315 DAY IN THE LIFE: RISKY AND REWARDING SPACEWALKS—SPACE SHUTTLE MISSION STS-120/ISS-10A CHAPTER 18 worse. While the array was parked in this partially deployed state, the flight control team tried to determine what to do next. Snag Snag caused array separation along the hinge line, like a tear along a crease on paper Figure 8. The 4B array damage. The crew and ground team aborted the deploy operation when the tear was noticed, thus the array did not get to the end of the deploy sequence where it would have been pulled taut. After looking at the video, it seemed clear the array could tear more if loads (i.e., forces) were put into it. One major concern was that vibrations and jet firings during the shuttle undocking or other visiting vehicles coming/going could cause the array to sway and rip further. It might not be repairable if it worsened. Further damage could lead to a need for the array to be jettisoned, meaning a future crew would have to dismantle it at the base and push the array to burn up into the atmosphere. This would present an extremely complex operation in which a jettison would leave the ISS with much-reduced power. Building another array would be costly. Future concerns aside, the near-term reality was that a ripped array could flap around and cause major damage to other components. This issue became the top problem to solve on this mission. 4.6 m (15.0 ft) 4.6 m (15.0 ft) 2.1 m (16.9 ft) Solar Array Blanket Blanket Box lower half Blanket Box upper half Guidewire Mechanisms Mast (partially extended) Mast Cannister (holds Mast prior to deploy) Figure 9. Solar Array Wing. Each array blanket was originally folded up, accordion-style, into a box. When the arrays were deployed, the boxes were commanded to unlatch, and the center mast that was folded up inside a canister was commanded to elongate. This effectively pulled the top half of the box (attached to the blanket) away from the bottom half of the box (attached to the other end of the blanket) and stretched out the folds in the blankets. (ISS Electrical Power Systems Training Manual - 01.04.05(0)T0005, Version 1.0, (supersedes TD9707)) Solar array retractions and deploys in the past had sometimes required EVA crew assistance, as the mechanisms to unfurl large blankets of solar cells are fairly complex. The blankets are huge, with each array “wing” composed of two blankets—each approximately 35 m (115 ft) tall. The blankets were originally folded up in an accordion fashion inside a blanket box that was only 0.5 m (20 in.) tall. Often, a fold would stick to an adjacent fold when the blankets were later stretched out (Figures 9-11), or some part of the delicate mechanism would need the ground team’s creativity to jostle or pull it, or it would need an EVA crew member to carefully expand the array. The P6 arrays were not expected to be sticky since they had been deployed previously however, the EVA crew had been trained, pre-mission, to handle other potential array issues