The International Space Station (ISS) is currently experiencing an issue with one of its two external ammonia coolant loops, with one loop operating much too cold due to the apparent failure of an ammonia temperature control valve. The issue, if not resolved via other means, could result in several spacewalks being required in the immediate future.
The coolant issue relates to a component known as a Flow Control Valve (FCV), which regulates the temperature of ammonia coolant in the ISS’ external cooling loops by mixing cool ammonia exiting the radiators with warm ammonia that has bypassed the radiators.
Early indications are that the FCV in the loop A coolant system is not closing properly, which is likely to be causing too much cool ammonia to enter into the cooling loop, which in turn has caused the loop A system to operate at a temperature which is much too low for normal operation.
The FCV itself is located outside the ISS inside a system known as the Pump & Control Valve Package (PCVP), which itself is located inside the Pump Module (PM) Orbital Replacement Unit (ORU).
While the PM acts to pump the ammonia coolant around the coolant loop, the specific job of the PCVP inside the PM is to provide flow control for the ammonia, which includes temperature regulation via the FCV.
The ISS External Thermal Control System (ETCS) has two separate external ammonia coolant loops – known as loop A and loop B – which together transport heat away from the ISS’ electronic equipment and toward their respective Heat Rejection Subsystem Radiators (HRSRs).
Click here for a list of news articles covering the ETCS: http://www.nasaspaceflight.com/?s=ETCS
Each of the two ETCS coolant loops have their own separate PM (making for two operational PMs on the ISS in total), and thus the issue with the FCV in the loop A PM PCVP does not affect the FCV in the loop B PM PCVP.
The ETCS is not to be confused with the Photovoltaic Thermal Control System (PVTCS), which is completely separate from the ETCS and is used only to cool the solar arrays and associated power generation equipment, and not the main ISS structure, cooling for which is provided by the ETCS.
The current FCV issue is located within ETCS loop A, and thus has no relation whatsoever to the Pump Flow Control Subassembly (PFCS) leak and associated replacement earlier this year, as that issue related to the PVTCS only.
According to L2 notes, on Wednesday (11 December) morning, the ETCS loop A PM unexpectedly shut down due to the Fault Detection Isolation and Recovery (FDIR) system detecting an under-temperature limit had been violated, essentially meaning that the coolant inside loop A got too cold.
Ground teams successfully repowered loop A, however at this time it was discovered that the FCV in the loop A PM PCVP was not operating correctly in that it would not close correctly, likely causing the ratio of cool-to-warm ammonia in loop A to be too high, in turn leading to the loop A ammonia becoming too cold for nominal operation.
The immediate response to this was to transfer all internal heat loads, including the US Destiny Laboratory, Airlock and Node 3, from loop A to loop B.
However, since one cooling loop alone cannot support all of the ISS’ heat rejection needs, and with the temperature of loop B rising, ground teams were forced to begin shutting down equipment in order to reduce the heat generation of the ISS’ systems.
The equipment that was shut down included half of the system in the Japanese Experiment Module (JEM), Columbus Orbital Laboratory (COL), and Node 2/Harmony, including the Node 2 Multimpexer/Demultiplexer (MDM) computer and a DC to DC Conversion Unit (DDCU). Smoke alarms and Caution and Warning (CW) systems were also powered off.
At the time of this article’s publication, the ammonia temperature in loop A was around -32 degrees centigrade, while the ammonia temperature in loop B was around 4.5 degrees centigrade.
The maximum nominal operating temperature for an ammonia loop is specified as 3.9 degrees centigrade – meaning that loop B cannot afford to get any hotter, hence the equipment shutdowns.
Ultimately, if the FCV issue cannot be resolved, then it will have to be replaced. However, since the FCV resides inside the PCVP, and since the PCVP resides inside the PM, this will mean that the entire loop A PM will need to be Removed & Replaced (R&Rd) via an Extra Vehicular Activity (EVA).
The loop A PM was only R&Rd fairly recently in space station terms, after the previous unit failed in August 2010.
Thus, the PM currently experiencing issues has been in service for just over three years, after being launched to the ISS on STS-121 in 2006.
The procedure to R&R a PM involves disconnecting four fluid QDs as well as electrical connectors, then driving four bolts to uncouple the old unit from its home, which in the case of the loop A PM is inside the Starboard 1 (S1) Truss.
The reverse procedure then needs to be completed to install a spare unit, with the failed unit then being stowed outside the ISS.
Thanks to NASA’s strategy to pre-position spares outside the ISS for the post-Shuttle era, there are currently three spare PMs outside the ISS – one located on External Stowage Platform-3 (ESP-3) on the Starboard side of the ISS, which was launched on STS-127 in July 2009.
Another PM is located on ExPrESS Logistics Carrier-1 (ELC-1) on the Port side of the ISS, and was launched on STS-129 in November 2009, and the third spare PM is located on ELC-2 on the Starboard side of the ISS, and was again launched on STS-129.
It is likely that the spare PM on ESP-3 would be utilised, as it is the oldest unit and is close to the location of the loop A PM on the S1 Truss.
Should EVAs be required – as is now expected – the EVA crew would likely consist of NASA astronaut Mike Hopkins, who arrived at the ISS in September, and NASA astronaut Rick Mastracchio, who arrived at the ISS in November.
Additionally, Japanese astronaut Koichi Wakata would also be able to perform an EVA if required.
However, complicating matters is that EVAs are currently suspended following a water leak inside Italian astronaut Luca Parmitano’s spacesuit helmet during an EVA in July, an event which could easily have led to him drowning.
The leak occurred inside Extravehicular Mobility Unit (EMU) number 3011.
Since then, NASA has been conducting extensive investigations into the water leak issue, with recent L2 notes stating that “the crew performed a series of tests on EMU 3011 as part of an ongoing effort for returning the suit back to service. The tests included water leak checks, communication checks, and suit pressure leak checks. EMU 3011 passed all tests.”
However, NASA had been planning to wait to return another EMU, serial number 3015, to Earth aboard a SpaceX Dragon vehicle and deliver a new EMU in its place before clearing EVAs to resume, however the next Dragon vehicle is not scheduled to arrive at the ISS until at least late February next year.
The first contracted resupply mission by Orbital’s Cygnus vehicle is also due to launch to the ISS in less than one week’s time, but it is unknown at this point how the FCV issue may affect planning for that flight.
UPDATE: Per L2 information, the Loop A PM R&R effort is pressing ahead. The planning calls for 3-4 EVAs, with the first targeted for December 19, due to the need to complete the EVAs before a high negative beta period starts on December 30.
(Images: via L2’s ISS Section – Containing presentations, videos, images and ungoing ISS status updates, with additional images NASA and ESA).
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