Shuttle Trouble & The Space Station

At almost the same time NASA managers were setting April 24 as the official launch date for shuttle Atlantis on a space station repair mission, engineers were running into a potentially significant problem with the spacecraft's dual-function rudder/speed brake, sources say. During a test of the rudder's actuator system at launch pad 39A late on Wednesday, engineers saw pressure readings of some 1,200 pounds per square inch instead of 400 psi as expected.

A senior shuttle engineer said the rudder's power drive unit, or PDU, likely will have to be replaced. But it's not yet clear whether the PDU can be replaced at the launch pad with Atlantis in a vertical orientation.

If the shuttle must be hauled back to its processing hangar for repairs, launch probably would be delayed at least two weeks and possibly longer. But that is strictly a worst-case scenario and engineers may, in fact, be able to make repairs - if needed - at the launch pad.

But they will have to be quick. NASA only has three days to launch Atlantis - April 24, 25 and 26 - before losing Air Force Eastern Range support until mid May. The Eastern Range provides radar tracking, photo documentation and self-destruct functions for all rockets launched from Florida. If Atlantis is not off the ground by April 26, the flight likely would slip to around May 11, after two military operations: a Titan 4 test and another classified operation. The launch of NOAA's latest GOES weather satellite is on May 3 and launch of a missile early warning satellite aboard a Titan 4B rocket is on May 8.

It typically takes two days to reconfigure the range to support another rocket launch and the next available slot for the shuttle comes on May 11, 12 and 13. After that, a Lockheed Martin Atlas 3 rocket has the range booked to launch a Eutelsat communications satellite on May 15.

Again, all of that is strictly worst case and the shuttle's rudder/speedbrake problem may end up going away or proving fixable at the pad in time to make an on-time April 24 launch attempt.

But until this issue is resolved, several issues involving Atlantis's launch date and the orbit of the international space station are important.

The space station currently is losing about 1.5 nautical miles of altitude per day due to atmospheric drag, a braking effect produced by the spacecraft's passage through the extreme upper atmosphere at a relatively low altitude. Atlantis's crew plans to use the shuttle's maneuvering jets to boost the station's altitude by some 30 nautical miles during the upcoming mission, using about 1,200 pounds of propellant. Three reboost rocket firings will be required.

NASA originally planned to boost the station's altitude by just 10 nautical miles in a single set of rocket firings. But that was before Atlantis's flight slipped from March to April and before strong solar activity acted to increase the effects of atmospheric drag.

If Atlantis's launch is delayed past the second week in May, ASA and the Russian Space Agency likely will be forced to boost the station's altitude by using the Russian Zarya module's maneuvering engines, burning up precious on-board fuel in the process. While that is not a problem per se, flight controllers want to protect the station's on-board fuel supplies as much as possible until launch of the Russian service module in July. The refuelable service module then will take over the station's propulsion chores.