Effect of motor length and propellant formulation on nozzleless solid rocket performance

Abstract

Experimental results are presented for four nozzleless motors of different length-diameter ( L/D) ratios using two different composite propellants. The experimental observations discussed are: the premature unchoking in motors of insufficient L/D ratios and the tendency for the propellant to extinguish under highly negative pressure gradient environment, both peculiar to nozzleless operation. A simple one- dimensional numerical scheme is presented to predict the performance of a nozzleless solid motor. Erosive burning, elastic grain deformation and L/D ratio-dependent combustion efficiency are considered in the scheme. A relatively simple procedure is followed to account for the coupling effect between port gas dynamics and elastic grain deformation. The experimental results are compared with those predicted by the numerical scheme. The predictions are in reasonable agreement with the experimental values. For a composite propellant the burning rate index n can significantly vary with respect to pressure. The application of this fact plays a very important role in accurately predicting the thrust-time trace of nozzleless motors.