Spacewalkers Heide Stefanyshyn-Piper (EV1) and Steve Bowen (EV2) have completed the third of four EVAs during STS-126, once again focusing on the starboard SARJ (Solar Alpha Rotary Joint). If required, a back-up option of a modified NOAX Gun is available for the remaining EVA tasks on EVA-4 – following the loss of two grease guns during EVA-1.
EVA-3 involved the changeout of the remaining five Trundle Bearing Assemblies (TBAs), the removal of the debris from the Race Ring, before greasing the metallic surfaces. One TBA task was outstanding.
The aim is to provide a temporary solution to the vibrations that were observed on the starboard SARJ, which will allow it to “autotrack” the sun and provide power to the ISS. A final solution will be carried out in 2010, via the installation of the SARJ XL replacement Race Ring. Due to the absense of the final TBA task, the autotrack test will have to wait until after EVA-4.
Tasks to be completed on this EVA were laid out by the SSP EVA Flight Readiness Review (FRR), although the usual pattern of being ahead of the timeline by the spacewalkers has led to several refinements of the EVAs.
“EV1 starts with covers 20 and 21, and TBA 5 first, followed by DLA (Drive Lock Assembly) cleaning. Then works covers 22 and 1, and TBA 6 (to line up with EV2). Next, covers 2 and 3, with TBA 7,” noted the presentation.
“EV2 starts with covers 15 and 16, with TBA 2. Then covers 17 and 18, with TBA 3. Next, cover 19 and TBA 4.”
This EVA was the longest to be carried out on this mission, which became a topic of discussion at the FRR to ensure safety limits were maintained.
“One violation will occur as part of nominal mission plan for EVA duration greater than 6.5 hrs (Shuttle Crew Scheduling Constraints, Section 3.2.a governs allowable EVA duration),” noted the FRR presentation. “EVA3 duration planned for 7 hrs to complete Starboard SARJ cleaning.
“Each section of the S3 SARJ estimated to take 1:45 to clean, lube, and R&R the associated TBA. Currently performing 3:00 hrs SARJ maintenance on EVA1, 3:45 hrs on EVA2, and 6:00 hrs on EVA3. EVA duration is based on 6:00 hrs of SARJ tasks + 30 min egress/setup + 30 min of cleanup/ingress.”
The long duration EVA was based on four training runs in the Neutral Buoyancy Laboratory (NBL) at the Johnson Space Center (JSC), which pre-empted the need for a flight rule that could be assessed in real time during the spacewalk.
“Consistent with data from repeated (4) NBL runs. CR (Change Request) documents exception. Flight Rule written to govern real-time EVA duration strategy.”
The Mission Management Team (MMT) also became involved with ensuring the spacewalkers had enough contingency via the tools being used to lubricate the SARJ hardware over the remaining EVA timeline, following the loss of two grease guns during EVA-1.
Using typical NASA ingenuity, engineers noted the spacewalkers could modify another gun – which is carried on the orbiter in the case of a repair being required on the RCC panels.
The nearest this came to being used was during STS-118, when Endeavour suffered a gouge on her underbelly, before it was cleared to re-enter without any required work.
The decision held some negative impacts – if the gun is used – due to the removal of some of the contingency related to a NOAX based repair. However, with a clean orbiter, and the highly unlikely event of damage being caused ahead of Late Inspections (post undocking), the gun was always likely to remain unused in its TPS repair role. One gun will remain unused regardless.
“Two NOAX Flight Tubes flown on every mission and stored independently of Manual NOAX Guns and application hardware. Supports early and late mission RCC Repair (Crit 1 task). Flight NOAX tubes are integrated IVA prior to Airlock egress; flight NOAX tubes cannot be integrated into NOAX gun EVA,” noted the MMT presentation on the decision.
“SARJ grease gun design was based on the NOAX manual gun design. Three modifications were made to the NOAX manual gun. 3/8” aluminum nozzle (NOAX) vs. 1/8” Teflon (SARJ). Zip tie added to SARJ Gun handle to minimize hand travel. Thermal cover on SARJ straight nozzle different than NOAX straight nozzle.
“Background: One of two Manual NOAX Guns and one NOAX nozzle proposed for possible use during SARJ EVA-3 and EVA-4. NOAX nozzle design is slightly different but can be configured for use on SARJ EVA.
“Concerns: NOAX nozzle not reusable for RCC Repair if used for SARJ EVA. Nozzle requires IVA disassembly and cleaning prior to use for NOAX repair. Risk of grease transfer/contamination on Manual Guns if gun is used later for NOAX repair
“Unknown effect of material incompatibility between NOAX and Braycote. Requires Dual Glove Box and arc-jet testing (5-6 days) to characterize effect. Risk of no redundancy for implementing a NOAX Repair during late mission contingency. Contaminated gun could be used as back-up.”
The MMT reviewed the request to use the TPS NOAX dispensing tool for EVAs 3 and 4 to assist with the lubrication of the port and starboard SARJ and decided to allow it to be placed in the crew lock area of the Quest airlock, ready to be used if required.
That gun may still be able to be used as a backup on a TPS repair, if absolutely required by replacing the cartridge.
“The MMT recognizes that there is a very low probability that the tool will be required for the contingency scenario of dispensing NOAX,” noted the MMT. “It was also observed that the tool could be cleaned up after it was used for SARJ lubrication and ready to serve as the backup to the prime NOAX dispensing tool, if needed.
“Since one of the primary objectives of this mission was to stabilize and restore the SARJ to fully functional status, the MMT concurred with the request to use the backup NOAX dispensing tool for EVAs 3 and 4.”
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.