Crack found in Discovery’s RMS

by Chris Bergin

An inspection of Discovery’s Remote Manipulator System (RMS) boom has revealed a surface crack that runs 1.25 inches diagonally the carbon composite, leading to the decision to remove the robotic arm from the orbiter today.

The crack runs through the outer four plys – about 1/3 of the boom layers – and is 0.15 inches deep. The arm cannot fly “as-is” on STS-121.

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The crack was originally missed by ultrasound tests that were carried out last week, but later confirmed after the section was magnified during evaluations.

The damage occurred following an incident in which a small area was ‘bumped’ by an access bucket, which was being used to clear up fragments of a broken heat lamp from a previous mishap.

‘The crack was found with a 10X magnifier that the ultrasound missed. After finding the crack, the ultrasound was used again to verify it,’ reported an acquired e-mail from KSC. ‘The crack measures approximately 1.25 inches long diagonally (running lower left to upper right), .015 inches deep. The carbon composite is .050 thick. The smaller indentation farthest aft was nominal.’

While no cracks were found on the inner surface of the arm, another report acquired by this site notes that the arm will be removed for further tests.

‘An MRB was held this morning and everyone agreed that the boom could not fly as-is,’ the report noted. ‘The arm will be removed from the vehicle Monday.

‘To determine if there is additional damage to other parts of the arm, measurements from the arm strain gauges will be taken before and after removal to compare against the previous baselines, and the arm will be inspected at MDA.’

As previously reported, one option would be to replace the damaged arm with RMS 201 – which is currently allocated to Endeavour, or re-install RMS 303, once it returns from further testing.

Both options impact on the ‘current’ processing flow towards a May launch window.

‘(Option One:) Replace the damaged boom segment and fly arm 303, which is preferred to keep the instrumented SRMS. The arm will be shipped from KSC to MDA by next Friday, will be back to KSC by April 4th, and will be ready to install in the vehicle by April 19th. This option is only possible if the launch date slips.

‘(Option Two:) Swap arm 303 with arm 201. KSC is still assessing the last point this option can be chosen and still make the current launch date.

‘There is a team working plans to inspect/test the MPMs to determine if they’re ok to fly, but those plans have not been finalized yet.’



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