*Spacewalk Monday to prepare station for more equipment
*Locking pin installed upside down on last spacewalk
*Urine recycling system shuts down during test run
By Irene Klotz
HOUSTON, March 22 (Reuters) - NASA called off tests of the International Space Station's urine recycler on Sunday after problems developed and revamped plans for Monday's spacewalk to fix an improperly installed cargo platform attachment.
Monday's spacewalk is the last of three planned during shuttle Discovery's mission at the space station, which is nearly completed after more than a decade of construction.
The primary goal of Discovery's flight was to deliver and install the station's last set of solar panel wings, which was accomplished on Thursday. The shuttle blasted off from the Kennedy Space Center on March 15 for a 13-day mission.
With the shuttle fleet due to be retired next year, NASA is counting on the Discovery astronauts to also complete several maintenance tasks that will ease the burden on future crews.
During the mission's second spacewalk, astronauts Steven Swanson and Joseph Acaba loosened connections on batteries that will be replaced during the next shuttle mission in June and installed a GPS navigation antenna needed to guide Japan's new cargo ship into its docking port.
But the spacewalkers ran into problems installing an attachment needed to mount a cargo platform. Lead spacewalk officer Glenda Laws-Brown told reporters at a briefing at the Johnson Space Center that a locking pin apparently was installed upside down.
"As you know, when you get to the International Space Station there is no up and down. And my guess is they thought they had it in the right configuration, but because up is down and down is up, it was actually 180 degrees out from where it should have have been," she said.
The astronauts used tethers to secure the attachment and moved on to other tasks.
Monday's spacewalk schedule was already tight due to the cancellation of a planned fourth outing. NASA shaved a day off the mission and canceled the last spacewalk when Discovery launched five days late due to a hydrogen fuel leak.
The shuttle needs to depart the station by Wednesday to avoid a schedule conflict with a Russian Soyuz capsule carrying the station's next crew.
In addition to fixing the cargo carrier attachment, astronauts Acaba and Richard Arnold are expected to reposition one of the station's rail carts and rewire a circuit breaker so that a failure of one of the station's gyroscopes does not take two devices out of operation.
Engineers also were trying to figure out why a water purification system that recycles urine and condensate into drinking water was not working properly. The Discovery crew delivered a new distiller for the centrifuge-type device that was successfully tested without liquids on Saturday.
Problems developed during the first test run Sunday using urine.
"Once you closed valve 3 it didn't seem to flow," station commander Mike Fincke radioed to ground controllers. "I have no idea why that could be, so good luck with the trouble-shooting and we're standing by to answer any questions or be of any use."
NASA wants to have the urine recycler working before expanding the station's live-aboard crew from three to six members in May.
The shuttle is due back at the Kennedy Space Center on Saturday.
(Editing by Jane Sutton and Philip Barbara)