STS-132′s OBSS cable snag sets up potential mitigation plan

Following the snagged cable incident on Atlantis’ Orbiter Boom Sensor System (OBSS) Pan Tilt Unit (PTU) during STS-132, the Program Requirements Control Board (PRCB) have outlined their investigation findings, and a potential mitigation plan. Later this month, engineers will test the addition of a permanent cable tie, in order to avoid future snags.

OBSS PTU Snag:

The OBSS and its sensor package are one of the unsung heroes of the Return To Flight (RTF) era, allowing for post-launch and pre-entry inspections of critical area of the orbiter’s Thermal Protection System (TPS).

Teamed with an arsenal of imagery footage and expert engineers with the Damage Assessment Team (DAT) in Houston, all post-Columbia missions have successfully returned with added – and justified – confidence in the health of the heatshield.

The evaluations into the TPS begins as soon as the shuttle launches, with ground-based cameras and radar tracking debris hazards, working in tandem with orbiter based assets, such as the ET cameras and crew photography footage of the tank – the main source of debris threats.

Flight Day 2 is highlighted by the first use of the OBSS during the mission, focusing on three primary areas; the nose cap and the port and starboard Wing Leading Edges (WLE) – ensuring the Reinforced Carbon Carbon (RCC) panels avoided damage during ascent.

STS-132′s FD2 inspections began without issue, with the OBSS unberthed, followed by the activation and calibration of the Laser Dynamic Range Imager (LDRI) on the boom’s sensor suite. A quick check for ice on the starboard T-0 umbilical was followed by a robotic translation to the start point for the starboard wing survey, which revealed an issue with the PTU.

“Background of Issue: During the beginning of STS-132 FD2 Inspection procedures, PTU readings and behavior were erratic and eventually lead to determination that it was not able to move the entire tilt range,” noted the PRCB presentation, available on L2.

Ground controllers asked the crew to reset the PTU in an effort to correct the issue, prior to the crew visually noticing a cable – known as the W601 cable – was snagged on the unit’s protruding reed sensor.

“All attempts to resolve were unsuccessful: Crew manipulation, ground control manipulation, change in PTU speed. PTU was unable to point to the tilt values required to support FD2 Inspection. A cable snag was confirmed by the crew viewing PTU motion from the Flight Deck windows.”

Despite losing the PTU’s full range of motion, most of the survey was completed, with a plan to utilize Flight Day 3′s R-bar Pitch Maneuver (RPM) – aided with additional ISS crewmembers, armed with cameras – to capture additional imagery footage for the DAT engineers.

Meanwhile, the Mission Management Team (MMT) asked KSC engineers to look into how the snag could have occurred, checking closeout photos of the OBSS PTU ahead of Payload Bay Door (PLBD) closure for flight, along with flight history and checks into replaced elements of the hardware.

“Cable Flight History: Sensor Pack 1 (SP1) with W601 Cable S/N (Serial Number) 1002 is the same unit that flew on STS-130. Prior to STS-132, SP1 assembly required a PTU change out – requires disconnect and re-connect of W601 cable (same cable, S/N 1002),” added the presentation.

“Full pan and tilt range was confirmed post re-assembly during the STS-132 Pre-Installation Acceptance (PIA) testing with no issues of a cable snag. Installation procedures and close out photos were reviewed – no indication of incorrect installation. S/N 1002 Cable has flown on STS-130, STS-128, STS-125, STS-123/124, STS-115, STS-114.”

With the MMT noting their preference for the full use of the PTU to be restored, a cable tie plan was implemented on EVA-2 of the mission, freeing and holding back the snagged cable, and ultimately allowing for a nominal Late Inspections to be carried out without issue.

“In-Flight Resolution: During EVA2, the cable was physically removed from the snag point and an additional cable tie was installed to preclude the snag from reoccurring. Full PTU pan and tilt motion was restored and Late Inspection was performed nominally.

“Photos of flight hardware post STS-132. Red circles are area of snag occurred.”

Post Flight Troubleshooting:

Once Atlantis was back inside her Orbiter Processing Facility (OPF-1), engineers began a hands-on inspection of the PTU by removing the unit from the OBSS, part of the nominal procedures for preparing the PTU for its next flight, although this time including the enforced changeout of the troublesome cable.

“Troubleshooting Plan: Post flight activities – remove from vehicle, post flight functional of LDRI, manipulate Pan and Tilt to verify proper cable movement with added cable tie, cut added cable tie, manipulate SP1 assembly to attempt and recreate snag,” the presentation continued.

“Replace W601 Cable S/N 1002 with S/N 1005, confirm W601 cable S/N 1005 cable characteristics during full pan and tilt motion. Per Pre-Installation Acceptance procedure pan/tilt values. Per on-orbit operational pan/tilt values, determine if additional cable tie required, perform STS-133 Pre-flight PIA in prep for OPF testing.”

Implementing part one of the plan, engineers cut the cable tie installed during EVA-2, and attempted to perform the movement of the PTU with the old W601 cable that snagged during FD2 inspections. Just like it did on orbit, the cable snagged several times during ground testing.

“Performed troubleshooting on 6/16/10 in lab at KSC. Verified proper cable movement during PTU motion with cable tie attached. Cut added cable tie. Cable retained some memory of the added cable tie adjusted configuration thus cable had to be manually manipulated to re-create the pre-launch configuration,” noted the presentation.

“Cable laid flush against the PTU tilt ring at +102, -252 (pan, tilt). Moved PTU pan to +85 and the cable remained against the tilt ring. When the PTU was tilted up, it was observed that the cable snagged against the fixed electronic stop (Reed Sensor).

“It was concluded that the cable had to have been laying flat against the tilt ring at +102, -252, when the crew began panning and tilting in slow rate in order for the cable to snag. Able to recreate this scenario multiple times using W601 cable SN 1002.”

Engineers then replaced the old W601 cable with a new spare, re-performed the movement of the PTU, and successfully avoided the cable snagging during the tests.

“With successful recreation of the issue, W601 cable S/N 1002 was changed out with S/N 1005. Motion of PTU and cable characteristic was verified – values per PIA procedures, which include full range of motion, and on-orbit operational values.”

Forward Plan:

As to what caused the old cable to snag on the reed sensor during STS-132, and how can such a situation be avoided in the future, engineers checked to see if the W601 cable was somehow being pressed against the PTU after the Payload Bay Doors have been closed – in other words, if hardware was breaching into the space (envelope) of where the cable was positioned.

“Envelope Measurement History: Each of the three fully assembled SP1 units were measured for dynamic envelope to confirm clearance to the radiators and to any stowed payload,” added the presentation.

“Current Active Sensor Pack 1s: S/N 1003 with W601 cable S/N 1005. S/N 1007 with W601 cable S/N 1003. During this activity, it was noticed that each W601 cable has its own characteristics with respect to routing, translation and memory.”

With those findings – showing it may be an uncontrollable issue if left as-is – the Orbiter Project Office (OPO) will test the potential of added an additional cable tie, in order to guarantee the W601 cable avoids being pressed into a position that threatens it subsequently being snagged on the reed sensor.

“OPO has requested to assess the envelope measurement with the additional cable tie. Pending confirmation of positive envelope impacts, cable tie will remain in place for flight,” confirmed the conclusions to the presentation.

Testing on the viability of adding a new cable tie will take place during the week of July 26.

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