From ISS On-Orbit Status Report for 01/08/2010 (Early Edition):
Thermal Loop A ETCS Pump Shutdown:
After losing one cooling system pump (of two), ISS is currently stable, although thermally on single string (no redundancy). Last night (at 7:48 PM EDT/11:48 PM GMT) RPC-1 (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 the 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 (Columbus Orbiting Laboratory) & the JPM are zero fault tolerant for C&W (Caution & Warning) annunciation and voice comm..); no video from Node 2, Node 3, COL & the JPM; HCOR (High-rate Communications Outage Recorder) is operating nominally; Russian UHF (Ultra High Frequency): 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, RPC-1 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 05/08 (Thursday) EVA with the first of two EVAs, followed by the second EVA on 07/08 (Saturday). The EVAs need special planning since the system is in a reduced power configuration.
---------- Post added at 06:46 PM ---------- Previous post was at 05:26 PM ----------
NASA's Main ISS Webpage said:
Mission Operations personnel and ISS Program management met Sunday morning to discuss the status of the loss of cooling loop A Saturday night on the International Space Station when a circuit breaker tripped just before 7 PM Central time Saturday night that resulted in the failure of the Pump Module for loop A that feeds ammonia to maintain the proper cooling for systems and avionics.
An attempt overnight Sunday to close the circuit breaker and restart the Pump Module was not successful.
The station is in a stable configuration with most systems receiving cooling and many systems operating with redundancy following the installation of jumper cables from the Destiny Lab's power system overnight. The crew is not in any danger and is monitoring systems and relaxing on an otherwise off duty day.
One of two Control Moment Gyroscopes (CMG-1) that was taken off line Saturday night was spun up once again this morning, enabling the station to operate with three of four CMGs to electrically control the orientation of the outpost. Temperatures on the Main Bus Switching Units, which route power to various systems, are a little higher than normal, but well within normal parameters and are stable.
The flight control and management teams today approved a preliminary plan to replace a planned spacewalk Thursday by Expedition 24 crewmembers Doug Wheelock and Tracy Caldwell Dyson with at least two spacewalks to swap out the failed Pump Module that resides on the station’s S1 truss. There are two spare Pump Modules on stowage platforms on the station's truss. The replacement module under consideration for replacement resides on External Stowage Platform 2, which is adjacent to the Quest airlock. The crew is being informed that replanning for alternate spacewalk activity is underway.
Although a final decision on a new spacewalk plan is still pending engineering and timeline analysis, the most likely scenario would call for an initial spacewalk no earlier than Thursday by Wheelock and Caldwell Dyson to replace the Pump Module and structurally bolt it into place on the S1 Truss, with an additional spacewalk by the duo two or three days later to mate fluid and electrical connections.
To protect for spacewalk activity later this week, the flight control team plans to vent residual ammonia in the lines between the Ammonia Tank Assembly on the S1 truss and the failed Pump Module Tuesday to prepare for the module's eventual replacement.
The tasks originally planned for Thursday's previously scheduled spacewalk by Wheelock and Caldwell Dyson to install a power extension cable to the Unity module prior to the delivery of the Permanent Multipurpose Module on the STS-133 mission in November and to install a Power and Data Grapple Fixture to the Zarya module to support future robotics work will be deferred to a later date.
A briefing to discuss the station's status and spacewalk replanning efforts is scheduled Monday on NASA Television at 3 PM Central time originating from the Johnson Space Center with Mike Suffredini, ISS Program Manager and Courtenay McMillan, Expedition 24 Spacewalk Flight Director. This briefing replaces a previously scheduled briefing on Tuesday.
