171 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL CHAPTER 9 the partnership, two major changes occurred in design modification from Space Station Freedom to the ISS that impacted the USOS solar arrays. First, the RS was added to the ISS design and included thrusters for vehicle attitude control. This eliminated the need for US Orbital Segment thrusters, which had been designed to limit plume impingement on the USOS solar arrays. The RS modules (and their associated attitude control thrusters) were based on elements of the Mir space station and did not take into consideration USOS solar array design or structural capabilities. Second, when Russia joined the ISS partnership, the inclination of the space station orbit was changed from 28º to 51.6º to make full use of Russian launch vehicle capabilities. This change in inclination also altered the beta angle range that the space station would see. At 51.6º inclination, the ISS would experience beta angles up to ±75º, which would cause times of no eclipses for days. In addition to exposing hardware to high temperatures at high beta angles, components of the ISS can cast shadows on other station equipment and thereby reduce solar power generation or create thermal gradients (i.e., longeron shadowing) that were not figured into the original design. Another constraint that has been applied to the USOS solar arrays is their use in reducing (or increasing) atmospheric drag on the ISS. Even at the orbital altitude of the ISS, there is enough of an atmosphere for the large surface area of the USOS solar arrays to cause drag on the ISS, especially when facing the direction of motion. This drag, although small, adds up over time, thus lowering the ISS attitude and causing the need for reboosts (see Chapter 7). To combat this constraint, the software used to calculate solar array angles to track the sun allows for biases to be applied. Biasing the solar array position can turn it more edge on to the velocity vector to reduce drag. Of course, this also turns it away from the sun, thereby reducing power generation and potentially causing longeron shadowing. This constrains when and how much bias can be used. As the ISS software developed over time, this biasing strategy was automated, which subsequently reduced the workload for ground controllers. Although uncommon, this strategy has occasionally been reversed to increase drag on the ISS to meeting visiting vehicle phasing constraints, as discussed in Chapter 14 (i.e., being in the right place at the right time for rendezvous or departure) without needing to burn propellant to deboost the ISS. Conclusion Operating the largest orbital solar power platform has been challenging, yet highly successful. Many design decisions and design changes have driven the need for automated software control and sophisticated analysis tools. The SPARTAN team continuously plans and adjusts EPS configurations to protect the ISS hardware and maintain power availability to critical systems and scientific payloads. The team must also be ready to respond to system failures or maintenance by adjusting plans or rerouting power—or both. Working with the ISS Program and engineering experts, SPARTAN will continue to maintain the ISS EPS in support of the crew, scientific research, and ultimately exploration.