Two astronauts aboard the International Space Station (ISS) conducted a spacewalk to continue the process of increasing the Station’s electrical power generation output. U.S. EVA-72 was conducted by NASA astronaut Kate Rubins and Japan Aerospace Exploration Agency astronaut Soichi Noguchi, who officially began the sojourn at 06:37 EST/11:37 UTC.
The spacewalk ended with the major tasks completed but the lesser tasks deferred to a future spacewalk when work at the solar array site took longer than planned and spacesuit battery limits mandated a conclusion to the EVA after 6 hours 56 minutes.
Solar array background:
The ISS is equipped with eight Solar Array Wings (SAWs), each of which were designed to produce around 30 kilowatts (kW) of power, for a total of about 250kW across all eight arrays.
The oldest array, attached to the P6 truss, was launched in 2000 on the STS-97 mission of the Shuttle Endeavour, with the additional arrays launching in 2006, 2007, and 2009 onboard the STS-115 and STS-117 missions of Atlantis and the STS-119 mission of Discovery, respectively..
Due to degradation — which was expected — the power-generating arrays now only produce around 160kW of power. At the same time, the Station’s users, including the recently added Nanoracks Bishop airlock and European Bartolomeo external platform, are requiring more power than was originally envisioned at the time of the outpost’s design.
Whilst the Station’s solar arrays were originally designed to be completely removed and replaced via the Space Shuttle, the retirement of that vehicle forced the ISS program to come up with an innovative solution to increase the amount of available power.
Specifically, six new ISS Roll Out Solar Arrays (IROSA) will be added. The IROSAs are a new type of array technology, which differs from the Station’s original arrays which were folded up and deployed in an accordion-like manner.
Developed by Deployable Space Systems of California, the IROSAs roll out in a mat-like manner from inside a cylindrical canister.
Its solar cells are mounted on a very thin, flexible blanket rather than traditional rigid panels, and it features a composite boom which adds rigidity and allows for deployment without the use of motors or complex mechanisms.
This allows the large arrays to be stowed very compactly — which is important given the current launch options available to the Station program.
The solar cells themselves are around 30% efficient, compared to the 14% efficiency of the original arrays. This allows the IROSAs to generate roughly the same amount of power as the original arrays, despite being only half their size.
The IROSAs will be installed over six of the Station’s existing solar arrays. Although the IROSAs will partly shadow the existing arrays, the original arrays will still continue to generate power alongside the IROSAs.
Installing the IROSAs over the top of the existing arrays allows them to utilise the existing sun-tracking motors — the Beta Gimbal Assembly — of the current arrays and allows each IROSA to be connected into the electrical system used by the current arrays to augment their power.
Each IROSA will produce 20kW of additional power, for a total of 120kW across all six arrays. The unshadowed portions of the original arrays will continue to produce 95kW, making for a combined total of 215kW of power which will be available to the Station, close to its original level.
The placement of the new IROSAs over the existing Station solar arrays. (Credit: NASA/Boeing)
A sub-scale version of the IROSA was successfully tested on Station in 2017, and the technology is already planned for use on the Power and Propulsion Element (PPE) of the Lunar Gateway, as well as on Dynetics’ Human Lander System (HLS), for the Artemis Moon program.
The six IROSAs will be launched two at a time on a pallet inside a SpaceX Cargo Dragon 2 trunk starting with the CRS-22 flight in June 2020. The new arrays will be installed via spacewalks
Previous spacewalk this week:
The IROSAs will be structurally mounted to the base of the Station’s existing Mast Canister Assemblies, which housed the current arrays during launch and now serve as their “base”.
However, as it was never envisioned that the IROSAs would be added to the ISS, no structure existed on which to mount them.
Constructing this was the goal of U.S. EVA-71.
A so-called “mod kit” was installed onto the 2B Mast Canister Assembly, consisting of several struts which were connected together to form a structural framework for the IROSAs to be mounted on in future.
The mod kit was physically bolted to the 2B Mast Canister Assembly using existing attachment points from unneeded ground handling fixtures and removed robotic pins.
The “mod kit” that will be constructed and installed onto the Mast Canister Assemblies. These kits will serve as the attachment platforms for the new arrays. (Credit: NASA)
The goal of spacewalk was to complete the build of the mod kit on the 2B Mast Canister Assembly and begin the build of the mod kit on the 4B Mast Canister Assembly — both of which are located on the P6 truss, the Station’s oldest solar arrays.
The first task for the spacewalkers after translating out to the P6 worksite, carrying some very large work bags with them, was to construct an A-frame structure called the upper bracket, which was in turn mounted to the center of the 2B Mast Canister Assembly.
A left lower strut and left mid strut were then connected between the upper bracket and the left side of the Mast Canister Assembly. A right lower strut and right mid strut were then installed on the opposite side.
This completed the 2B mod kit installation.
Another upper bracket was then constructed and attached to the 4B Mast Canister Assembly, as was the right lower strut.
This is where the spacewalk concluded.
Completion of the 4B Mast Canister Assembly mod kit was performed on U.S. EVA-72.
EVA-72:
For U.S. EVA-72, Kate Rubins and Soichi Noguchi translated out to the P6 truss worksite.
Once there, they began their first task, which was to complete the build of the 4B mod kit which was begun on the previous EVA. Some tricky bolts presented a challenge on that spacewalk, so the duo attempted to tighten those bolts along with completing the assembly of the support bracket structure for the future IROSA.
Similar completion work was accomplished on the 2B side.
The location of part of EVA 71 & 72 at the 2B and 4B arrays on the P6 truss. (Credit: NASA)
The next task was to have been the venting of the Early Ammonia Servicer jumpers so they can be safely stowed without the risk of rupturing due to thermal expansion. However, the IROSA work took longer than planned, and battery limits on one spacesuit caused Mission Control to ask the pair to return to the Quest Airlock to evaluate further tasks.
Once at the airlock, Mission Control concluded that a planned task of replacing a failed Wireless External Transceiver Assembly — which is an external wireless video data relay — at Camera Port 12 on Node 1/Unity could not proceed due to battery limits; however, Soichi did have time to retrieve an APFR (Articulating Portable Foot Restraint).
Of the tasks left uncompleted were venting of the ammonia servicer, the above-mentioned wireless transceiver work, and routing of two ethernet cables along the aft side of the P1 truss to Camera Port 9 in preparation for future wireless access point and High Definition external video camera installations.
Angle showing how the new IROSAs will be deployed over the current arrays. (Credit: NASA)
Troubleshooting the P1 and P2 truss PAPOS (Payload Parking Position Interface) power connectors on the new Bartolomeo external platform on the European Columbus module was not completed as well.
These connectors were unable to be properly mated on a previous EVA, and the crew will attempt further steps to mate them, which involves inserting a pen-like tool to spread apart the pins in the connectors, which are believed to be too tight.
It was initially planned that an external 360-degree Virtual Reality camera called ISS Experience would be used to capture external imagery of the EVA; however, the device was unable to be powered up.
Despite being brought back inside the ISS and troubleshooted by the crew, the issue was unable to be resolved, meaning the camera will not be available for the EVA.
A small, pinpoint hole was discovered in one of Kate’s gloves during the spacewalk; however, no leak was detected from her suit and the small hole was not the reason for concluding the spacewalk with taks left to accomplish.
(Lead image credit: NASA)