For the third time in 9 days, Thomas Pesquet and Shane Kimbrough donned their spacesuits and ventured outside the International Space Station (ISS), this time to install the second of six new Boeing-built ISS Roll Out Solar Arrays (IROSAs) to the outpost — part of a program to increase the station’s electrical power capability as its science and research demands increase and future expansion plans continue.
The Extravehicular Activity (EVA) – officially known as US EVA-76 – began at 11:52 UTC / 07:52 EDT when Thomas from the European Space Agency (ESA) and Shane from NASA took their extravehicular mobility units (EMUs, or spacesuits) to battery power before exiting the Quest Airlock.
This was the third — and final — EVA to install the second of the first two sets of new roll-out solar arrays, this time to the 4B channel on the Port 6 (P6) truss segment of the ISS.
The EVA was largely a copy of the originally-planned US EVA-74 (see below for details), which encountered multiple EMU suit issues for Shane and an interference issue with the unfolding of the first set of new arrays. That EVA was originally supposed to see the complete installation of array #1, but that did not come to pass.
The interference issue was studied and corrected on the ground, with Thomas and Shane successfully getting the first new solar array installed on a re-planned US EVA-75 (which was originally slated to mark the installation of the second array — the one installed during EVA-76).
US EVA-76 incorporated those successful changes. For this EVA, Thomas and Shane first prepared and released the new solar array from the carrier on which it arrived aboard the CRS-22 SpaceX cargo Dragon earlier this month.
Thomas then attached himself to the end of Canadarm2; Shane and Thomas then maneuvered the array out of the carrier and, with Thomas holding on to the array, Megan McArthur, working inside the ISS at the Robotics Work Station, then drove him as far out on the station as the arm would reach. There, he passed the array back off to Shane, repositioning himself, and then took the array back for its final installation.
The duo then worked together to install it onto a mounting bracket deployed earlier this year on a previous spacewalk, rotated it to its deployment location, positioned the mounting bolts, installed the electrical cables, and drove the final two bolts to extend the array to its fully deployed position.
With the start of Quest airlock re-pressurization at 18:37 UTC / 14:37 EDT, the EVA officially came to end after 6 hours 45 minutes.
Overall, this was the 241st EVA for station construction and maintenance, the ninth spacewalk this year from the ISS, Shane’s ninth spacewalk, and Thomas’ fifth.
Previous EVA issues and successes
US EVA-74 encountered numerous issues, primarily with Shane’s suit and a hardware interference with the solar array unfolding that brought an end to the spacewalk well before the main objectives could be accomplished.
After exiting the Quest Airlock, the pair translated out to the IROSA Flight Support Equipment (FSE). However, in a somewhat sign of things to come, the hatch covering would not close at first, and Shane had to spend more time than planned getting the airlock’s fabric hatch covering configured properly.
The FSE, pallet on which the IROSAs are attached, was removed from cargo Dragon’s trunk by Canadarm2, also known officially as the Space Station Remote Manipulator System (SSRMS) — part of the overall Mobile Base System on the station. Canadarm2 then installed the FSE onto the Mobile Base System (MBS) Payload ORU Accommodation (POA).
After translating to the FSE, the duo began setup of the worksite and released launch restraint bolts on the IROSA. However, before the EVA could progress, two issues were noted with Shane’s suit. First, a sensor in the suit’s sublimator — which provides pressure — registered a spike. Shortly thereafter, the Display and Control Module (DCM) in his suit malfunctioned, necessitating an immediate return to the Quest Airlock to connect back to Station umbilicals to attempt a restart of the unit.
The “warm restart” of the DCM meant that Shane’s suit momentarily lost its cooling and CO2 scrubbing capabilities; however, this is an acceptable condition, per EVA procedures, when attempting to “warm restart” a DCM. A failure to restart the unit would have meant a premature end to the EVA.
The restart was successful, and ground teams sent Shane back out to Thomas while managers and engineers continued to discuss the sublimator issue — which itself could have also stop the EVA early. Fortunately, through a series of suit configuration tests, ground teams were able to determine that the sublimator was functioning properly and that a faulty sensor likely triggered an erroneous pressure increase reading.
With his suit good to go, Shane translated out to the P6 truss installation site — specifically the 2B Integrated Electronics Assembly (IEA) — to begin more setup while Pesquet – mounted to the end of Canadarm2 – held on to the IROSA while he was “flown” out toward the P6 truss.
Inside the ISS in the Robotics Work Station in the Cupola viewing module, NASA astronaut Megan McArthur controlled Canadarm2; she was the one to physically drive Thomas out toward the P6 truss.
Due to the fact that P6 is at the very outboard end of the station, Canadarm2 cannot reach all the way to the worksite, meaning Thomas had to hand-off the IROSA to Shane, who then in turn held on to it whilst Thomas dismounted the arm and repositioned.
Once Thomas was in position, Shane handed the IROSA back to him. The duo then aligned the IROSA onto the mounting bracket of the “Mod Kit” — which was installed during a spacewalk earlier this year — at the base of the 2B Mast Canister Assembly (MCA).
The IROSA was first soft-docked onto the mounting bracket before an attempt to unfolded it into its deployment configuration stalled due to interference/blockage from a nearby structure. At this point, already at the six hour mark into the spacewalk, ground teams decided to have the duo photograph the interference and firmly secure the IROSA as is and end the spacewalk.
US EVA-75 was then re-planned and occurred on 20 June to successfully complete the main objectives of US EVA-74 as well as “get ahead” tasks to reduce the amount of work needed for the next spacewalk.
After leaving the Quest airlock, the pair worked to unfold the new solar array on the 2B power channel. This time, the mitigation strategies developed on the ground worked, and Thomas and Shane were able the secure the IROSA properly before running electrical and data connections between the new array and the station’s power system.
The crew members then successfully deployed the solar array from its flight support structure — marking the first time since 2009 that a new array was unfurled on the station. Until this spacewalk, the newest solar arrays for the station were those on the S6 truss, delivered by the STS-119 crew of the Space Shuttle Discovery.
The eight original Solar Array Wings (SAWs) on the ISS, which each produce around 30 kilowatts (kW) of power for a total of about 250kW are beginning to show signs of degradation, with the oldest array now having been in space since 2000 when the P6 truss and associated arrays was delivered to the station by Shuttle Endeavour’s STS-97 crew.
With over 20 years of use, and normal degradation of solar arrays, the eight SAWs now only produce around 160kW of power – against a backdrop of rising power demands from the station’s increasing users.
This led the Station program to develop the ISS Power Augmentation (IPA) plan, which called for adding six additional solar arrays to the station in order to restore the outpost’s power generation to its original levels.
Under the IPA program, six new ISS Roll Out Solar Arrays (IROSAs) will be added. Whilst the station’s original arrays were folded up and deployed in an accordion-like manner, the IROSAs are a new type of array technology which roll out in a mat-like manner from inside a cylindrical canister.
The IROSAs will be installed on top of six of the station’s existing solar arrays, which will allow the IROSAs to utilize the same sun-tracking motors and be connected into the same electrical system as the current arrays.
With the IROSAs being around 30% efficient, compared to the 14% efficiency of the original arrays, the IROSAs will generate roughly the same amount of power as the originals despite being only half their size.
Each IROSA will produce 20kW of additional power, for a total of 120kW across all six arrays.
However, because the IROSAs are smaller, they will not completely cover the half of the six SAWs they’ll be installed over. Instead, portions of the original arrays will still be power positive.
The unshadowed portions of the original arrays will continue to produce 95kW as a result, making for a combined total of 215kW of power available to the ISS — an increase of nearly a third compared with the outpost’s current levels.
The first EVA encounter numerous issues, primarily with Shane Kimbrough’s suit and a hardware interference with the solar array deployment that brought an end to the spacewalk well before the main objectives could be accomplished.
(Lead image: Placement of the new IROSAs over the existing station solar arrays. Credit: Mack Crawford for NSF L2)