at 357 feet (108 meters) and wider than a 777-300 wingspan at 240 feet (73 meters), the International Space Station has encountered no major structural issues, an important gauge for extended use. Its thick trusses remain robust, McCann said. In 2009, it shook violently when a main engine “reboost” misfired, but no damage resulted. The reboost regularly maneuvers the station to its proper orbit. McCann likened the episode to a commercial jetliner making a hard landing—the airplane is built to withstand that sort of stress; it just shouldn’t happen repeatedly. When the Starliner docks with the station, the coupling will be less demanding on the station’s structure than dockings that involved the much larger space shuttles, which are no longer operational, said Matthew Duggan, Boeing’s manager of ISS integrated analysis. “We track everything obsessively,” Duggan said. “Every single vehicle docking has a measurable stress on its physical life.” While a meteor or debris strike is always a possibility, safeguards keep the probability low for surface penetration. Five times per year, on the average, the space station is moved in its orbit to avoid space debris. Thick shields protect sensitive elements. Leak patch kits are accessible if something invasive happens. Crew members are installing an ultrasonic system that uses small microphones to assist in pinpointing leaks. “We got very lucky on the structure,” said Brad Cothran, Boeing director of sustaining engineering for ISS. “They’re beefy, large trusses, naturally designed to survive the ascent.” To ready itself for the coming Starliner flights, the space station took delivery of 31 modification kits. They have been used to reconnect power to different sources, move certain elements into orbit and reattach them, replumb the system, add new antennas, and install docking adapter parts, Clemen said. Installation of the International Docking Adapter will require a spacewalk and a delicate operation, he said. The 1,021-pound (463.1-kilogram) part will be removed robotically from a cargo trunk and positioned within 10 inches (25 centimeters) of the connection, at which point crew members will attach it manually and carefully—something as small as a human hair could prevent a proper fit, Clemen said. The space station is no stranger to this type of work; visitors have performed 192 spacewalks, according to NASA. The space station’s eight power channels face as much scrutiny as any system, according to Jeffrey Donoughue, Boeing’s manager for ISS avionics. Each channel consists of a solar array, which is a wing-like collection of solar panels, and three batteries. The solar arrays power the space station and regenerate the batteries when facing the sun; the batteries take over when the station is in the dark. The arrays, composed largely of fabric and glass, were damaged early on in 2000 but were effectively repaired and are closely monitored through photographs. Controller boxes, which determine the amount of power the arrays need to provide, still fail periodically and require a risky fix, Donoughue said. This year, the space station will begin replacing its original nickel hydrogen batteries with those made of lithium ion. The new battery, Donoughue said, offers twice the energy density and half the size of the nickel hydrogen battery, which weighs several hundred pounds, resembles a two-drawer filing cabinet and is approaching the end of its life span. The station’s environmental control and life-support system, which controls air, water and waste, is crucial to its longevity. The original system was not regenerative. The current system has the ability to turn condensation and urine into drinkable water and reuse carbon dioxide to help generate oxygen, among other sustaining functions, all someday critical to deep-space exploration. Transformation was not an easy MARCH 2016 | 21
Frontiers March 2016 Issue
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