Shuttle launch delayed again amid station arm mystery
Posted: June 7, 2001

Shuttle Atlantis is lowered into position for attachement with the external fuel tank inside the Vehicle Assembly Building. Photo: NASA-KSC
NASA officials have again postponed the launch of space shuttle Atlantis on the next international space station construction mission while engineers struggle to understand problems with the outpost's new robotic arm.

Liftoff of Atlantis carrying the $164 million airlock for Alpha is now set for no sooner than July 7 at 7:03 a.m. EDT (1103 GMT) from Kennedy Space Center, Florida.

The launch could slip further, however, if more work is needed to get the arm fully functional before Atlantis' crucial visit. The Canadian-made arm will be used to hoist the 12-ton airlock from the shuttle's payload bay and mount it to station.

NASA has until July 17 to get Atlantis off the ground before a solar constraint prohibits the shuttle from visiting the station through Aug. 3 because temperatures would be too high for the shuttle's systems.

Engineers have spent the last few weeks studying two problems that have struck the arm during testing by the station's Expedition Two astronauts.

The first one with the arm's Wrist Roll Joint caused the brakes on suddenly engage without being commanded to do so. But that glitch has never repeated itself despite dozens of attempts to recreate the problem.

The more serious trouble is an intermittent loss of communications between the arm's backup controller unit and the Should Pitch Joint electronics.

The communication dropouts occur on the redundant side of the arm's power and control string. The primary side has worked flawlessly.

When the problem manifests itself, the arm's entire backup side is rendered useless.

Animation shows Canadarm2 hoisting the airlock from the shuttle and mounting it to the international space station. Photos: NASA
The arm was tested again this week after a diagnostic software patch uplinked to the station. The problem reoccurred during the first attempt to operate the arm on the backup string.

But as mysterious as the problem itself, the arm worked just fine during the next 12 tests.

"We are not declaring victory," NASA flight director John Curry told reporters Thursday. "We want to get more and more confident that there's not some inherent problem with the arm and that everything is okay."

So now NASA and the Canadians have to figure out why the problem cleared up, as well as ensuring it won't occur while the arm is used during the delicate task of installing the airlock.

"When we get these situations that occur where a failure manifests itself for some period of time and then all sudden just goes away magically -- well, nobody likes magic in the flight control world," Curry said. "We want to understand what's causing the problem."

There are three options being worked on by engineers:

  • Developing a software patch that will "mask over" the communications dropouts between the Arm Controller Unit located near the crane's elbow and the Joint Electronics Unit. That would be designed to prevent the arm from being sent into safe mode when the failure occurs.

  • A second software "mask" would make the backup computer system believe the Should Pitch Joint does not exist, allowing the arm to continue to function on just six of its seven joints. This would allow the airlock installation go forward with the normal plan with all seven joints on the primary string, but if the backup side were employed the Should Joint would never be activated so the communications glitch wouldn't occur.

  • The last ditch effort would be mounting a repair mission to replace the Should Pitch Joint before the airlock is launched. Such a repair would call for the next two shuttle missions to flip-flop with Discovery launching on Aug. 5, followed by Atlantis with the airlock in the Sept. 22-to-27 time frame.

Atlantis remains inside Kennedy Space Center's Vehicle Assembly Building awaiting rollout to the launch pad. NASA doesn't want the shuttle moved to the oceanside pad until it is clear when Atlantis will lift off, particularly if the next two missions are reversed in the line up.

Station officials are reluctant to flip the missions since the Discovery mission's main goal will be ferrying the Expedition Three crew to the station and returning Expedition Two to Earth.

That would leave the airlock installation job to the Expedition Three astronauts, who have not been trained for the task.