X-Ray Computed Tomography for Solid Rocket Motors

In 1995, the U.S. Air Force ICBM System Program Office (SPO) opened its new Computed Tomography (CT) facility employing the Advanced Research & Applications Corporation, ARACOR, ICT 2500 CT system, to enhance the SPO’s Nondestructive Inspection (NDI) capabilities. The system was characterized by using various material and resolution phantoms. Determining how the CT system responds to each material in the solid rocket motor was critical. Equally important, is understanding how flaws in each material are manifested in the CT data. Solid rocket motor inspection procedures were developed using critical flaw size in each section of the motor. Initially the system was used for flaw detection only. Today CT inspection has been integrated into our solid rocket motor Aging and Surveillance program because it provides quantitative measurements of material characteristics in terms of density and dimension. CT is able to detect subtle changes in material properties and growth of flaws over time to identify any adverse aging trends. This requires baseline CT data on each rocket motor, precise material characterization, and CT system repeatability. An automated analysis is performed using the Automated NDE Data Analysis System (ANDES). ANDES detects flaws in the various solid rocket motor materials by using density (CT number) threshold values. While the CT analysts excel in pattern recognition and feature interpretation, computers excel in measurement and record keeping. ANDES complements human analysis and provides quantitative nondestructive information, allowing informed engineering decisions. Analytical tools and techniques available to the CT technicians include color mapping, contrast variation, and advanced image manipulation such as morphological filtering. Before and after each rocket motor inspection, system performance is measured by performing a CT scan of an Aluminum disk per American Society Testing & Materials, ASTM E 1695 specification. This data is used to derive Modulation Transfer Function (MTF), Contrast Discrimination Function (CDF), and Contrast-Detail-Dose (CDD) curves from the image. MTF, CDF, and CDD measurements are used to determine total system performance; defect phantoms are used to ensure CT technician’s analysis meets standards.