Superb EVA-2 highlights FD6 – ODS issue cleared for contingency redock

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STS-129 Spacewalkers Michael Foreman and Randolph Bresnik have concluded the second of three EVAs, which has been slightly replanned due to the success of EVA-1 and alarms on Station. Meanwhile, engineers have cleared Atlantis for a contingency redocking with the ISS – in the event of damage being found during post-undock Late Inspections – following an incident relating to the ring alignment on the Orbiter Docking System (ODS).

EVA-2:

The crews of STS-129 and the ISS were woken for the second consecutive night by a false depressurization alarm, caused by a troublemaking Russian Poisk module (MRM-2). The newly arrived module is believed to be sending out the false indication, although engineers are still working on the cause.

The alarm sounded at 9:53 p.m. EST, more than two hours after Atlantis’ crew went to sleep for the night. Emergency procedures required the spacewalkers to move out of the airlock while teams on the ground verified that the alarms were false.

As a result of the rude awakening, Foreman and Bresnik’s campout procedures were changed, in order to prepare their bodies for what is currently planned to be a six hour EVA. This involved the duo hitting exercise equipment on station, to aid the removal of nitrogen from their bodies.

Around 30 minutes of get-ahead tasks were deleted from the timeline as a result. Regardless, EVA-2 still managed to throw the timeline out of the airlock, as the duo flew through the tasks in had, which already included the deployment of a Payload Attach System (PAS) planned for EVA-3, still on the second spacewalk’s tasks, along with the pre-planned work.

A3The original PAS deploy – which was planned for EVA-2 – has already been conducted via the opening spacewalk of the mission, after the spacewalkers managed to complete all their original tasks with two hours to spare.

“Install AIS/ARISS GATOR antennas on Columbus. Close Node 2 Zenith window flap. Relocate FPMU and VSSA from CP2 on S1 to CP6 on P1. Install WETA #3 at CP1 on S3. Install HPGT Guide Post,” noted the Flight Readiness Review (FRR) preset schedule of tasks on EVA-2.

With all the tasks completed after about four hours, controllers asked the spacewalkers to press ahead with the one remaining PAS on the inboard Nadir section of the Station. That task was also completed with time to space.

“Put Mike Foreman of the US Navy done for three PAS’ on two EVAs,” Bresnik was heard saying, as Foreman went on to check connector pins on the SGANT cables that were installed on EVA-1. An Articulated Portable Foot Restraint (APFR) was also located, as the duo completed the beefed-up EVA in just over six hours in a brilliant spacewalk.

Ahead of the EVA, robotic work was completed for the installation of ELC-2 on the ISS via the Space Station Remote Manipulator System (SSRMS), following handoff from the Shuttle Remote Manipulator System (SRMS), that had removed the payload from Atlantis on Friday.

ODS Clearance:

Atlantis nominal docking on Wednesday came after engineers cleared a minor ring alignment issue on Atlantis’ Orbiter Docking System (ODS). No issues have been reported since, although for the purpose of due diligence, engineers have presented their findings to the Mission Management Team (MMT).

A2“Description of Anomaly: Docking ring lost alignment during ring extension at approximately 23 percent ring linear advance,” outlined the MMT presentation (L2). “During Docking Ring Extension, Ball Pair Ball Screw No 3 Linear Advance lagged behind the other ball screw pairs at initiation of ring drive (MET 1/02:00:43) and appeared to be “stuck”. The maximum delta position between petals 1 and 3 was 2 percent, causing the ring to lose alignment.

“All three petal base positions tracked together until MET 1/02:01:35. The docking system experienced a similar signature where Ball Pair Ball Screw No 3 Linear Advance lagged behind Ball Pair Ball Screw No 1 Linear Advance. The maximum delta position between petals 1 and 3 was 2.4 percent, which again caused the ring to lose alignment.”

“First time occurrence of misalignment above 20 percent linear advance during ring extend. Centering springs on mechanism assisted realignment per design. Ring drive continued nominally and stopped at Initial Position.”

The ODS hardware, which is the original system that was designed for Atlantis’ ISS missions, was built with the assistance of Russian engineers – who are regularly involved in any engineering challenges related to the system. They were consulted after the ring alignment issue was noted on orbit.

“Discussion: Russian engineers supporting STS-129 consulted after ring extension completed. No concerns expressed over loss of alignment during ring extend. Ball screw joint centering springs assisted realignment as per design,” added the MMT presentation.

“Ring achieved initial position in nominal two motor drive time, no issues with ringdrive capability were observed. Misalignment may be caused by several factors including residual friction in ball screw assembly or ring drive differential mechanism. Recommend performing normal inspections of hardware at KSC during next flow.”

A4Docking proceeded without issue, and undocking will not be of any concern – as ring drive operations will not be involved. However, engineers also cleared the system in the event of redocking with the ISS – which would only be initiated in the event Atlantis has to return to Station in a Safe Haven scenario.

Such a scenario would only occur if serious damage was found during the post-undock Late Inspections, meaning Atlantis would need to return her crew to Station to wait for the Launch On Need (LON) vehicle, Endeavour, to launch next year to pick up the STS-129 crew.

Such a scenario is extremely unlikely, especially due to the excellent condition of Atlantis’ Thermal Protection System (TPS), via FD2 and FD3 inspections.

“Orbiter Docking System performance was nominal during docking sequence operations. No signs or indications of ball screw mechanism divergence during ring retract operations and especially during ring drive from 30 percent to 20 percent linear advance,” added the presentation.

“Ring drive operations are not performed during undock sequence. No issue for Undock. No issue for contingency re-dock. Successful operation of mechanism during docking. Same procedures apply for ring extension prior to contingency operations.”

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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