Each of NASA's Space Shuttles employs 38 position reaction control system (PRCS) thrusters to control attitude and position while on-orbit. If a chip occurs in its protective ceramic coating, burn-through could occur, thus creating a potentially hazardous situation. Currently, these thrusters are manually inspected using conventional visual methods. When a chip is found, a rubber replica is made and measured by hand in order to determine the depth and overall size of the feature. This process is laborious, time-consuming and subject to human error. NASA has conducted a program to significantly improve the technology by which the PRCS thrusters are inspected. The result is a fully automated, laser-based inspection system that is capable of three-dimensional mapping of the most inaccessible areas of the thrusters, the combustion chamber and throat region. The two-phase program began with the development of a miniature, precision laser-triangulation sensor. Laboratory tests confirmed the sensor to be capable of mapping features to an accuracy of better than 0.0127 mm. Phase two included the development of a robotic scanning system that provides four degrees of motion for negotiating the complex geometry of the thruster throat and combustion chamber. Each scan provides NASA with a photograph-quality image and profile map of the interior surface of the thrusters. Chips as small as 0.25 mm diameter can now be reliably located and characterized using this automated, laser-based mapping technology. This new NDT capability will allow NASA to quantitatively monitor the condition of each of its many PRCS thrusters while eliminating human error and subjectivity in this critical inspection task.
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