Abstract: A thin frangible cork barrier conforming to the contour of the burning surface of individual solid propellant grains in a multiple grain rocket motor prevents premature ignition of the protected grain by the heat of combustion of previously ignited adjacent grains and is expelled through the rocket motor nozzle upon ignition of the protected grain. In a second embodiment, the thermal protection afforded by the frangible cork barrier is augmented by bonding to a thermally-protective elastomer layer which is in turn bonded to the propellant grain burning surface. By virtue of the composition and structure of the embodiments, thermal protection is afforded. Upon ignition of a protected grain the thermal barrier breaks up so as to pass readily through the throat of a small motor nozzle.