Abstract: Slow cook-off protection for rocket motors is achieved through the use of a slow cook-off trigger mechanism in which a mechanical response is thermally initiated. A metallic housing contains and is thermally coupled to a bimetallic snap action disc spring. A firing pin is connected to the disc spring and telescopically guided by the housing toward an expolosive booster charge. The booster charge is coupled in a conventional manner to a linear shaped charge. The linear shaped charge is disposed adjacent to the rocket motor case in order to cut a stress riser in the casing when activated. The circular bimetallic spring is designed to snap between a first and second configuration to thereby drive the firing pin into the explosive booster charge at a predetermined temperature below the expected ignition temperature of the rocket propellant. After the trigger is activated and the linear chaped charge cuts a stress riser in a rocket casing, continued temperature rise results ignition of the rocket propellant.