Laser-based Mapping Technology for the Inspection of Rocket Thrusters

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 threedimensional 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, laserbased 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. Introduction: The primary reaction control system (PRCS) thrusters on NASA’s Space Shuttle (Figure 1) are subject to chipping in their protective ceramic coating. Technicians use a flashlight and mirror to inspect the combustion chamber and throat of the thruster after each flight. In the event that a chip is detected (Figure 2), measurement of flaw size is accomplished by creating a mold impression and measuring the features using an optical comparator. This visual inspection method is operator dependent and subject to human error. As part of its ongoing program to improve the inspection methods used on safety-critical Space Shuttle components, NASA recently funded a project to develop an automated, laser-based inspection system for PRCS thrusters. The objective of this program is to reduce the dependence upon the operator’s visual acuity and judgment through the development of an automated, highly accurate and repeatable thruster scanning system. Figure 1. The Space Shuttle has 38 PRCS thrusters located around the orbiter. Figure 2. Chip in throat of PRCS thruster. A Proof-of-Concept study was funded by NASA for the development of a miniature, highperformance sensor that could fit through the 50 mm diameter thruster throat and operate in the tight confines of the thruster combustion chamber. A prototype laser profile sensor (Figure 3) was designed, built and demonstrated to measure features to an accuracy of better than 0.01 mm (1). The sensor was evaluated on a retired PRCS thruster and was demonstrated to be capable of negotiating its geometry and the repeatable detection of chips in the ceramic coating. The successful demonstration of the prototype sensor resulted in the funding of a Phase II program, in which a full-scale scanning system would be developed and delivered to White Sands Test Facility in New Mexico, USA. Figure 3. Miniature laser profile sensor developed for NASA Theory: The principle of optical triangulation (2) employs the use of a light source, imaging optics, and a photodetector (Figure 4). The light source and focusing optics are used to generate a focused laser beam that is projected onto a target surface. An imaging lens captures the scattered light and focuses it onto a lateral effect photodetector, which generates a signal that is proportional to the position of the spot in its image plane. As the distance to the target surface changes, the imaged spot shifts due to parallax. To generate a three-dimensional image of the part surface, the sensor is scanned in two dimensions, thus generating a set of distance data that represents the surface topography of the part. Figure 4. Principle of optical triangulation. Laser-based profiling (LP) sensors are used to detect a variety of defects such as deformation, corrosion and pitting. In addition to generating a proximity-related signal, laser-based sensors also generate a signal that is associated with the reflectivity of the part surface. We refer to these data as the as LaserVideoTM images (LVI). By using a highly focused excitation laser and high data sampling resolution, these LVI images can take on near photographic quality. Through the process of mapping the variations in total reflected laser intensity signal, sensors can be used to detect very fine features such as small scratches, surface roughness and discoloration. LTC has developed laser-based inspection systems for a wide variety of NDE applications, including: • Mapping denting and ovality in nuclear steam generator tubes; • Measuring erosion in boiler tubes, heat exchangers and pipes; • Measuring erosion and pitting in high-performance gun tubes; • Mapping and categorizing corrosion pitting in solid rocket booster gas generators and other components. In each of these cases, the objective was to rapidly and accurately detect and quantify potential flaws in high-value, safety-critical components. Figure 5 shows an example laser-profile mapping of pitting in a component surface Figure 5. Laser-generated profile of corrosion on a surface. To illustrate the near-photograph quality of LaserVideoTM image, Figure 6 shows a pair of coins that were scanned with a laser sensor. The spatial sampling resolution used for this scan was 0.025 mm (0.001 inch) with a laser spot size of approximately 0.0125 mm (0.0005 inch) diameter. The LVI images reveals very fine features such as surface scratches that might, otherwise, be undetectable using other measurement methods. Figure 6. LaserVideoTM images of coins. Results: Several challenges had be overcome in order to provide NASA with an inspection system. A second-generation laser profiling sensor had to be designed that could the demanding requirements imposed by NASA. This sensor was approximately one-third smaller than the prototype sensor, and had to meet the following specifications: • Measuring Range: 4.0 mm