Spacewalkers reconfigure station cooling system
Two astronauts aboard the International Space Station (ISS) conducted a spacewalk lasting seven hours and 48 minutes on Friday to return the cooling system on the outpost back to its original configuration.
The spacewalk, designated as Extravehicular Activity (EVA) 33, was performed by NASA astronauts Kjell Lindgren and Scott Kelly on Nov. 6, 2015. Lindgren was the lead spacewalker, known as EV-1, wearing the spacesuit with red stripes. Kelly was EV-2, wearing an all-white suit. They switched their suits to battery power at 5:22 a.m. CST (11:22 GMT), signifying the start of the planned six-hour thirty-minute spacewalk to reconfigure the cooling system back to its original configuration. By the end of EVA-33, most tasks were completed, but due to time constraints, a radiator, which was to be stowed, was left extended.
The reconfiguration comes a few years after a leak on in the port cooling system necessitated an EVA to extend the Trailing Thermal Control Radiator (TTCR) known as the “ticker”. The ticker is located on the P6 truss segment on the far port side of the station. It was originally launched in December 2000. It was temporarily placed on top of the Z1 truss segment just above the Unity module.
P6 is unique because it actually has three radiators on it, whereas its later launched sibling, the S6 truss – located on the starboard side of the Integrated Truss Assembly – only has one. This was because P6 was intended as a temporary all-in-one power and cooling system for ISS in its early stages of construction. In 2007, the truss segment was relocated to the port side of the P5 truss segment and two of the radiators, including the ticker, were stowed, as they were no longer needed.
A slow leak in the P6 truss was first noticed in 2006, but in June 2012, flight controllers noticed a sharp increase in the leak rate, but still relatively small. This leak was still large enough that the ammonia channel 2B cooling loop would have hit its minimum quantity limit within six months. This prompted engineers to believe that this leak might actually be a second leak, possibly the result of a debris strike.
In November 2012, an EVA was conducted to isolate the leak by deploying the ticker and isolating the primary radiator, called the Photovoltaic Radiator (PVR). However, this did not stop the leak and in May 2013, a large increase in the leak rate from channel 2B occurred prompting yet another EVA to replaced the channel 2B Pump Flow Control Subassembly.
This fixed the problem, but the ticker continued to provide the primary cooling for the P6 truss. For NASA engineers, this was not desirable as this leaves both the PVR and the ticker, which serves as a backup, exposed and at risk for debris strikes.
When the astronauts left the Quest airlock, Kelly brought a tool bag from inside the airlock and Lindgren retrieved a vent tool bag, which was stowed on the zenith side of the airlock exterior, before they both started translating to the P6 truss.
Once they arrived, the first task was for the two to remove a cover on the Fluid Quick Disconnect Coupling (FQDC) box, which was near the PVR on P6. They were to drive a bolt to open a valve in order to allow channel 2Bs ammonia coolant to flow into the PVR. Afterward, the FQDC cover was replaced.
Kelly then translated over to the P3/P4 truss segment where the port Solar Alpha Rotary Joint (SARJ) is located to begin installing an ammonia fill jumper cable to allow ammonia to flow from the Ammonia Tank Assembly (ATA) on the P1 truss to the port side past the port SARJ. This completed the fill pipeline which cannot be connected during normal operations as the rotation of the SARJ prevents that.
Lindgren stayed behind to begin breaking the connection between the PVR and the ticker by closing the valve between the two. After that, he began setting up the vent tool to allow excess ammonia in the fill line to prevent over-pressurization of the PVR system.
Once all the excess ammonia was vented, Lindgren connected the P5 jumper cable to the P6 truss to complete the fill line. Although there was enough ammonia in the system, the refill is intended to increase margins in case there are future losses. This fill took about 20 minutes.
While the fill was ongoing, Lindgren took a Pistol Grip Tool, which is an EVA version of a cordless drill, and began turning a bolt that would retract the ticker. After he completed that task, the ammonia fill was completed. He returned to the jumper cables to begin disassembling the pipeline and stowing it. This took about 17 minutes.
Lindgren then moved to the P5 jumper cable to begin configuring it to vent more excess ammonia in the fill line before stowing it as well.
While Lindgren was working on stowing the ammonia pipeline, Kelly began working to reconfigure the Starboard Crew Equipment Translation Aid (CETA) cart by folding and tying two brake handles. This CETA cart reconfiguration will allow the Mobile Transporter (MT), with attached CETA carts, to translate to another work site while still allowing for SARJ rotation.
After that, both spacewalkers moved to the port SARJ and drove bolts on two struts to reduce vibration signatures that ground teams were seeing. They believe the cause to be due to a loose strut.
Both Kelly and Lindgren were to then translate back to the ticker to attach cinches on the retracted radiator before installing a thermal cover to protect it from the space environment. However, due to time constraints, ground teams felt that it would be quicker to re-extend the ticker rather than leave it in an unprotected state.
After completing those tasks, the duo started heading back to the airlock with all their bags. On the way back, Lindgren reconfigured the ATA jumper to remove it from the fill line and connected it to the vent position. This would allow for a future venting if an over-pressurization event occurs.
By the time the crew arrived back at the airlock and reconnected their batteries to Quest’s systems, the total duration of the EVA had swelled to nearly eight hours. The hatch was closed at 1:04 p.m. CST (19:04 GMT) with the re-pressurization of the airlock beginning a few minutes after that. This completed the 190th spacewalk in support of assembly and maintenance of ISS. Crews have now spent more than 1,192 hours working outside the orbital laboratory.
Video courtesy of NASA
Derek Richardson has a degree in mass media, with an emphasis in contemporary journalism, from Washburn University in Topeka, Kansas. While at Washburn, he was the managing editor of the student run newspaper, the Washburn Review. He also has a blog about the International Space Station, called Orbital Velocity.