Shuttle program manager Ron Dittemore holds a tip seal segement. Photo: NASA TV/Spaceflight Now

Workers will replace one of space shuttle Atlantis' main engines before the ship launches to the International Space Station next month because of another paperwork snafu.

NASA officials decided Wednesday to replace Atlantis' center main engine with a spare because defective nickel-alloy tip seals are probably installed inside the powerplant's high-pressure liquid hydrogen turbopump.

Each turbopump contains two tip seals amounting for six segments per engine. The seals run the circumference of the pump housing, about 18-thousandths-of-an-inch away from each of the two turbine blade wheels. The seals prevent airflow above the turbine blades to direct super-hot gases into the turbine machinery. The turbine shaft spins at 37,000 rpm.

The space agency says a paperwork mixup allowed two faulty seal segments to be used in the Atlantis engine instead of being thrown away.

The seal manufacturer was supposed to scrap the two segments after finding flaws when the pieces were produced.

However, the segments thought to be junk were recently found lying around and engineers tested them. The ultrasonic inspections showed the segments were fine, leading NASA to determine the bad segments were accidentally sent to the space agency.

Paperwork traced the defective parts to Atlantis' engine.

Officials uncovered the mistake during a recent inventory of about 100 tip seal segments in the shuttle fleet.

A main engine awaits replacement on shuttle Endeavour last December inside the Vehicle Assembly Building. Photo: NASA

The investigation was prompted after a seal segment delaminated and was rubbed by the turbine blades during shuttle Discovery's launch last December. Discovery's faulty segment should have been tossed into the trash too, but accidentally flew on three shuttle flights.

NASA managers discussed the issue at length before clearing Endeavour for launch last month knowing that paperwork could not be found for one seal segment aboard that shuttle. The documents were later located.

Officials are still debating whether to replace Atlantis' engine inside the Vehicle Assembly Building or once at the launch pad. If the engine swap is performed in the VAB, rollout will occur around March 24; if the work is done at the pad, rollout would take place on March 21.

Around sunrise today Atlantis will be backed from Orbiter Processing Facility bay 3 and rolled a quarter-mile atop a 76-wheel transporter to Kennedy Space Center's mammoth Vehicle Assembly Building.

Once inside the 52-story VAB, the shuttle will be lifted vertically and attached to an awaiting external fuel tank, twin solid rocket boosters and mobile launching platform.

The launch of Atlantis with a seven-person crew is expected no earlier than April 17 at 2302 GMT (7:02 p.m. EDT). A firm date will set during the traditional Flight Readiness Review that starts on April 4.

The 10-day flight will service the International Space Station and deliver a ton of supplies to the fledgling outpost.

The new crew patch for STS-101. Photo: NASA

The mission was recently revamped to perform maintenance on the Russian-built Zarya module, the first piece of ISS launched in November 1998. The work will extend Zarya's certified life to the end of 2000.

The laundry list of things the six Americans and one Russian will do includes replacing at least one and possibly as many as four of Zarya's batteries, along with other electronics for the module's power system. Astronauts will also change out fans, smoke detectors, fire extinguishers and a power distribution box forthe U.S.-built Unity node.

One spacewalk is planned to replace a communications antenna assembly mounted outside Unity that has not worked properly, attach the rest of the Russian Strela crane to the station's exterior and reseat a U.S. crane that has become loose over the past few months.

The flight will mark the first for Atlantis since undergoing an extensive tune up, including installation of a state-of-the-art glass cockpit.