MODULAR PAYLOAD WARHEAD

Abstract

A warhead casing and method of inserting compartments into a warhead casing. The system includes a nose section, a tail section, and a tubular body section therebetween. The sections define a lengthwise axis of the casing. The system further includes a first plurality of compartments radially arranged along a perimeter of the tubular body section. The first plurality of compartments form a first cross-sectional layer. The system further includes a central cavity extending along the lengthwise axis of the casing, the central cavity being defined at least by an inner surface of the first plurality of compartments or the tubular body section.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of U.S. Provisional Patent Application Ser. No. 62/281,792 filed on Jan. 22, 2015, the entire disclosure of which is hereby incorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to warhead casings, and more particularly to warhead casings for penetration-type warheads, and related methods of loading.

BACKGROUND

For penetration-type warheads, the warhead casing remains generally intact after first impact with the target until detonation occurs. For penetration-type warheads, it is beneficial to keep the combustible components separate until the desired time of detonation, possibly after first impact. While mixing the combustible components at this later time is beneficial, this may result in a higher percentage of unexploded ordnances. An unexploded ordnance does not detonate when desired and, if not removed, the unexploded ordnance could create a hazard to civilians and others for decades after the intended detonation. To address the humanitarian effects caused by unexploded munitions, countries began to negotiate a protocol to minimize the dangers of explosive remnants of war.

The Convention on Cluster Munitions (“Convention”) is an international treaty that prohibits the use, transfer, and stockpile of cluster bombs. Cluster bombs are a type of explosive weapon which scatters submunitions, also known as bomblets, over an area. Cluster bombs can be incendiary, chemical, anti-personnel, anti-tank, anti-electrical, mine-laying, or leaflet dispensing. The Convention was adopted on May 30, 2008 in Dublin, and was entered into force on Aug. 1, 2010, six months after it was ratified by 30 states. As of September 2015, 108 states have signed the treaty and 96 have ratified it or acceded to it. Countries that ratify the Convention are obliged never under any circumstance to: use cluster munitions; develop, produce, otherwise acquire, stockpile, retain or transfer to anyone, directly or indirectly, cluster munitions; and assist, encourage or induce anyone to engage in any activity prohibited to a State Party under the Convention. The Convention allows certain types of weapons with submunitions that do not have the indiscriminate area effects or pose the same unexploded ordnance risks as cluster munitions. Further, the Convention permits weapons containing fewer than ten submunitions, with each submunition weighing more than 4 kilograms (8.8 lb). Further, each submunition must have the capability to detect and engage a single target object and contain electronic self-destruct and self-deactivation mechanisms.

The cluster munitions ban process, more commonly known as the Oslo Process, began in February 2007 in Oslo. At this time, 46 nations issued the Oslo Declaration, committing themselves to: conclude by 2008 a legally binding international instrument that prohibits the use and stockpiling of cluster munitions that cause unacceptable harm to civilians and secure adequate provision of care and rehabilitation to survivors and clearance of contaminated areas.

Currently, the United States is not a signatory to either the Convention or the Oslo Process. However, the Pentagon's cluster munitions policy stipulates that by the end of 2018, the U.S. Department of Defense will no longer use cluster bombs that result in more than 1% of the ordnance remaining unexploded. As a result, any cluster munitions purchased, used, or sold by the Department of Defense after 2018 must meet this standard.

Thus, there exists a need for munitions that decrease the percentage of unexploded ordnances. Because a wide variety of guidance and control devices exist and have been proven in use for the dimensional parameters (size, weight, center of gravity) of existing bomb types, replacing the existing bomb types with a completely new bomb design would require redesign and replacement of all the supporting systems—an undertaking that would both be costly and require significant development time. Accordingly, improvements are needed in the technology areas relating to warhead casings to address the deficiencies of prior known casing designs while, if desired, maintaining functionality with existing support systems.

SUMMARY

In one embodiment, the warhead casing includes a nose section, a tail section, and a tubular body section therebetween. The sections define a lengthwise axis of the casing. The warhead casing also includes a first plurality of compartments. The first plurality of compartments includes inner and outer surfaces. The first plurality of compartments are arranged along an inner perimeter of the tubular body section and form a first cross-sectional layer. The warhead casing also includes a central cavity that is defined at least by the inner surface of the first plurality of compartments. The central cavity can extend along the lengthwise axis of the casing.

In another embodiment, the warhead casing includes a nose section, a tail section, and a tubular body section therebetween. The sections define a lengthwise axis of the casing. The warhead casing also includes a first cross-sectional layer including one or more compartments positioned along an inner perimeter of the tubular body section. The first cross-sectional layer includes inner and outer surfaces. The warhead casing also includes a central cavity that is defined at least by the inner surface of the first cross-sectional layer. The central cavity extends along the lengthwise axis of the casing.

A method of inserting compartments into a warhead casing is also shown and described. The method includes providing a tubular body section defining a lengthwise axis of the casing. The method further includes inserting a first plurality of compartments along an inner perimeter of the tubular body section, the first plurality of compartments forming a first cross-sectional layer. The method further includes inserting a second plurality of compartments along the inner perimeter of the tubular body section. The second plurality of compartments form a second cross-sectional layer adjacent to the first cross-sectional layer. The first and second plurality of compartments form at least a portion of the central cavity extending along the lengthwise axis of the casing.

Other features, benefits, and combinations will be apparent from the various figures of the drawings and the following detailed description of the illustrative embodiments herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Like reference numerals are used to indicate like parts throughout the various figures of the drawings, wherein:

FIG. 1 is a schematic view generally showing a warhead casing.

FIG. 2 is a cross-sectional schematic view taken along line 2-2 of FIG. 1, showing a warhead casing according to an embodiment of the invention.

FIG. 2A is a detail view of the circular portion 2A of FIG. 2.

FIG. 3 is a partial cross-sectional perspective view showing the details of the warhead casing of FIG. 2.

FIG. 4 is a cross-sectional schematic view similar to FIG. 2, showing the warhead casing according to another embodiment of the invention.

FIG. 4A is a detail view of the rectangular portion 4A of FIG. 4.

FIG. 5 is an isometric view of the first plurality of compartments according to an embodiment of the invention.

FIG. 6 is a cross-sectional schematic view of the first plurality of compartments of FIG. 4.

FIG. 7 is an isometric view of a compartment according an embodiment of the invention.

FIG. 8 is an isometric view of a separate compartment according to another embodiment of the invention.

DETAILED DESCRIPTION

This invention is applicable to a wide variety of different warheads, including the BLU (Bomb Live Unit) series and SDB (Small Diameter Bomb) series warheads. As used herein, a “warhead” is intended to include a payload bomb of any type. This invention is also applicable to warhead casings that use GBU (Guided Bomb Unit) series guidance systems, such as electro-optically guided or laser guided units. These are merely illustrative examples with which this invention may be utilized, as one skilled in the art would appreciate the applicability of this invention to other warhead casings. The warhead casing 10 according to the embodiments of this invention shown and described herein include an overall exterior profile shape and weight distribution substantially similar to those of prior known designs, such as the BLU-108, which provides advantages for implementing the invention with existing systems developed for use with prior designs. It has been proposed to manufacture a warhead casing similar that used for the BLU-109, but to cut helical grooves on the interior of the tube to make it frangible such that, upon detonation, the casing wall will shatter into fragments. This approach is likely to result in a warhead casing that will not withstand the mechanical forces it is likely to undergo during normal handling and transport both on the ground and when suspended from an aircraft wing.

Referring to the figures, and to FIGS. 1, 2, and 3 in particular, a warhead casing 10 according to an embodiment of the invention is shown. The warhead casing 10 includes three general sections: a nose section 12, a tubular body section 14, and a tail section 16, which extend adjacent to one another along a lengthwise axis A. As shown, the nose section 12 and tubular body section 14 are structurally symmetric about the lengthwise axis A, and collectively define a central cavity 18 configured to carry an explosive payload (not shown). Some characteristics of the warhead casing 10 may incorporate features similar to those shown and described in U.S. application Ser. No. 14/291,822, filed May 20, 2014, and entitled WARHEAD CASINGS AND METHODS OF MANUFACTURE, assigned to the assignee of the present invention and incorporated herein by reference.

FIG. 2 shows the nose section 12 being formed from a unitary piece 20, while FIG. 4 shows the nose section 12 as including a nose interior 22, a nose exterior 24, and a nose retainer 26. FIGS. 4 and 4A show an annular nose compartment 28 being formed by the nose interior 22, the nose exterior 24, and the nose retainer 26. This nose compartment 28 may be filled with a combustible fluid or solid materials and may be designed to better distribute payload materials at a shortened blast radius or to the area directly below the warhead 10 (as it falls and explodes in a nose down orientation). In the embodiments shown in FIGS. 2 and 4, a traditional guidance system may be attached to the nose section 12 for properly directing the warhead to the desired location/coordinates. These guidance systems are well known in the art.

FIG. 2A shows a detailed view of the nose section 12 joined to the tubular body section 14 using a welding process, such as a fillet weld, to form a first weld joint 30. After joining the nose section 12 and the tubular body section 14, the first weld joint 30 may be processed to provide a smooth and continuous connection between the nose section 12 and tubular body section 14. To that end, the joined unit may be heat-treated, and burrs or other irregular surface features created during joining of the nose section 12 with the tubular body section 14, may be removed. Thereby, a smooth and continuous component may be created from the nose section 12 and the tubular body section 14. Alternatively, the fore body end 32 of the tubular body section 14 may include an engagement feature (not shown) configured to releasably engage a corresponding engagement feature (not shown) provided on the aft nose end 34 of the nose section 12. For example, the engagement features may include threaded portions, whereby a threaded portion of the nose section 12 may be threadably engaged with a threaded portion of the tubular body section 14. The engagement features may include other suitable mechanical fastening elements that enable the nose section 12 to be securely, yet releasably, joined to the tail section 16.

The tubular body section 14 extends from the nose section 12 along the lengthwise axis A. As shown, the tubular body section 14 has a generally straight-walled tubular shape. One skilled in the art would appreciate that the wall thickness may vary depending on the warhead and the intended application. The tubular body section 14 includes an interior surface 36 and an exterior surface 38. The interior surface 36 may be coated with an adhesive, such as an asphaltic adhesive, well known in the industry. The adhesive may secure the compartments and/or explosive material in place, as is discussed in greater detail below.

The tail section 16 extends from the tubular body section 14 along the lengthwise axis A and has a generally straight-walled or slightly flared tubular shape. After the first weld joint 30 has been processed as described above, the tail section 16 may be joined with the tubular body section 14. More specifically, as shown in FIGS. 2, 3, and 4, the fore tail end 40 of the tail section 16 may be permanently joined with the aft body end 42, such as by rosette welding, to form a second weld joint 44.

As shown in FIGS. 2 and 3, the tail section 16 includes an aft closure 46 (shown schematically), attachable to the tail section 16 to enclose the central cavity 18 defined by the tail section 16. The aft closure 46 may be attached using a threaded engagement and/or held in place by a separate threaded retainer. The aft closure 46 would typically be a smaller diameter than the inside diameter of the tubular body section 14 and/or the tail section 16. The aft closure 46 is removably attached to an inner circumference 48 of the tail section 16. While not shown, it is also envisioned that the nose section 12 and tubular body section 14 may be together integrally formed as a unitary piece. Similarly, the tubular body section 14 and the tail section 16 may be together integrally formed as a unitary piece.

As shown in FIGS. 2, 3, and 4, eight cross-sectional layers are shown, each formed by a plurality of compartments as discussed below. However, more or fewer layers, each having one or more compartments, may be used. As shown, the first cross-sectional layer 50 is formed by a first plurality of compartments 52a-h (with compartments 60a, 60e-h being shown in FIG. 2) and is adjacent to the nose section 12 and a second cross-sectional layer 54. The second cross-sectional layer 54, formed by a second plurality of compartments 56a-h (with compartments 56a and 56e-h being shown in FIG. 2), is adjacent to the first and third cross-sectional layers 50, 58. The third cross-sectional layer 58, formed by a third plurality of compartments 60a-h (with compartments 60a and 60e-h being shown in FIG. 2), is adjacent to the second and fourth cross-sectional layers 54, 62. The fourth cross-sectional layer 62, formed by a fourth plurality of compartments 64a-h (with compartments 64a and 64e-h being shown in FIG. 2), is adjacent to the third and fifth cross-sectional layers 58, 66. The fifth cross-sectional layer 66, formed by a fifth plurality of compartments 68a-h (with compartments 68a and 68e-h being shown in FIG. 2), is adjacent to the fourth and sixth cross-sectional layers 62, 70. The sixth cross-sectional layer 70, formed by a sixth plurality of compartments 72a-h (with compartments 72a and 72e-h being shown in FIG. 2), is adjacent to the fifth and seventh cross-sections layers 66, 74. The seventh cross-sectional layer 74, formed by a seventh plurality of compartments 76a-h (with compartments 76a and 76e-h being shown in FIG. 2), is adjacent to the sixth and eighth cross-sections layers 70, 78. The eighth cross-sectional layer 78, formed by an eighth plurality of compartments 80a-h (with compartments 80a and 80e-h being shown in FIG. 2), is adjacent to the seventh cross-sectional layer 74 and the tail section 16. As shown, first, second, third, fourth, fifth, sixth, seventh, and eighth plurality of compartments 52a-h, 56a-h, 60a-h, 64a-h, 68a-h, 72a-h, 76a-h, 80a-h may be positioned along the inner perimeter of the interior surface 36 of the tubular body section 14, and may collectively define the central cavity 18 inward of the compartments 52a-h, 56 a-h, 60a-h, 64a-h, 68a-h, 72a-h, 76a-h, 80a-h.

FIGS. 5 and 6 more clearly show the first plurality of compartments 52 arranged in a cylindrical cross-section. However, FIGS. 5 and 6 could very easily instead represent the second, third, fourth, fifth, sixth, seventh, and eighth plurality of compartments 56a-h, 60a-h, 64a-h, 68a-h, 72a-h, 76a-h, 80a-h. The first plurality of compartments 52 includes compartments 52a, 52b, 52c, 52d, 52e, 52f, 52g, 52h. As shown in FIG. 6, the first plurality of compartments has three distinct shapes. Compartment 52a has a “Type A” shape, compartments 52b and 52h have a “Type B” shape, and compartments 52c-g have a “Type C” shape. All of the shape types may have curved inner walls 86 and outer walls or lids 84 that are substantially concentric with the central axis A and the tubular body section 14. The specific radius of these curved walls will depend on the diameter of the tubular body section 14 and the desired volume of each compartment 52a, 52b, 52c, 52d, 52e, 52f, 52g, 52h and/or the central cavity 18. Forward and aft walls may be substantially straight and radial. Type C compartments (52c-g) may have side walls that are radially oriented, as best illustrated in FIG. 6. Type B compartments 52b and 52h may have one side wall that is radially oriented and an opposite wall that is not. A single Type B compartment construction may be used in either position 52b or 52h simply by reversing the orientation of the unit end-to-end. The Type A compartment 52a may be formed with sidewalls that are substantially parallel to each other such they will engage the non-radial wall of adjacent Type B compartments 52b, 52h. In this manner, the compartments may be assembled inside the tubular casing 10 by stacking the units in place and inserting the Type A compartment 52a with parallel sidewalls last. Unlike a keystone, the Type A compartment shape allows it to be inserted or removed from the inside of the circumference. One skilled in the art would appreciate that the compartments 52a-h may each have the same shape, different shapes, or a combination of the same and different shapes (as shown).

While the following features and characteristics are referenced with respect to the first plurality of compartments 52, the features and characteristics apply equally to the second, third, fourth, fifth, sixth, seventh, and eighth plurality of compartments 56, 60, 64, 68, 72, 76, 80, which may have respective compartments 56a-h, 60a-h, 64a-h, 68a-h, 72a-h, 76a-h, 80a-h. As shown in FIGS. 7 and 8, the first plurality of compartments 52 (such as compartment 52a, 52b) may have a first plurality of openings 82 that extend radially toward the interior surface 36 along the lengthwise axis A of the tubular body section 14. As shown, the first plurality of compartments 52a-h include a first plurality of lids 84 that accommodate the respective shape of each compartment 52a-h. The first plurality of lids 84 seal the contents within each of the first plurality of compartments 52a-h. Specifically, the lids 84 are arcuate to accommodate the arcuate cross-section or inner surface of the tubular body section 14. The first plurality of compartments 52a-h include an inner surface, specifically an inner radial surface 86, and an outer surface, specifically an outer radial surface 88. The first plurality of compartments 52a-h are arranged along the inner perimeter of the interior surface 36 of the tubular body section 14. As shown in FIGS. 5 and 6, compartments 52a and 52b are contacting each other. Specifically, as shown in FIG. 6, a first longitudinal surface 90 of compartment 52a contacts a second longitudinal surface 92 of compartment 52b. Adhesive or various attachment mechanisms (not shown) may be used to connect the compartments 52a, 52b to the other adjacent compartments 52a-h.

The first plurality of compartments 52a-h may be made from a variety of inexpensive and easily formed materials, such as plastics, fibers, or metals. Further, the thickness of the individual compartments 52a-h may vary if desired. For example, one or more of the compartments 52a-h may be a strongback positioned and used to connect the warhead casing 10 to an aircraft (not shown). The strongback may be composed of any suitable material such as a solid block of tungsten or other suitably strong material. Likewise, a variety of contents (not shown) may be inserted into the compartments 52a-h. For example, each of the compartments 52a-h may include liquids, solids (such as shrapnel, incendiary chemicals, electronics). Further, it may be desirable to have certain compartments 52a-h remain empty, such as for reducing the overall weight or shifting the center of gravity. One skilled in the art would appreciate the range of materials and contents that may form or may be inserted into each respective compartment 52a-h. While not shown, it is also appreciated that the plurality of compartments 56, 60, 64, 68, 72, 76, 80 (or their respective individual compartments) may have various lengths, measured along the length wise direction A.

The modular design, shown through the use of compartments, provides many benefits. For example, the modular design allows for customization resulting in a variety of different loading patterns. Further, each compartment may be color coded possibly according to the respective contents or intended position in the warhead casing 10. Alternatively, the compartments may be categorized by another discernable marking (such as by a letter, number, shape, barcode or RFID tag). This allows the specific desired compartments to be assembled on-site, which decreases the amount of inventory needed to be retained on-site. For example, tests could be performed off-site to determine the preferred loading pattern based on the particular mission and purpose of the warhead. This is especially beneficial where storage space is limited (such as on a ship) or where shipping is expensive and/or dangerous. Another benefit is time. Each of the compartments may be “qualified”, such as by Applied Research Associates. This qualification may include the weight, center of gravity, and shape of each compartment. This qualification allows the compartments to be used interchangeably, without adverse effects on the aerodynamic characteristics of the warhead casing 10.

Methods of inserting the compartments into the tubular body section 14 of the warhead casing 10 according to an embodiment of the invention will now be described. In this regard, like reference numerals refer to like features. The method includes obtaining a tubular body section 14 defining a lengthwise axis A of the warhead casing 10. The method further includes inserting a first plurality of compartments 52 along an inner perimeter of the interior surface 36 of the tubular body section 14. The first plurality of compartments 52 may be placed in the tubular body section 14 individually or inserted together as a first cross-sectional layer 50. The first plurality of compartments 52 may be slid within the interior surface 36 of the tubular body section 14 toward the nose section 12. Thereafter, a second plurality of compartments 56 may be loaded along an inner perimeter of the tubular body section 14. The second plurality of compartments 56 form a second cross-sectional layer 54 adjacent to the first cross-sectional layer 50, but closer to the tail section 16. Subsequent compartments may be inserted in the same manner until the warhead casing 10 is full or the desired number of compartments have been successfully inserted. The first and second plurality of compartments 52, 56 (and subsequent plurality of compartments thereafter) form at least a portion of the central cavity 18 extending along the lengthwise axis A of the warhead casing 10.

Alternatively or in addition to the plurality of compartments 56, 60, 64, 68, 72, 76, 80 being contained within the interior surface 36 along the inner perimeter of the tubular body section 14, the plurality of compartments 56, 60, 64, 68, 72, 76, 80 may be located along an outer perimeter, adjacent the exterior surface 38, of the tubular body section 14 and may be contained within a sheath (not shown) made of metal, composite, or polymeric material.

While the present invention has been illustrated by the description of specific embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of the general inventive concept.

Claims

1. A warhead casing, comprising:

a nose section, a tail section, and a tubular body section therebetween, the sections defining a lengthwise axis of the casing;

a first plurality of compartments arranged radially along a perimeter of the tubular body section, the first plurality of compartments forming a first cross-sectional layer; and

a central cavity extending along the lengthwise axis of the casing, the central cavity being defined at least by an inner surface of the first plurality of compartments or the tubular body section.

2. The warhead casing of claim 1:

wherein the first plurality of compartments are arranged along an inner perimeter of the tubular body section, and

wherein the central cavity is defined at least by the inner surface of the first plurality of compartments.

3. The warhead casing of claim 1:

wherein the first plurality of compartments are arranged along an outer perimeter of the tubular body section, and

wherein the central cavity is defined the tubular body section.

4. The warhead casing of claim 2, wherein the first plurality of compartments have a first plurality of openings that extend radially along the lengthwise axis of the casing.

5. The warhead casing of claim 4, wherein the first plurality of compartments have a first plurality of lids configured to seal contents within the first plurality of compartments.

6. The warhead casing of claim 5, the first plurality of lids are arcuate to accommodate an arcuate cross-section of the tubular body section.

7. The warhead casing of claim 2, wherein the inner surface is arcuate and defines respective bases to the first plurality of compartments such that the central cavity is generally cylindrical extending along the lengthwise axis of the casing.

8. The warhead casing of claim 2, wherein the tubular body section has an interior surface and the warhead casing further comprises:

adhesive applied to at least a portion of the interior surface.

9. The warhead casing of claim 1, wherein one compartment of the first plurality of compartments is a strongback.

10. The warhead casing of claim 2, further comprising:

a second plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the first plurality of compartments, the second plurality of compartments forming a second cross-sectional layer.

11. The warhead casing of claim 10, further comprising:

a third plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the second plurality of compartments, the third plurality of compartments forming a third cross-sectional layer;

a fourth plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the third plurality of compartments, the fourth plurality of compartments forming a fourth cross-sectional layer.

a fifth plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the fourth plurality of compartments, the fifth plurality of compartments forming a fifth cross-sectional layer.

a sixth plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the fifth plurality of compartments, the sixth plurality of compartments forming a sixth cross-sectional layer.

a seventh plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the sixth plurality of compartments, the seventh plurality of compartments forming a seventh cross-sectional layer.

an eighth plurality of compartments arranged along the inner perimeter of the tubular body section and adjacent to the seventh plurality of compartments, the eighth plurality of compartments forming an eighth cross-sectional layer.

12. A warhead casing, comprising:

a nose section, a tail section, and a tubular body section therebetween, the sections defining a lengthwise axis of the casing;

a first cross-sectional layer including one or more compartments positioned along an inner perimeter of the tubular body section, the first cross-sectional layer including inner and outer surfaces; and

a central cavity, defined at least by the inner surface of the first cross-sectional layer, extending along the lengthwise axis of the casing.

13. A method of inserting compartments into a warhead casing, the method comprising:

providing a tubular body section defining a lengthwise axis of the casing;

inserting a first plurality of compartments along an inner perimeter of the tubular body section, the first plurality of compartments forming a first cross-sectional layer, the first plurality of compartments forming at least a portion of the central cavity extending along the lengthwise axis of the casing

14. The method of claim 13, further comprising:

inserting a second plurality of compartments along the inner perimeter of the tubular body section, the second plurality of compartments forming a second cross-sectional layer adjacent to the first cross-sectional layer; the first and second plurality of compartments forming at least a portion of the central cavity extending along the lengthwise axis of the casing.

15. The method of claim 14, further comprising:

inserting a third plurality of compartments along the inner perimeter of the tubular body section, the third plurality of compartments forming a third cross-sectional layer adjacent the second cross-sectional layer.

16. The method of claim 15, further comprising:

inserting a fourth plurality of compartments along the inner perimeter of the tubular body section, the fourth plurality of compartments forming a fourth cross-sectional layer adjacent the third cross-sectional layer.