1. Field of the Invention
The present invention relates generally to lightweight mortar projectile guns and, more particularly, to an improved architectural design for a motor base plate.
2. Description of the Related Art
Lightweight gun systems are being widely used in current military operations. The lighter the gun, the greater the mobility and versatility in maneuvering the military force has. This is of an advantage where the gun systems must be transported quickly, or to difficult terrains or climates. Modern mortar systems have been an effective means of force suppression since the trench warfare of WWI. Their evolution over subsequent years has led to improvements in safety, range, and lethality. A recent unclassified debriefing from soldiers in Afghanistan noted that mortars were “essential” due to the distance and elevation to targets in the mountainous terrain, and that “mortars were responsible for many kills”.
Mortars and their ammunition are smaller and lighter than other artillery, making them ideal for support of movement to contact, ambush, retrograde, and other deployment maneuvers. Their “lobbing” trajectory can place their munitions on target over obstacles and to positions at higher elevations than the position of the mortar. The smooth bore of the mortar tube eliminates loads created by the rifling of a gun tube, allowing mortar rounds to carry higher payloads in thinner skins than artillery shells, thus providing a greater explosive source than similar sized artillery.
Unlike artillery pieces such as howitzers and cannon, mortars need no complex recoil equipment and are usually smoothbore and muzzle-loaded. The recoil force of the mortar is transferred directly into the baseplate and from there into the ground. The metal baseplates are relatively heavy:
Bipod 6 functions as a support and means to adjust the angle of trajectory. This is achieved by adjusting the angle that barrel 4 makes with the ground. It also provides the means to hold barrel 4 at a proper angle. Base-plate 8 is a heavy welded steel dish. It has socket 10 at the center to take the breech piece. This provides the capability to rotate the barrel 4 around a full 360 without shifting the base-plate.
Similar to base-plate 8, barrel 4 and bipod 6 are also made of steel. Current mortars take advantage of important attributes of steel. However, there are disadvantages associated with the use of steel as the main material for manufacturing the mortars. For example, 81 mm and 120 mm mortars made of steel are very heavy and require a team to transport each piece. Typical prior art 120 mm mortars weigh between 272 kg and 341 kg in the traveling configuration. This creates problems when these mortars can no longer be carried by machine and must be carried by humans. In these situations, the 120 mm mortars must be dismantled and transported part by part. This requires at least 3 to 4 people to carry all the parts. Furthermore, in situations where time is of the essence and the rounds must be fired continuously, dismantling and re-assembling the mortars may not be practical.
Another problem with the current 120 mm mortars is that there is no mechanism to reduce the recoil force and absorb the recoil energy of the mortar assembly after each round is fired. Presently, sand bags are placed under and around base-plate 8 to absorb the recoil movement of mortar 2. Despite this, present 120 mm mortars on a non-absorbing surface may jump as high as 3 to 4 feet off the ground. This poses a clear danger to the mortar operators. As a consequence, mortars are either placed on absorbing surfaces such as soft ground or sandbags and may have extra bags placed on the mount to reduce rebound effects. The recoil problem is even greater with a light mortar such as the mortar of the present invention.
Previous attempts at achieving weight reduction have focused on simple material substitution, without significant structural changes in design to effect a meaningful weight reduction. In addition, the materials presently used in the mortar base plates are susceptible to stress corrosion cracking due to the combination of operating environments and residual stresses created during manufacturing processes or stresses encountered during operation.
Consequently, a need has been felt for providing an mortar gun system having a light weight, portable base plate capable of withstanding repeated, substantial recoil force and absorb the recoil energy of the gun system caused by firing rounds.
It is therefore an object of the present invention to provide an improved motor base plate capable of handling recoil impact stress to achieve all the military qualification stress levels for all available mortar base plates.
Features of the present invention are provided in an architecturally design mortar base plate that is light weight, and cost effective and size insensitive such that it can be adapted to meet any available mortar base, such as 60 mm, 81 mm, 120 mm or any size between.
Briefly described according to one embodiment of the present invention, an architecturally design mortar base plate is provided having a forged metal frame structure having a center receiving hub and socket made of one piece machine from solid maranging steel, 4140 Steel, or any structural material for socket to absorb the impact over repeated cycling. A plurality of metal ribs extend radially out therefrom to distribute the impact strength from the socket regime to throughout the whole base volume to consume the energy that created due to firing. The metal ribs interconnect in an interlocking, nested, mounted, bonded or welded manner to support the socket in a position that is repeatable after firing. The ribs further are architectural designed to minimize weight while maintaining maximum desired strength.
Advantages the present invention result from the optimizing of strength in a manner in which the overall material volume is reduced, thereby reducing the total weight of the mortar base plate itself. The required strength comes from the choice of material and composite design. The whole design is based on eliminating stress corrosion or any weakening mechanism of the existing bulk materials that are used for such hardware. The mortar base is designed such way that the critical cracks are engineered not to grow and controlled by the geometry, several and different material choice and engineered design to eliminate or minimize the crack formation and crack growth during life of the base plate.
Further, the use of the “spider” configuration and stabilizer pad provides a platform that dynamically attenuates the force of the mortar recoil over time in much the same manner as recoil adapters on aircraft mounted gun systems. This reduction of load during the firing period will provide a more stable firing platform in soft ground or snow conditions.
Further still, the structural design and material properties of the improved baseplate will substantially increase the useful life of this mortar system component compared to the existing aluminum and other potential substitute materials by utilizing a more impact compliant structure with a configuration that eliminates high stress-concentration members such as gussets.
Additionally, it is a goal of the program that the proposed mortar baseplate design, with replaceable, interchangeable 60 mm and 81 mm ball sockets, will be strong enough and light enough to replace both the M3A1 and M7 baseplates, thus significantly reducing logistics costs.
The advantages and features of the present invention will become better understood with reference to the following more detailed description and claims taken in conjunction with the accompanying drawings, in which like elements are identified with like symbols, and in which:
The best mode for carrying out the invention is presented in terms of its preferred embodiment, herein depicted within the Figures.
Referring now to
The primary support of the baseplate consists of a structurally engineered multi-legged unit. The design concept shown in
The structural design of the support leg structure 10 gives it the characteristics of a spring, which will absorb and distribute the recoil force of the mortar. The legs will be fabricated from materials that optimize the weight to stiffness ratio for the application. While maraging steel is known to possess superior strength (typically 3 times greater than that of heat-treated alloy steel, such as 4140), toughness, and malleability, characteristics that provide the least weight for a given strength. Non-stainless varieties of maraging steels are moderately corrosion-resistant, and resist stress corrosion and hydrogen embrittlement. Additional corrosion protection is gained by cadmium plating or phosphating. Another unique characteristic of the maraging steel is that even though it has a very high yield strength and fracture toughness, the fracture mechanism is ductile which is very unique property of the material. This characteristic of the steel enables engineers to design critical structures—such as ballistic missile skins—that are significantly lighter and stronger than with other materials.
Fabrication of the support leg structure will consist of machining a flat pattern of the structure from steel plate. The part will then be press-forged into its final shape.
It has been found that the functional requirements for the mortar baseplate include: transmitting the recoil force to ground in a manner that does not shift the mortar cannon prior to exit of the projectile; withstanding the recoil of a minimum of ten thousand rounds without permanent deformation, fracture, or other failure modes; and, operating in specified environmental extremes of temperature, humidity, precipitation, particulates, etc. A typical force-time curve for a projectile launched by a non-progressive propellant is shown in Table 1.
The force-time curve can be generated analytically if the projectile and cannon properties are known, along with the propellant type and amount of charge, by using the law of combustion, energy conservation law, and the equation of motion for the projectile. Alternatively, it can be established from test results. The area under the force-time curve represents the linear impulse generated by the firing of the mortar and must be absorbed by the mortar baseplate without interfering with the ballistic path required to place the round on target.
As shown best in conjunction with
Referring now to
Forging results in metal that is stronger than cast or machined metal parts. This stems from the grain flow caused through forging. As the metal is pounded the grains deform to follow the shape of the part, thus the grains are unbroken throughout the part.
The forged ribs 120 distribute the recoil and impact strength from the socket 112 to throughout the whole base volume to consume the energy that created due to firing. Alternately, as taught by the related applications referenced above, Kevlar® or structural fabric webs 124 can be affixed additionally between the ribs 120 shares the recoil impact energy to each rib 120 and polymer body.
Referring now to
It is anticipated that the stabilizer pad 200 is made of a polymeric material, including synthetic rubbers, thermoplastic rubbers, urethanes, etc. In addition to the physical properties required to withstand environmental and operational conditions, key material characteristics will also include the necessary coefficients of stiffness and damping that will lead to an effective mortar baseplate unit.
Given the present teachings and findings, additional functional elements are anticipated within the present invention. Horizontal leg stringers may be added to the leg support structure in order to distribute the transverse force created by the elevation angle of the mortar to each of the support legs and ultimately to the stabilizer pad. The leg stringers will be fabricated from flexible high strength, low elongation material, such as aramid, to minimize weight. The locations, material requirements, and effectiveness of the stringers will be evaluated during design analysis.
In accordance with a preferred embodiment of the present invention, the ribs 20 are connected into a joint 26 within the hub 12. The joint 24 is designed to allow for a transfer of motion along with any impact force such that the ribs 10 have a ‘shock absorber’ effect in cushioning the hub 12. The ribs 20 thereby distribute and dampen this force throughout the structure, and allow for the hub 12 to move back to its original position after firing.
The loading on the baseplate assembly under such conditions is shown in
F-Reaction(t)=knzn(t)+cnvn(t) (1)
F-Reaction(t) is the reaction force between the baseplate system and ground (This force will be variable over time based on the linear impulse from the mortar, and will have a planar distribution on the stabilizer pad);
kn is the equivalent spring constant of the support legs and stabilizer pad;
zn(t) is the displacement of the support legs and stabilizer pad over time;
cn is the damping fact& of the support legs and stabilizer pad; and
vn(t) is the velocity the support legs and stabilizer pad over time.
In the transverse, or x-direction, the spring-damper characteristics are expected to be less of a factor, and the resistance to the recoil force and overturning moment will be reacted by friction between the stabilizer base and ground, and the weight of the mortar system.
A preliminary finite element analysis performed on the initial concept shows the level of the von Mises stress in one support leg in
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. Therefore, the scope of the invention is to be limited only by the following claims.
The present application incorporates subject matter that was first disclosed in U.S. Provisional Application 61/182,455 filed on May 29, 2009 and U.S. Provisional Application 61/239,219 filed on Sep. 2, 2009, which are incorporated by reference herein as if fully rewritten. There are no previously filed, nor any co-pending applications, anywhere in the world.
Number | Date | Country | |
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61182455 | May 2009 | US | |
61239219 | Sep 2009 | US |