STS-134: ISSP Approves DDO – Soyuz Flyabout Returns to Mission Planning

by Pete Harding

International Space Station (ISS) program managers have approved the use of Dual Docked Operations (DDO) during the upcoming STS-134 mission, the first time such an operation has ever been cleared for use. Managers are also planning to conduct a Soyuz flyabout during the STS-134 mission, per its official return to the mission plan following previous deletion from the timeline.

Dual Docked Operations:

DDO is the docking or undocking of a Russian Soyuz or Progress vehicle to the ISS, while the Space Shuttle orbiter is docked to the ISS. Docking or undocking a Soyuz or Progress while a Space Shuttle is present at the ISS presents a unique set of challenges, which do not apply for Russian vehicle dockings or undockings during periods when Space Shuttles are not present at the station.

The challenges associated with DDO stem from issues relating to proximity operations between Soyuz or Progress vehicles, and the Space Shuttle. In the past, concerns have centred around Soyuz and Progress trajectories and their clearances with the Shuttle, in the event that an aborted docking was to occur.

Analysis conducted from previous Shuttle missions determined that the SM Aft, DC-1 Nadir and MRM-2 Zenith ports were acceptable for DDO dockings and undockings, however the FGB Nadir port was only approved for undockings due to its proximity to the Shuttle, and thus increased risk of a collision between a Soyuz or Progress and the orbiter in the event of an aborted docking.

Another concern with DDO regarded thruster pluming of the Shuttle by the Soyuz or Progress vehicles. During dockings, Soyuzes and Progresses control their attitude and trajectory using thrusters, which expel plumes of particles which can contaminate and damage the Shuttle – specifically windows, camera lenses, and Thermal Protection System (TPS) tiles.

Past analysis showed that the Shuttle’s TPS would not be at risk from thruster pluming, and recent analysis conducted in support of Soyuz a flyabout determined that the windows would also be safe.

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A procedure also exists to place the Orbiter Boom Sensor System (OBSS) – with its sensitive optics – behind the Shuttle’s TPS heatshield using the Shuttle Remote Manipulator System (SRMS), in order to shield the OBSS from any thruster pluming.

In spite of past analysis, DDO has not been approved for use on any Space Shuttle mission prior to STS-134, with NASA managers instead opting to slip launch dates in order to deconflict Shuttle docked periods with Soyuz or Progress dockings or undockings. However, recent Shuttle and Soyuz schedule changes have necessitated the need for DDO during STS-134.

Under original plans, Soyuz TMA-20, which launched to the ISS on December 15th 2010 carrying Russian Cosmonaut Dimitri Kondratyev, American Astronaut Cady Coleman, and Italian Astronaut Paolo Nespoli, was to undock from the MRM-1 Nadir port on May 16th, just two weeks ahead of the arrival Soyuz TMA-02M/27S on May 30th.

However, due to the fact that the Soyuz TMA-02M launch slipped to June 7th because of problems associated with the new 700 series “digital” Soyuz, the Soyuz TMA-20 undocking and landing slipped to May 23rd. The STS-134 slip from April 29th to May 16th then put the Soyuz TMA-20 undocking right in the middle of Endeavour’s docked mission (currently Flight Day-8).

STS-134 Specific Articles:

Due to the already lengthy delays to STS-134, mission managers did not want to delay the mission further to accommodate the Soyuz TMA-20/25S undocking, instead opting to approve DDO for use.

Several factors made DDO more acceptable for use on STS-134, including the fact Soyuz TMA-20 would be undocking, not docking, and so the risks associated with abort trajectories and thruster pluming would be less.

Also, with MRM-1 now installed on the FGB Nadir port and the PMM on the Node 1 Nadir port, more clearance and shielding between the Soyuz and the orbiter are available now than on previous Shuttle missions. Thus, with the aforementioned past and recent analysis concluded, the use of DDO on STS-134 was approved.

The use of DDO on STS-134 carries with it a few concerns, relating to the consequences of any launch slips to the mission flight plan. If the STS-134 launch slips past its current date of May 16th, then the Soyuz TMA-20 undocking will slip out of the currently free Flight Day-8 (FD-8) and into an earlier mission Flight Day.

Any launch slip of Endeavour would facilitate the need for replanning of the mission timeline, in order to deconflict the Soyuz TMA-20 undocking with other mission activities. This would also create issues since the ISS and Shuttle crew will be operating under separate sleep shifts for the STS-134 mission, in order to allow the ISS crew to participate in the Soyuz TMA-20 undocking activities while the STS-134 crew sleep.

This separate sleep shift option was ruled out of all previous DDO scenarios due to the problems it created with the flight plan, since it complicates operations for the ground crews who have to support separate Shuttle and ISS activities.

The difference in sleep schedules between the Shuttle and ISS crews on the STS-134 mission is at the limits of acceptability, as noted during the May 9th STS-134 press conference. Thus, any delay in the STS-134 launch may push the limit out of the acceptable range.

As noted during the ISS crew Weekly Planning Conference (WPC) on May 14th, another potential problem is that fact that the ISS change of command ceremony, where Expedition 27 Commander Dimitri Kondratyev will hand over command of the ISS to Expedition 28 Commander Andrey Borisenko, will occur during a Shuttle crew sleep period.

Status information on L2 also noted that the Alpha Magnetic Spectrometer (AMS) team was concerned with how the Soyuz TMA-20 undocking may affect the AMS-02 installation activities in the event of a launch slip. Soyuz undockings are prohibited during times when robotics activities are occurring at the station, as would be the case during the AMS-02 installation.

The status information mentions that providing Endeavour launches prior to May 20th, then AMS-02 will be installed on the ISS prior to the Soyuz TMA-20 undocking.

If, however, Endeavour launches on May 20th, then AMS-02 will be being installed on the same day as the Soyuz TMA-20 undocking (May 23rd). A launch on May 21st would mean that Endeavour would be docking to the ISS on the same day as the Soyuz TMA-20 undocking. A launch on May 22nd or after would mean that Soyuz TMA-20 would be departed from the ISS before Endeavour arrived.

Soyuz flyabout:

Following its previous cancellation from the STS-134 mission, the Soyuz flyabout has now returned to the STS-134 flight plan, per a Mission Evaluation Room (MER) note obtained by L2, which states that “The Soyuz Flyabout has moved to ULF6”.

The Soyuz flyabout was originally planned for the STS-133 mission in February, but was deferred to STS-134 following the decision to cancel the STS-133 Soyuz flyabout due to issues with new “digital” Soyuz TMA-01M spacecraft, which was making its first test flight.

The STS-134 Soyuz flyabout using Soyuz TMA-21 was changed to use Soyuz TMA-20 due to a potential ISS de-crew situation if Soyuz TMA-21 fail to re-dock to the ISS within weeks of the impending Soyuz TMA-20 departure.

In order to accommodate this flyabout the Soyuz TMA-20 departure would have needed to be moved up, but due to the negative impacts on the STS-134 mission, and ISS stage operations in light of the Soyuz TMA-02M launch delay, the Soyuz TMA-20 undocking and flyabout during STS-134 was cancelled. Soyuz TMA-20’s undocking was delayed to May 23rd in order to minimise ISS crew disruptions caused by the Soyuz TMA-02M delay.

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Soyuz flyabout planning was then switched to STS-135, using Soyuz TMA-21. Fitting the Soyuz flyabout into the packed STS-135 mission timeline presented many challenges to mission planners, due to the busy nature of the STS-135 flight plan. However, the STS-134 launch slip to May 16th presented an opportunity since it aligned Endeavour’s docked mission with the delayed Soyuz TMA-20 undocking.

Following an insistence in the May 9th STS-134 press conference that a Soyuz flyabout would not occur on STS-134, NASA managers have now decided to take advantage of the opportunity presented by the alignment of Endeavour’s docked mission with the Soyuz TMA-20 undocking.

(Image left from set of L2 hi res images, photoshopped by NSF member Lee Jay Fingersh).

There are many benefits to performing a Soyuz flyabout on STS-134, which comes from the fact that no re-docking of Soyuz TMA-20 to the ISS is required, due to the fact that the Soyuz TMA-20 crew would be returning to Earth following the flyabout. This eliminates the risks associated with a failed re-docking and subsequent loss of ISS crewmembers, and also reduces some of the concerns with thruster pluming of the orbiter.

An additional benefit of an STS-134 Soyuz flyabout is that the ATV-2 spacecraft will be present at the ISS, which would not be the case for an STS-135 flyabout. Finally, performing the Soyuz flyabout on STS-134 will eliminate the proposal from the STS-135 mission, thus making the STS-135 mission much easier for mission planners to work with.

Previous articles on the Soyuz Flyabout Evaluations:

It is not yet known precisely how the Soyuz TMA-20 flyabout will occur. All previously planned flyabout trajectories have assumed an undocking from the MRM-2 Zenith port, not MRM-1 Nadir. However, a flyabout manoeuvre may not be necessary at all, due to the fact that Soyuz TMA-20 will not need to re-dock with the ISS, and so a manoeuvre to re-align with the docking port will not be necessary.

It may be possible for Soyuz TMA-20 to undock and retreat from the ISS, then hold position while the ISS changes attitude to allow for photography of the Shuttle/ISS stack, then continue with de-orbit preparations.

However, a known problem with this is that Soyuz undockings occur in automatic mode, and this new procedure may require the use of manual mode. This creates problems as it will create lots of replanning work for the Russian ground teams, and also may present risk since the Soyuz TMA-20 crew have not been trained to conduct such a manoeuvre.

Other problematic factors in a Soyuz TMA-20 flyabout are that, for a normal undocking, the Soyuz crew are strapped into the Descent Module, preventing access to the forward-facing window in the Orbital Module. Communication passes over Russian Ground Sites (RGSs) will also need to align with the right orbital lighting conditions to perform photography.

However – to reiterate – at this time, no information is available as to how the Soyuz TMA-20 flyabout will occur. A final Go/No-Go decision on the Soyuz TMA-20 flyabout is expected to be made during the STS-134 mission.

(All images via L2 Presentations and videos). The Launch Day article will be published ahead of tanking, with extensive coverage provided on the news site, forum and L2 special sections – the latter of which is the world’s best front row seat to Shuttle launches and missions, breaking every major event over the last 10 missions – including STS-134’s first attempt – before any other site, including NASA sites/TV).

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