ISS astronauts complete 200th station EVA for maintenance tasks

by Pete Harding

Two astronauts aboard the International Space Station (ISS) completed the 200th spacewalk in the station’s history, in order to perform a number of maintenance and upgrade tasks outside the orbital laboratory. The spacewalk, designated US EVA-42, got underway at 13:08 PM GMT and was shortened to just over four hours due to a battery life concern.

EVA-42 procedures:

EVA-42 was performed by NASA astronauts Peggy Whitson as EV-1, wearing the suit with the red stripes, and Jack Fischer as EV-2, wearing the all-white suit. It was Whitson’s ninth EVA, and Fischer’s first.

The EVA was shortened due to an issue ahead of the spacewalk where the spacewalkers had to share a SCU (Service Cooling Unit) line after the umbilical for Fischer’s EMU suffered from a leak. His suit was in good shape, but the line could not be used. The impact on the EMU battery life meant the full EVA duration couldn’t be conducted.

The first order of business, and the primary task for the EVA, was the replacement of an avionics box on ExPrESS Logistics Carrier-4 (ELC-4) on the S3 Truss, called an ExPrESS Pallet Controller Assembly (ExPCA), pronounced “ex-pecker”.

The ExPCA provides power, commanding, and data distribution capabilities to the ELCs and their attached ORUs and experiment payloads. This particular ExPCA on ELC-4 is considered to be failed, and so will be replaced with a new unit which was flown to the ISS on the recent Cygnus OA-7 mission.

It is for this reason that EVA-42 was delayed by over a month from its originally planned date, as Cygnus needed to arrive at the ISS before the EVA could be conducted.

Following egress of the Quest airlock, EV-1 Whitson translated out to the ExPCA worksite on ELC-4. Meanwhile, EV-2 Fischer installed an Articulating Portable Foot Restraint (APFR) on the SSRMS and ingress it, whereupon he was “flown” out to join Whitson at the worksite.

The act of replacing the ExPCA was a fairly simple affair, even though it is the first time that this has ever been performed on-orbit. Firstly, two “scoop” handing aids were installed onto the old ExPCA, following which two bolts were driven in order to remove the old unit, which was then be temp-stowed prior to being brought back inside the ISS.

The new ExPCA was then installed onto ELC-4, again via the driving of two bolts, with all needed electrical connection being made automatically as the unit was bolted in place.

Once complete, Whitson then headed to the second task of the EVA on the Alpha Magnetic Spectrometer (AMS-02), which involved installing a small feedback terminator connector between two 1553 data ports.

The task was necessary due to the fact that some cooling pumps inside AMS-02 have been showing signs of degradation, as AMS was originally designed to be a short-term ISS experiment that would be returned to Earth after use, rather than a long-term experiment.

This has led the AMS teams to begin developing a plan to potentially install new active-control pumps in the future, which would need command and control ability.

The AMS 1553 ports, which were designed for use during AMS-02’s launch on the Space Shuttle, could possibly be used for this purpose. However, ground teams would first like to test that the now unused 1553 lines are still in good condition before proceeding with any plan along this line.

As such, a loop-back connector was installed between two 1553 ports on AMS, essentially connecting an output port to an input port in order to check the continuity of the 1553 line.

Whitson was also going to take pictures of the AMS, and the newly installed SAGE-III Earth monitoring experiment on ELC-4. While this was happening, Fisher was set to install a new wireless external HD camera onto Camera Port-3 (CP-3) on the S1 Truss.

However, this and another task of the EVA were canceled due to the shortened EVA. The other task involved three wireless communication antennas that were to be installed onto handrails on the Nadir side of the Destiny laboratory module, which will be used to downlink future wireless HD video from the new external HD cameras.

However, thanks to the spacewalkers completing the primary task in double quick time, insulation was re-installed on to the Japanese Robotic Arm during the second half of the EVA.

The duo also met up at Pressurised Mating Adapter-3 (PMA-3), recently relocated to the Zenith port of Node 2, to install a shield (known as the “bowtie”) around the forward side of the exposed Common Berthing Mechanism (CBM) berthing collar, in order to protect this critical area from debris.

In all, the majority of the EVA tasks were completed, despite the shortened EVA duration.

EVA-42 comes hot on the heels of a recent, highly successful robotic operation to remove and replace the failed Main Bus Switching Unit-2 (MBSU-2) on the S0 Truss.

An MBSU is a unit which takes the primary power feeds from two of the station’s power channels and distributes in onto the secondary power generation equipment, which in turn powers the station’s loads.

This means that if any of the station’s four MBSUs fail, a quarter of the station’s total power will be lost, and as such an MBSU failure is considered to be a critical occurrence.

Back in late April, MBSU-2 experienced a Loss of Communication (LOC) with the ISS. This meant that no commanding or data insight to the MBSU was available, although it continued to distribute power without issue. This concern, however, was whether the LOC was indicative of a pending total failure of the unit.

As an MBSU failure is highly undesirable, the decision was made to remove and replace the MBSU with a spare unit already on-orbit. However, in a break with past practice, where an EVA was required to replace an MBSU, this operation was performed entirely via robotic assets.

Specifically, the ground-controlled Special Purpose Dextrous Manipulator (SPDM) “Dextre” was used, aided by the fact that a thermal cover had been previously removed from the spare MBSU on a prior EVA, allowing for robotic access.

Once the crew had installed two jumpers inside the station to provide power to MBSU-2’s loads, the SPDM began the process by relocating the MBSU Flight Releasable Attachment Mechanism (FRAM) Flight Support Equipment (FSE) from ESP-2 to the SPDM’s Enhanced ORU Temporary Platform (EOTP).

The SPDM then proceeded to remove the failed MBSU from the S0 Truss via the driving of two bolts, and replaced it with the spare MBSU which itself was removed from its FRAM FSE. The failed MBSU was then installed onto the FRAM FSE, which in turn was placed back on ESP-2.

With the new MBSU powered up and all loads successfully transferred, the operation was an outstanding success, serving as a great example of how the ISS is silently pushing the boundaries of space exploration, proving out new robotics capabilities which in the past would have required human interaction via a spacewalk.

Another issue currently under evaluation, however, relates to a tear in Solar Array Wing (SAW)-3B on the S6 Truss and the very outboard end of the station.

According to L2 notes: “During a review of imagery that was taken during a routine solar array inspection, ground teams identified a ~6-7 inch “tear” on one of the hinges of the 3B solar array blanket”.

“Follow-up imagery will likely be requested to continue to assess the level of damage incurred and determine whether any additional actions will be required to address the area of concern.”

This is not the first time that a solar array tear has occurred on the ISS – back on STS-120 in 2007, the 4B solar array was torn, requiring spacewalkers to go and “stitch” it back together via makeshift tools.

On that occasion, the Orbiter Boom Sensor System (OBSS) was used as an extension for the SSRMS, in order to provide the required reach to access the torn array location.

The OBSS, now called the Enhanced Integrated Boom Assembly (EIBA), has since been left behind on the ISS to fulfil this purpose again should it ever be needed, and has been modified with an additional grapple fixture on its outboard end to provide additional reach.

In the case of the recent 3B tear, it is understood that the tear is around a quarter of the way up the array from its base, which puts it in the area which is right at the limit of what the additional reach via the EIBA can provide.

However, it is not yet known whether a fix will be required at all, since the 3B tear is smaller than the 2007 4B tear, and there are no concerns with the tear increasing once the array is fully unfurled, since the 3B array is already fully extended, unlike the 4B array 2007.

Analysis is ongoing as to the forward plan regarding the array –  alongside ongoing analysis into a small ammonia leak in the Loop B external cooling system.

(Images via NASA).

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