EVA-3 checks SARJ, relocates NTA – Endeavour APU changeout

by Chris Bergin

The third and final EVA of STS-124 has ended, after Mike Fossum and Ron Garan were tasked with the removal and replacement of the Nitrogen Tank Assembly (NTA) , and further detective work on the port SARJ (Solar Alpha Rotary Joint).

Meanwhile, the removal and replacement of Auxiliary Power Unit (APU 3) on Endeavour will lead to a hot fire of the system, ahead of her launch on either LON (Launch On Need) STS-400 or STS-126.

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STS-124 Flight Day 9:

With Flight Day 9 marking the final spacewalk of the mission, the vehicles continue to perform well, with no significant issues under evaluation by the Mission Management Team (MMT).

EVA-3 began around 45 minutes ahead of the timeline, with the first task involving the relocation of the spare NTA located on External Stowage Platform (ESP) to the Starboard 1 (S1) Truss, replacing an old NTA.

During this operation, the crew on the International Space Station (ISS) will not be allowed to undergo any un-isolated exercise.

Pre-empting the mission, the spacewalkers also had the option of donning the Overgloves – which protect their gloves from cuts – for the spacewalk, which would have added difficulty to the NTA element of the EVA.

‘Overgloves may be removed per crew preference for the following NTA prep tasks: QD (Quick Disconnect) ops and thermal cover installation and electrical connector ops,’ noted the STS-124 EVA Flight Readiness Review Presentation on L2.

‘Do not wear overgloves while holding the NTA on the SSRMS (Space Station Remote Manipulator System).

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‘Safety issue: Lack of tactile feel during SSRMS maneuvers with NTA increases risk of losing NTA and possible subsequent vehicle damage. Overgloves will be worn at all other times during the EVA unless crew fatigue/safety is at risk.’

However, thanks to the successful use of the modified EMU gloves, the use of the Overgloves have not been required.

Following the S1 NTA install, the SRMS (Shuttle Remote Manipulator System) was moved to the final JEM (Japanese Experiment Module) RMS deploy viewing position.

Though documented ahead of the mission as a get-ahead, the requirement to reinspect the port SARJ has gained importance, following observations of what appears to be grease on the race ring.

At present, engineers do not know where the grease is coming from, though it may be leaking from the associated Trundle Bearing Assembly (TBA).

However, the port SARJ does not appear to be suffering from the same problems as noted on the starboard SARJ, though samples will be taken for further evaluations once Discovery returns home. Currently the port SARJ is operating without issue.

Those evaluations will continue to focus on the forward plan for the starboard SARJ, which has suffered a divot – itself a big problem – as a repair will be required to get that SARJ working again.

Evaluations into the starboard SARJ has been ongoing since inspections back on STS-120’s mission, which found metallic shavings – consisting of 1505 Nitride material – that had been grinding away from the race rings.

Other tasks included the JEM RMS Wrist/Elbow Vision Equipment (WVE/EVE) MLI and Launch Lock removal, the JEM Aft Window Launch Lock removal, R&R of the External Television Camera Group (ETVCG), and deployment of MMOD (Micro-Meteoroid Orbital Debris) shields.

Endeavour Status:

A large amount of work in ongoing inside Endeavour’s Orbiter Processing Facility (OPF), as she undergoes the removal and replacement of her APU 3.

Stood down from primary LON responsibilities associated with STS-124 (major article relating to this on site Monday), Endeavour is being processed for her role as the rescue orbiter (LON/STS-400) to support STS-125, and her primary mission of STS-126 – the latter currently scheduled for November 10.

While her booster set are being ‘built up’ ahead of stacking in the Vehicle Assembly Building (VAB) around July 14, work began on replacing APU 3 on Saturday.

‘APU 3 lube oil offload and purge was accomplished Thursday; preps continue Friday in support of the SCAPE (Self-Contained Atmospheric Protection Ensemble) ops Saturday to R&R the APU,’ noted the latest processing report on L2.

Due to the changeout of an APU, Endeavour will be required to undergo a hot fire to validate the system.

Each orbiter has three APUs – which produces pressure for the orbiter’s hydraulic system that controls the vehicle’s wing flaps, rudder, body flap and landing gear, among other elements of the vehicle.

She was also powered up this past week for SSPTS (Station to Shuttle Power Transfer System) testing, while work continued with the Standard Mixed Cargo Harness (SMCH) configuration in her midbody.

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