Veteran spacewalkers Christina Koch and Drew Morgan wrapped up a multi-hour session of Extravehicular Activity (EVA) yesterday to begin replacing old nickel-hydrogen batteries in the P-6 truss of the International Space Station (ISS) with upgraded lithium-ion units and associated adapter plates. The two astronauts—both of whom had completed one EVA apiece before—ventured outside the station’s Quest airlock for U.S. EVA-56 at 7:39 a.m. EDT Sunday, 6 October, to begin a series of five U.S. spacewalks over a three-week period to attend to the complicated P-6 battery changeout.
Although similar battery replacement work was done on the space station’s S-4 and P-4 trusses in January 2017 and March 2019, neither of those tasks required more than two EVAs. However, both the S-4 and P-4 trusses are located “inboard” on the station’s expansive Integrated Truss Structure (ITS) and thus more readily accessible both for spacewalkers and the station’s robotic assets, notably the 57.7-foot-long (17.6-meter) Canadarm2. For the “outboard” S-6 and P-6 trusses, greater distances need to be traversed, far further than Canadarm2 can reach, and this requires a longer period—and multiple EVAs—to get the job done.
The station’s electrical system carries eight power channels and giant Solar Array Wings (SAWs) on each of the four truss segments. These massive structural members were installed by Space Shuttle crews over a period of almost a decade: the P-6 truss launched to the space station in November 2000, followed by P-4 in September 2006, S-4 in June 2007 and most recently S-6 in March 2009. Aboard each truss, Integrated Equipment Assemblies (IEAs) housed 12 nickel-hydrogen batteries to store electrical power from the solar arrays and route it through Battery Charge/Discharge Units (BCDUs) and Direct Current Switching Units (DCSUs) to feed on-board ISS systems. These batteries were designed for a functional lifetime of 6.5 years and approximately 38,000 charge/discharge cycles, but they actually lasted almost a full decade and some 50,000 charge/discharge cycles.
Although shuttle spacewalkers replaced the aging batteries in the P-6 truss with like-for-like nickel-hydrogen units in July 2009 and May 2010, the other three trusses retained their “original” battery sets for many more years. However, plans were in work to build new lithium-ion batteries, which were smaller, greatly exceeded the storage capacity of their predecessors and could function for a decade with 60,000 charge/discharge cycles. And whereas the ISS required 48 of the large nickel-hydrogen batteries, only 24 of the smaller lithium-ion units were needed; six per truss, rather than 12. Essentially one lithium-ion battery takes the place of two nickel-hydrogen batteries, with an adapter plate added to complete the circuit.
“In place of two of the nickel-hydrogen batteries, we now put in one lithium-ion battery to take the place,” said Lead Spacewalk Officer Kieth Johnson in last week’s press briefing at the Johnson Space Center (JSC) in Houston, Texas. “But you have to complete the circuit, so [for] the two batteries we’ve taken out we’ll put in a new lithium-ion battery and we’ll replace the other one with an adapter plate and we’ll hook the wiring up between the two.”
The new batteries are being ferried into orbit aboard four Japanese H-II Transfer Vehicles (HTVs), the first set arriving aboard HTV-6 in December 2016. A month later, Expedition 50 spacewalkers Shane Kimbrough, Peggy Whitson and Thomas Pesquet supported a pair of EVAs to install the first set of six lithium-ion batteries into the S-4 truss. Another set followed aboard HTV-7 in September 2018, which were installed into the P-4 truss in March 2019 across two more EVAs by Expedition 58 spacewalkers Anne McClain, Nick Hague and Christina Koch. Since Canadarm2 could easily reach the S-4 and P-4 worksites, much of the battery installation task was done with ground-commanded robotics, leaving the spacewalkers to physically hook up electrical connectors.
The third set of batteries for P-6 arrived late last month aboard HTV-8, with the final set for S-6 slated to arrive on HTV-9 in May 2020. Following HTV-8’s successful capture and berthing at the station on 30 September, Canadarm2 ground controllers extracted its External Pallet (EP)—which holds the six new lithium-ion batteries and their six adapter plates—and positioned it as far “outboard” as possible, about 20 feet (6.6 meters) from the P-6 worksite. This aided Koch and Morgan, who were faced with a long translation from the Quest airlock on the far-starboard side of the station to the far-port side of the truss.
Koch performed an EVA back in March to install batteries in the P-4 truss, also on the port side of the station, and her familiarity with this worksite directly led her to be designated “EV1”—the lead spacewalker—for today’s U.S. EVA-56. And with Morgan having completed his own EVA as recently as August, Astronaut Office Deputy Chief Megan McArthur remarked at last week’s press briefing that having this particular pairing start the EVA marathon “maximizes efficiency” within the Expedition 61 crew. They will also complete U.S. EVA-57 on 11 October, although they will switch roles, with Morgan taking the EV1 mantle for the first time in his career. According to Mr. Navias, all EVA crew members have been trained for both EV1 and EV2 roles.
For her part, Koch characteristically took the challenge in stride. “All hands on deck,” she tweeted on 4 October, sharing a photograph of herself and Expedition 61 crewmate Jessica Meir performing Extravehicular Mobility Unit (EMU) and airlock checks. “Suited up this week in preparation for 5 spacewalks in a row to upgrade the @Space_Station solar array batteries,” she wrote. “Work begins on Sunday!”
And for the Expedition 61 crew, commanded by Italian astronaut Luca Parmitano, Sunday commenced with an early breakfast, after which Koch and Morgan headed directly for the Quest airlock. Assisted by Parmitano and Meir, the duo performed 60 minutes pre-breathing on masks, during which time Quest’s inner “equipment lock” was depressed from its ambient 14.7 psi to 10.2 psi. Koch and Morgan purged their suits and the atmosphere was repressurized back up to 14.7 psi, enabling them to head into a nominal pre-breathing regime of about 50 minutes in length. They then completed 50 minutes of In-Suit Light Exercise (ISLE) to rapidly remove nitrogen from their bloodstreams, before Parmitano and Meir manhandled them into the outer “crew lock”.
Hatches between the two locks were closed and the process of depressurization got underway. Finally, Koch cranked open the hatch and U.S. EVA-56 officially commenced at 7:39 a.m. EDT, as the ISS orbited high above Mongolia. After performing customary buddy checks of each other’s suits, the spacewalkers proceeded crisply to the P-6 worksite, setting up tools, tethers and Articulating Portable Foot Restraint (APFR) hardware. By two hours into the spacewalk, the first old nickel-hydrogen battery had been extracted and installed back into the EP for eventual disposal. And less than an hour later, at 9:30 a.m. EDT, the first new lithium-ion battery had been plucked from the EP and installed in its place.
Working more than an hour ahead of schedule, Koch and Morgan labored to “shepherd” the batteries between the worksite and the EP. And although these batteries carry hefty weight on Earth—365 pounds (165 kg) for each of the old nickel-hydrogen units; 420 pounds (190 kg) for each of the new lithium-ion ones—they can be manipulated with a fingertip in the microgravity of low-Earth orbit. That said, the properties of mass and the problems of overcoming inertia remain the same, and Koch and Morgan were required to take extreme care when manipulating these half-refrigerator-sized batteries back and forth. Their unwieldy size, noted today’s Flight Director (and former astronaut) Timothy “T.J.” Creamer, renders it exceptionally difficult to “corral” them into a small area without causing damage.
Nevertheless, Koch and Morgan managed to press deeply into their get-ahead tasks, removing a third nickel-hydrogen battery. Returning to the airlock after seven hours and one minute, Koch and Morgan have therefore completed a major task previously assigned to them for U.S. EVA-57, coming up on Friday, 11 October.