The International Space Station: Operating an Outpost in the New Frontier Page 316 (350 of 438)

CHAPTER 18 DAY IN THE LIFE: RISKY AND REWARDING SPACEWALKS—SPACE SHUTTLE MISSION STS-120/ISS-10A 316 such as freeing up small snags along the guidewires that help keep the array straight (Figures 11-12). Guidewire (vertical) Hinge Solar Cells (gray) Blanket (folded up like an accordian) Figure 10. A solar array blanket located in a ground facility. It is shown still mostly folded up in the lower half of the blanket box, as if the ground had just started to command a deployment. The active side of the array (shown) is where the solar cells (gray) are located to gather energy from the sun. “Guidewire” that runs between top and bottom halves of blanket box that guides the array during deploy and helps keep the array flat while deployed Guidewire Hinge Grommets Figure 11. Passive side of the array (solar cells are on the other side of the array). The blankets are hinged. When they deploy, small grommets move up the guidewire. The guidewire keeps the blankets from swaying and bowing too much when the array is fully deployed. Team 4 and the console team were still working on the SARJ inspection EVA but had to start reprioritizing an array repair in short order. On Flight Day 9, the team decided to change EVA 4 from the SARJ EVA to a solar array repair EVA. Parazynski and Wheelock were chosen to perform the spacewalk, with Parazynski performing the repair since he was the most experienced EVA crew member on board. The Challenges Repair of the array would be a high- degree-of-difficulty spacewalk if a fix was even possible. The flight control team did not have much time to create and execute such an EVA before the shuttle had to undock. Under the direction of the Team 4 flight director, multiple flight control and engineering teams—EVA, robotics, and power, to name a few—worked around the clock to identify each roadblock and devise a fix. Below are some of the challenges that the ground team and crew faced going into the EVA. 1. The Damage Might Be Just Out of Reach The solar arrays were far, far away from the crew modules. P6 was no longer in the center of the truss as it was at the beginning of the mission. The most obvious technical issue was going to be reaching the damage. The snag could not be reached by an EVA crew member on the robotic arm, even with the arm fully outstretched. The team looked at the potential for retracting the array so the damage was near the base, but it was too risky to move the array that much. The damage could get worse, and

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CHAPTER 18 DAY IN THE LIFE: RISKY AND REWARDING SPACEWALKS—SPACE SHUTTLE MISSION STS-120/ISS-10A 316 such as freeing up small snags along the guidewires that help keep the array straight (Figures 11-12). Guidewire (vertical) Hinge Solar Cells (gray) Blanket (folded up like an accordian) Figure 10. A solar array blanket located in a ground facility. It is shown still mostly folded up in the lower half of the blanket box, as if the ground had just started to command a deployment. The active side of the array (shown) is where the solar cells (gray) are located to gather energy from the sun. “Guidewire” that runs between top and bottom halves of blanket box that guides the array during deploy and helps keep the array flat while deployed Guidewire Hinge Grommets Figure 11. Passive side of the array (solar cells are on the other side of the array). The blankets are hinged. When they deploy, small grommets move up the guidewire. The guidewire keeps the blankets from swaying and bowing too much when the array is fully deployed. Team 4 and the console team were still working on the SARJ inspection EVA but had to start reprioritizing an array repair in short order. On Flight Day 9, the team decided to change EVA 4 from the SARJ EVA to a solar array repair EVA. Parazynski and Wheelock were chosen to perform the spacewalk, with Parazynski performing the repair since he was the most experienced EVA crew member on board. The Challenges Repair of the array would be a high- degree-of-difficulty spacewalk if a fix was even possible. The flight control team did not have much time to create and execute such an EVA before the shuttle had to undock. Under the direction of the Team 4 flight director, multiple flight control and engineering teams—EVA, robotics, and power, to name a few—worked around the clock to identify each roadblock and devise a fix. Below are some of the challenges that the ground team and crew faced going into the EVA. 1. The Damage Might Be Just Out of Reach The solar arrays were far, far away from the crew modules. P6 was no longer in the center of the truss as it was at the beginning of the mission. The most obvious technical issue was going to be reaching the damage. The snag could not be reached by an EVA crew member on the robotic arm, even with the arm fully outstretched. The team looked at the potential for retracting the array so the damage was near the base, but it was too risky to move the array that much. The damage could get worse, and

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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

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317 DAY IN THE LIFE: RISKY AND REWARDING SPACEWALKS—SPACE SHUTTLE MISSION STS-120/ISS-10A CHAPTER 18 the accordion aspect could not be expected to work correctly. However, the Space Shuttle had a long 15 m (49.2 ft) boom called the Orbiter Boom Sensor System (OBSS) that the shuttle had been flying since the Columbia accident. The purpose of the OBSS was for up-close inspection of damage on the underside and wings of the orbiter. A shuttle EVA crew member could stand on the tip of the boom while the boom was grasped by a robotic arm to perform a repair. Use of the boom grasped by the ISS robotic arm (17.6 m [57.7 ft]) looked feasible for reaching the array damage—albeit barely—according to virtual reality and software models of the ISS. The ground team used this as the concept for the EVA. The ground team set up the robotic systems ahead of the EVA, optimistic that the computer models were accurate in their conclusion that Parazynski could reach the array damage site. The team prepared for the EVA by translating the Mobile Transporter with the ISS robotic arm on it from the end of the truss to the center of the ISS. The SSRMS was now ready to take the OBSS out of the Discovery cargo bay. When the time came, the ISS arm would grasp the boom and the Mobile Transporter would move back to the end of the truss to point the tip of the boom toward the truss structure so that Parazynski could put his feet in a restraint at the tip for the ride out to the array. These complicated robotics operations were not pre-planned. A fairly large effort was required to develop the procedures and perform the analysis to ensure the operation was safe. 2. The Damage Was Not Well Understood Even with the most powerful zoom lenses, the damaged portion of the array was too far away from the crew inside the Space Shuttle and the ISS for a detailed look at the problem. Using the best crew-taken photos, the flight control team tried to map out the damage to the extent possible to develop a good repair (Figure 12). In the end, there were some “if you see this then do that” steps in the crew repair procedures on the day they performed the EVA. As always, the flight control team tried to prepare for every imaginable scenario. Upper Blanket Box Lower Blanket Box Panel 37 Panel 38 Panel 36 Panel 35 Panel 34 Large Damage Hinge Loops and Doubler – Hinge Wire is inside the Doubler ~ 8–12 inches Small Damage Hinge Wire from Lower Hinge (sketched in) ~ 8–12 inches Location of Grommets Guide Wire Stbd Port Figure 12. This was the best close-up photo of the damage available in the days prior to the EVA. The ground team marked it up with white lines and red markings, as shown, and uplinked it to the crew in this training documentation.

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