Iii-Flight Measurements of the Vibration History of Spin-StabilizedProjectiles

Abstract

M spin-stabilized projectiles commonly contain solid but nonrigid or liquid pay loads. These types of pay loads often produce projectile/pay load interactions that are undesirable. Vibrations of the projectile/pay load assembly could impair the performance of military fuzing, especially if that fuzing is mechanical in nature. In some instances, the timer is mechanical, but normally all fuzes have mechanical arming and safety devices. Projectile designers during test sequences often report fuze malfunctions, but little is known about the vibration environment of an artillery projectile during flight. In response to this need, an instrumentation package consisting of a strain gage bridge/attitude sensor and a telemetry unit was built and successfully tested. Several types of units were flight tested and data showing undesirable vibration histories during the flight of a projectile have been recorded.' Data from 155mm projectiles with liquid payloads are discussed. A standard yawsonde body was modified to accommodate a Wheatstone bridge. The bridge was located on a flat machined onto the cylindrical portion of the yawsonde body which extends into the ogive of a parojectile (Fig. 1). This flat was located 90 deg from the optical sensors of the yawsonde. Wires that provide voltage input and resistance measurements were located in a groove. These wires were brought through a hole in the groove to the interior of the yawsonde. The strain gages were semiconductor types and required amplification for telemetering. The output of the bridge circuit was not calibrated in such a fashion as to provide meaningful measurements of strain. Rather, the outputs of all units were adjusted for maximum sensitivity to variations in strain, i.e., vibrations. The output of the bridge circuit will be directly affected by the spin and yaw of the projectile; hence, yawsonde data are needed to interpret the vibration history of a particular projectile flight. The angular and spin motions of a projectile are traced by a yawsonde. The projectile motion is presented in terms of oN and 0 vs time. The complement of the angle between a vector drawn to the sun and a vector aligned with the spin axis of the projectile is aN, while is the derivative of the projectile's Eulerian roll angle with respect to the sun plane, i.e., the plane containing the missile's axis and the sun. The output of the strain gage bridge is an indicator of a fluctuating strain, and a spectral analysis can reveal the frequency content of these fluctuations. The presence of a particular frequency component implies that the projectile is experiencing a measurable mechanical strain at that frequency. For example, all projectiles experienced strains produced by the spin and yaw motions at the spin and yaw rates. As seen from Fig. 2, the Fourier transform of the bridge