ASSIST: Predicting Flaw Initiation and Propagation in Aged Solid Rocket Motors

ABSTRACT: When a solid rocket motor propellant grain contains one or more flaws (cracks, debonds, casting voids) or develops them as it ages, the risks of testing that motor is significantly increased. The responsible authority must decide between testing a motor that may destroy expensive testing infrastructure and relegating a motor for demilitarization that may not have failed from the flaw. Both circumstances waste precious resources. Such decisions demand a more accurate assessment of flaw propagation, so a tool that can provide that capability is critical. No such tool currently exists. ASI proposes to develop an innovative new tool, called ASSIST, that will provide that accurate assessment. We propose to couple fully transient, fully three-dimensional fluid and structural dynamic codes that can be used to determine what happens to a motor that contains a flaw. ASSIST will contain a detailed model for the combustion inside the flaw volume that determines, in large part, the stress/strain field induced in the crack tip. It will also contain a detailed propellant fracture mechanics model to determine whether the flaw will propagate, based upon the stress/strain field at the crack tip. We will validate ASSIST with an analysis of a large, flawed motor. BENEFIT: The Air Force will benefit directly from ASSIST through its application analyzing various strategic and tactical solid rocket motors in inventory that have been in service for long periods of time. In fact, one of the critical flaws that limit the service life of the Minuteman III first stage motor is the cracking that occurs in certain portions of the grain as the propellant ages. The ASI tool can be used to determine how much cracking can be tolerated before catastrophic failure will occur. The tactical motor inventory arguably has a more difficult service life, since thermal cycles and rough handling/flight loads are not well defined for any particular motor. That makes for a high likelihood of encountering a flawed propellant grain during routine service life testing. The use of ASSIST in those situations is of significant benefit to decision makers. Other DoD components can likewise benefit from ASSIST. Army and Navy weapon systems also have challenging life cycle environments that increase the likelihood that flaws will develop as the motors age. Again, the decision of whether to use or test such a system demands an accurate solution that ASSIST can provide. The Missile Defense Agency (MDA), in particular, faces the aging problem every day. That Agency uses aged Navy strategic assets (in excess of twenty five years in age) as the propulsive components of their strategic targets. They have got to know which motors are safe to fly and which ones need to be demilitarized. The cost of a single strategic missile defense test is in excess of $25M, so an unexpected failure from one of those aged motors is incredibly expensive. ASSIST will benefit those decision makers. In fact, any government agency that flies its own aircraft can likewise benefit from this analysis tool, because it is fundamentally an aeroelastic analysis tool. Repeated dynamic aeroelastic dynamic loading of aircraft wings can result in catastrophic failure. The ASI tool can benefit those agencies through a better understanding of those aeroelastic loads. Commercial entities can likewise benefit from ASSIST. Rocket-deployed parachute developers, such as BRS, Inc., use motors that age with time. The environments in general aviation aircraft that use their systems are benign compared to military environments, but the motors age nonetheless. They face exactly the same issues in determining the safe useful lives of their motors. As with the government agencies, commercial aircraft makers and even Formula 1 race car manufacturers can benefit through static and transient aeroelastic analysis of their aerodynamic wings. And, commerical jet engines need aeroelastic analyses of their turbine blades, as do those used in commercial gas turbine engines. ASSIST provides ASI with ample commercial opportunity, so we plan to pursue those opportunities vigorously.