Removed connector key to finding root cause of STS-122’s problems

by Chris Bergin

A major effort is continuing on pinpointing the specific root cause of the Engine Cut Off (ECO) sensor system issues, responsible for the delay of Atlantis’ launch on STS-122.

Data continues to be evaluated, alongside ongoing testing at various NASA centers, on a fault that occurred on an unspecific location of the LH2 Feed-through Connector inside the External Tank. The connector assembly is to be removed and sent away for testing.

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Troubleshooting Effort:

NASA engineers managed to rule out various ECO hardware – such as the sensors themselves – from the fault tree, via the Time-Domain Reflectometry (TDR) equipment installed into the stack during Tuesday’s tanking test.

While the TDR data showed that the fault is associated with the Feed-through connector system, it is still not known exactly which element of the hardware is at fault, or what is causing the issue to occur during tanking.

‘Feed-through High Resistance Due To: Recessed Pin/Socket. Improper Mate. Corrosion. Contamination. Defective/Oversized Socket. Bent Pin. Defective/Undersized Pin. Connector Shell/Insert Separation Or Locking Ring Separation…

‘Faulty Shell, Insert, Or Backshell. Faulty Connector Grommet Or Interfacial Seal. Tolerance Stackup. Improper Wire Termination (Harness),’ noted documentation on the candidates, before also noting that: ‘Harness failures near Feed-through connector are still possible until ruled out.’

Tests have already been carried out on another connector, previously removed from ET-120, which underwent an array of investigative techniques, including X-rays and cryo testing under lab conditions. Strangely no issues were found.

Removing the connector from Atlantis’ ET will be key to finding the exact cause of the open circuit, spotted via the TDR testing this week. However, engineers already have theories in place, mirroring those assumptions made prior to the tanking test.

‘Scenarios associated with open circuit at Feed Through Connector: Contamination of contacts between external/internal plug and feed through pins. Contamination includes process induced (i.e., leaktec or Krytox), moisture, and liquid air,’ noted one of several presentations that have been created since the tanking test.

‘Internal movement of plug sockets could induce failure: Movement can be caused by internal or external forces acting on connector. When connector is subjected to cryo conditions, the internal or external harness induces tension on the associated sockets, which disengages the contact from the feed through connector pin.

‘External Foam could contract increasing load on socket. Tolerance build-up in connector harness assembly leads to insufficient pin to socket contact. Connector Shell/Insert Separation or Locking Ring Separation as a result of separation forces (1) or release of locking mechanism (2) from foam/moisture barrier/ET contraction under cryogenic thermal conditions.’

TPS (Thermal Protection System) foam will be removed from around the connector on the tank, ahead of removing the hardware – which will then be sent off to the Marshall Space Flight Center (MSFC) for tests.

United Space Alliance (USA) engineers have been preparing access to the area of the tank, ahead of today’s arrival of a MAF (Michoud Assembly Facility) supervisor, who’ll guide the team through the delicate operation of removing foam from around the exterior of the feed-thru box area of the tank. A larger MAF team will arrive at KSC on the 27th of December for the full R&R procedure.

With testing of the current data from the tanking test still being evaluated, a full plan of action is yet to be drawn up.

This sees the R&R process being carried out in two steps, with foam removal taking place prior to Christmas, and the actual removal of the Feed-through assembly to be mapped out by managers on the 27th of December. It is still to be decided just how much of the hardware will be removed from the tank.

‘KSC to perform feed-thru connector box exterior foam removal before Christmas,’ noted processing information on Friday, which also came with a methodical set of instructions for the R&R process. ‘MAF crew will arrive on site ready to dissect internal foam and remove feed-thru on 12/27.’

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Interestingly, it is uncertain if NASA managers will make any decision on a new launch date target for STS-122 at the next PRCB (Program Requirements Control Board) meeting on Dec 27, due to volume of data that is still being evaluated.

It is likely some data will be outstanding until ET-125’s connector has been put through testing at MSFC’s Cryo Lab – which won’t be until after the meeting.

However, the current plan is to continue to push ahead with ensuring STS-122 is in the best possible posture for the nearest achievable launch date, with documentation showing the recommendation to go ahead with the ‘cherry picking’ of the best available Feed-through components, ready to be installed into ET-125, and ET-126 – the latter currently inside the VAB (Vehicle Assembly Building) for a now-delayed mating with Endeavour’s STS-123 stack.

‘Further Define MSFC Cryo Lab Test with Feedthru Configuration Based on Tanking Test Results. Cherry Pick Feedthru components for possible use in STS-122,’ noted the latest forward plan overview. ‘Determine Access potential for ET-126 Feedthru in Integration Cell. Constraint to ET-126 Mate.

‘Continue To Refine Tanking Test Data – Better understand Failure mechanism with High Speed voltage measurements. Refined TDR Results. Partner ECO Troubleshooting Team with ET Project to Recommend Repair/Fix/Flight Rationale for ET-125 and ET-126. Reorganize ECO Near Term Team to Efficiently look at Repair/Fix options and implementation at KSC.

‘Next Checkpoint expected at a Special PRCB Dec 27.’

Further updates/articles will follow…

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