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[i]Thermal Loop A ETCS Pump Shutdown[/i]
After losing one cooling system pump (of two), ISS is currently stable, although thermally on single string (no redundancy). Last night (at 7:48pm EDT) RPC1 (Remote Power Controller 1) in RPCM (RPC Module) S11A_D tripped open, powering off the Loop A ammonia pump of the S1 ETCS (External Thermal Control System), resulting in the loss of one half of the cooling to ISS.
This required a number of powerdowns (i.e., turning off selected systems for thermal protection), including redundant power to four CQs (Crew Quarters), three in Node-2, one in Kibo JPM, with both fans in each CQ remaining functional but zero fault-tolerant (crew is still Go for CQ use). Due to loss of heater power, MBS (Mobile Base System), SSRMS (Space Station Remote Manipulator System), and SPDM (Special Purpose Dexterous Manipulator) are currently zero fault-tolerant.
T2/COLBERT treadmill alignment guides are installed and should remain installed during crew sleep periods to protect against LOAC (Loss of Attitude Control) while attitude control is degraded; the latter also renders use of unisolated exercise (Russian Velo) No-Go to reduce momentum perturbations; all other exercise is currently permitted.
S-band is operating nominally (on String 2, with String 1 in hot backup); Ku-band is operating nominally; audio subsystem is operating nominally (Node-2, COL and JPM are zero fault tolerant for C&W/Caution and Warning annunciation and voice comm); no video from Node-2, Node-3, COL and JPM modules; HCOR (high-rate communications outage recorder) is operating nominally; Russian UHF: no issues, both radios are deactivated.
Possible causes of the RPC trip include an electrical fault in the cabling between the RPCM and ammonia pump, electrical fault in the EMI (electromagnetic interference) filter, electrical fault in the pump control board, or an electrical fault in the pump.
The RPC tripped because of an overcurrent; this points toward hardware components that do not have internal current limiting, such as the EMI filter, pump control board, and the pump itself (they get their current limiting function from the RPCM). A mechanical failure of the pump is considered unlikely.
After a pump restart attempt this morning, RPC1 tripped again. There will be more attempts.
There are two spare pumps on orbit. It will take two EVAs to remove and replace the pump. ISS is stable, but single string, and it is desired to get Loop A back as soon as possible.
The teams are looking at replacing the Thursday (8/5) spacewalk with the first of two EVAs, followed by the second EVA on 8/7 (Saturday). The spacewalks need special planning since the system is in a reduced power configuration.
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