NASA hard at work on complex array repair plan

by Chris Bergin

With EVA-4 now moved to Saturday, NASA engineers are continuing to work their P6 4B solar array repair plan effort, covering all the bases, including the possibility of a docked late inspection.

Presentations show that the main focus is being placed on the robotics and training involved, along with evaluations on how long the OBSS (Orbiter Boom Sensor System) sensor package can survive via its thermal limitations.

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P6 4B Repair option latest: 

STS-1 commander John Young would describe the plan as ‘sporty’ – with the EVA repair noted as ‘performing the repair activity following a few days of development and little to no operator training is not recommended,’ on NASA documentation, which points towards the reason of another one day delay to the spacewalk. 

The repair needs to be carried out, given the array cannot be left in its current configuration past STS-120’s mission, for fear of further damage on its power generating panels.

The amount of work required to provide a procedure, one that that gives spacewalker Scott Parazynski the best chance of carrying out a successful repair at the first attempt, is huge, with documentation showing NASA engineers are working through potential problems the EVA may come up against – in order to be prepared to have immediate solutions.

This was outlined in non nonsense outlines, which clarify why the extensive planning of this repair is the absolute priority, ahead of Parazynski’s ride out to P6 2B.

‘SSRMS (Space Station Robotic Manipulator System) configuration provides limited maneuverability around the damage site due to singularities and structural clearances,’ noted one large presentation, with graphical representations of Parazynski’s positioning at the array.

‘ROBO estimates ~5 bays of motion in each direction (TBC). FOR mode control is lost when extended arm singularity is reached. Reconfiguration in Single joint mode required to return SSRMS to non-singular configuration

‘Pre-planning and training are required to develop recovery trajectories to ensure that SSRMS and OBSS remain clear of each other and the arrays. Executing unplanned reconfiguration trajectories in real-time is not a safe thing to do in this particular situation

‘Significant uncertainties remain with developing full trajectories that maintain adequate clearances.’

‘Maneuvering an EV crew member on the OBSS is very different from maneuvering with EV attached directly to SSRMS. Kinematic and dynamic sensitivity and behavior are not understood at this time. Training is required to develop the intuitive feel for SSRMS behavior in the proposed configuration.

‘This requires additional time to plan the maneuvers. Clear rules on how the results of the characterization will affect the execution of the EVA need to be developed.’

Parazynski will be monitored throughout his EVA, by a variety of cameras, mainly onboard Discovery, ensuring he remains at a safe distance from the array during the repair.

‘This kind of coordination is not difficult but requires practice and training. Things that seem trivial like camera placement on the different RWS monitors are critical to safe execution and require training to refine,’ noted the presentation, adding that lessons have been learned by the sun obscuring the view of the cameras, as seen during the problematic deployment of the array.

Still a big issue, the ‘thermal clock’ on the OBSS sensor package continues to be evaluated. The mission’s current high beta angle is believed to be a bonus for the survival of the package, but Flight Day 10 presentations point to this still being evaluated.

‘Thermal constraints: OBSS cameras and sensors are not powered when grappled by SSRMS. FD3 and FD5 handoff maneuvers had a 90 minute thermal clock. SAW repair operations using SSRMS and OBSS will violate the 90 minute clock.

‘Options for developing thermally favorable positions for OBSS while on SSRMS may be available. Multiple iterations are often required to find those configurations.’

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While Late Inspection can still be carried out on the orbiter after undocking, NASA would prefer to have the OBSS available for this operation. Should it appear that the OBSS package would suffer damage during the array repair, an option of carrying out a part-Late Inspection, while docked to the ISS, is being considered.

‘Any late inspection requirements would have to be performed prior to the repair EVA if there is uncertainty about the sensors,’ added a presentation, which could be available, now that EVA-4 has been delayed until Saturday.

MMT and Team 4 presentations now uploaded into L2.

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