Venting cap system

The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon.

The invention relates generally to safety systems for warheads or rockets, and more particularly to a mechanically-strong venting cap system that prevents a cook-off explosion in warheads or rockets.

The term “cook-off” as it relates to military ordnance refers to the unwanted exploding of ordnance when subjected to high temperatures caused by fire or other sources of heat. A well-known approach to mitigating cook-off effects involves opening or “venting” both ends of the ordnance to prevent the ordnance from affecting, or from being affected by, adjacent explosive components. A particularly hazardous explosive component adjacent to a warhead is the warhead's detonating fuze. Accordingly, it is highly desirable to separate a fuze from its warhead when ambient temperatures approach those capable of starting a cook-off event.

One type of well-known cook-off venting system places a meltable (plastic) adapter between an end of a warhead and a warhead's fuze. As ambient temperatures approach those temperatures associated with a cook-off event, the adapter melts thereby allowing the fuze to fall away from the warhead while simultaneously venting the warhead. Unfortunately, plastic adapters are not very strong and are subject to cracking and breaking under impact loads, e.g., the ordnance is dropped during handling.

Accordingly, it is an object of the present invention to provide a venting cap system for a warhead.

Another object of the present invention is to provide a venting cap system for a warhead that supports the warhead's fuze, and provides for the release of the fuze and venting of the warhead in the presence of temperatures associated with a cook-off event.

Still another object of the present invention is to provide a venting cap system that is mechanically strong enough to withstand handling conditions.

Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.

In accordance with the present invention, a venting cap system includes a first open-ended sleeve and a second open-ended sleeve. The first open-ended sleeve is made from a first material that melts at a selected temperature. The first open-ended sleeve has an internal region commencing at a first end thereof defining a diameter D S . The first open-ended sleeve has an externally-threaded region commencing at a second end thereof with the externally-threaded region defining a minor diameter D T . The second open-ended sleeve is made from a second material that does not melt at the selected temperature. The second open-ended sleeve is coaxially fitted in the first open-ended sleeve. The second open-ended sleeve has a first end that is flush with the first end of the first open-ended sleeve. The second open-ended sleeve has a portion that axially overlaps at least part of the externally-threaded region of the first open-ended sleeve by an amount L that satisfies the relationship L<(D T 2 −D S 2 ) 1/2 . The coefficients of linear expansion of the first material and the second material are approximately equal.

Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the exemplary embodiments and to the drawings, where corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:

FIG. 1 is a cross-sectional view of a venting cap system in accordance with an embodiment of the present invention; and

FIG. 2 is a cross-sectional view of a venting cap system in accordance with another embodiment of the present invention.

Referring now to the drawings and more particularly to FIG. 1 , a venting cap system in accordance with an embodiment of the present invention is shown and is referenced generally by numeral 10 . Venting cap system 10 is illustrated as it would be when coupled to an end of a warhead 100 , the particular size and shape of which are not limitations of the present invention. Further, it is to be understood that warhead 100 is not part of the present invention and is, therefore, illustrated using dashed lines.

Venting cap system 10 is a two-piece system that is essentially two coaxial sleeves, an outer sleeve 12 and an inner sleeve 14 . Outer sleeve 12 is an open-ended sleeve (e.g., substantially cylindrical in the illustrated embodiment) made from a material that will melt at a temperature associated with the onset of a cook-off event for warhead 100 . One end of outer sleeve 12 is configured to be coupled to the illustrated end of warhead 100 . For example, outer sleeve 12 may be externally-threaded at 12 A near one end 12 B to couple, threadably, to an internally-threaded region 100 A of warhead 100 . The minor diameter of externally-threaded region 12 A is designated D T . The opposing end 12 C of outer sleeve 12 has an internal diameter designated D S .

The material used for outer sleeve 12 may be a variety of thermoplastic materials to include, but not limited to, polycarbonate, compositions of polycarbonate and glass, nylon, and tetrafluoroethylene. Suitable compositions of polycarbonate and glass are disclosed in U.S. Pat. No. 7,331,292, the contents of which are hereby incorporated by reference.

Inner sleeve 14 is an open-ended sleeve (e.g., cylindrical in the illustrated embodiment) made from a material that will not melt at the temperature(s) associated with the onset of a cook-off event for warhead 100 . Further, inner sleeve 14 is made from a material that enhances the mechanical strength of venting cap system 10 . For example, inner sleeve 14 may be made from metal, the choice of which is predicated on the material used for outer sleeve 12 as will be explained further below. Inner sleeve 14 extends coaxially into and overlaps some or all of externally-threaded region 12 A for a distance L. For purpose of the present invention, L must be less than (D T 2 −D S 2 ) 1/2 . In this way, when outer sleeve 12 melts away, inner sleeve 14 can freely fall away (i.e., pivot down) from the illustrated end 100 B of warhead 100 that forms the pivot point for inner sleeve 14 . This relationship is important because the warhead's fuze (not shown in FIG. 1 ) is coupled to inner sleeve 14 . By way of example, an internal region of inner sleeve 14 may be threaded at 14 A near outboard end 14 B of inner sleeve 14 to accommodate coupling of a fuze thereto. To maximize the mechanical strength of venting cap system 10 , inner sleeve 14 is sized such that outboard end 14 B is flush with end 12 C of outer sleeve 12 , and such that the maximum value for L is used that satisfies the above-noted relationship.

As mentioned above, inner sleeve 14 may be a metal that is selected and predicated on the choice of material for outer sleeve 12 . More specifically, to minimize the risks of de-bonding (i.e., outer sleeve 12 is generally molded about inner sleeve 14 ) at the inner faces of outer sleeve 12 and the outer faces of inner sleeve 14 , the materials for both sleeves should be closely matched in terms of their coefficients of linear expansion. For example, if glass-filled polycarbonate (with a coefficient of linear expansion of 12.0×10 −6 in/in ° F.) is used for outer shell 12 , aluminum (with a coefficient of linear expansion of 12.3×10 −6 in/in ° F.) is a good choice of material for inner sleeve 14 . Additionally, some or all of the external radial surfaces of inner sleeve 14 may be knurled (e.g., small crosshatched score lines) to enhance the bonding grip between outer sleeve 12 and inner sleeve 14 . Note that inner sleeve 14 is not generally threaded into outer sleeve 12 as the threads could introduce unwanted stress concentrations.

As mentioned above, the present invention is not limited by the configuration of the warhead and is not limited by the means of coupling/bonding the inner and outer sleeves. Accordingly and by way of an illustrative example, another embodiment of the present invention is shown in FIG. 2 and is referenced generally by numeral 20 . Elements that are similar to those described in the previous embodiment are referred to by common reference numerals. In this embodiment, the illustrated end of warhead 100 is tapered as indicated at 100 C. Outer sleeve 12 continues the taper on its external radial surface 12 D forward of externally-threaded region 12 A to blend into the external diameter of a fuze 102 . Inner sleeve 14 is partially embedded in outer sleeve 12 at end 14 C thereof to aid in the retention of inner sleeve 14 . The partial embedding of inner sleeve 14 in outer sleeve 12 as illustrated could be formed, for example, by using insertion molding techniques.

The advantages of the present invention are numerous. The venting cap system will prevent a catastrophic cook-off event. At the same time, the venting cap system is mechanically strong enough to withstand rough ordnance-handling conditions.

Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.

Finally, any numerical parameters set forth in the specification and attached claims are approximations (for example, by using the term “about”) that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be at least construed in light of the number of significant digits and by applying ordinary rounding.