Final STS-128 EVA completed – PMA3 clocking issue under evaluation

by Chris Bergin

ESA astronaut Christer Fuglesang and NASA astronaut John Olivas have completed the final spacewalk of the STS-128 mission, with the successful deployment of a payload attachment pallet the primary task for EVA-3. Meanwhile, engineers are continuing to look into why the Pressurized Mating Adaptor 3 (PMA3) is out of configuration.

Discovery’s Status:

The veteran orbiter continues to perform without any major issues, with all of her Thermal Protection System (TPS) cleared for entry – pending one final check during Late Inspections after docking – and all of the orbiter systems working within their required parameters.

neptecOnly the Fuel Cell 2 KOH Transient Limit Violation is being monitored as an item of interest at the Mission Evaluation Room (MER).

“Have observed intermittent (instantaneous) drops in the FC2 inlet KOH concentration below the 29 percent concentration rule. Transients seen multiple times since SSPTS (Station to Shuttle Power Transfer System) activation and correspond to transients in shuttle vehicle power level often associated with cryo tank O2 heater cycles,” noted Mission Management Team (MMT) documentation on L2.

“Flight Rule limit of 29 percent provides an additional 5 percent protection for instrumentation error above the actual cell concentration limit of 24 percent. Allows for worst case sensor bias in telemetry used to calculate concentration. Error measured during ATP for this fuel cell was less than 1 percent on all parameters used in computing cell KOH concentration.

“Impact: Transients below 29 percent not a concern. CHIT 7740 submitted and concurred with to update the limit to 24 percent average (not instantaneous); consistent with STS-127 experience for FC3 operation and limits, If the KOH concentration approaches 24 percent EGIL will increase fuel cell 2 load in an attempt to raise the KOH concentration.”

Transfers between Discovery’s middeck, the attached MPLM and Station have been proceeding on the timeline, with well over half the supplies now removed and installed into the Station’s storage.

“Transfer Status: Approximately 61 percent complete overall; on schedule,” added the MMT documentation. “Middeck: 76 percent complete (97 percent resupply complete), MPLM: 61 percent complete (90 percent resupply complete). 118 hours in the timeline overall; on schedule to complete.”


Scheduled to last six and a half hours – but ending after seven hours – the two spacewalkers worked on the deployment of an attachment system that will be used to hang spare parts on the International Space Station’s truss, before replacing a failed rate gyro assembly.

The S3 Zenith Outboard PAS (Payload Attachment System) Deploy task work was similar to the Unpressurized Cargo Carrier Attachment Systems (UCCAS) deployment that proved to be troublesome during its deploy on STS-119.

A2During the UCCAS 2-P3 Nadir deployment, the platform stuck in the detent position. The platform was successfully deployed on STS-127, after engineers created a contingency of a detent tool – which is available for the PAS deploy.

“Plan for deploying the remaining four PAS sites: Prevent the PAS platforms from reaching the detent position using a tether. Crew training use of a tether and crew vigilance to control platform position,” noted the Flight Readiness Review presentation on L2 associated with the detent tool.

“With detent tool mockup, no lubrication on test article – (result): Platform released from Detent at approximately 30 lbf static load (applied on Yoke).”

Thanks to the smooth deployment, no contingency tools were required. The spacewalkers also completed the changeout of the Rate Gyro Assembly 2 (RGA2), along with work on the installation of two GPS antennas, a Remote Power Control Module and the installation of two large avionics cables as a get-ahead for the arrival of Node 3.

The spacewalkers had planned to complete additional preparations for the arrival of the European-built Tranquillity node in 2010. However, during the second STS-128 spacewalk, Olivas and Fuglesang found that the cables appeared to be in an incorrect configuration to reach properly for relocation.

A3“PMA3 Configuration: Clocked 90 degrees off from expected orientation for EVA task. Discovered during EVA2 when crew attempted the get ahead on PMA3 heater cable routing,” noted Mission Management Team (MMT) documentation on L2. “Under investigation.”

The issue is not a problem for the upcoming arrival of the Japanese HTV cargo ship, allowing engineers on the ground to evaluate a forward plan via simulations in JSC’s Neutral Buoyancy Laboratory (NBL).

“PMA3 orientation meets HTV rndz ops and ventilation/IMV hookup expectations. Heaters required for long-term stowage,” added the MMT presentation.

“CHIT 7746 submitted to request the data needed to complete this task on EVA3: Analysis for new path to route the cable has been worked. NBL called up for a run this morning to verify this new path.

“Bolt torque data required to ensure release of the PMA3 cable harness stanchion brackets and handhold bracket; these were expected to have been released back in 5A.1 timeframe but are still in place.”

With the evaluations continuing, managers decided to defer the associated tasks with PMA3.

The STS-128 crew is scheduled to go to sleep around 3 am Central Saturday and wake up at 10:59 am, as they close in on undocking early next week.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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