The primary goals of the second spacewalk are to:

• Install a video camera on the hull of the Unity node;
  
  
• Connect fluid lines leading to the S1's ammonia tank assembly; the truss has two tanks, each loaded with about 300 pounds of ammonia. Connecting these lines will permit ground controllers to run ammonia through the coolant system when it is activated next year;
  
  
• Finish removing launch locks holding the CETA cart in place;
  
  
• Install 25 of 31 "spool positioning devices," or SPDs, on all of the ammonia line quick-disconnect fittings currently aboard the station as well as the ones that will be used to connect ammonia lines between S1 and S0. The goal is to prevent potentially crippling problems on future assembly missions.

STS-112 lead EVA officer Oscar Koehler demonstrates installation of a spool positioning device to a quick-disconnect. Photo: NASA TV/Spaceflight Now

"What we determined in ground testing is that there are two seals in the fluid QDs," said lead spacewalk planner Oscar Koehler. "You can get a build-up of pressure between those QDs because if one of the seals leaks, the other seal will stop that fluid and you'll build up a pressure in there. You have to have a certain number of thermal cycles to raise that pressure to a level where it will basically lock up the QD so when you come to do maintenance later on, you can't get that QD off.

"So the work around is to basically take one of those seals out of the loop so if you have a leak - and we're talking very small leaks - you just leak past the other seal, you won't build up that pressure so when we come back to do maintenance, you can get the QD off."

The SPDs Wolf and Sellers will attach to each quick-disconnect fitting lock the handle, or bale, used to draw the two sides of a connector together so that only one of the internal seals can engage.

"They position the bale, the handle, so the valve is partly open and it's only open enough to take one of those seals out," Koehler said.

Additional SPDs will be installed on the next two shuttle missions to protect all of the station's planned coolant line quick-disconnect fittings. For this flight, 24 one-inch-wide SPDs will be installed, six 1.5-inch SPDs and one 0.5-inch SPD. All but two will be installed during the second spacewalk with the final pair installed during the crew's third excursion.

Two of these quick-disconnects will be fitted with the spool positioning devices on STS-112. Photo: NASA TV/Spaceflight Now

The work is important because successfully separating ammonia quick-disconnect fittings could be critical in the event of a future failure.

"If at some point we have a radiator leak or we have a radiator get damaged, we can actually remove that radiator and replace it with a new one," Koehler said. "And the QD is what allows you to disconnect your fluid lines from that ORU. Almost all the ORUs that have ammonia going to them outside have the same type of QDs."

Ammonia will not flow through S1's thermal control system until a later mission. But six QDs connecting coolant lines running between the Destiny lab module and the station's current radiators on the P6 solar array atop the Z1 truss are pressurized with ammonia.

If Wolf and Sellers have any problems installing SPDs on these connectors, they will simply press on with other tasks and astronauts on a future assembly mission will use a special tool to pry any stuck QDs apart as required.

Because of the possibility of a leak, both spacewalkers have practiced procedures to remove any residue of the chemical before re-entering the space station.

"Ammonia is the primary external coolant in the coolant loops of the space station and there is the potential for it to escape in the various operations we do," Wolf said. "First of all, it is not dangerous immediately to the space suit. The spacesuit can tolerate large amounts of ammonia. We don't want to contaminate the internal atmosphere, of course, when we come in. So it will sublimate or bake off.

"We try to get out in the sun first of all. We try to do those tasks early in the EVA so if we get contaminated there will be more time for it to bake off into the vacuum. We have some special brushes to brush it off. We even have detection techniques in the airlock."

If any residue is suspected, the astronauts will partially repressurize the airlock, take an air sample, and if necessary, depressurize the airlock again to allow additional time to bake out the chemical.

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