NASA’s Doug Wheelock and Tracy Caldwell Dyson have completed their eventful opening EVA to changeout a failed ammonia coolant Pump Module (PM) on the International Space Station’s (ISS) External Thermal Control System (ETCS) “Loop A”. Lasting eight hours and three minutes, the spacewalk will be followed by a second EVA next Wednesday.
The main task of EVA-1 was to remove the current PM – which suffered from a power spike which tripped a circuit breaker on July 31 – and replace it with a spare during the second EVA next week. However, issues with one of the Quick Disconnect (QD) lines resulted in the old PM remaining in place for the meantime.
The ETCS provides cooling to the entire United States On-Orbit Segment (USOS). The ETCS consists of two independent loops, designated Loop A and Loop B – the latter of which is currently working without issue.
The system is designed to collect heat from the Internal Thermal Control Systems in the US Lab, Node-2, COL, JEM, and Node-3, via IFHXs (Interface Heat Exchanger). The PM provides the motive force to transport the ammonia around the loop.
The opening EVA was delayed twice, as engineers on the ground worked through refining the EVA timeline, while training runs were carried out inside the NBL (Neutral Buoyancy Laboratory) at the Johnson Space Center (JSC).
The EVA-1 tasks will run in the following order: “Demate connections and QDs (quick disconnects) on the old PM (Pump Module); Install PM jumper to bypass PM and connect ATA (Ammonia Tank Assembly) at S1 to Loop-A; Install AGB (Adjustable Grapple Bar), currently on ESP-2 (External Stowage Platform 2), on old PM.
“Remove old PM from S1 (starboard) truss and stow on POA (Payload ORU Accommodation) on the MT (Mobile Transporter) at WS2 (Worksite 2); Prepare spare PM (on ESP-2) for installation, and; Install spare PM on S1 and make electrical & data connections.”
Several leaks of ammonia – seen as snowflakes – were observed during the demating of QDs on the PM, half way through the EVA’s duration. However, the main issue was with one of the QDs that refused to demate.
With two alternative QDs demated with no issues, Wheelock returned to M3, which eventually released as planned – much to the pleasure of ground controllers, who burst out into applause. However, time was running out, leading to the removal of the PM being moved to EVA-2.
A larger leak of ammonia was then observed during M3’s demating, as the spacewalkers appeared to be working in a snowstorm, as the fluid leaked as flakes. Although no serious contamination was found during suit inspections, a 30 minute bake out inside the airlock ensured none of the hazardous material could return inside of the Station with the suits.
Ahead of Saturday’s spacewalk, the last US Stage EVA on the ISS was conducted in January, 2008.
Refer to the live thread for further details, and a review of the live play-by-play text and image coverage.
A Week Of Planning:
A large part of the evaluations have related to the Failure Investigation Team (FIT) efforts to overview what is known as the Next Failure Response – part of a week of extensive evaluations into supporting both the ISS and the EVAs.
“FIT for failure response actions dealt mainly with items related to supporting EVA with the next worst case failures that take down the 2nd cooling loop,” noted one a several expansive memos and overviews on the evaluations (L2).
“Actions that were noted for the crew include completing the EVA battery charging as soon as possible, looking for the adapter for the US LiOH cartridges for use on the Russian side, and when approval is gained from ISS Program management to request the crew to begin assembling materials to build a second contingency jumper via pin kits with the OGS jumper.
“The crew will need to be timelined appropriately to allow the associated hardware to be located for this spare jumper preconfiguration effort. Most of the discussion dealt with how to get the USOS configured to best support comm during an EVA since the Internal Audio Controller (IAC) required for UHF comm to the EV crew has a very short time to overheat (less than 30 minutes) that would result in the termination of the EVA.
“A scenario was discussed to recover cooling for an MBSU (Main Bus Switching Unit) failure but installation of power jumpers by the crew and thermal cooling reintegration would take approximately 1.5 hours. Improving air cooling via ducts, removing closeouts, and rotating racks was discussed where applicable to gain more thermal clock time.”
A long list of actions included work to determine the best configuration for removing closeout panels and rotating racks to ensure the best cooling capability in the modules, and bringing the spacewalkers safely back into the airlock – in the event of another failure before, during and after the opening EVA – full list on L2.
“The team had originally developed a pump module ammonia vent plan that could be completed using ground commanding of various isolation and vent valves in the ETCS,” added another overview.
“Due to the length of the tasks the crew would be required to perform during a single eclipse pass, coupled with a highly detailed choreography between the EV crew and ground to support this task there is no way to fit the necessary QD manipulations and Pump Module venting within a single eclipse period.
“The team decided to proceed with the manual vent option which has no eclipse constraints but does require the crew to position and use the EVA vent tool. In support of this the nominal EVA 1 timeline will, in accordance with the Flight Rules, protect for 2 hours and 20 minutes of consumables at the end of the EVA for EMU decontamination protocols.
Pending the results of this analysis, it may not be required to vent the PM during EVA 1 as long as the NH3 in the pump will not freeze and potentially prevent post failure analysis from determining what caused the original pump failure.”
A third EVA was being discussed on some memos during the week (L2), relating to an issue on a Remote Power Control Module (RPCM), which – it was feared – may have suffered from wiring damaged. By late this week, a third EVA to changeout the RPCM had become “less likely”.
A full overview of the status of the RPCM will be provided in the EVA-2 article.