Two astronauts aboard the International Space Station (ISS) have completed the replacement of a failed Pump Module outside the station. The spacewalk, designated US EVA-25, took place on the backbone of the hugely successful EVA-24, allowing for the completion of all spacewalk tasks in just two EVAs.
Three EVAs were originally planned to Remove & Replace (R&R) the failed loop A Pump Module (PM) on the S1 Truss of the ISS – the first EVA to disconnect the failed PM from the ISS, the second EVA to remove the failed PM and replace it with a spare PM, and the third EVA to connect the spare PM and stow the failed PM.
However, the first EVA (US EVA-24 on December 21) managed to not only disconnect the old PM, but also remove it from the ISS – essentially completing around half of the tasks originally scheduled for EVA-25.
This increased efficiency can be attributed to ammonia Quick Disconnect (QD) lines that were much easier to handle than they have been in the past for this EVA, which could be due to the fact that the pressure of the ammonia lines was reduced prior to the disconnection of the QDs.
Not only was more work completed than was scheduled, but EVA-25 was also around one hour shorter in duration than planned, since the EVA ended early due to spacewalker Rick Mastracchio apparently experiencing some discomfort inside his spacesuit.
While some media reports have claimed that water was found inside the spacesuit upon return to the Airlock, such reports are in fact false – instead it is believed that, upon return to the Airlock, a switch for a sublimator on Mastracchio’s suit was inadvertently activated, causing a water build-up on the suit.
A sublimator is a device which cools spacesuits in the vacuum of space by venting a small amount of cooling water into space, which in turn evaporates (since liquid water cannot exist in a vacuum, it evaporates from a solid into a gas in a process known as sublimation).
Via this process heat in the cooling water can be vented into space, thus providing cooling for the suit.
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However, the process will not work in a pressurised environment (such as the pressurised Airlock), and so when the sublimator was accidentally activated inside the Airlock, this caused water to build-up on the suit.
It is for this reason that EVA-26 was postponed by 24 hours, from its originally scheduled date of December 23, to today, in order to allow more time for suit drying.
It is also this reason that, for EVA-26, Mastracchio used EMU suit number 3011, whereas on EVA-25 he used EMU 3010.
This allowed time for EMU 3010 to fully dry out, since any residual water entering the vacuum of space could freeze and cause the sublimator to cease to function.
US EVA-25 began slightly ahead of schedule at 11:53 AM GMT, and lasted seven and a half hours. The EVA crew were astronaut Mike Hopkins as EV-1 wearing EMU 3005, and Rick Mastracchio as EV-2 wearing EMU 3011.
Originally, Hopkins was planned to be EV-2 for EVA-25 and EV-1 for EVA-26, however since many EVA-26 procedures will now be performed on EVA-25, Hopkins took the EV-1 designation for EVA-25 in order to reduce the procedure changes needed.
While the original purpose of EVA-25 was to remove the failed PM from the truss and install the spare PM, since the failed PM was removed during EVA-24, instead EVA-25 focused on installing the new PM into the ISS, and also connecting its ammonia and electrical lines – a task originally planned for EVA-26.
Thus, after egressing the Quest airlock, the two spacewalkers moved out to the Starboard 3 (S3) Truss, to External Stowage Platform-3 (ESP-3), which is the location of the spare PM that will be installed into the ISS in place of the failed PM that was removed from the S1 Truss on EVA-24.
Once at ESP-3, the two spacewalkers worked to remove Multi Layer Insulation (MLI) coverings from the spare PM, which was launched into space on the STS-127 mission in July 2009.
The duo began to disconnect four electrical connectors from the spare PM, and drove four bolts to remove the spare PM from its adapter plate housing on ESP-3.
The bolts – as can be expected – were slightly troublesome, requiring the torque multiplier on the Pistol Grip Tool (PGT) to allow for the spare PM to come free from ESP-3.
EV-1 Hopkins then attached a handling aid to the PM known as a “scoop”, and hitched a ride on the end of the Space Station Remote Manipulator System (SSRMS) over to the PM installation site at the S1 Truss, whereupon the spare PM was inserted into its slot inside the Truss structure itself.
Four bolts were driven to attach the new PM to the ISS. At this point, the two spacewalkers began to mate the four ammonia QD jumpers to the new PM.
M1 provided no problems, before M2 required a little extra work to complete its mating to the PM. However, M3 took some time to be demated from the Jumper Box, requiring an array of tools – and some clever thinking on ground – before it too finally released.
At the same time, some ammonia flakes touched the EMUs of both spacewalkers, requiring some consideration on the decontamination procedures ahead of re-entering the Station.
After the electrical connections were also made – without issue – clean up tasks were the final element of the spacewalk.
With the new PM installation e complete, ground controllers were able to turn it on and begin initial tests and commissioning, with a view to soon recovering full nominal cooling capability on loop A.
Ahead of loop A’s re-integration with the Interface Heat Exchangers (IFHXs), in order to allow for cooling of internal ISS equipment via loop A to resume, a “bump test” provided the initial confidence in the new system. A full checkout will take place later on Tuesday.
While a third EVA (EVA-26) was originally planned to connect the new PM and stow the failed PM, since the new PM should now be fully connected by the end of EVA-25, it will likely not be necessary to conduct EVA-26 immediately following EVA-25.
The only objective remaining should be to stow the failed PM, which is not a critical task.
Instead, it is likely that the failed PM will remain attached to its current temporary stowage location on the Payload ORU Accommodation (POA) on the Mobile Base System (MBS) until at least January, whereupon a short EVA would be performed to transfer the failed PM to the previous home of the spare PM on ESP-3.
Leaving the failed PM on the POA long-term is not desired since engineers believe the failed PM could be re-used in the future, since only one small component inside it (the FCV – Flow Control Valve) is failed, and thus an external FCV could be fabricated in the future, and connected to the PM in order to return it to service.
For this reason, it is desirable to attach the failed PM to ESP-3, where it will have access to an MLI housing for protection from debris and electrical connections for heating, as no MLI protection or electrical power is available to the PM while on the POA.
(Images: via L2’s ISS Section – Containing presentations, videos, images and ongoing ISS status updates, with additional images NASA and CSA).
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