Endeavour undocks from a healthier ISS – heads to Late Inspections

by Chris Bergin

Shuttle orbiter Endeavour has undocked from the International Space Station (ISS) on Flight Day 15 of STS-126. Endeavour and her crew depart the Station in a much healthier state than when they arrived, notably via the maintenance on the Solar Alpha Rotary Joints (SARJs), with early results showing the downstream multi-mission plan – which ends with SARJ XL – may not be required.

The docked phase of the mission involved four spacewalks, all of which focused on the cleaning and lubrication of the starboard SARJ – with EVA-4 carrying out the same process on the port SARJ. With the starboard SARJ being the problem, it also saw its Trundle Bearing Assemblies (TBAs) replaced.

A two orbit Autotrack test was carried out on the starboard SARJ, following EVA-4, with initial data proving to be positive on the improvements to the rotating joint’s health – via less vibration and lower currents required to drive the motors.

“Port SARJ returned to autotrack operation post-EVA. Drive current levels slightly less after lubing,” noted Mission Management Team (MMT) documentation on L2.

“Starboard SARJ autotrack test performed; required significantly less current to drive: currents were 0.7 – 0.9A prior to lube, currents were 0.17A average and 0.35A peak after lube.”

While the data will take up to two months to fully analysis, managers are already hopeful of the possibility that the starboard SARJ won’t require renewed cleaning and lubrication on a regular basis. More so, a long term plan for the SARJ – which was mapped out during the summer – may not be required.

That plan involved several downstream missions including SARJ ‘clean and lube’ requirements, before the long-term solution of replacing the Race Ring – a plan known as SARJ XL – was initiated.

SARJ XL is classed as “an innovative scheme to potentially take the single spare race ring and insert it between the existing spalled inboard race ring, and the other spare ‘outboard’ race ring.”

At present, engineers are pressing ahead to launch SARJ XL on one of the CLF (Contingency Logistics Flight) missions in 2010, with that mission’s EVAs dedicated towards its installation.

“Trade Study: (ISS Manager) Mr. (Mike) Suffredini had asked about accelerating SARJ XL to ULF4 from ULF5 (ULF5 is the last mission in the “current” manifest). Due to various reasons, many due to hardware availability, the SSPCB directed the SARJ XL to stay on ULF5,” noted the latest news via a MOD memo, on L2, prior to the mission.

“However, (Mr Suffredini) did indicate that he wanted hardware delivery to stay in the same May 09 time period. If for some reason the assembly sequence slips, he wants the have other mission options (ie: hardware delivery should not slip with launch date). One option discussed is whether ULF4 (Russian MRM) may end up slipping with ULF5 taking its place in the sequence.

“Spare Race Ring Viability Inspection: Boeing looked at the ring that will be used for SARJ XL. Although there were several microscopic corrosion “features”, the ring was described as being pristine.”

Due to the complexity of the downstream manifest scheduling – which is already being pushed to into a full 2010 of shuttle flights, even before the now-likely shuttle extension kicks in – removing the need for SARJ XL has numerous benefits, ranging from cost savings to allowing other tasks to take priority on the CLF missions.

The best case scenario envisioned involves a requirement to “clean and lube” the starboard SARJ – and possibly its port counterpart as aging starts to take effect – once every year or so. Such tasks could be included during space station increments, as opposed to becoming shuttle mission EVA tasks.

Once the data from STS-126’s get-well tasks is fully analyzed, engineers will have a better understanding on both their ability to use autotrack on the starboard arrays, and the maintenance timeline. The potential deletion of SARJ XL can then be decided on.

Meanwhile, back on Flight Day 15 of STS-126, another key event is the required Late Inspections of Endeavour’s TPS (Thermal Protection System). The orbiter’s heatshield has already been deemed as completely clear of any notable damage via Flight Day 2 inspections, and Flight Day 3’s RPM imagery.

While the orbiter is protected by the Station during the docked phase – thus the likelihood of any damage is very small – one final inspection is important after docking, due to the contingency of returning to the ISS, should imagery find serious damage on the orbiter.

In such a scenario. the orbiter would be re-docked with the ISS, and her crew taking up residence on the ISS until Discovery could be launched on a rescue mission. Endeavour would undock ahead of Discovery’s arrival, where she would either be commanded through either a destructive – tail first – disposal re-entry, or RCO’ed through to landing.

Such LON (Launch On Need) scenarios are very unlikely, but NASA don’t take chances when it comes to the safe ability of an orbiter to re-enter since Return To Flight – since when the TPS inspections were included into shuttle missions, inclusions that involve the use of the OBSS (Orbiter Boom Sensor System).

The OBSS instrumentation package – which rides on the end of the 50 foot boom – consists of visual imaging equipment, the Laser Dynamic Range Imager (LDRI), and the Laser Camera System (LCS). The post-Columbia modification has sensors that can resolve at a resolution of few millimetres, and can scan at a rate of about 2.5 inches per second.

One small issue with the “tilt” direction of the camera suite on the OBSS was noted early in the mission, though this will not affect the ability to gain the required scans during Flight Day 15.

Classed as “OBSS ITVC Tilt Angle Offset”, the issue relates to the PTU (Pan Tilt Unit), which was noted to be offset by nine degrees in tilt and 1.5 degrees in pan. A similar issue was noted on STS-123.

“Bad PTU reset – incorrect reset could be caused either by operator error, or by a cable jam/snag,” noted one of several MMT presentations (L2) on the probable causes.

“2) Cable snag on cable tie – this is the first flight for the cable tie. The tie was incorporated in order to prevent a cable to sensor blanket snag that was observed in ground testing. Remote possibility with position of cable tie

“3) Hardware malfunction -Most unlikely cause since there isn’t any other reports of off-nominal PTU function.”

The bad reset was ruled out, following a failed solution via the reset that was carried out later in the mission as part of the troubleshooting.

“Performed Test 2 from 12:55 am to 2:13 am on Flight Day 12. Offset at -260 degrees tilt agrees with offset of ~9 degrees seen on FD2. Expect OBSS PTU behavior for Late Inspection to be similar to what was observed on Flight Day 2.”

However, with the issue known, Late Inspections can be carried out as planned, without any problems.

“Regardless of Troubleshooting: Additional Pan/Tilt resets will not pose additional risk to the hardware,” added one presentation. “Resets are performed numerous times on ground. Not a life cycle issue. Post Flight ground activities include an inspection and basic functional test.”

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.

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