Spacewalk Success as second EVA installs troublesome MBSU-1

by Pete Harding

International Space Station (ISS) astronauts Suni Williams and Aki Hoshide have successfully installed the Main Bus Switching Unit-1 (MBSU-1) that proved to be troublesome during US EVA-18 last Thursday. The EVA, known as US EVA-19, got underway at 11:06 GMT and concluded at 17:34 GMT.

US EVA-19 procedures:

After spacewalkers Williams and Hoshide performed the In Suit Light Exercise (ISLE) protocol, US EVA-19 got underway around one hour earlier than the start time for US EVA-18 last Thursday, due to the need to reduce solar radiation exposure from a Coronal Mass Ejection (CME).

The main focus of EVA-19 will be to install the new MBSU-1 onto its coldplate atop the S0 Truss, which was unable to be installed following removal of the old MBSU-1 during US EVA-18, due to a bolt that as unable to be driven.

Since the MBSU electrical connections cannot be made until it is fully seated on the coldplate – meaning its bolts must be fully driven – the ISS has been without one of its four MBSUs since last Thursday.

Specifically, two of eight power channels – 1A and 1B – have been unavailable due to MBSU-1 being disconnected, meaning the ISS has been running on only 75 per cent of its normal power.

The situation was further complicated by the apparent failure of Direct Current Switching Unit-3A (DCSU-3A), which took down power channel 3A, and thus a further 12.5 per cent of the ISS’ power, meaning ISS has been running on only 62.5 per cent of its normal power over the past few days.

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While EVA options to possibly replace DCSU-3A are still being analysed, the ISS is still in a safe configuration despite the loss of over one third of its normal power. The situation however is far from ideal, with the Alpha Magnetic Spectrometer-02 (AMS-02) losing power for three hours due to the DCSU-3A failure, despite the careful power management plan currently in effect.

The procedures for the EVA will consist of removing the partially installed MBSU-1 from the S0 Truss, by undoing its H2 bolt – which is longer than the completely disengaged H1 bolt – by 4-5 turns. Opening attempts to remove the bolt appeared to be troublesome, prior to a successful bolt removal by Hoshide.

The removed MBSU-1 was then be temporarily stowed on what is known as a Multi Use Tether (MUT) ballstack. At this point, just over two hours into the EVA, Williams broke the world record for a female spacewalker’s total EVA time, breaking Peggy Whitson’s record of 39 hours and 46 minutes.

Following a full visual inspection of MBSU-1 and its S0 Truss coldplate by the spacewalkers, efforts turned toward cleaning the MBSU-1 bolts and the bolt receptacles on the S0 Truss, which during EVA-18 were observed to be filled with metal shavings, possibly evidence of a galled bolt.

Specifically, these cleaning procedures consisted of cleaning the H2 bolt with a magnet, checking the H1 bolt tolerance by hand, lubricating the H1 bolt stanchion, and checking the thread quality of the H2 bolt by greasing an ACME bolt and installing it into the H2 stanchion.

The ACME bolt in question was scavenged from a Photovoltaic Controller Unit (PVCU) Multiplexer/Demultiplexer (MDM) bracket that resides inside the ISS, since it was identical to the MBSU H2 bolt. By installing the bolt into the H2 stanchion, the spacewalkers were able to check for damage to the threads, and also “chase” the threads, plue remove any debris from the stanchion.

The crew built special tools to drive the bolt and perform cleaning, using items scavenged from aboard the ISS including a toothbrush, a wire brush, and a bolt-stanchion-cleaning “chimney sweep” tool.

Following cleaning and inspection, the crew attempted the re-install MBSU-1 onto its coldplate on the S0 Truss, driving its H2 bolt, followed by its H1 bolt, which will make an electrical connection between MBSU-1 and the ISS.

With the successful installation of the H2 bolt the key point of the EVA, applause was heard ringing out across the ISS Flight Control Room (FCR), earning the spacwalkers – per a call from the CAPCOM – a “slice of awesome pie”.

Had MBSU-1 not been installed by the four-hour “bingo” point, it was to be returned to the Quest airlock to be brought back inside, so that its bolts can be removed, prior to it being tied-down to the S0 Truss during a future EVA. Despite being slightly over the four hour point, the decision was made the press ahead, leading to the success.

The spacewalkers then moved on to replacing a Space Station Remote Manipulator System (SSRMS) end B Camera/Light Pan/tilt Assembly (CLPA), which was successful.

Some get-ahead tasks options – such as installing a thermal/debris cover on Pressurised Mating Adapter-2 (PMA-2), removing a Mobile Base System (MBS) mast camera, and performing troubleshooting on a grounding wire on the Functional Cargo Block (FGB) Power Data Grapple Fixture (PDGF) – were not taken.

The spacewalkers are returning to the Quest airlock to conclude EVA-19 after six hours and 28 minutes.

(Images: Via NASA, NASA TV and L2 Content.
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