Ball motion in a ball-obturated tubular projectile
Abstract
When a ball with a concentric hole through it is suspended within a spinning tubular projectile the ball will rotate in such a way that its hole is aligned with the axis of spin of the projectile. The ball thus serves as an
automatic obturator—plugging the projectile while the two bodies are within the gun barrel and rotating to open the tubular passage following exit from the muzzle. The motion of the ball is gyrodynamic in nature and highly dependent
upon the external moments on the ball that arise because of its motion relative to the spinning projectile.
This report presents the results of an analytical and experimental
study to provide an understanding of such motions in as complete a sense as
is possible. The general equations of motion are formulated together with
appropriate mathematical expressions for the external moments. These moments
have their origins in the forces acting on the ball (gravity and aerodynamic
forces are considered) which, in the presence of relative motion between
the ball and the projectile, lead to sliding friction torques. Torques
due to fluid shear are also considered but these are found to be relatively
insignificant.
Exact solutions are obtained using standard numerical techniques. In
addition, a linear form is developed and these solutions lead to useful
approximations that are valid over a broad range of operating conditions.
Experiments are described in which the general validity of the theoretical
models (exact and linear) is demonstrated for cases in which there is
little or no aerodynamic load. The experiments, together with the linear
approximation, lead to a semi -empirical method for determining the effective
coefficient of sliding friction for such systems. A model is proposed to account for aerodynamic loading and sensitivity
studies are conducted to determint the nature and scope of the influence
of various design parameters upon systme performance. The linear approximation
is proposed as a useful design guide when applied with due awareness
of its limitations. In addition, a design criterion is presented by means
of which it is possible to avoid designs that lead to operation in a
"hovering" region. Such operation leads to prolonged delays in ball opening
time. Application of the design criterion leads to ball/projectile
designs that are optimum from the point of view of minimum ball opening time.
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.NPS Report Number
NPS69-81-001BRelated items
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