The results of a combined experimental and analytical investigation of the first three natural frequencies and mode shapes of a four-stage solid-propellant research vehicle are presented. A full-scale vehicle was assembled, with fuel mass simulated, and placed on a suspension sys¬tem to simulate the free-free condition of flight. The agreement between the experimental 1y and analytically determined values for the mode shapes for all three modes and the natural frequency for the first mode was found to be very good. Differences of the order of 8 and 12 percent were noted between the calculated and measured values for the natural fre¬quencies of the second and third modes, respectively.nThe various stages of the vehicle were connected by screw-type Joints and an experimental study was conducted to determine the effect of Joint' stiffness on the first natural frequency of the vehicle. This was done by unwinding individual Joints between stages of the vehicle;The ini¬tial loosening of the Joint between the second and third stages resulted in a 20-percent reduction of the first natural frequency. Further unscrewing of the Joint did not cause further reduction of the frequency.
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