Following a meeting between NASA and Russian Space Agency (Roscosmos) managers on Friday, a decision was made to call off the planned Soyuz documentary flyabout of the International Space Station (ISS) and Endeavour during the STS-134/ULF-6 mission, due to concerns with crew impacts in the event that the Soyuz should fail to re-dock with the ISS. Flyabout planning efforts are now shifting to the STS-135/ULF-7 mission.
STS-134 Flyabout Cancelled:
Under original plans, the recently arrived Soyuz TMA-21/26S would have undocked from Mini Research Module-2 (MRM-2) “Poisk”, conducted the flyabout, and then re-docked to MRM-2.
However – according to meeting notes acquired by L2 – a major concern with this proposal stemmed from the possibility that Soyuz TMA-21 could fail to re-dock with the single fault tolerant Russian docking mechanism on MRM-2 – the same docking mechanism found on all other Russian modules of the station. Such a failure would have had major impacts for the ISS crew.
Should TMA-21 have failed to re-dock to MRM-2, the only option would have been for the crew to return to Earth, since no other Soyuz-compatible docking ports would have been available on the ISS. This would have resulted in a situation where a complete de-crew of the ISS was necessary, due to the impending departure of Soyuz TMA-20/25S.
Under a 29th April launch of Space Shuttle Endeavour, the Soyuz TMA-21 flyabout would have occurred on Flight Day-13 (FD-13) of the mission, which would have been 11th May. Should TMA-21 have failed to re-dock on this date, a serious situation would have been placed on the ISS, considering there would only have been five days remaining until the planned departure of Soyuz TMA-20/25S from MRM-1 “Rassvet” on 16th May.
Soyuz TMA-20’s departure with its three crewmembers, necessary due to the impending arrival of Soyuz TMA-02M/27S at MRM-1 on 1st June, would have resulted in no crewmembers being present on the ISS for the two week and two day period between TMA-20’s departure and TMA-02M’s arrival.
Aside from the loss in crew science activities, a de-crew situation is a very risky prospect for the ISS, due to the fact that a crew would not be present to deal with any on-board problems or emergencies that may arise.
The meeting overview notes that NASA and Roscosmos managers discussed potential remedies for such a de-crew situation, including the prospect of having Soyuz TMA-20 wait until Soyuz TMA-02M arrived before departing. This proposal, however, also carried its own problems, due to the fact that the Soyuz TMA-02M launch is likely to be slipped by just over a week, from 30th May to 7th June.
This slip is due to the fact that Soyuz TMA-02M is the second flight of the new “digital” (700 series) Soyuz, which recently concluded its first flight on the Soyuz TMA-01M mission on 16th March.
Due to several anomalies seen with the new digital Soyuz systems during the TMA-01M mission, post-flight data will likely lead to modifications being made to TMA-02M, which will cause the launch date to slip to the right.
The potential one week slip to Soyuz TMA-02M would mean that TMA-20 would need to remain on-orbit for almost an extra three and a half weeks in order for the two missions to overlap, thus averting a de-crew situation. This length of delay to the Soyuz TMA-20 landing was deemed unacceptable by Roscosmos managers.
Not wishing to be beaten, the notes add Roscosmos managers developed a new plan for a Soyuz flyabout utilising Soyuz TMA-20, docked at MRM-1. Under a nominal mission plan, with a 29th April launch and no Soyuz TMA-21 flyabout, Endeavour would undock from the ISS on 11th May, just five days shy of the planned Soyuz TMA-20 departure.
Thus, Roscosmos managers proposed either delaying Endeavour’s launch, or moving Soyuz TMA-20’s departure up, so that Endeavour would be present at the ISS during the Soyuz TMA-20 undocking. Under this plan, once TMA-20 undocked from MRM-1, the flyabout would be conducted, and TMA-20 would then return to Earth as normal.
The major benefit of using Soyuz TMA-20 for the flyabout over Soyuz TMA-21 is that no re-docking would have been required, thus eliminating the risk of a re-docking failure and re-docking thruster pluming of the Shuttle by the Soyuz.
The flyabout would have taken on a different form from the planned TMA-21 flyabout, due to the fact that Soyuz TMA-20 is on the opposite side of the ISS (Nadir) from TMA-21 (Zenith), and the fact that a re-docking would not be required.
Dual Docked Operations (DDO) constraints would also have needed to be reviewed in light of Soyuz operations in close proximity to the Shuttle’s Payload Bay.
Ultimately, the decision to slip the STS-134 launch yet further was not desired, nor was the decision to move up the Soyuz TMA-20 departure, due to the fact that the undocking and associated preparation activities, which would have taken many days during the STS-134 mission, would have had too large of an impact on crew timelines and activities during STS-134.
The accelerated undocking would also have added at least five days to the already widening gap between the Soyuz TMA-20 undocking and the delayed Soyuz TMA-02M launch – thus impacting the ISS crew following Endeavour’s departure. In light of this, the Soyuz flyabout during STS-134 was cancelled.
It is now expected that the Soyuz TMA-20 undocking and landing will slip by a similar amount to the Soyuz TMA-02M launch slip, in order to minimise ISS crew time impacts. Thus, if the TMA-02M launch slips by one week, the TMA-20 departure will also slip by one week. In spite of this, NASA managers are still reviewing DDO constraints, in case launch delays cause Endeavour to find herself present at the ISS during the Soyuz TMA-20 undocking.
Previous articles on the Soyuz Flyabout Evaluations:
Managers Pursuing STS-135 Flyabout:
Following the cancellation of the STS-134 flyabout, NASA and Roscosmos agreed to switch all flyabout planning efforts to the STS-135 mission, which will be the final flight of the Space Shuttle Program.
The STS-135 Soyuz flyabout would use Soyuz TMA-21, docked at MRM-2, with a similar attitude/trajectory to those planned for STS-133 and STS-134.
The use of Soyuz TMA-21 is acceptable for the STS-135 flyabout, as a failure to re-dock will not result in a de-crew situation, since Soyuz TMA-22 would arrive at the ISS before the next Soyuz landing (TMA-02M). However, if TMA-21 failed to re-dock, it would still leave the ISS crew at three people for an extended period of time.
The biggest hurdle to the STS-135 Soyuz flyabout will be squeezing it into the already jam-packed STS-135 mission timeline.
Mission planners are already struggling to fit all primary transfer and EVA objectives into the docked mission timeline, which is more challenging than most Shuttle mission timelines due to the fact that STS-135 will only have four crewmembers, and the fact that Shuttle Atlantis does not possess Shuttle to Station Power Transfer System (SSPTS) capability, which limits the amount of time that Atlantis can spend at the ISS and eliminates the possibility of a mission extension.
In spite of Russian crewmembers being drafted in to help with mission activities, NASA managers are already in the process of deciding what items to drop from the STS-135 mission timeline, and an STS-135 flight plan timeline status get-well presentation, available to download on L2, listed a Soyuz flyabout as a “threat” to the mission timeline.
Further complicating matters is the fact that Ron Garan, who would be a crewmember on Soyuz TMA-21 during the STS-135 flyabout, is also scheduled to perform and EVA during the STS-135 mission. Squeezing Garan’s EVA preparation, flyabout preparation, and cargo transfer activities into the STS-135 mission timeline will likely be a challenge to mission planners.
If the current schedule holds, Europe’s Automated Transfer Vehicle-2 (ATV-2) spacecraft will depart from the ISS prior to the STS-135 mission, meaning that it will not be visible in any flyabout images. There are rumours that the ATV-2 mission may be extended in order to maximise ISS reboost propellant capability, but it is not yet known whether this extension, if approved, would place ATV at the ISS during the STS-135 mission.
STS-135 Specific Articles: http://www.nasaspaceflight.com/tag/sts-135/
STS-135 will also bring a Multi Purpose Logistics Module (MPLM) to the ISS, however this module will likely have been returned to Atlantis’ Payload Bay by the time the flyabout occurs the day prior to undocking.
(Images used, all via L2 presentations, imagery and L2 acquired flyabout video and accredited within the article).