Fracture Considerations for Surveillance Programs

Abstract

The structural capability of solid propellant grains has been defined in terms of grain safety margins. However, many rocket motors with structural flaws (bore cracks, unbonds, etc.) have been fired successfully within ballistic tolerances. This indicates that conventional grain safety margins do not define adequately conditions which limit structural response of a solid propellant gram. Use of fracture mechanics techniques has been included in some motor service life predictions and eventually will become a routine characterization and evaluation requirement for all solid rocket motors. Some of the available fracture mechanics techniques are summarized, and laboratory fracture test methods, which should be considered for motor surveillance programs, are reviewed. Use of these fracture mechanics tools, together with crack combustion analysis recently developed at the University of Utah, provides a realistic assessment of structural defects and motor ballistic performance.