With the U.S. Operational Segment (USOS) of the International Space Station (ISS) newly replenished to three members, following the recent arrival of Dragon Endeavour crewmen Doug Hurley and Bob Behnken to join Expedition 63 Commander Chris Cassidy, the first in a series of critical spacewalks to complete power system upgrades is expected to kick off on 26 June and 1 July. Two sessions of Extravehicular Activity (EVA), featuring veteran spacewalkers Cassidy and Behnken, will set to work removing aging nickel-hydrogen batteries from the station’s S-6 truss segment and replace them with smaller, lighter and more capable lithium-ion units.
In doing so, the astronauts will enter the homestretch of a three-year EVA/robotics campaign to remove and replace batteries from all four power-generating segments of the station’s expansive Integrated Truss Structure (ITS).
The four massive truss segments—P-6 and P-4 on the “port” side and S-6 and S-4 on its “starboard” side, together with adjoining structural members—were launched and installed by successive Space Shuttle crews between November 2000 and March 2009. Each truss houses a pair of giant Solar Array Wings (SAWs) and two power channels to afford the station with electrical energy, power storage and cooling. And within each truss, Integrated Equipment Assemblies (IEAs) originally housed 12 nickel-hydrogen batteries to store electrical power from the SAWs and route it through Battery Charge/Discharge Units (BCDUs) and Direct Current Switching Units (DCSUs) to feed on-board systems. The batteries were designed for a functional lifetime of 6.5 years and 38,000 charge/discharge cycles, but actually lasted almost a full decade and 50,000 charge/discharge cycles.
Several of the aging batteries in the oldest truss (P-6) were replaced with like-for-like units by shuttle spacewalkers in July 2009 and May 2010, but the other three trusses retained their original batteries for far longer. However, plans were in work to install new lithium-ion batteries, which were smaller, greatly exceeded the storage capacity of their predecessors and could function for a decade at 60,000 charge/discharge cycles. One lithium-ion battery takes the place of two nickel-hydrogen ones, with an adapter plate serving to close the circuit. As a result, 48 large nickel-hydrogen batteries would be removed from the four truss segments and replaced with just 24 lithium-ion units.
In January 2017, during two sessions of EVA, Expedition 50 spacewalkers Shane Kimbrough, Thomas Pesquet and Peggy Whitson pulled out the first 12 nickel-hydrogen batteries from the S-4 truss and replaced them with six lithium-ion ones. And in March 2019, Expedition 58 spacewalkers Anne McClain, Nick Hague and Christina Koch did likewise for the P-4 truss. Since both the S-4 and P-4 trusses lie relatively close to the center of the space station—and afford relative ease of access for the spacewalkers and for the 57.7-foot-long (17.6-meter) Canadarm2 robotic arm—no more than two EVAs were needed for each replacement task.
However, the P-6 and S-6 segments lie much further “outboard” on the truss and the greater distances Canadarm2 and the spacewalkers would need to travel to move batteries and tools out to the worksites promised a correspondingly greater workload. As such, between four and six EVAs were timelined to replace the batteries on each truss. Between October 2019 and January of this year, four spacewalks by Expedition 61 astronauts Drew Morgan, Christina Koch and Jessica Meir successfully changed out the P-6 batteries. That left only S-6—the “youngest” of the four trusses, having arrived at the station aboard shuttle Discovery during STS-119 in March 2009—as the only ITS component still in need of an upgrade. Batteries for its two power channels (identified as “1B” and “3B”) were originally scheduled to be replaced in March, but slipped into the summer.
“Like every other battery replacement set, there are two EVAs per power channel,” NASA’s Rob Navias told AmericaSpace, “with an option for a third, if needed.” This raises the possibility that at least four and perhaps as many as six spacewalks could be needed for the S-6 battery upgrade. The initial two EVAs on 26 June and 1 July will tend to batteries in Power Channel 1B, although it remains to be seen if the work to replace the Power Channel 3B batteries will also be replaced whilst Hurley and Behnken remain aboard the station. “Dates for the EVAs for the 3B power channel battery work are under review,” Mr. Navias told us, although NASA expects the Dragon Endeavour crewmen may remain on the ISS until at least August.
“Whether we press ahead with the third and fourth EVAs for the 3B power channel for the S-6 truss will not be determined until after the second EVA,” NASA’s Megan Sumner told AmericaSpace, “when the ISS and CCP programs discuss options for the DM-2 mission duration.”
Mr. Navias noted that the upcoming EVAs will feature the “same choreography” as was adopted for the recent P-6 EVAs. The exact choreography and a step-by-step timeline of the spacewalks will be finalized during an EVA briefing at 2 p.m. EDT 24 June.
Cassidy (designated “EV1”) and Behnken (“EV2”) are already among the Astronaut Office’s most seasoned spacewalkers, having each racked up six EVAs earlier in their careers. Cassidy logged 31 hours and 14 minutes during shuttle mission STS-127 in July 2009 and during his Expedition 35/36 increment in May-July 2013. Notably, during those spacewalks he participated in the replacement of a suspect Pump Flow Control Subassembly (PFCS) following indications of an ammonia leak and was at hand during the infamous “water-in-the-helmet” incident which hit Italian astronaut Luca Parmitano. Behnken, for his part, has logged 37 hours and 33 minutes during shuttle missions STS-123 in March 2008 and STS-130 in February 2010, during which he worked to install the first element of Japan’s Kibo lab, the Dextre robot for Canadarm2, the Tranquility node and the station’s multi-windowed cupola.
With Cassidy and Behnken both trained for EVAs, Hurley is a qualified Canadarm2 robotic arm operator and will assist the spacewalkers with their suit-up activities in the Quest airlock. His backup will be Russian cosmonaut Ivan Vagner. Last week, Hurley and Behnken worked inside Quest to configure the airlock itself and the space suits for the upcoming activity. And on Thursday, Cassidy and Behnken spent several hours in the airlock resizing their suits. At present, it is expected that the two forthcoming spacewalks will total around seven hours, potentially adding up to 14 hours more EVA hours to the cumulative totals for both men. This may position Behnken squarely in the Top Ten most experienced spacewalkers in the world, with Cassidy sitting somewhere in the Top Twenty.
Designated “U.S. EVA-65” and “U.S. EVA-66”, the spacewalks—as their names imply—are the 65th and 66th excursions from the Quest airlock in U.S.-built space suits in the absence of the Space Shuttle, to be conducted since February 2002. The first-ever so-called “Stage EVA” was performed by Expedition 4 astronauts Carl Walz and Dan Bursch and lasted five hours and 47 minutes. The two men tested the newly-installed airlock and performed preparatory work ahead of an upcoming shuttle construction mission. Since then, ISS-based spacewalkers have replaced lithium-ion batteries, repaired the Alpha Magnetic Spectrometer (AMS-2), installed International Docking Adapters (IDAs) for Commercial Crew and lubricated the Latching End Effectors (LEE) on Canadarm2. Others have reconfigured electrical and fluid lines to support the arrival of new station components and replaced failed ammonia coolant systems and electrical system components.