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I'm currently following US EVA-24 with Rick Mastracchio (EV1) and Mike Hopkins (EV2) tasked to replace a failed Pump Module (S1 PM), one of two external pump modules of the US section of the International Space Station (ISS). The schedule is also visible on this screen taken off live stream a few minutes before the EVA, scheduled to start on December 21, 2013 at 12:10 UTC (which it did):

       enter image description here

       Spacewalks overview for the US EVA-24 as broadcast on NASA TV (credit: NASA)

Currently, good half an hour into EVA 1, Mike and Rick are already 15 minutes ahead of schedule, and it seems they were even asked to perform a few additional checks (e.g. gloves check) that were not on the checklist as read to them by Koichi Wakata inside the station. So this naturally begs the question:

Why did NASA assess that they will require so much time (3 spacewalks each over 6 hours long) to replace a single Pump Module? Is this merely scheduling procedures with a good measure so astronauts don't have to hurry? Or do they always prepare flexed schedules like this for some other reason?

Update: A good hour into EVA, the two astronauts are roughly 25 minutes ahead of schedule. Here's another screen captured off the live stream, showing Pump Module design:

   enter image description here

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NASA scheduled these three EVA based on previous experience. In 2010, the International Space Station (ISS) suffered similar problems that also required one of the two Pump Modules (PM) be replaced. Douglas Wheelock and Tracy Caldwell Dyson of Expedition 24 took three EVA to complete that, each lasting over 7 hours. To be more precise:

  • Expedition 24 EVA 2 on 7 August 2010 lasted 8 hours and 3 minutes (11:19 - 19:22 UTC)
  • Expedition 24 EVA 3 on 11 August 2010 lasted 7 hours and 26 minutes (12:27 - 19:53 UTC)
  • Expedition 24 EVA 4 on 16 August 2010 lasted 7 hours and 20 minutes (10:20 - 17:40 UTC)

Reason for such long EVAs was stuck Quick Disconnects (QD) on the PM that would not release.

The QD task was considered an easy one as the hardware was built for frequent and fast mating/demating, but when Doug Wheelock tried to remove the QDs from the S1 Pump Module, the QDs showed how hard it was to get them detached. Teams battled with the hardware for hours before calling it a day and heading back inside the airlock to re-group for a second EVA that used modified procedures to get the stubborn QDs demated.

And from the same source:

After the trouble encountered when performing a Pump Module R&R in 2010, this task is now known as a major undertaking and NASA expects that two to four EVAs will be needed.

Source of both quotes: Spaceflight101: ISS Expedition 38 - US EVA-24 Updates

Problems during the first US EVA-15 egress by Doug Wheelock and Tracy Caldwell-Dyson are described in detail in this NASA's ISS On-Orbit Status 08/07/10 update.

So as Rick Mastracchio (EV1) and Mike Hopkins (EV2) have an added advantage of being able to study in detail the 2010 PM R&R EVAs, and they also had videos of those previous EVAs available, this schedule is computed based on previous NASA's experience with replacement of PM and they're even shortened a bit to account for better preparedness of the two Expedition 38 spacewalkers.

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  • $\begingroup$ BTW I still didn't find an answer for this one that is related: With one out of two Thermal Control Systems on ISS malfunctioning, how long can Loop B keep up with increased load? $\endgroup$
    – TildalWave
    Dec 21, 2013 at 16:42
  • $\begingroup$ Update: EVA 1 finished successfully a good hour ahead of schedule and lasted 5 hours, 28 minutes and 11 seconds. EVA 2 will be on Monday. $\endgroup$
    – TildalWave
    Dec 21, 2013 at 17:33
  • $\begingroup$ Any idea why the QD refused to QD? Was it a design issue? $\endgroup$
    – Everyone
    Dec 22, 2013 at 7:26
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    $\begingroup$ @Everyone Yes, it's covered in ISS On-Orbit Status 08/07/10: "US EVA-15 ... was terminated without Loop A PM (Pump Module) removal because of a leak at one (M3) of the four QDs (Quick Disconnects) connecting the ammonia lines to the failed PM..." They also had to do decontamination "bake-out" to boil any ammonia off their suits before ingress, and during next EVAs the ammonia lines were at full pressure before reattachment which made them stiff and difficult to handle. EVA-24 is now better prepared, so faster. ;) $\endgroup$
    – TildalWave
    Dec 22, 2013 at 8:07

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