Certain large bore projectiles, such as artillery shells and recoilless rifle bullets fired from gun systems or rocket propelled grenades, missiles and rockets fired from tube launchers, typically comprise radially extending fins that are sized to be loaded within the gun barrel of the artillery system or tube launcher. The fins stabilize the projectile in flight maintaining the alignment of the projectile with the aimed trajectory. Many large bore projectiles also include a rocket motor or similar means for self-propelling the projectile or supplementing the primary propellant means for the projectile. With these projectiles, the fins are a necessary feature that prevents the self-propelled projectile from veering off course due to the thrust generated by the motor or the propellant means.
The inherent challenge with finned projectiles is positioning the fins such that they extend radially outward past the outer diameter of the main projectile body during flight to maximize the engagement of the fins to the air as the projectile flies while still fitting the projectile within the gun barrel or tube. A contravening concern is engaging the larger exterior surface of the projectile body to the gun barrel or tube allows the barrel or tube to more efficiently aim the projectile than with the smaller surface area provided by the tips of the fins. Fixed fins create a tradeoff between superior aerodynamic qualities of fins that extend beyond the outer diameter of the primary projectile body and the superior accuracy of sizing the projectile body to engage the barrel or tube. Accordingly, many finned projectiles comprise deployable fins that retract prior to firing to allow the projectile body to engage the barrel or tube and deploy upon exiting the barrel or tube to extend radially outward past the outer diameter of the projectile body.
Deployable fin projectiles typically comprises a main projectile body that engages the walls of the barrel or tube to axially align the projectile with the barrel or tube and engage the projectile to the rifling of the barrel when fired from a gun system. Prior to firing, the fins are retracted behind or hidden within the projectile body such that the projectile body defines the maximum outer diameter of the projectile. Upon leaving the barrel or tube, the fins deploy radially outward from behind or within the projectile body to engage the air. The projectiles typically comprise mechanical assemblies having springs or other similar biasing means that retain the fins in the retracted position during loading and deploy the fins as the projectile leaves the barrel or tube.
As both fixed and deployed fins are typically positioned at the rear of the projectile to maximize the aerodynamic advantage of the fins, the mechanical assemblies for deploying the fins are also positioned at the rear of the projectile and thereby proximate to any initial propellant charge for firing the projectile from guns. The heat and pressure from the ignited propellant gases can damage the mechanical assemblies resulting in failure of the mechanical assemblies to deploy or fully deploy the fins. In addition, the rapid acceleration of the projectile coupled with the heat and pressure can also cause the mechanical systems to fail.
Similarly, many tube launched rocket assisted projectiles have starter motors or propellant charges that launch the projectile from the tube before the primary motor ignites. The starter motor or propellant charge can also damage the mechanical fin deployment assemblies. Similarly, the mechanical fin deployment assemblies can often comprise additional assemblies for synchronizing deployment of the fins. The additional synchronization systems further increases the likelihood that some or all of the fins will fail to deploy as a result of damage during firing.
Although the mechanical assembly can be shielded to reduce the likelihood that the mechanical assemblies will be damaged during launch, the shielding increases the weight and bulk of the projectile. As the chamber of the gun or the tube launcher is fixed volume governed by the caliber of the projectile or standard missile or rocket size, any additional bulk to any portion of the projectile, such as from increased shielding, must result in a corresponding reduction in size in another portion of the projectile. Specifically, the additional shielding often reduces the possible projectile volume allotted to the projectile motor and/or payload.
Similarly, the fins themselves can reduce the size of the projectile volume allocated to the motor or payload or alter the shape of the projectile. As depicted in U.S. Pat. No. 4,334,657 and U.S. Pat. No. 7,083,141, the fins typically scissor between the retracted position and deployed position within a single plane transverse to the central axis of the projectile to minimize any potential disturbance to the trajectory of the projectile from the deploying fins. However, the scissoring of the fins into the projectile reduces the internal space of the projectile that can be allocated for the payload or the motor or creates an irregularly shaped space within the projectile. The reduction in motor size and/or payload reduces the range and overall effectiveness of the projectile.
The inherent tradeoff between preserving the mechanical function of the fin deployment mechanisms and reducing the performance of the projectile creates a need for a means of consistently deploying the fins without reducing the projectile volume allocated to the payload, primary motor or other projectile systems.