Flight Instabilities of Spinning Projectiles Having Nonrigid Payloads

Abstract

Severe flight instabilities were experienced by an Army spin-stabilized projectile which had a partialsolid/partial-liquid pay load. Characteristic of this flight instability was a sharp increase in projectile yaw angle accompanied by an abrupt loss in projectile spin rate. It was known that this instability was due to movement of the nonrigid payload, but the exact mechanism causing the effect was not understood. A special laboratory test fixture was used to force a full-scale projectile payload to simulate the combined spin and simple coning motion of the projectile in flight. The fixture was used to determine critical factors influencing the payload-induced despin moment, and from this the associated flight stability was inferred. Candidate payload configurations intended to eliminate the instability were evaluated on the fixture, culminating in a payload design which provided the desired functional and flight performance. Subsequent fixture tests with homogeneous, viscous liquid fills produced similar despin characteristics to those obtained with the partial-solid/partial-liquid payloads. The despin data indicated increasing instability with increasing liquid viscosity with a maximum effect at a kinematic viscosity of about 105 CS whereupon the instability decreases with increasing viscosity. Instrumented flight tests showed good correlation with fixture results. The similar instability characteristics of the homogeneous, viscous liquids and the partial-liquid/partial-solid payloads could aid in developing a general theory describing this instability.