The International Space Station: Operating an Outpost in the New Frontier Page 52 (86 of 438)

CHAPTER 3 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON 52 and some electrically controlled— serve to reduce the exposure of the outer window pane to contamination from jet firings or material off- gassing, provide debris shielding for that outer pane, and block sunlight from entering the ISS cabin. How big are the windows? Size (diameter) of common ISS windows: n US hatch: 20.3 cm (8 in.) n US Laboratory: 51 cm (20 in.) n JEM (two): 51 cm (20 in.) n Cupola center: 70.6 cm (27.8 in.) n Cupola trapezoidal (six): 0.22 m area (322 in2) n Service Module Window #9: 48.3 cm (19 in.) None of the windows on the ISS are composed of a single pane of glass, nor is each pane as thin as one found in a home window. Rather, two panes of relatively thick glass maintain the pressure integrity of the module and, typically, there are additional thinner protective panes on the inside and outside of the window. That makes for a total of four panes. The inner protective pane (called the “scratch pane,” as it is intended to prevent the crews from inadvertently scratching the glass pressure pane) can be removed if necessary. A vacuum is drawn on the inter-pane space to prevent condensation from forming between the two panes of window glass. That is the purpose of the flex hose that Mike Foale is inspecting in Figure 19. That flex hose also proved to be an inviting hand hold for early ISS crew members over time, this resulted in the development of a small leak in that hose. A protective box has now been installed over these flex hoses to ensure they do not get bumped and start leaking, as shown in Figure 20. Figure 19. Expedition 8 Commander Mike Foale using the Ultrasonic Leak Detector in 2004 to pinpoint a small leak that had developed in one of the seals of the hose used to maintain a vacuum between the panes of glass within the large window of the US Laboratory. Once the leak point was identified, the hose was disconnected (which stopped the leak to space) until a replacement hose could be flown to the space station. Figure 20. The US Laboratory window with its shutter closed (noted by white cover visible through the window) and protective box covering its vacuum flex hose. Clockwise from the box is the handwheel crews use to open and close the shutter.

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CHAPTER 3 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON 52 and some electrically controlled— serve to reduce the exposure of the outer window pane to contamination from jet firings or material off- gassing, provide debris shielding for that outer pane, and block sunlight from entering the ISS cabin. How big are the windows? Size (diameter) of common ISS windows: n US hatch: 20.3 cm (8 in.) n US Laboratory: 51 cm (20 in.) n JEM (two): 51 cm (20 in.) n Cupola center: 70.6 cm (27.8 in.) n Cupola trapezoidal (six): 0.22 m area (322 in2) n Service Module Window #9: 48.3 cm (19 in.) None of the windows on the ISS are composed of a single pane of glass, nor is each pane as thin as one found in a home window. Rather, two panes of relatively thick glass maintain the pressure integrity of the module and, typically, there are additional thinner protective panes on the inside and outside of the window. That makes for a total of four panes. The inner protective pane (called the “scratch pane,” as it is intended to prevent the crews from inadvertently scratching the glass pressure pane) can be removed if necessary. A vacuum is drawn on the inter-pane space to prevent condensation from forming between the two panes of window glass. That is the purpose of the flex hose that Mike Foale is inspecting in Figure 19. That flex hose also proved to be an inviting hand hold for early ISS crew members over time, this resulted in the development of a small leak in that hose. A protective box has now been installed over these flex hoses to ensure they do not get bumped and start leaking, as shown in Figure 20. Figure 19. Expedition 8 Commander Mike Foale using the Ultrasonic Leak Detector in 2004 to pinpoint a small leak that had developed in one of the seals of the hose used to maintain a vacuum between the panes of glass within the large window of the US Laboratory. Once the leak point was identified, the hose was disconnected (which stopped the leak to space) until a replacement hose could be flown to the space station. Figure 20. The US Laboratory window with its shutter closed (noted by white cover visible through the window) and protective box covering its vacuum flex hose. Clockwise from the box is the handwheel crews use to open and close the shutter.

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51 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON CHAPTER 3 staged hardware usually covers the window of that particular hatch. That leaves three primary window locations for viewing events outside the US Segment of the ISS for research, educational events, or crew viewing and photography. The US Laboratory has a single, large- diameter window (Figures 19-21). Research experiments are mounted into the Window Observation Rack Facility, which is installed over the top of this window, thus enabling detailed observations of Earth. The JEM has two large-diameter windows on its port bulkhead these windows are used to monitor operations of the JEM robotic arm, activities on the JEM Exposed Facility, and operations of the JEM Airlock. The Cupola is a module of windows attached to the bottom of Node 3 (Figure 18). It has one large, round window at its center and six trapezoidal windows around its perimeter to provide a breathtaking bay-window view of Earth. The Cupola is used by the crew not only for Earth viewing, but also to monitor the arrival and departure of visiting vehicles as well as EVAs and robotics occurring on the bottom of the ISS. Figure 18. “The ‘Cupola’, attached to the nadir side of the space station, gives a panoramic view of our beautiful planet,” said Expedition 25 Commander Doug Wheelock (shown in photo) about this module that has seven large windows. The Earth-facing science window in the US Laboratory was specially designed and manufactured to support scientific investigations. This was accomplished through specific and fine polishing specifications, and through application of coatings on the glass surfaces (as well as specific decisions on which coatings to leave off the window). This detail on the US Laboratory window enables excellent optical qualities that allow the use of various cameras and telescopes that operate in both the visible and non-visible light wavelengths. The windows in the Laboratory, JEM, and Cupola, as well as many of the windows in the Service Module, are also protected with shutters to ensure as little debris as possible makes its way onto the high-quality optical glass. These shutters—some manually controlled

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53 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON CHAPTER 3 The design concept of these windows is that any high-velocity debris would hit the outermost debris pane, followed by the outer pressure pane. If the debris were going fast enough to break through both panes, the particles would be slowed enough such that they would not penetrate the inner pressure pane. As shown in the diagram in Figure 21, this design is similar to the Whipple Shield design of the module debris shields described earlier. The smaller windows on the ISS, such as the US hatch windows, have the same dual- pressure pane design concept as well as inner and outer protective covers. Window Assembly Components Window Panes Figure 21. Overview of the US Laboratory window. Note that the flex hose was the source of the leak discussed in Chapter 16. The windows of the US hatches, US Laboratory, JEM, and Cupola are designed to be replaceable in the event a window pane breaks. An astronaut would install an external pressure cover over the window, via an EVA, to replace the windows exposed to space. The window would then be removed from the inside of the ISS. The windows themselves cannot be removed by a spacewalking astronaut the removal must be done from the pressurized environment of the ISS cabin. Thus, if debris were to damage both panes of a window and cause a module to depressurize, it would not be possible to replace that damaged window. Instead, the crew would install the external pressure cover, via EVA, and repressurize the evacuated module. The crew could then go into that module and remove the window. With the window removed, the crew would install an internal pressure cover over the window’s hole until the new window was ready to be installed. The pressure covers are on orbit to provide a means to respond to a broken window pane however, no spare windows are kept on board the ISS. A spare would need to be manufactured and flown after the failure.

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