Two U.S. astronauts will venture outside the International Space Station (ISS) on Friday to begin the homestretch in a three-year-plus sequence of spacewalks to remove outdated nickel-hydrogen batteries from the four trusses of the Integrated Truss Structure (ITS) and install smaller, more capable lithium-ion ones in their place. Expedition 63 Commander Chris Cassidy and Flight Engineer Bob Behnken will float outside the station’s Quest airlock shortly after 7:35 a.m. EST and their Extravehicular Activity (EVA) is expected to last up to seven hours.
Another EVA on 1 July will complete the replacement of batteries in Channel 1B, one of two power channels on the station’s S-6 truss segment. It remains to be seen if Cassidy and Behnken will perform at least two additional EVAs to replace batteries for the S-6’s second power channel, Channel 3B. Earlier today (Wednesday), NASA briefed the media on the nature of the forthcoming EVAs.
As previously described in a recent AmericaSpace article, the four massive trusses—P-6 and P-4 on the “port” side, S-6 and S-4 on the “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 contribute to the station’s requirement for electrical energy, power storage and distribution and cooling.
And within each truss, 12 nickel-hydrogen batteries stored and routed electrical power from the SAWs to feed on-board systems. The batteries were designed for a functional lifetime of 6.5 years. Although those of the oldest truss (P-6) were replaced with like-for-like batteries by shuttle spacewalkers in July 2009 and May 2010, the other retained their original units for much longer.
Early in the last decade, plans were laid to fit new lithium-ion batteries, which were smaller, greatly exceeded the storage capacity of their predecessors and could a larger number of charge/discharge cycles over a full decade. Each lithium-ion battery takes the place of two nickel-hydrogen ones, with an adapter plate closing the circuit. As a consequence, 48 nickel-hydrogen batteries could be replaced with just 24 lithium-ion ones. In January 2017, spacewalkers Shane Kimbrough, Peggy Whitson and Thomas Pesquet conducted two EVAs replaced batteries in the S-4 truss. Then, in March 2019, spacewalkers Anne McClain, Nick Hague and Christina Koch did likewise for the P-4 truss. More recently, between October 2019 and last January, spacewalkers Koch, Drew Morgan and Jessica Meir tackled the P-6 truss.
Since the S-4 and P-4 trusses reside near the “center” of the space station, they could be reached more easily and the robotic arm’s Dextre “hand” could do much of the heavy lifting of the batteries. However, P-6 and S-6 sit much further “outboard”, beyond the reach of Dextre and the 57.7-foot-long (17.6-meter) Canadarm2, which places added complexity in terms of getting the spacewalkers, their tools and the batteries out to the distant worksites.
As such, whilst the S-4 and P-4 battery swaps required only two EVAs apiece, the P-6 and S-6 campaigns were predicted to require as many as five EVAs per truss: two for each power channel, plus an “extra” spacewalk if needed. (Actual experience on the recent P-6 battery swap allowed the entire task to be done in four EVAs.) Replacement batteries for the two S-6 power channels—identified as 1B and 3B—arrived at the space station last month aboard Japan’s H-II Transfer Vehicle (HTV-9).
These will be the 65th and 66th spacewalks to be conducted out of the station’s Quest airlock, in U.S.-made Extravehicular Mobility Units (EMUs), and in the absence of a docked Space Shuttle, since the pioneering EVA-1 by Expedition 4 astronauts Carl Walz and Dan Bursch in February 2002. Efforts to prepare for the spacewalks have entered high gear in recent days, with Cassidy and Behnken performing suited “fit-checks” in the Quest airlock on Tuesday, assisted by Expedition 63 crewmates Doug Hurley and Ivan Vagner. Hurley will also provide Canadarm2 robotic support during the EVAs, with Vagner helping with crew suit-up activities.
Cassidy and Behnken are already among the astronaut office’s most seasoned spacewalkers, having each racked up six prior EVAs in their careers. Cassidy has logged 31 hours and 14 minutes during shuttle mission STS-127 in July 2009 and Expedition 35/36 in May-July 2013, during which he helped install and activate the final component of Japan’s Kibo lab, 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. Interestingly, Cassidy led the choreography to develop the S-6 battery EVAs and is ideally suited for this task. Behnken logged 37 hours and 33 minutes of spacewalking time 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.
Earlier today (Wednesday), NASA managers at the Johnson Space Center (JSC) in Houston, Texas, provided a comprehensive briefing for the two EVAs to conduct the Channel 1B battery replacements. Kenny Todd, deputy ISS program manager, outlined the “wash, rinse, repeat” nature of these battery-swap EVAs—having done so many of them in the past—and pointed out that the new lithium-ion infrastructure puts the space station in a position which is “good on batteries for a number of years to come”.
Kicking off at 7:35 a.m. EDT Friday, the opening EVA will see Cassidy (designated “EV1”, with red stripes on the legs of his space suit) and Behnken (“EV2”) spend approximately six hours and 40 minutes outside. Guided by robotics officer Hurley and aided from the ground by a Mission Control team led by Flight Director Royce Renfrew and Capcom Jasmin Moghbeli, the veteran spacewalkers will pluck out the old nickel-hydrogen batteries from Channel 1B and replace them with new lithium-ion units and adapter plates from the HTV-9 Exposed Pallet (EP). The two men will swap roles for the second EVA on 1 July, with Behnken in the EV1 mandle and Cassidy as EV2.
Should a third EVA be needed to close out the Channel 1B work, Mr. Todd anticipates it might happen around 6 July and noted that—barring any unforeseen circumstances—the program is “trying to nail down those dates” in the hope of possibly doing the Channel 3B EVAs whilst Hurley and Behnken remain aboard the ISS. His estimate was that the second pair of spacewalks are “on a path” to occur as soon as mid-July.
One of the drivers in this capability, of course, is how long Hurley and Behnken’s Dragon Endeavour can remain aloft. Launched on 30 May, it was predicted that the performance of its solar arrays would produce a maximum mission duration of 119 days. And from remarks provided by Commercial Crew Program Manager Steve Stich, it would appear that the arrays’ limits are operating right on prediction at present.
Dragon Endeavour herself continues to chug along in fine form, running mainly in a “quiescent” state, although earlier Wednesday she was briefly powered up for a series of propellant system checks. A “safe haven” capability to use the roomy spacecraft for up to 24 hours in case of an emergency has been successfully trialed, as has the testing of voice and data exchange between the Crew Dragon and the station. Mr. Stich pointed out that current expectations are that Hurley and Behnken will return to Earth around 2 August, producing a mission duration for Demo-2 of about 64 days.
Their return to a parachute-assisted splashdown off the East Coast of the United States will make this the first oceanic landing by a U.S. crew of astronauts since the Apollo-Soyuz Test Project (ASTP) in July 1975. Mr. Stich explained that three landing sites are under consideration: a “prime” to the east of Cape Canaveral (appropriately known as “Capeside”), a backup off Pensacola and another off Jacksonville. However, he stressed that the final decision of which will be the primary and backup landing zone will be made in due course, based on factors such as the orbit and weather conditions.
Following the return of Hurley and Behnken, Mr. Stich anticipates around a six-week “iron bar” of time before the first Post-Certification Mission (PCM-1) of Crew Dragon—also designated “Crew-1”—sets off. Their Crew Dragon spacecraft is presently as SpaceX’s facility in Hawthorne, Calif., where it will continue to undergo propulsion system checkouts until the end of June. Its unpressurized “trunk” is also deep into acoustic testing, with hopes high that the complete vehicle will ship to the Cape in late July. This six-week iron-bar process, which would see Crew-1 flying in the mid-September timeframe, has been dictated by the need to run through complex data reviews from Dragon Endeavour’s departure, re-entry and landing.
FOLLOW AmericaSpace on Facebook and Twitter!