NASA managers today opted to replace suspect seals in the hydraulic system of the shuttle Discovery's right main landing gear strut, work that could delay launch on a space station assembly mission by a few days.

In an unrelated development, agency managers today also officially approved the addition of a fifth spacewalk for Discovery's mission to test a heat-shield repair tool that could prove useful in the event of damage like the tile gouge experienced during the last shuttle mission.

The additional spacewalk makes Discovery's mission one of the most ambitious space station assembly flights yet attempted. Along with delivering a new multi-hatch 31,300-pound module to the lab complex, the astronauts also will detach a set of stowed solar arrays, move it to the far end of the station's main power truss and then re-extend the huge panels.

Discovery's mission sets the stage for delivery of European and Japanese research modules scheduled for launch late this year and early next. Those modules will be attached to Harmony and as such, Discovery's mission is a long-awaited milestone for NASA's international partners.

But the schedule for launching the European Space Agency's Columbus module aboard the shuttle Atlantis in early December was already tight and with the unexpected landing gear work on Discovery, NASA will not have much, if any, contingency time left to handle additional problems with either mission.

The right main landing gear hydraulic leak was discovered during routine testing in preparation for the shuttle's rollover from its processing hangar to the Vehicle Assembly Building this week for attachment to an external fuel tank and solid-fuel boosters.

The allowable leak rate for the seals in question is one drop of hydraulic fluid per hour. When jacks holding Discovery off the hangar floor were lowered and the full weight of the orbiter was put on the landing gear, engineers initially noted a "weight-on-wheels" leak rate of 285 drops per hour.

After cycling the landing gear multiple times, the calculated leak rate decreased to just 23 drops per hour. But that was still far beyond specification and after reviewing the issue today, LeRoy Cain, manager of shuttle integration at the Kennedy Space Center, ordered both seals replaced.

The work will require engineers to remove the right-side tires and brakes and disconnect the hydraulic system and instrumentation before replacing the seals.

"The vendor (Goodrich) arrives tomorrow, the replacement seals arrive Wednesday," said a NASA spokesman. "Between now and then they'll have the vehicle prepped and ready to change those out."

Rollover to the Vehicle Assembly Building had been planned for Wednesday with rollout to pad 39A on tap Sept. 27 and launch Oct. 23.

The landing gear work now is expected to delay rollover a week or so, to around Sept. 25 or 26. The launch team had five days of contingency time built into the processing schedule. Taking that time into account, the seal replacement could delay launch by two to three days.

But officials said today it's too early to predict what impact the work will actually have. Initial NASA and contractor schedule estimates in cases like this tend to be conservative and engineers frequently do better than initially expected. But that remains to be seen.

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