Expansive plan drawn up for leaving OBSS on ISS after STS-123

by Chris Gebhardt

With processing of space shuttles Endeavour (STS-123) and Discovery (STS-124) continuing on a smooth course, plans for leaving the OBSS on the International Space Station (ISS) after Endeavour’s mission has being finalized.An expansive plan details on-orbit TPS (Thermal Protection System) refined late inspections for Endeavour, ahead of the arrival of Discovery, which cannot launch with an OBSS due to clearance issues with the primary research lab for JAXA’s Kibo module complex.

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With Endeavour’s targeted launch date on STS-123 now just days away, the challenging construction mission is set to feature five planed EVAs, one of which will be dedicated to the attachment of Endeavour’s OBSS to the Station for use by Space Shuttle Discovery during its STS-124 mission later this year.

Because of Payload Bay clearance issues with the primary Kibo lab module, Discovery will be unable to launch with an OBSS. As a result, NASA has drawn up a series of plans to ensure OBSS access for Discovery while on orbit.

To accommodate this unique situation, Endeavour’s flight plan has undergone a series of exclusive changes that involve a docked late inspection on Flight Day 12 (FD-12) and the attachment of the OBSS to the Station on FD-13.

Docked Late Inspection:

Because of the need to leave the OBSS at the Station, Endeavour’s crew will perform the now-customary late inspection of ship’s TPS while docked to the Station.

Thanks to procedures developed for the STS-121 mission in July 2006, the port wing inspection procedures have already been tested in the docked configuration. Similarly, nose cap survey procedures were drawn up prior to the STS-117 mission last year.

‘New Starboard survey procedure developed for STS-123. Full coverage requires half the survey with the OBSS above the Payload Bay and the other half with the OBSS under the Orbiter,’ notes an expansive MOD presentation on L2 – dedicated to the procedures involved.

To accomplish the starboard wing survey, the crew will make use of several cameras on the Station, to ensure that the OBSS remains at a prescribed distance from the RCC (Reinforced Carbon Carbon) panels.

‘Currently have one clearance violation,’ added the presentation, which listed the complex challenges that will be refined during the procedures. ‘During (starboard) survey pass 4, OBSS clearance to structure (RCC) is 43 inches (60 inches required for auto-sequence). Violation lasts for around six minutes and adequate clearance monitoring exists.’

Following the late inspection, ground engineers will begin processing the data as soon as it is downlinked. ‘Typically takes 34 hours to complete image processing plus eight hours for DAT (Debris Analysis Team) assessment.

(OBSS) transfer will take place approximately 24 hours into the analysis cycle. Will have completed around 80 percent of Level 2 image processing (around 95 percent of hot regions)’ by the time the OBSS is attached to the Station.

While the potential of discovering TPS damage after the OBSS is stowed on the Station is small, the crew can retrieve the OBSS (via an additional EVA) to perform a focused inspection or repair.

‘If OBSS is needed for follow-up focused inspection or repair, an EVA would be required to retrieve the boom. May require back-to-back EVA’s for OBSS retrieval and repair. Post repair, there may not be capability remaining to re-stow OBSS on ISS.’

EVA-5/OBSS Transfer to ISS:

The fifth and final scheduled EVA of Endeavour’s mission will see astronauts Robert Behnken and Michael Foreman attach the OBSS to the S1 truss via Orbital Support Equipment (OSE) as well as install a keep-alive umbilical (KAU) which will provide electrical power for the OBSS sensor packages.

The OSE, installed by spacewalkers during the STS-118 mission last August, will provide physical attachment points for the OBSS as well as attach points for the keep-alive attach device (KAD).

‘(KAU) consists of an avionics box on a camera support stanchion (on S1 truss), around 31 foot umbilical and keep-alive attach device (KAD).’

In addition, the spacewalkers will have to manage a critical OBSS thermal clock which dictates how long the OBSS sensor packages can be un-powered without adverse affects.

While the Shuttle Remote Manipulator System (SRMS) will provide power for the sensor packages during the initial part of the EVA, a hand-off of the OBSS to the Space Station Remote Manipulator System (SSRMS) will have to be performed in order to maneuver the OBSS to the proper position near the S1 truss.

Once this hand-off occurs, the OBSS will be in an un-powered state until the KAU can be attached. Currently, the EVA timeline indicates that the OBSS will be in an un-powered state for around one hour 45mins. While this time is acceptable, it leaves little margin with the thermal clock estimates.

To mitigate this issue, the OBSS sensor packages will be pre-heated using power provided from the SRMS. Additionally, the EV crew will also have the option of installing a thermal cover over the sensor packages prior to the hand-off maneuver.

Once this hand-off occurs, the SSRMS will move the OBSS to the installation site where the two spacewalkers will attach the KAU. Once the KAU is verified to be in proper working order, the spacewalkers will take manual control of the OBSS and physically berth it to the OSE without the assistance of the Station arm.

Depending on how the EVA timeline unfolds, and whether the OBSS thermal cover is installed prior to the transfer activities, the spacewalkers will install the cover prior to the completion of the EVA so the cover can provide protection from micro-meteoroid orbital debris (MMOD) and thermal stability.


Like Endeavour’s STS-123 mission, Discovery’s mission timeline for STS-124 will see several unique changes designed to mitigate the lack of an OBSS until the vehicle reaches the ISS.

FD-2 TPS Inspections:

While the normal FD-2 TPS inspections will not be possible, a limited check of Discovery’s TPS will be performed using the End Effector (EE) of SRMS. While the use of the SRMS will help provide insight on the health of Discovery’s TPS post-launch, the capability of the SRMS is limited.

Due to the same clearance issues that prevent Discovery from carrying an OBSS to orbit, the elbow camera of the SRMS will be locked into a position that will prevent it from contacting the Kibo module and/or the port radiator during launch, thus making it virtually unusable until the restraint is removed during EVA-1.

‘Flight-specific Starboard and Port wing RCC survey auto-sequences have been developed from existing EE surveys. Starboard survey expanded to include more area on wing glove and wing tip,’ added the FRR presentation.

‘Limited by SRMS reach and clearance to payload bay cargo, so only the upper surface can be scanned. Port survey covers more area on wing tip, but at an increased range than the generic survey. The increased range is primarily due to limited clearance viewing under the payload bay door without the elbow camera.’

In addition to the physical constraints of the SRMS, lighting will play a key role in the timing of the TPS inspections. Unlike the OBSS which can effectively operate during orbital night, the End Effector of the SRMS does not have this capability.

‘Earthshine required for survey lighting is accommodated by nominal LVLH attitude; surveys must be performed during daylight,’ notes the FRR presentation.

In addition to the SRMS inspections, the traditional Rbar Pitch Maneuver (RPM) will be performed prior to docking on FD-3.

EVA-1/OBSS Retrieval:

Once Discovery arrives at the Station, preparations will begin for the retrieval of the OBSS with the SSRMS grappling the boom shortly after docking.

The next day, two spacewalkers will remove the thermal cover from the sensor packages, physically release the OBSS from the OSE, and disconnect the KAU. Once this is complete, the SSRMS will maneuver the OBSS to a predetermined position where the Shuttle arm will grab hold of the boom.

As with the transfer activities on STS-123, a thermal clock will govern the acceptable amount of time the OBSS sensor packages can go without power.

Furthering this delicate dance of power with the OBSS is the need to temporarily shutdown power to the sensor packages after the boom has been transferred back to the Shuttle arm. The reason for this temporary shutdown is for the Launch-to-Activation (LTA) operations with the Kibo module. Once these activities are completed, power will be reapplied to the sensor packages.

‘Maximum power off constraint is currently around 49 minutes; current expected off time 35-45 min. Additional analysis in progress to verify/improve the allowable max power off time. Post-SRMS grapple, power can be cycled for short periods of time to protect the hardware, if required for EVA-related LTA disconnect issues. Overall objective is to minimize time without OBSS powered.’

In addition to the OBSS and Kibo activities on EVA-1, the spacewalkers will remove the launch restraint from the elbow camera of the SRMS, allowing full mobility of the system for the duration of the mission.

OBSS Inspections:

Following the retrieval of the OBSS, the flight crew will perform a serious of checkout operations on the sensor packages similar to what was seen during the STS-120 mission last fall.

‘Following EVA-4 during STS-120, Imagery Ops requested specific positioning for LCS and IDC sensor checkout prior to Late Inspection. OBSS was unpowered for around nine hours during EVA-4. TPS community has requested checkout similar to that performed during STS-120.’

This checkout will be performed on FD-5 to verify that all sensors are functioning normally ahead of Focused Inspection activities on FD-7 (if required) and Late Inspection operations on FD-11.

Late Inspections:

The customary late-inspection operations will see several changes to accommodate the need to conduct a complete scan of the Discovery’s TPS.

‘Inspection procedures will be executed as if they were performed on FD2. Wing glove and crew cabin portions of survey will be performed (usually FD2 only). Crew requested that the post-undock surveys not be referred to as Late Inspection to avoid inadvertently performing callouts to skip steps for ‘Late’ inspection.’

In addition to Late Inspection procedures, an extra flight day has been added to the mission timeline to provide enough time to analyze the OBSS data.

While several open items remain for the STS-124 portion of the operation, including the timed removal of the thermal cover from the OBSS on EVA-1 and the checkout techniques of the sensor packages on FD-5 (and the potential impacts to the mission if they are not performed), no show stoppers to the complex OBSS transfer operation have been identified by NASA’s engineering community.

L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.



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