As STS-132 reaches its half-way point, astronauts Steve Bowen and Mike Good have ventured outside the Station/Shuttle complex to conduct the second of three planned EVAs of the mission on Flight Day 6, with the OBSS (Orbiter Boom Sensor System) PTU (Pan-Tilt Unit) cable repair and the beginning of the P6 battery R&R (Removal and Replacement) taking center stage.
EVA-2 / OBSS PTU Cable Issue Overview:
Facing a slightly altered EVA timeline due to the need to repair the cable routing issue on the OBSS PTU, Steve Bowen and Mike Good awoke along with the rest of the STS-132 crew at 0249 EDT.
After a brief hygiene break, Bowen and Good returned to the Quest Airlock where they spent the night performing an EVA campout.
Spacesuit purge was then undertaken before spacesuit prebreathe began. The ISS/Shuttle crew then began depressurizing the Quest Airlock ahead of the start of EVA-2 operations.
Flight crews generally work ahead of the scheduled timeline, as was seen with EVA-2, which was scheduled to begin at 0714 EDT – with Bowen and Good taking their spacesuits to battery power at 06:38 EDT.
Egress from Quest was followed by astronaut’s translation from Quest to Atlantis’ Payload Bay for work on the OBSS PTU cable snag issue.
Following FD-2’s failure of the PTU on the OBSS to properly position the LDRI (Laser Dynamic Range Imager) for inspection of Atlantis’ WLE (Wing Leading Edge) RCC (Reinforced Carbon-Carbon) panels, engineering teams at the Kennedy Space Center attempted to recreate the cable routing problem experienced by Atlantis’ crew on orbit.
According to KSC engineering documents acquired by L2, “We evaluated theories for how the cable might have become snagged in the first place in order to have some idea how hard it might be to release the cable. We were not able to duplicate the issue exactly, and couldn’t even get the cable jammed in there by physically trying to do so.”
However, it appears that the unit at Kennedy undergoing this testing is different than the unit on Atlantis.
“On the unit at KSC the cable memory always popped it right out. It appears by comparison that for the unit on-orbit the cable memory holds the cable close to or flat against the ring that surrounds the connector that goes into the side of the camera.”
On the flight unit on Atlantis, a screw on the ring in question that could have restrained the cable against the ring as the camera was tilted up. The screws on the flight unit then acted as a “wedge” for the cable between the Reed sensor.
“The cable seems to be flatter in one direction and fatter in the other based on the way the wires lie inside the outer covering, and the on-orbit cable appears to have the flat side lined up more with the snag point such that it would be easier to get it stuck than the one we looked at on the ground,” notes the KSC engineering notes.
While the KSC document notes that it would have been helpful to know if the cable was lodged only when the LSRI instrument was pointed up, they believe that pointing the camera in a downward, position roughly 30-degrees or more, will enable the cable to become dislodged – or dislodged enough that an EVA crew could move the cable out of its wedged location.
“In other words if this is the case we don’t think the crew would have to ‘unwedge’ the cable unless it has damaged the outer cable covering or something else that is causing a more real snag,” notes the engineering report.
“With proper cautions for fragile hardware & no touch areas, the crew should be able to manipulate the cables by hand to free the snag.”
Additionally, the crew on the ground did not identify the other cable in the area as the cause of the issue; in fact, they could not see how that cable could cause the problems in question.
Also, there does not appear to have been a misrouting of the cable.
STS-132 Specific Articles: http://www.nasaspaceflight.com/tag/sts-132/
Furthermore, in addition to their investigation work, the KSC team also identified several methods an EVA crew could use to free up the PTU.
“We evaluated 4 different restraint methods,” notes the report. “Cables need to be able to move/slide relative to each other with any restraint method – otherwise it puts drag on the PTU joints and may prevent full range of motion.”
In order to release the snagged cable, the PTU needs to be panned down 30 to 75 degrees from the nominal “stow” position. “This will ensure that the tilt magnet adjustment screw is not engaging the cable and forcing it into the read sensor groove,” notes the KSC presentation, available for download on L2.
However, “The PTU tilt must not be at the stop (down direction) as this will increase the likelihood of breaking the stop magnet during cable manipulation and cable tie installation.”
In all, the KSC team does not expect the PTU cable activities to take more than 15-minutes once set up of the area is complete. However, the team did not time all of the activities in detail.
Additionally, the KSC team noted that time at the end of the task should be set aside to redo some of the procedures in case the configuration the EV crew places the cables in is unacceptable or requires rework.
For the repair, the crew were able to move the PTU even though it is in a powered configuration. The tie mitigation effort did prove troublesome, but ended with engineers confident that movements of the PTU are now acceptable.
Following this, Bowen and Good spent the majority of the EVA time replacing the first three of six P6 batteries – or as many as the allotted EVA time will allow given the addition of the OBSS PTU objective.
After battery R&R is complete for the day, Bowen and Good headed into cleanup the work area and then reenter the Quest Airlock. The SSRMS (Space Station Remote Manipulator System) will then be maneuvered to a SARJ (Solar Alpha Rotary Joint) clear position to allow for rotation of the Port SARJ in between EVAs 2 and 3.
With a bonus get-ahead of completing work to secure the bolting of the SGANT antenna, the full and successful EVA ended after a duration of 7 hours 9 minutes.
An EVA tag up will be completed about one and one-half hours after the completion of the EVA. Then, the ISS crew will head to bed around 1749 EDT followed by the Shuttle crew at 1819 EDT.