Tough Repair Job Continues

Expedition 24 Flight Engineers Doug Wheelock and Tracy Caldwell Dyson spent Tuesday completing preparations for their second spacewalk Wednesday out of the Quest airlock to replace the failed ammonia coolant Pump Module on the S1 truss of the complex while the International Space Station Mission Management Team gave its final approval to proceed with the spacewalk.

NASA managers decided to begin Wednesday's spacewalk one hour later than originally planned. The additional time will allow teams to fine-tune robotic procedures and get some extra rest. NASA TV coverage now will begin at 7 a.m., and the crew is scheduled to switch to battery power at 7:55 a.m.

Overnight, ground controllers activated the Gaseous Pressure Regulator Valve (GPRV) for the Loop A Nitrogen Tank Assembly, lowering the overall pressure in the plumbing for the inactive cooling loop in advance of Wednesday’s spacewalk by Wheelock and Caldwell Dyson. The GPRV’s pressure reduction will assist in the closure of quick disconnect valves at the S0/S1 truss interface Wednesday by Wheelock and Caldwell Dyson as part of the isolation of the line for the removal of the fourth and final fluid line connector holding the failed pump module in place. The lowered pressure will also facilitate the remate of all of the fluid lines once the new pump is installed. That activity is targeted for the third spacewalk no earlier than Sunday.

Once outside on Wednesday, Wheelock and Caldwell Dyson will isolate the Loop A ammonia cooling line upstream from that final connector by closing quick disconnect valves, then will use a tool to vent residual ammonia from the failed Pump Module before the final cooling line is disconnected. That will lead to the disconnection of five electrical and data cables and four bolts from the old pump so it can be extracted from the truss through the use of a grapple bar and parked on a payload bracket on the station’s Mobile Base System. If all goes as planned, the spare pump will be installed during the third spacewalk.

Wheelock and Caldwell Dyson conducted final timeline reviews Tuesday with flight controllers and moved into the Quest airlock just before 4 p.m. EDT to begin their overnight “campout” to reduce the nitrogen in their bloodstreams. They will be awakened at 2 a.m. Wednesday to complete spacewalk preparations and to suit up for their day’s work. This will be the fifth spacewalk for Wheelock, who will wear the suit bearing the red stripes, and Caldwell Dyson’s second spacewalk. She will wear the unmarked suit.

FMI: www.nasa.gov