The International Space Station (ISS) may live on until 2028, following confirmation by the Space Station Program Control Board (SSPCB) that partner agencies have been asked to produce an extension blueprint for continued operation until 2025, with the potential to push as far as 2028. As part of their opening evaluations, NASA managers have examined the health of the two Solar Alpha Rotary Joints (SARJs).
Previously due to be deorbited in around 2016, the ISS gained an extension to 2020 – pending the approval of the FY2011 budget proposal. It is understood there is little risk of the extension being reversed, should the US Congress decide to refine President Obama’s future plan for NASA.
As far as extending the ISS’ lifetime deep into the 2020s, the SSPCB minutes (available on L2) noted an “ISS Life Extension Internal Technical Integration effort” – a plan which will involve all ISS partners assessing the viability of safely operating the orbital outpost to at least 2025, pending Program-level approval.
“This (overview) is a status of the ISS Life Extension Internal Technical Integration thus far and the proposed work plan. The life extension analysis was split into 7 categories (structure, hardware, spares, etc.) with an itemized schedule,” noted the SSPCB, contained within the MOD 8th Floor News.
“Based on the preliminary plan, all analysis/procurement/facility updates should be complete by all partners by 2016. All partners have agreed to analyse out to 2025 at this time (with possible extension to 2028 later).”
Monthly meetings will be utilized to update the SSPCB four times a year, with the Board meeting also likely to benefit from Station partner updates on their assessments.
“The overall goal of the analysis is to show that we are safe to operate, not an entire vehicle re-certification. From an MOD perspective, this issue impacts us with respect to facility updates and infrastructure to extend operations lifetime – the need to eventually rehost MCC (Mission Control Center) and update user applications.
“Will lead monthly integration meetings and report quarterly to the SSPCB, and analysis will not commence until the program has approved it. Once the baseline plan is approved, charts will be established (within two months) to track the progress.
“(ISS Program manager) Mr. (Mike) Suffredini wants to determine what life extension certification means – (if) it is different from initial certification. The Program will need to document these differences.”
SARJ Forward Plan:
The SSPCB also provided an update and forward plan for the Station’s two SARJs – used to rotate the large power generating solar arrays on either side of the ISS as they track the sun.
Evaluations into problems with the starboard SARJ began after vibrations and power fluctuations were noted by ground controllers and ISS crewmembers, which led to an inspection of the hardware during STS-120’s EVAs.
With observations of metallic shavings – consisting of 1505 Nitride material – on the hardware, engineers concluded the debris was the result of grinding on the Race Rings.
While a similar issue was noted on the Port SARJ, concerns grew with the Starboard SARJ when Mike Fossum observed a depression, or pit, on the Race Ring during his EVA inspection.
Plans were then created to replace the Trundle Bearings Assemblies (TBAs) and grease/lube the Race Ring during STS-126’s EVAs. The results were encouraging.
“Starboard SARJ TBA replacement and grease results: Status briefing on the initial quick look results of the Starboard SARJ rotations and disturbances seen after the ring was lubricated and the TBA’s were changed out,” noted managers in an update after the work.
“The disturbances seen by starboard SARJ motion have greatly decreased based on the two orbits of autotrack that were performed immediately post R&R.
The team requested an extended autotrack to obtain more data and determine if the disturbances drop even lower after a long period of operation and distribution of the grease due to rotation.”
Managers then created a forward plan to lubricate the the Race Rings over upcoming missions – with up to 10 EVAs involved – with an end goal of replacing the Starboard Inner Ring with what was known as SARJ XL – set to fly on one of the final shuttle missions.
“After the STS-126 crew cleaned and lubricated the stbd SARJ, even though the race ring is damaged, the current required to rotate the joint was much lower than previously required,” added a note ahead of STS-119. “Because of this situation, there is higher confidence that this inboard race ring will be available for future operations.”
While the positive results negated the need for future Shuttle EVAs to include lubrication tasks into their mission content, the latest SSPCB has again updated the status of the SARJs, with regard to the ISS operating until 2020.
Their conclusions point to a Stage EVA to lubricate the Race Rings on each of the two SARJs in 2015 – two EVAs in total – being sufficient for the healthy operation of the hardware through to 2020. An ISS extension past that date would result in a repeat task on both SARJs in around the 2020 timeline.
“SARJ Status and Forward Plan: On-orbit telemetry indicates both SARJs are operating well and are described as healthy. Both drive motor current and ISS acceleration levels remain low,” noted the update to the SSPCB (L2).
“SARJ LITE testing has concluded that a 5 year lube interval is sufficient. It is assumed that a lubrication EVA would take one EVA per SARJ (2 EVAs total). A lubrication in 2015 will enable SARJ lifetime to make 2020. Lifetime is counted from start of autotrack after lubrication, not just from last lubrication EVA.
As far as SARJ XL, as expected – due to the aforementioned positive results, and lack of any mentions in the STS-133, STS-134, or even in the notional STS-135 mission planning documentation – will not fly, and will remain on the ground “until needed”.
It is not clear as to what vehicle would be able to launch the relatively large piece of hardware to the ISS after the shuttle is retired.
“The Program accepts that the SARJ outboard full redundancy be reserved as a contingency and not pre-planned for implementation. SARJ Recovery Team can be phased out as the current team architecture can handle any forward SARJ work,” added the notes.
“The Program decides to leave spare race ring on the ground until needed; the SARJs will remain on inboard ops indefinitely noting that it would be 5 EVAs needed to reconfigure to outboard ops.”
Confidence in the SARJ’s continued good health for an extended period of time has been aided by results on the ground via a test rig – which will continue to be funded throughout the lifetime of the ISS, or until it fails.
“SARJ Test 4 (Debris on Ring) Results: Test objective was to simulate a TBA roller passing over a lubricated race ring contaminated with metallic debris for an equivalent of 15 years of on-orbit SARJ cycles. Used actual debris from returned TBA as debris for test,” the SSPCB notes continued.
“Testing showed significant deformation indicating a high localized load. Testing setup used worst-case conservative conditions. Visual indication of pitting was observed early-on in testing, but problems did not propagate into full deterioration of the surface, leading to higher confidence that lifetime can be extended.
“The Program decides to continue to run the SARJ test rig to failure to get additional SARJ lifetime and lubrication data. Testing will conclude when the test rig fails – could be tomorrow or 10 years from now. The team will periodically present findings and data to the SSPCB.”