Troubleshooting plan created for STS-122 – includes tanking test

by Chris Bergin

Tuesday’s Program Requirements Control Board (PRCB) meeting has created a troubleshooting plan, one they hope will allow STS-122 to launch in early January.

The plan works on the basis of identifying the cause of the ECO (Engine Cut Off) sensor system issues during Atlantis’ two launch attempts. The recommendation of a tanking test – to be carried out next Tuesday – was presented to managers.

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NASA’s plan was drawn up to ‘determine the troubleshooting plan required to support the next launch attempt.’ In total three methods will be used to gather the required data and generate supporting rationale for the next launch attempt.

‘Flight hardware testing, STS-122, ETs, Orbiters, Flight Spares, Off-Line testing to understand possible causes, Test Beds, NDE, etc,’ noted the presentation. ‘Analysis of hardware configurations and environmental effects.’

The root cause of the problems is still unknown, though an opening candidate list has been drawn up, in order to try and pinpoint the problem that is downstream of the sensors themselves.

‘ET Sensor Failure Due To High Resistance Above False Dry Value Due To Broken Lead Wire. ET LLCO (Low Level Cut Off) Harness High Resistance Due To Broken / Damaged Conductor. ET Connector High Resistance Due To Recessed Pin or Socket,’ listed the 33 page presentation, dated December 11, and available on L2.

‘ET Connector High Resistance Due To Contamination that causes an Insulation Between the Pins and Sockets. ET Wire Splice High Resistance Due To Broken / Damaged Conductor. Tolerance Stack Up in feed-thru connector.’

Two options were laid out in the presentation. The second option worked on the basis of relying on data to try and find the component that is at fault, replacing it, and reconfiguring the vehicle for launch.

However, it was the first option, which includes carrying out a tanking test on the ET out at the pad, that was recommended to the PRCB. This option also includes the use of a Time-Domain Reflectometer (TDR), injecting a pulse into a line and watching for reflections.

TDR utilizes a method of recording the time between sending a pulse – and receiving a reflection – that will give engineers the length of cable between where they injected the pulse and the reflection occurred.

Because reflections occur at changes in impedance, by recording the amplitude of the reflection, the approximate change in impedance can be determined. This will allow for a plot of impedance, versus length along the cable, hopefully aiding the pinpointing of the open circuit.

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‘Perform cryogenic cycle of at least LH2 tank to develop a thermal profile across suspect components in order to induce and capture a failure. Develop a real time Time Domain Reflectometry (TDR) test to be performed during the cryogenic cycle. May require personnel access to the pad during a stable cryogenic condition,’ the presentation noted as option one.

‘TDR test location will be at a created splice point in the Orbiter AFT as to not disrupt vehicle connector configuration. TDR methodology requires characterization of the flight hardware, like flight hardware and TDR connections. Point sensor data will be available to the test team except during TDR.

‘Cryogenic cycle approach will be directed as a part of IPR (Interim Problem Report) troubleshooting. The IPR will be used to define test parameters. The vehicle will have the minimum configuration required to perform the cryogenic cycle troubleshooting.’

A ‘collaboration’ of tests is also noted by the presentation, which includes testing of old components – such as the LH2 feedthrough connector – at various levels of cryogenic levels.

‘Review ET-120 flight hardware past test results and determine any new cryogenic testing on components. Develop cryogenic bed testing of a standard feed thru connector configuration with varying moisture levels.

‘Develop cryogenic bed testing with off nominal feed thru connector configurations. Develop NDE (x-ray) capability for on pad ET feed thru/ET harness location. Develop hardware stock sweeps and lot date relationship of components. Determine tolerance analysis of open potential causes.’

The recommendation of utilizing option one is aimed at providing a launch opportunity ‘very early’ in January of next year. Currently the NET (No Earlier Than) launch target is January 2.

‘Key Decisions: Proceed with Option 1: Test under Cryogenic Thermal conditions recommended to most likely capture failed condition,’ added the presentation.

‘TDR plan to splice in the Orbiter AFT upstream of the New production break connector and take data during Cryogenic testing. Characterization of TDR process required prior to Cryogenic testing. Procedure development will drive readiness to test. IPR will be used to document configuration, parameters, requirements, etc.

‘Collaborative Test Suite being developed to enhance STS-122 testing and provide supporting data for potential causes. Upon review of both the Orbiter and ET options to break into the ECO circuit, the engineering community recommends the Orbiter option number 1 (which includes the splicing of associated wiring in the orbiter aft).’

Meanwhile, Shuttle manager Wayne Hale praised the troops for their efforts over the two launch attempts, from the engineers involved with the vehicle, to the managers involved with the two ECO-related Mission Management Team (MMT) meetings.

Transcripts of the two MMT meetings have been acquired by this site’s L2 sections, which gave a fascinating insight into a passionate, but highly professional debate.

‘Mr. Hale appreciates the team’s hard work over the last week. Are proud that at every step of the way, every position was presented and considered. We acted with only a very measured and thoughtful process.

‘In the end, it wasn’t a good plan for us to launch, so we didn’t. We’re on a path to find the cause of the problems in this particular system that’s given us so much trouble in the past. Have set a NET launch date of January 2, 2008.

‘That really is operationally the first date we could launch in the next launch period. Don’t intend to drive teams to solve it by then, but need a date to work against.’

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