Three phenomenons，occurring in the experiment of φ90 subscale engines，have been studied，i.e，a) unbounding of the grain to plastic tube to insulate; b) rising of the aft section of the pressre-time history，and c) the precipitating of Al₂O₃ particles on the throat of the nozzle. The paper derives for mulae of estimating the effects of the precipitate on the history as the burning grain is in smoking. The designs to overcome the unbounding and precipitating have been shown.