Discovery Space Shuttle Faulty Rudder Speed Brake system
During the space shuttle recertification process, that took place for 2 years and 3 months after the space shuttle Columbia disaster, NASA discovers numerous problems that could have lead to more catastrophic failures resulting in loss of space vehicle and crew members. One mistake that was uncovered was when space shuttles Discovery's rudder speed actuator system was opened for the first time 20 years.
The rudder speed brake is a critical sequence to the space shuttle landing procedure.
Landing procedures of the space shuttles
When a space shuttle returns from space, it is in fact a high tech glider during descent and landing. With no power, slight course corrections and deceleration maneuvers are done with using the wing and body flaps and a rudder speed brake. The rudder speed brake is controlled with actuators in the tale sections.
As the space shuttle approaches the landing strip, the rudder speed brake is used for aerodynamic braking in order to slow down the space shuttle. At a descent angle 7 time steeper than a commercial jetliner, and traveling up to 220mph, right after the rear wheels touch down a drag shut is deployed followed by the nose gear touchdown then the rudder speed brake is fully deployed. 30 seconds after deployment of the rudder speed brake, all of the lading systems and maneuvers aid in the space shuttle slowing down to 68mph. At 68mph the drag shut is released and the space shuttle coast to a stop.
The tale section contains 4 speed brake rudder actuators controlled by a mechanical power drive unit. Each actuator contains a set of 9 inboard planetary gears and a set of 9 outboard gears. Two shafts connect power drive unit to the the actuators, which work together to open and close the speed brake flaps. If the rudder speed brake fails the space shuttles, Could be destroyed upon landing.
Inspection of space shuttle Discovery's brake actuators.
During recertification all 3 space shuttles, were at the Kennedy Space Center orbiter processing facility. When the actuators system is opened up on space shuttle Discovery, engineers discover corrosion in the 4th actuator and the planetary gears wear installed in a reversed direction from how they should have been. The outboard gears are slightly smaller than the inboard gears, and when installed correctly the 9 smaller outboard gears are in the top half of the actuator, and the larger 9 inboard gears are in the bottom section. NASA launched a investigation to find out how the speed actuator planetary gears are reversed as this mistake could also be present on space shuttles Atlantis and Endeavor.
Space shuttle faulty rudder speed brake
Incorrectly installed space shuttle brake actuator gears could be disastrous
When installed incorrectly, the gears reduce the over strength of the actuator because full gear tooth engagement is impaired. Only part of the teeth were engaging, therefore it results in a higher stress state for the part of the gear that is engaged and could result in a failure during flight. The orbiters would travel much faster than normal at touch down. When the drag shoot deployed the air resistance would have been much getter, the force would slam nose section into the runway. The massive high speed stress would make the front landing gear slam into the runway, with the front landing wheel braking off, resulting in loss of control.
Even if the drag shoot was not deployed to avoid front landing wheel failure, the space shuttle would run out of runway before it would come to a stop.
The deployment of the rudder speed brake is a critical sequence to the landing procedure. Discovery at time had flown 30 missions and landed successfully each time. The other 3 gears were installed correctly. It was the lowest load actuator that was put together incorrectly but it could have been just a matter of time before the gear failed.
Both Endeavour and Atlantis brake actuator assemblies were also inspect and no other gears were found to be installed incorrectly. When NASA changes out the actuator, they discover the same problem with the spare actuator. NASA engineers have since than developed solution to make sure the situation never happened again.
Assembly difficulties of the space shuttle brake actuators
During the original assembly of the gears into the actuators, technicians may have become confused for 2 reasons. The gears in each the 4 actuators are all installed differently as each actuator supports different loads. The top and bottom gear sets look almost identical in size. Technicians designed a special set of tools for each actuator, a bottom set of tools can only be used with bottom gears and a top set of tools can only be used with top gears. As long as the technicians use the new tool design, the gears could never be installed incorrectly again.
The initial requirement for the rudder brake system was to be certified for 100 missions with a space shuttle operational time of 10 years. However, The space shuttle flight schedule never was as aggressive as projected when the space shuttles began space missions in 1981. As space shuttle Discovery had only done 30 mission so it was still considered under the original certification. Engineering and operational certifications seem to dominate the design without considering all the different applications of assembly.