A variable stability projectile using an internal moving mass

Weapon designers have known for some time that projectiles with low static margin have proven to be more susceptible to launch perturbations than comparable projectiles with high static stability, and that low static margin allows greater control authority. The work reported here examines a mechanism for active static margin control in flight through mass centre modification, demonstrating that such a system has significant impact on required manoeuvre control force to achieve a given control authority. A system is developed wherein a mass translates aft during flight inside a cavity aligned with the projectile centre-line, thereby altering the mass centre. This in turn decreases the projectile's static margin after launch and allows greater control authority later in flight, while at the same time decreasing initial throw-off errors. A seven-degree-of-freedom flight dynamic model is used to predict the performance of the system. Results show that by decreasing static margin after launch the projectile is less susceptible to launch perturbations and has increased control authority through the remainder of flight, leading to a smart projectile that outperforms rigid projectiles that are highly stable or highly manoeuvrable. This smart weapon feature is particularly attractive when the maximum control force and moment are small, and therefore is developed specifically for controllable munitions rather than missiles, which often exhibit ample control authority.