No return of the leak – ISS teams clear concern as false signature

by Chris Bergin

The International Station Station (ISS) team have confirmed they still have an air tight cooling system on the P6 Truss Photo Voltaic Thermal Control System (PVTCS), following the observation of leak indications during the recent Soyuz TMA-07M undocking. The indications were blamed on a lack of data on the entire system, as the ISS maneuvered for the departure of the Russian vehicle.

Recent Ammonia Leak:

Following recent events, the ISS controllers are continuing to keep a very close eye on the system that uses ammonia to dissipate heat from the electrical power systems on the truss.

The system leaked last week, when controllers observed data that indicated a large increase in a previously known small ammonia leak in the cooling loop of power channel 2B. At the same time, the crew inside the station noted that they were able to see flakes of ammonia originating from the channel 2B area, which were floating away into space.

Marshburn and Cassidy EVAConfirmation of a serious leak ultimately resulted in both the teams on the ground and in space to spring into action, resulting in an unplanned EVA taking place less than two days after the leak was spotted.

Spacewalkers Chris Cassidy and Tom Marshburn ventured out of the Quest Airlock on Saturday, successfully inspecting the hardware, before swapping out the old 2B Pump Flow Control Subassembly (PFCS) with one of the two spares located on the truss.

With the system recharged, controllers successfully returned the flow of ammonia without any anomalous indications, allowing the spacewalking duo to return back inside the ISS an hour ahead of schedule.

Marshburn then prepared for his return back to Earth onboard Soyuz TMA-07M, along with departing Expedition 35 commander Chris Hadfield and Russian cosmonaut Roman Romanenko. They successfully landed on the steppe of Kazakhstan, southeast of Dzhezkazgan on Monday.

Leak Returns?

However, controllers noticed an issue during the period surrounding the departure of the Soyuz, which appeared to indicate the ammonia leak on the 2B system had returned.

PVTCS “Post the Soyuz undock, there were several indications that the gross 2B PVTCS leak was still present. The 2B PVTCS was shutdown to preserve consumables as the team continues to monitor the system,” noted a flash on L2’s rolling ISS Update Section.

“The channel 2B primary power equipment has powered down to a dormant configuration. Channel 2A continues to power all downstream loads of channel 2B.”

Controllers positioned cameras on the ISS to take a closer look, in order to see if they could spot ammonia flakes departing from the region. No such observations were seen.

With evaluations taking place on the ground, the focus switched to a possible false signature indication on the system, which was immediately backed up by the ISS’ requirement of maneuvering to different attitudes to cater for the Soyuz’s departure.

ISS Attitude during Soyuz undockingAs a result of the PVTCS troubleshooting performed on EVA 20 in November, the modified 2B loop includes four separate accumulators as compared to one accumulator in the unmodified system,” added notes.

“During the Soyuz undock, telemetry was only available for a single accumulator. After the maneuver to the undocking attitude, the quantity reading of that accumulator began dropping.”

As part of the written response plans, the pump was shut down and the power was removed from the 2B power channels.

Once the ISS was back into its regular attitude, additional data became available that showed the quantity in the other accumulators had actually been increasing or remaining stable while the one accumulator had been decreasing. This backed up the theory the ammonia fluid was shifting through the system, as opposed to leaking – a theory supported by the absence of snowflakes seen with the earlier leak.

The team continued to investigate the decreasing accumulator quantities and pressures – a trend of about 9.6 percent per day over a six hour period – with a focus on the attitude maneuvers and associated changes to the passive thermal environment.

“However it continued at the same rate through the undock, comm attitude maneuver, and return to TEA with all Arrays in Autotrack, and for a full orbit after this unabated, and we are not currently able to explain it by other means,” notes added, showing the evaluations were continuing throughout Tuesday. “This signature and its magnitude are very comparable to the fast leak rate observed on Thursday.

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“We do not have the benefit of crew eyes on it this time because they entered crew sleep since the signature became more definitively non-transient. We do have high definition video of the area in question, but it does not show anything observable at this time.”

Truss SystemsThankfully, data for the Early External Thermal Control System (EETCS) system became available later on Tuesday, confirming stable quantities in the starboard and trailing radiator accumulators and the EETCS PFCS accumulator, allowing for the 2B power channels to become reactivated.

“Based on further data review, ground teams determined that there is no gross leak of the 2B PVTCS system, and that the ammonia quantity downward trend was caused by the 2B PFCS experiencing cold conditions in the Soyuz undocking attitude,” concluded the latest notes.

“An Anomaly Resolution Team (ART) meeting was held, and the 2B PVTCS was later reactivated per the ART recommendation.”

The lack of a leak will come as a relief, not least because the potential for another EVA would only become available in a few week’s time. The ISS will only return to a six member crew – with two NASA astronauts – when Soyuz TMA-09M docks at the end of the month.

(Images: L2’s ISS Section and NASA)

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