This invention further relates to motor assemblies comprising the above-mentioned case assemblies loaded with propellant and including a nozzle assembly, and to rocket assemblies comprising said motor assemblies and a projectile, warhead, or payload assembly.

2. Description of Related Art FIG. 7 represents a perspective exploded view of the case assembly, which is generally designated by reference numeral 770. The case assembly 770 includes a case sleeve 772, such as a resin-impregnated-filament-wound pressure vessel, a forward end closure 780, and an aft end closure 784.

A forward receptacle end portion 774 of the case sleeve 772 accommodates a mating portion 782 of the forward end closure 780, whereas an aft receptacle end portion 776 of the case sleeve 772 accommodates a mating portion 786 of the aft end closure 784. The forward and aft end closures 780 and 784 are secured to the case sleeve 772 with adhesive bonds formed between opposing surfaces of the end closure mating

portions 782 and 786 and their associated receptacle end portions 774 and 776.

During normal operation, shear stresses develop at the opposing surfaces as the result of axial load differentiations (applied in the direction designated by arrow La in FIG. 7) between the case sleeve 772 and end closures 780 and 784 caused by, among other things, internal pressure, acceleration of the rocket assembly 770 during lift-off, and sudden deceleration. Tensile stresses also develop at the opposing surfaces; the tensile stresses are the result of radial displacement differentials between the case sleeve 772 and the end closures 780 and 784 caused by, among other things, internal pressure, which causes the case sleeve to circumferentially expand.

Conventional end closures are designed not to deform or yield until the case sleeve reaches a maximum expected operating load well above normal operating loads of the rocket assembly. Typically, deformation and yielding are obviated by providing the end closure mating portions with relatively thick walls. As a result, the conventional end closure mating portions are stiff and do not track axial movement and radial deformations of the case sleeve.

Consequently, radial and circumferential expansion of the case sleeve transfers peel loads to the adhesive between the case sleeve and the end closure mating portions, causing tensile stresses to be localized in the adhesive. However, the strength of the adhesive material often is not sufficient to tolerate stresses encountered at normal operating loads, much less maximum expected operating loads. Thus, the adhesive

bonds represent the weakest link of the case assembly 770.

In order to compensate for inherent weaknesses in these adhesive bonds and avert premature failure of the motor assembly, conventional case assemblies 770 are usually provided with supplemental mechanical fasteners (not shown) to reinforce the adhesive bonds and account for peel loads. Conventional supplemental fasteners include blind fasteners such as pop rivets, standard nuts and bolts, or bolts that extend through the tubular piece and thread into the end closure.

Notwithstanding the reinforcement characteristics that the conventional supplemental mechanical fasteners contribute to the case assembly, the use of such fasteners in small tactical rocket motors generates at least two problems. First, conventional supplemental mechanical fasteners are often very expensive. The reality of marketplace demands on minimizing costs associated with the preparation of small tactical rocket motors and the expense of supplemental mechanical fasteners have, to a large extent, made the use of composite motor casings cost prohibitive for many applications.

Second, when case assemblies are subjected to fire or other unexpected sources of intense heat, the adhesive bond between the case sleeve and the end closure mating portions desirably is designed to fail well before the heat causes the rocket propellant stored in the pressure vessel to undergo auto- ignition. The adhesive bond thus possesses an inherent high temperature relief capability, also known as an inherent Insensitive Munitions (IM)

capability. This IM capability makes polymeric adhesives particularly advantageous for use in case assemblies. However, the IM capability of the adhesive bond is negated by the provision of supplemental mechanical fasteners, which mechanically retain the assembly together even after the adhesive bond has undergone its inherent high temperature relief failure.

SUMMARY OF THE INVENTION It is therefore an object of this invention to overcome the problems outlined above.

It is yet another object of this invention to provide a case assembly in which an adhesive material bonds at least one end closure structure to a case sleeve structure, and in which the adhesive bond is designed to be insensitive to internal operating pressures encountered during rocket motor operation, yet provide high temperature relief to the case assembly prior to propellant auto-ignition.

Another object of this invention is the provision of a case assembly in which an adhesive material bonds an end closure structure to a case sleeve structure, and which is free of mechanical fasteners or other devices that impede or destroy the IM capability of the case assembly.

In accordance with the principles of this invention, these and other objects are attained by the provision of a case assembly comprising at least one end closure structure and a case sleeve structure with an inner receptacle surface. The case sleeve structure receives a mating portion of the end closure structure. The mating portion comprises a

tubular wall, which is coupled to the case sleeve structure via a bond comprising an adhesive material disposed between an outer surface region of the tubular wall and a region of the inner receptacle surface of the case sleeve structure. Although various materials can be selected for preparing the end closure structure, the tubular wall of the end closure structure is preferably made of a metal or alloy, and more preferably is made of aluminum or an aluminum alloy. In a preferred embodiment, the tubular wall of the mating portion has a reduced thickness portion and a transition portion, wherein the transition portion tapers in thickness. The reduced thickness and transition portions are collectively constructed and arranged to deform or yield under longitudinal and radial operating loads to track variations of the inner receptacle surface due to such loads and maintain the peel load of the adhesive material below the tensile capability of the adhesive. (The tensile capability as referred to herein means the ultimate tensile stress that the adhesive can be subject to prior to peeling from one of the opposing surfaces.) Since the tubular wall of the end closure structure deforms to move substantially in unison with (or substantially tract) the deformation of the inner receptacle surface of the case sleeve structure, the longitudinal and radial loads are transferred away from the adhesive bond to the end closure structure so that localized tensile and shear stresses at the adhesive bond are significantly reduced and substantially eliminated.

Thus, under normal process conditions, such as at temperatures between -500F and +1500F, structural

failure occurs in the end closure structure, rather than in the adhesive bond line. Theoretically, the tubular wall thickness can be adjusted to cause failure of the end closure structure at a desired or predetermined pressure. This performance of this object is especially attainable when the average shear stress (P/A) is maintained less than 50% of the ultimate strength of the adhesive.

As opposed to conventional designs for securing end closures to a case sleeve, in the preferred embodiment of this invention the end closure structures may be fixed to the case sleeve structure without mechanical fasteners. Further, the adhesive material may be selected so that the temperature beyond which it melts, decomposes, or otherwise becomes structurally weakened to such a point that the integrity of the case assembly becomes compromised at normal operating pressures is below the temperature at which propellant to be loaded into the case assembly ignites spontaneously. Because no mechanical fasteners are used to augment the adhesive bond at high temperatures and the adhesive possesses poor structural properties at high temperatures, the pressure vessel has inherent high temperature relief capability, which imparts IM capability in rocket motor applications. Should the case assembly or any component of the rocket assembly catch on fire or otherwise subject the adhesive bonds of the case assembly to intense heat, the adhesive bond between the end closure structure and case sleeve structure predictably fails, well before the heat causes the propellant to ignite spontaneously, i.e., auto- ignite.

It is another object of this invention to provide a motor assembly comprising the foregoing case assembly loaded with a propellant and including an igniter and an aft nozzle end assembly integrally formed with, constituted by, or otherwise secured to the end closure structure.

It is another object of this invention to provide unguided rocket assemblies and guided missile assemblies (collectively referred to herein as rocket assemblies) comprising the foregoing case assembly loaded with a propellant, and including a nozzle assembly, igniter, and projectile, warhead or payload assembly, in which at least one of said assemblies is integrally formed with, constituted by, or otherwise secured to the end closure structure.

These and other objects, features, and advantages of this invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of this invention.

BRIEF- DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate embodiments of this invention. In such drawings: FIG. 1 is a perspective sectional view of an end closure structure in accordance with an embodiment of the present invention; FIG. 2 is a sectional view of a case assembly comprising the end closure structure of FIG. 1; FIG. 3 is a sectional view of an end closure structure especially suitable for securing an

aft nozzle end assembly to a case assembly in accordance with another embodiment of this invention; FIG. 4 is a sectional view of another embodiment of an end closure structure especially suitable for securing a forward projectile end assembly to a case assembly in accordance with yet another embodiment of this invention; FIG. 5 is a Table comparing performance characteristics of various adhesives and surface preparation methods; FIG. 6 is a perspective exploded view of a rocket assembly including the forward and aft end closures of this invention; and FIG. 7 is a perspective exploded view of a case assembly including conventional forward and aft end closures.

DETAILED DESCRIPTION OF THE INVENTION Referring now more particularly to the drawings, one embodiment of a part of a case assembly of this invention is generally designated by reference numeral 100 in FIG. 2. As shown in FIG. 2, the case assembly 100 comprises a case sleeve structure 102, such as a carbon-epoxy composite wound cylindrical vessel, with a receptacle inner surface 108, and an end closure structure 104.

Referring now to the isolated sectional view of the end closure structure 104 in FIG. 1, the end closure structure 104 has a substantially tubular configuration defined by an annular wall 106. The annular wall 106 comprises a base portion 110, a reduced thickness portion 112 axially spaced from the base portion 110, and a transition portion 114

interposed between the base portion 110 and the reduced thickness portion 112. The thickness of the transition portion 114 tapers from a given (or predetermined) thickness to that of the reduced thickness portion 112.

The outer surface of the end closure structure 104 comprises a mating outer surface region 120 associated with the reduced thickness and transition portions 112 and 114, and an enlarged outer surface region 122 associated with the base portion 110 and having a diameter larger than that of the mating outer surface region 120. The sudden increase in diameter between the outer surface regions 120 and 122 defines an annular shoulder 124.

In the illustrated embodiment, the end closure structure 104 has an axial slot 126 formed along a substantial portion of the reduced thickness portion 112 and a transverse bridge portion 128 partitioning an inner chamber of the end closure structure 104.

This embodiment may be modified, for example, by omitting the bridge portion 128.

In the case assembly 100 of FIG. 2, a region of the inner receptacle surface 108 of the case sleeve structure 102 opposes the mating outer surface region 120 (FIG. 1) of the end closure structure 104 to define a bond line therebetween in which adhesive material is disposed. The axial positioning of the case sleeve structure 102 relative to the end closure structure 104 is determined by the annular shoulder 124, which one end of the case sleeve structure 102 abuts to inhibit axial movement of the case sleeve structure 102 towards the end closure structure 104.

During operation, the case sleeve structure 102 undergoes dimensional deformation due to internal operating pressure and external forces. Such dimensional deformation is usually manifested by circumferential expansion of the case sleeve structure 102 and, in conventional assemblies, results in a peel load being applied to the adhesive material, which peel load promotes separation of the adhesive material from the inner receptacle surface of the case sleeve structure and/or the mating portion of the end closure structure. This invention overcomes the problems associated with peel loads by the provision of the reduced thickness portion 112, which is constructed and arranged to permit the end closure structure 104 to deform or yield under normal operating pressures so that the mating outer surface region 120 moves substantially in conjunction with (i.e., tracks the movement of)-the inner receptacle surface region 108 as internal pressure circumferentially expands the case sleeve structure 102.

Moreover, the ultimate tensile stress and shear stress that the adhesive can tolerate are not independent; to the contrary, alleviation of the amount of peel load acting on the adhesive permits the adhesive to tolerate a greater amount of shear stress. Accordingly, an ancillary benefit of this invention is that the construction and arrangement of the reduced thickness portion 112 and the transition portion 114 permits greater axial load transfer between the case sleeve structure 102 and end closure structure 104 (which manifest as shear stresses) to be tolerated during rocket motor operation.

Based on experiments, a relationship between the load on the pressure vessel (P) and the inner and outer diameters (Do and Di) of the reduced thickness portion has been hypothesized, and is set forth in equation (1) below. Although it is believed that this equation assists in explaining the principles of this invention, it is understood that this invention is not limited to this hypothesis or equation (1), which states: a, [(D,/2)2- (Di/2)2] (1) p = (1) (Di/2 ) 2 wherein: Do is the outer diameter of the reduced thickness portion of the end closure structure; Di is the inner diameter of the reduced thickness portion of the end closure structure; and ay is the yield stress of the closure (for 7075-T7351 aluminum alloy at=60000 psi).

The stress in the adhesive is < 50% T'iit.

The determination of ay and Tuit can be performed by standard test specimens, or in some cases are available in the literature, such as Mill- HDBK-5F Handbook. Generally, these determinations when viewed in reference to this disclosure, would be apparent and obtainable to the skilled artisan without undue experimentation. A discussion of such tests is set forth in ASTM D3165-73.

The presence of optional axial slot 126, which may be provided in the plurality, reduce the stresses in the adhesive bond during both pressure and temperature loading; however, testing has indicated that axial slots 126 are not necessary for

satisfactory performance. Also a peripheral bevel 130 may be provided at the end of reduced thickness portion 112 to create a locally thicker bond line; however, testing has also indicated that bevel 130 is not necessary for satisfactory performance of the case assembly 100.

A rocket motor assembly comprising the case assembly similar to that of FIG. 2 is illustrated in FIG. 6. The rocket motor assembly, generally designated by reference numeral 660, includes an aft end closure structure 662 and a forward end closure structure 664 designed in substantially the same manner as the end closure structure 104 discussed above. The aft end closure structure 662 can constitute, be formed integrally with, or otherwise be firmly secured to at least one nozzle assembly 663, while the forward end closure structure 664 can constitute, be formed integrally with, or otherwise be firmly secured to an assembly 665 comprising at least one aerodynamic conical portion, warhead, and/or payload assembly. The rocket assembly 660 illustrated in FIG. 6 is depicted in a loaded state, meaning that the case assembly is loaded with a propellant grain 666. The rocket assembly 660 further comprises an igniter 668. Among mechanisms for securing nozzle and forward assemblies to the end closure structures are, for example and without limitation, welds, bolts, lock wires, retaining rings, adhesive joints, threaded surfaces, the like, or a combination thereof.

Alternative embodiments of this invention will now be described by referring to FIGS. 3 and 4.

In order to facilitate an understanding of the

structure and operation of these embodiments, and in the interest of brevity, the structural elements of the embodiments of FIGS. 3 and 4 corresponding in structure and/or function with elements of the embodiment in FIGS. 1 and 2 have been designated by similar reference numerals to those used to designate the corresponding elements of the embodiment of FIGS.

1 and 2, with the substitution of the prefix numeral 3 or 4 in FIGS. 3 and 4, respectively. For example, the corresponding structure of the end closure structure 104 shown in FIG. 1 is designated by reference numeral 304 in FIG. 3.

The end closure structure 304 shown in FIG.

3 is especially suitable for securing a nozzle assembly to a case assembly. Although not shown in FIG. 3, the end closure structure 304 can constitute, be formedintegrally with, or otherwise be firmly secured to a nozzle assembly. Mechanisms useful for securing the nozzle assembly to the end closure structure 304 may include, among others, welds, bolts, adhesive joints, screw threads, a lock wire, the like, or a combination thereof.

The end closure structure 304 includes base portion 310, reduced thickness portion 312, and transition portion 314. The reduced thickness portion 312 and a part of the transition portion 314 define a mating outer surface region 320. The base portion 310 and the remainder of the transition portion 314 define an enlarged outer surface region 322, which has larger diameter than that of the mating outer surface region 320. The sudden increase in diameter between the outer surface regions 320 and 322 defines an annular shoulder 324.

Disposed on the mating outer surface region 320 are peripheral shim rings 340 and 342 extending circumferentially around the outer surface region 320 and spaced from each other in the longitudinal direction. The shim rings 340 and 342 can be formed integrally with the end closure structure 304, or can be bonded or otherwise connected to the end closure structure 304 or the case sleeve structure (not shown in FIG. 3). A substantially annular cavity is thereby formed having outer and inner surfaces defined by the inner receptacle surface region (not shown in FIG. 3) and the mating outer surface region 320, respectively. The ends of the annular cavity are defined by the shim rings 340 and 342. The radial dimensions of the shim rings 340 and 342 and the longitudinal spacing between the shim rings 340 and 342 can be selected to give the cavity a desirable thickness and length, respectively. The annular cavity contains one or more resins or adhesives, which can be introduced into the annular cavity and cured to form the adhesive bond joint.

The end closure structure 304 is preferably formed from an aerospace-aircraft metal alloy, such as an aluminum alloy, such as 7075 aluminum alloy with a selected temper, such as a T7351 temper, but may be formed from another metal alloy, composite or plastic. The end closure structure 304 also includes portion 350 having structure adapted to permit a nozzle end assembly (not shown) to be coupled therewith.

The end closure structure 404 shown in FIG.

4 is especially suitable for bonding a projectile assembly to a case assembly. Although not shown in

the FIG. 4, the end closure structure 404 can constitute, be formed integrally with, or otherwise be firmly secured to a projectile assembly as noted above.

The end closure structure 404 includes base portion 410, reduced thickness portion 412, and transition portion 414. The reduced thickness portion 412, the transition portion 414, and a part of the base portion 410 collectively define a mating outer surface region 420. The remainder of the base portion 410 defines an enlarged outer surface region 422, which has larger diameter than that of the mating outer surface region 420. The sudden increase in diameter between the outer surface regions 420 and 422 defines an annular shoulder 424. Peripheral shim rings 440 and 442 extend circumferentially around the mating outer surface region 420 and are spaced from each other in the longitudinal direction. A substantially annular cavity is defined by the inner receptacle surface region of the casing sleeve structure (not shown in FIG. 3), and the outer surface region 420, and shim rings 440 and 442. The annular cavity contains one or more resins or adhesives, which can be introduced into the annular cavity and cured to form the adhesive bond joint.

The end closure structure 404 can be formed from alloys, composites, and/or resins, as discussed above, yet is preferably formed from 7075-T7351 aluminum alloy. The end closure structure 404 also includes portion 450 having structure adapted to permit a projectile assembly (not shown) to be coupled therewith.

The case assembly of this invention does not require supplemental mechanical fasteners to mate the end closure structures to the case sleeve structure, and the walls of the case sleeve structure may be configured as a simple prefabricated straight walled cylinder. The preparation of the bond surfaces preferably uses environmentally friendly chemicals. The cooperative relationship between the end closure structures and case sleeve structure of this invention results in case assemblies that encounter failure in the end closure structures or case sleeve structure prior to failure at the adhesive joint. Accordingly, case assembly failure can be predicted and tailored by proper choice of end closure structure and composite case assembly parameters, such as diameters and wall thicknesses, rather than at the adhesive bond due to peel loads, where failure is much less predictable.

Where it is desirable for the adhesive bond to possess an inherent high temperature pressure relief capability or IM capability, the resins and adhesives selected for the adhesive joint preferably break down at temperatures about 2000F (i.e., glass transition temperature Tg « auto-ignition temperature of the rocket propellant), so that the adhesive joint is likely to weaken well before propellant auto-ignition, thereby precluding a violent case pressure burst. Suitable adhesives include, but are not limited to, epoxies, polyimides, polyesters, and polyamides. Since the adhesive bond can be provided with adequate strength without the use of mechanical fasteners at the adhesive junction, the pressure vessel is provided with inherent high

temperature IM relief caused by degradation of the adhesive material.

Adhesive selection is also preferably based on strength and compatibility of the adhesive with the selected mating surface materials.

Processibility of the adhesive is another important consideration. For aluminum end closure structures, the preferred surface preparation method is that described in U.S. Patent No. 5,520,768, the complete disclosure of which is incorporated herein by reference. Should it become desirable to employ a titanium end closure structure, a method of surface preparation of titanium structures is described in U.S. Patent No. 5,660,884, which is thereby incorporated herein by reference.

A cavity (or bondline) thickness of about 0.030 inch between the end closure structure and the case sleeve structure is preferred, although other thicknesses may be satisfactory and even preferable, depending on the application. The preferred method of placing adhesive into the cavity is injection.

Other placement methods such as films, troweling, or brush or spray applications are possible. Low viscosity is desired for adhesives which are injected. Adequate working potlife is highly desirable. The adhesive should be selected such that the temperatures associated with cure (i.e., the heat required to cure the adhesive and the heat generated by the adhesive during cure) does not damage the case assembly or cause the propellant to auto-ignite.

Generally, the temperatures associated with cure should not exceed about 1800F for these reasons.

In developing the method and design of this invention, a variety of composite surface preparations were examined. It was found that a release cloth (commonly referred to as "Peel Ply") incorporated into the surface of the composite prior to cure, then peeled off prior to bonding, produced a uniform and reproducible surface for bonding. The combination of the structural adhesive Duralco 4525 and peel ply yielded both high strength and process insensitivity (robustness) compared to other surface preparations and adhesives examined. Duralco 4525, which is manufactured by Cotronics Corp. of Brooklyn, N.Y., has a viscosity similar to hot maple syrup.

This adhesive does not appear to have excessive exothermic reactions and possesses an adequate potlife. Accordingly, peel ply unprimed Duralco 4525 is preferred although other surface preparations, such as for example, the surface preparations listed in FIG. 5, may be satisfactory, depending on the application. (EA9394 is manufactured by Dexter Hysol Aerospace Materials Division. The primer of the surface preparations is UF 3332, available from Thiokol Corporation.) The case sleeve structure can be made of any structural material and be made by any method suitable for rocket motor use, but in a preferred embodiment comprises a composite material.

Preferably, the composite case sleeve is constructed using carbon tow, such as M30S tow manufactured by Toray, pre-impregnated with a suitable resin.

Suitable resins include, by way of example and without limitation, epoxy, polyimide, polyester, and/or polyamide formulated resins. The composite

fibers can be, for example and without limitation, Kevlar, glass, and/or carbon. A chemoheologically tailored matrix resin described in U.S. Patent No.

5,011,721, and variations and applications of which are described in U.S. Patent Nos. 5,356,499, 5,545,278, and 5,593,770, is preferred. The complete disclosures of each of these United States patents are incorporated herein by reference.

The wall thickness may vary for different applications. The stiffness of the composite case sleeve, which is dictated by wall thickness and laminate lay-up) should be set to substantially match the flexibility and yielding strength of the mating portion of the end closure structure so that, under internal pressure, both the composite case and the end closure structure flex substantially the same amount to prevent the adhesive joint from being subject to such peel loads that failure occurs at the adhesive joint. The above-mentioned 0.030 inch preferred cavity thickness is thick enough so as to increase the compliance of the joint without sacrificing joint strength.

EXAMPLES Sample case assemblies with bonded closures according to the present invention were constructed for testing as follows. The case sleeve was filament wound on a mandrel to a wall thickness of 0.07".

Prior to winding, the mandrel was covered with peel- ply, such as Green Release Cloth (commercially available from Airtech Bleeder-Lease "A") or non- transferring release coating on nylon fabric A888 (circumference of the forward and aft portions of the

bond length of the closure. The shims assured a uniform bond gap of preferably 0.030". (Shims need not be made of brass. In addition, shim structures are preferably integrally formed on the outer surface of the bond length of the closure.) The closure was then inserted into the case. Four holes -1/16" diameter were pre-drilled approximately 1" from the end of the case. Adhesive was injected into the holes at a pressure of -80 psi until adhesive was observed to spew from the adjacent holes. After injecting the adhesive, the holes were covered with tape and the specimen was placed in an oven at 1700F for 1 hour. After cure, the specimen was removed from the oven and the process was repeated on the other end.

It is to be understood that the presently described process for manufacturing test specimens is for illustrative purposes only and is not intended to be limiting.

After curing the closures, a liner of RP6401-1 (rubbery urethane) was poured into the case.

The case was laid on it side and rolled on a roller overnight. The resulting bladder seals the porous composite during hydroburst.

Testing was performed by injecting hydraulic fluid into the case at a rate of approximately 5000 psi/min. To determine the effect of temperature, the specimens were conditioned to the desired temperature and wrapped in insulation to maintain temperature during the test. Thermocouples indicated the temperature of the case wall prior to testing.

Example 1: Test at -500F -- Bond length was 3.5n Hydroburst pressure 10,440 psi Example 2: Test at 750F -- Bond length was 3.5" Hydroburst Pressure was 10,400 psi Example 3: Test at +1500F -- Bond length was 3.5" Hydroburst Pressure was 11,270 psi From the results of Examples 1-3 it was concluded that the invention has high pressure capability. The average hydroburst pressure was over 10,000 psi (10,700 psi with a percent of coefficient of variation (C.V.) of 4.5). This data supports the process and temperature insensitivity of the joint.

This application claims priority of both provisional application 60/039,570 entitled Design of Bonded Closures for Low Cost Process Insensitive High Pressure Composite Pressure Vessels with Insensitive Munitions Capability, filed on February 28, 1997, and provisional application 60/039,842 entitled Bonded Closures for Low Cost Process Insensitive High Pressure Composite Pressure Vessels with Inherent High Temperature Pressure Relief Capability and Method of Forming the Same, filed on March 4, 1997, each of which is assigned to the assignee of the present application, and the complete disclosures of each of which are incorporated herein by reference.

The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or

to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.