Dynamic Behaviour of Composite Structures Containing Viscoelastic Materials, with Applications to Solid Fuel Rocket Motors

Abstract

The object of the research was to study the effect of forced harmonic vibration on solid fuel rocket motors. A rocket motor consists of the motor case (which contains the propellant) and the blast tube attached to the aft end. The propellant is a viscoelastic material, no reliable data on the complex modulii of the propellants were available so a method was developed to measure it - the admittance method. This consists of vibrating a rod longitudinally, the applied force is kept constant the acceleration at the free end and the phase difference between force and acceleration are measured. The frequency is varied and a set of measurements obtained for each frequency. These data are then used in a computer program to iterate to the complex modulus necessary to give the particular acceleration and phase at the particular frequency. Thus values of complex modulus are obtained for each frequency at which measurements are made. The modulus thus measured was used to calculate (i) the acceleration and phase at the forced end of the rod in longitudinal vibration (ii) the acceleration and phase at both ends of the rod in transverse vibration (iii) the acceleration along the length of the full scale sample. The calculations compared very well with measured data.T  he rocket motor was then analysed as rigidly connected beams. The equations used were the Timoshenko beam equations which include the effect of shear deformation and rotary inertia. The unknown coefficients were found from the end conditions of the beams. Analyses were carried out on four different motor designs, experimental data were available for three of them. The calculated and measured results were in good agreement even though the data were not detailed enough to give an accurate representation of the constraints.