STS-117 Rollout – NASA to solve Atlantis SRB problem at the Pad

by Chris Bergin

Shuttle Atlantis has arrived at Pad 39A on Thursday, but the problem with the right SRB (Solid Rocket Booster) has not yet been solved, according to NASA memos acquired by this site.

The issue with erratic chamber pressure in one OPT (Operational Pressure Transducer) in the RH forward skirt area may require all six transducers – three on each booster – being removed and replaced while Atlantis and the STS-117 stack are sat at the Launch Pad.

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NASA and United Space Alliance workers allowed rollout to proceed with only a 24 hour delay after evaluating that work could be carried out at the Pad, as opposed to keeping Atlantis and the stack inside the Vehicle Assembly Building (VAB).

‘Here’s a brief summary of where we stand on the open issues,’ noted a NASA memo acquired by this site’s L2 sections late on Wednesday night. ‘Presently the Orbiter is powered down working rollout preps and platform retraction. The engineering community is comfortable rolling to the pad with the problems listed below.’

Although the OPT is not yet fully confirmed as the problem, it is thought that is the cause of the issue that arose during the Shuttle Interface Test, which also caused a few other minor electrical and sensor issues, tagged as a dropout of SRGA #4 SMRD indications.

‘Both of these anomalous events happened at the about same time during S0008 shuttle interface test,’ added the memo. ‘The apparent cause (subject to failure analysis confirmation) is that one of the RH SRB chamber pressure transducers – an OPT- shorted out internally and caused enough of a dip in voltage in the RH SRB forward skirt to cause the RGA SMRD indications to dropout.

‘The defective OPT has already been removed. A new one will be installed at the PAD (Note: The failure of this OPT may eventually lead to the R&R (Remove and Replace) of all six (3 per SRB) OPTs). Several internal SRB cables were demated as a result of T/S on this incident and will thus require retest.’

Plenty of contingency remains in the flow for Atlantis, ahead of her STS-117 NET (No Earlier Than) launch date of March 15. This will allow other work to be conducted, such as the replacement of the right SRB’s camera recorder, which was unrelated to the OPT short.

‘RH Camera recorder 2 indicated ‘off’; S/B.’ Will do an R&R of the camera recorder at the PAD. Not related or electrically liked to the other RH SRB IPRs (In Process Reviews).’

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As to exactly what has caused the shorts and faults in the right SRB is still being evaluated, but engineers noted that it could be related to ‘leaky voltage’ in an electronics box in the aft of Atlantis, possibly – although not claimed – in areas related to the Point Sensor Box (PSB), which also gained some attention during the ECO (Engine Cut Off) sensor issues on previous flights.

No issues with the ECOs were noted on STS-116, and it is believed that error was related to a bad batch of sensors on older External Tanks.

‘Indication that the Orbiter is supplying RH SRB Bus A Backup power toggled ‘ON’ for 1 sample when commands to supply this backup power were ‘OFF,” added the memo.

‘Measurements taken by EPD in Orbiter aft show that this erroneous indication is attributable to a phenomenon known as ‘RPC leakage’ which means that an electronic, solid state switch is not shutting off fully but is instead providing a small ‘trickle’ of voltage/current.

‘This particular ‘leaky’ switching device is located in Orbiter Aft-PCA-3. At this time, the local USA/NASA engineering community feels that the box is flight-worthy as-is, but this viewpoint is subject to PRT and engineering management concurrence.’

This is an ongoing evaluation and further tests will be required at the launch pad, including the possibility of more changeouts of related systems.

‘The affected systems have already been alerted as to the possibility of this R&R occurring at the PAD,’ ended the memo. ‘More to come.’

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