KSC Contact: Joel Wells
KSC Release No. 41-00
With only two weeks remaining in Space Shuttle Atlantis’ tight processing schedule, Shuttle managers asked the KSC workforce to change out a critical Shuttle component. The 320-pound power drive unit for the orbiter’s rudder/speed brake had failed a routine test, but replacing it would be anything but routine.
A team of KSC work planners, engineers and technicians assembled on April 10 to finalize the strategy for a task that they had never before executed at the launch pad. Disconnecting the bulky power drive unit with Atlantis in a vertical orientation posed the threat of air intrusion into the orbiter’s hydraulic system ñ an unacceptable condition for flight. Successful replacement required a tremendous amount of coordination. Cryogenic development, heavy lift operations, new work procedures and seven straight days of effort rested squarely on the shoulders of KSC’s workforce along with the standard launch preparations that remained.
“We weren’t that worried about each technical task,” explained Ed Mango, NASA Shuttle project engineer. “I think the most challenging part of this job was integrating the multiple tasks into one smooth plan.”
The rudder/speed brake is hinged to the orbiter’s vertical tail and allows the ship’s commander or pilot to control right and left yaw, as well as air speed during critical entry and landing maneuvers. The hydraulic power drive unit or PDU is located inside the tail, at the base of the hinge, and its job is to push a system of drive shafts and mechanical actuators. The PDU rotates the two-ply rudder/speed brake panels together for orbiter yaw control and flares them apart for air speed control. Without a good PDU, the rudder/speed brake will not function.
“It became clear that the part had to be changed out in order to fly, so we asked the team to develop a safe plan and to take their time implementing it,” said Dave King, KSC Director of Shuttle Processing. “With safety as our foremost goal, our folks made every effort to meet the scheduled launch date.”
While NASA and United Space Alliance flow managers labored over the PDU replacement plan, engineers at KSC’s Cryogenic Testbed Facility had to prove that a crucial part of that plan would work. To prevent air intrusion into the hydraulic system, workers would have to freeze the six titanium hydraulic lines that lead to and from the PDU before removing it. In only three days, KSC’s cryogenic test team designed, fabricated and tested a copper manifold used to freeze the PDU lines. Testing confirmed the feasibility of the process and proved that it would not harm flight hardware.
“After the first test, I had no doubt that we would be successful,” said Andreas Dibbern, NASA hydraulic systems engineer. “But our excitement grew as each step brought us closer to accomplishing something that we had never done before.”
Once the concept was proven, engineers at the launch pad wrapped the 6-foot long, º-inch diameter copper lines around the 5/8-inch diameter PDU hydraulic lines on Shuttle Atlantis. With liquid nitrogen flowing through the copper manifold at -320 degrees F, a 4-inch plug of fluid was frozen solid inside each line in only minutes. Workers could now disconnect the 2 feet long by 2 feet wide by 2 feet high PDU from the ship.
The burden then shifted to a team of technicians and heavy equipment operators who actually removed the faulty unit and replaced it with one that had been pulled from Shuttle Columbia at Boeing’s processing facility in Palmdale, CA. Managers mobilized a 250-ton crane, a 40-ton crane, and two cherry pickers capable of lifting about four workers each at Launch Pad 39A.
“We deal with heavy lift operations all the time – including lifting the orbiter. So we didn’t perceive this effort as a problem at all,” recounted Fred Pearson, vertical operations manager for United Space Alliance. “From the crane operators to the technicians, I have a very experienced crew. We just all came together to execute the plan. The system engineers really removed a lot of obstacles and made our job much easier.”
On April 12 at about 7:30 p.m., with workers positioned on the pad surface preparing ground support equipment, crane operators at the ready, supervisors and safety personnel on the Mobile Launcher Platform, and technicians in the cherry pickers, more than 16 people were postured to support. The actual PDU replacement took less than 4 hours and electrical/mechanical connections were completed the next day.
Test engineers in Firing Room No. 1 of KSC’s Launch Control Center conducted a retest of the Shuttle’s entire hydraulic system on April 15 and 16. Testing confirmed that the power drive unit had been replaced successfully with no air intrusion. Shuttle engineers at Johnson Space Center led a thorough evaluation of the initial PDU failure and determined that it was not a constraint for this flight. Additional PDU testing may be required prior to other flights. The failure and consequences will be fully evaluated at the prelaunch Mission Management Team Review scheduled for Saturday. The launch of the Space Shuttle Atlantis on mission STS-101 remains scheduled for April 24 at about 4:15 p.m. EDT.
NOTE TO EDITORS: Video of the PDU replacement effort and interviews with related personnel is available at the KSC Press Site. Still photos are also available at the Press Site or at Publishers Photo Corner on KSC’s website at www.ksc.nasa.gov
