Ordnance – Barrels
Utility Patent
1998-12-07
2001-01-02
Ark, Darren W. (Department: 3643)
Ordnance
Barrels
C089S014100, C042S076010
Utility Patent
active
06167794
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to the field of ballistics, and it particularly relates to a vibration absorber for use on a gun barrel, in order to enhance the structural stability of a weapon system which is subject to vibrational disturbance prior to the firing dynamics of launch. The absorber will increase the accuracy of the weapon system by reducing the variation in the initial conditions of the weapon, at the commencement of the highly non-linear dynamic equations that govern launch dynamics.
BACKGROUND OF THE INVENTION
Numerous attempts have been made to improve the accuracy of weapon systems, particularly those subject to vibrational disturbance. The vibrational disturbance concern has gained increasing importance and visibility with the advent of longer, more slender, gun barrels as typified by the XM291 tank gun system.
The reason for the current focus on this problem is two fold. First, decades of dedicated research and development have increased the accuracy of weapon systems in many areas. As the accuracy of the weapon systems has increased, the role of the vibrational disturbance has become more pronounced. Second, with the ever-increasing need for higher projectile exit velocities, impetus for longer and longer barrels is resulting in weapon systems that are more susceptible to flexural vibrations.
Conventional attempts to improve the accuracy of weapon systems can be generally categorized as follows:
Extension of the Gun Mount/Cradle
One means of reducing the receptance of a gun barrel to flexural vibrations is to decrease the effective cantilevered length of the gun system. This may be achieved by increasing the length of the supporting structure that holds the gun barrel. This effectively increases the ratio of stiffness to inertia of the system. The square of the ratio of stiffness to inertia is indicative of the resistance of a gun barrel to low frequency vibrations.
A variation on the extended mount approach has been to utilize a traditional mount to support the gun barrel, but to then incorporate damping pads via a mount extension, that couples the barrel to the cradle with low stiffness, but high damping. The result is that the mount extension need not be as solid, since increased stiffness is not the primary objective of the approach. An example of this approach is the British 30 mm, L21A1, system commonly called the RARDEN. (See Geeter et al, “Low Dispursion Automatic Cannon System (LODACS) Final Report (U),” ARDEC Technical Report ARSCD-TR-82011, Picatinny Arsenal, N.J., August 1982).
Although the extension of the gun mount/cradle has succeeded in reducing vibrations, it can present a negative impact of increasing the imbalance of several weapons systems, since the center of gravity of typical weapon systems is forward of the trunnion bearings. This imbalance necessitates the application of control torques, equal and opposite to the weight of the weapon system, multiplied by the horizontal offset of the center of gravity from the pivot point. These requirements place a heavy burden on the pointing system.
Further, for many weapon systems, extension of the gun mount/cradle becomes ungainly as the ratio of in-mount barrel length to overall barrel length increases. It would be a challenging endeavor to package such support structures in a fielded weapon system.
Increase of Gun Barrel Thickness
Gun barrels may be constructed with thicker walls. Since the stiffness is a function of the outer radius to the fourth power, and the inertia is a function of the outer radius to the second power, significant increase to the ratio of stiffness to inertia of the system can be made.
Thicker gun barrels increase the ratio of stiffness to inertia, but they require a significant ratio between the inner radius (the radius of the bore) and the outer radius. If the wall thickness, that is the difference between the inner and outer radii, is reasonably small relative to either radius, a thin walled approximation would have the inertia and stiffness increase proportionally to each other, thus no net gain. For example, a Taylor series expansion of the ratio of stiffness to inertia as a function of the outer diameter is dominated by the linear term for barrels whose wall thickness is a fraction of the bore radius. The second term exists, but it doesn't dominate until the wall thickness becomes impractical.
A related problem with this approach is that increased weight of the barrel is a direct consequence. This exacerbates both the extension of the center of gravity of the gun further out from the trunnions, and increases overall weapon weight which is supposed to be minimized.
Composite Barrel Construction
Gun barrels may be constructed of materials with a higher stiffness to inertia ratio, such as carbon fiber reinforced epoxy, or composite over-wraps of traditional gun steel barrels. The goal is to increase the net ratio of stiffness to inertia of the system, and this can be achieved. Reference is made to Hasenbein et al, “Metal Matrix Composite-Jacketed Cannon Tube Program,” ARDEC-Benet Technical Report ARCCB-TR-91027, Watervliet Arsenal, N.Y., August 1991).
Composite barrel construction is a viable alternative to enhance the structural stability of weapon systems. It is however challenged by the need to protect the barrel from the hot and erosive action of the propellant gases. This typically results in a composite over-wrap incarnation over a thin-walled steel barrel. A remaining challenge is to maintain the bond between the base material and the composite over-wrap during both manufacture, especially the autofrettage process and the firing loads which create concurrent radial dilation of the barrel and axial recoil loads. This firing dynamic challenge is exacerbated by the pressure discontinuity that travels behind the obturation of the projectile with a speed that may resonate a traveling radial dialation wave of the bore surface. Other challenges include impaired heat transfer across the insulating composite and increased recoil velocity of the cannon during operation.
Fluted Gun Barrels
Gun barrels may be constructed with flutes that look like fins emanating from the center of the gun. In analogy with design of an “I-Beam” the general design concept is to get the steel at a greater radius for an increased stiffness, without increasing the inertia in proportion. An example of this approach is the British 30 mm, L21A1, system commonly called the RARDEN. (See Geeter et al, “Low Dispursion Automatic Cannon System (LODACS) Final Report (U),” ARDEC Technical Report ARSCD-TR-82011, August 1982). However, fluted gun barrels are expensive to manufacture, and they may compromise a desirable static stress distribution that is manufactured into most large caliber gun barrels using a process called autofrettage and they increase system weight.
Application of Active Controls: Feed-Forward Cancellation or Feed-Back Vibration Cancellation
If the input excitation can be anticipated, a control signal can be applied through an actuation system to preempt the disturbance energy. An example for a tank gun system while traversing rough terrain would be the use of a sensor to detect the vertical acceleration of the tank hull, and to apply immediate counteraction force via the elevation actuator system. In many tank guns the center of gravity extends forward of the trunnion bearings. This is a result of the limited working volume within the armor protected turret. Thus, a vertical heave upwards applies a torque to the gun system that may be cancelled by an applied downward force at the elevation coupling, behind the trunnions.
For current systems, this concept of feed-forward cancellation treats the gun barrel as a rigid body, and ignores flexural modes, and in particular the first flexural mode which the vibration absorber of the present invention is designed to attenuate significantly. Inclusion of the inverse plant dynamics in the open loop control law could reduce this source of disturbance vibration energy, but would not usurp the vibration absorber.
The c
Ark Darren W.
Moran John F.
Sachs Michael C.
The United States of America as represented by the Secretary of
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