21.06.2021
Two astronauts have ventured outside the International Space Station (ISS) to attempt to continue installation of the first of six new Boeing-built solar arrays — part of a program to increase the station’s electrical power generation capacity as its science and research demands increase and future expansion plans continue.
The Extravehicular Activity (EVA) – officially known as US EVA-75 – began at 11:42 UTC / 07:42 EDT when Thomas Pesquet from the European Space Agency (ESA) and Shane Kimbrough from NASA took their spacesuits to battery power before exiting the Quest Airlock to begin their work.
IROSA background
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.
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 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.
EVA-74 issues
US EVA-74 encountered numerous issues, primarily with Shane Kimbrough’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 first task for the pair was to translate 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 pair of 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).
For Pesquet and Kimbrough, 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 further, 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.
Angle showing how the new IROSAs will be deployed over the current arrays. (Credit: NASA)
With his suit good to go, Kimbrough 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 Pesquet 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 Pesquet had to hand-off the IROSA to Kimbrough, who then in turn held on to it whilst Pesquet dismounted the arm and repositioned.
Once Pesquet was in position, Kimbrough 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.
Teams will now evaluate a path forward to unfold the array, which must happen before it can be unfurled.
Thomas and Shane also did not mount four electrical connections between the IROSA and the 2B MCA in order to connect the IROSA into the electrical system of the current 2B solar array as they ran out of time.
The ISS once the new arrays are installed – via Mack Crawford for NSF L2.
After returning to the Quest Airlock, the duo took their suits off battery power at 19:26 UTC / 15:26 EDT, concluding the 7 hour 15 minute spacewalk.
EVA-75
Sunday’s spacewalk will now attempt to complete the main objectives of US EVA-74, with Thomas and Shane moving out to the P6 truss worksite.
After leaving the Quest airlock, Thomas will take the lead moving to the worksite. The pair will both work to unfold the new solar array on the 2B power channel, and Shane will drive in the two remaining bolts to secure the solar array in place.
The two installed the new iROSA into its mounting bracket during th 16 June spacewalk, but an interference associated with the array’s hinge created an alignment issue and prevented a full roll-out.
Ground teams have since identified the solution related to the sequence of deployment that will be employed on this spacewalk.
The crew members will work together to deploy the solar array from its flight support structure. Once they complete unfolding and the new array and driving the final two bolts into place, the duo will install cables for connection to the station’s electronics.
If successful, Thomas and Shane will then turn their attention to get-ahead work for the second iROSA installation, currently scheduled for US EVA-76 on 25 June.
Overall, US EVA-75 is the 240th EVA in support of station construction and maintenance and the eighth spacewalk so far this year outside the outpost.
(Lead image: Placement of the new IROSAs over the existing station solar arrays. Credit: Mack Crawford for NSF L2)
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