be rotated to point at and track the sun as the ISS orbits the Earth. Two separate kinds of rotary joints are used to position USOS solar arrays (Figure 8), due to the changing alpha and beta angles and the potential need to change ISS attitudes (see Chapter 7). SARJs ß ß ß ß ßß ß ß α α BGAs BGAs Rotation SARJs Rotation Figure 8. US Segment solar array angle rotation showing the rotation of the SARJ and the Beta Gimbal Assembly (BGA), as indicated. The BGAs allow the arrays to compensate for the β angle (see Chapter 7, Figure 7), which changes slowly over the year. The SARJ nominally rotates 360° as the space station rotates around the Earth to always keep the solar cells facing the sun. This angle is called the “α” (alpha) angle. Solar Alpha Rotary Joints The SARJs rotate the PVMs—entire truss segments—to provide alpha angle array pointing capability (i.e., when the ISS is in the nominal +XVV attitude). The port and starboard SARJs are located at the outboard end of the P3 and S3 truss segments, respectively, and provide 360° continuous rotational capability to the segments outboard of P3 and S3. The rotary joints normally complete one 360º revolution during each 90-minute orbit of the ISS around the Earth. Essentially, the SARJs are large gears rings with supporting bearings and drive motors. Each SARJ features two redundant control strings, one powered from each power domain. Each string consists of firmware controllers that include sensors to monitor the position and rotational speed of the SARJ, along with drive/ lock assemblies that house the motor, gear teeth, and locking racks used for positioning. See Figure 9.

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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

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