NASA change EVA-4 for array repair, via ambitious plan

by Chris Bergin

NASA are working a plan to conduct the repair of the two rips in the P6 4B Solar Array on a 24 hour delayed EVA-4. Since delayed again to Saturday.

The plan is still being worked, though it will involve a spacewalker being sent to the damaged area on the end of the Space Station Remote Manipulator System (SSRMS) and the OBSS (Orbiter Boom Sensor System) arm. Major evaluations are taking place on how long the OBSS’ sensor package can survive during the operation.

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P6 4B Repairs: 

Flight Day 8’s deployment of SAW 4B, consisting of two photovoltaic blankets, each made up of 31 individual segments (bays), was aborted at 12:25pm when one of the bays showed a tear at its corner. Another smaller tear can be observed in hi res images of the damage.

While the array is providing good power, structural integrity is the key factor for carrying out a repair.

The plan is to work on a method of repairing the array ahead of EVA-4. Delaying EVA-4 and cancelling EVA-5 will allow NASA to work on a procedure. This proceedure – which is still at the planning stage – may take engineers a while to figure out, and has since seen EVA-4 delayed to Saturday.

As far as how the array suffered the tear during deployment, this is still being evaluated. However, it appears one of the guidewires snagged in a grommet.

‘We believe one of the three guidewires snagged on one of the grommets, which started tearing out the hinge,’ said ISS program manager Mike Suffredini. ‘We retracted one bay to relieve some of the stress.’

If EVA-4 fails to repair the array, the program holds the option of added another docked day to STS-120, allowing for another spacewalk to try and find a solution.

‘If EVA-4 is not successful, which is a possibility, we would like to have the option to pursue EVA-5 on Sunday. We would have to delay departure of Discovery by another day,’ added Suffredini.

‘This is our priority as a program. We need this array. It could gain further damage if we leave it in this configuration.’

The main area of conversation at the MMT relates to the trade-offs that would be required to allow a spacewalker to ride up to the damaged area, on top of the SSRMS and OBSS.

The plan to use the SSRMS and OBSS in tandem (as seen in the image associated with this article) has now been confirmed.

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While the challenge of having a spacewalker on the end of both booms is obvious, the plan has been deemed as having no expected structural issues for the two arms.

However, the main focus relates to potential damage to the OBSS’ sensor package, which would compromise the full capability of Late Inspections of Discovery’s Thermal Protection System (TPS) after undocking.

The OBSS sensor package is at risk of suffering a failure during the EVA repair, due to its ‘thermal clock’, which ranges from two hours for most instruments, to five hours for LDRI if warmed up and covered with a large ORU bag ahead of the EVA. 

At present, the OBSS is expected to survive, due to the high beta angle, which will allow the sensor package to stay ‘warm enough’ during the operation.

Should damage occur to the OBSS, Late Inspections would be conducted only via the Discovery’s RMS, which has slightly higher risk numbers, especially after – and still under consideration – the ‘large’ MMOD strike that was picked up by the WLE (Wing Leading Edge) sensors on Tuesday.

The 2.74 GRMS indication is classed as a ‘significant hit’ on the port (left) panel 1. The indication was triple the maximum ever observed, noted NASA information. However, should there be MMOD damage on the panel, this would at least be spotted without the need of the OBSS’ sensors.

Regardless, NASA are working on contingencies for the OBSS, including the removal of certain sensors, to be left in an unpressurized airlock during the EVA, and the potential swapping around of cameras.

‘If the Sensor Package 1 Pan Tilt Unit is not operational after the Solar Array Repair task, the PTU could possibly be swapped out for an Orbiter Payload Bay Camera PTU,’ noted one MMT presentation.

‘There are 2 other flight LCS units available to the Program currently in flow for other flights.

‘The next need date for this particular unit would be mid Feb 2008 for STS-125. However, if the LCS fails due to thermal overstress, there are spare parts available for repair or for an additional unit build.’

With the changes to the mission timeline, the inspection of the starboard SARJ (Solar Alpha Rotary Joint) has been deleted from STS-120.

MMT presentations now uploaded into L2. Work in progress repair plan also uploaded.

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