Ignition element

An ignition element is provided for integration into a primer assembly. The ignition element includes an electrode housing including at least one cavity and an electrode disposed in the electrode housing. The electrode housing and electrode are electrically isolated from each other using insulators. A bridgewire and a primary ignition charge are located in the cavity such that upon application of current to the electrode the pyrotechnic ignition charge is ignited. A headstock is provided, and the electrode housing is threadably disposed within the headstock. Burst disks cover the cavity and separate the ignition charge from a primer charge containing powder disposed in a single booster holder.

FIELD OF THE INVENTION

The present invention relates to the field of pyrotechnics and propellants. In particular, the present invention relates to ignition elements used in primer assemblies.

BACKGROUND OF THE INVENTION

In general, an explosive is a material that reacts chemically to generate gases at high temperatures and pressures. The expansion of these gases is used to produce a wide range of effects including destructive blast effects, propellant effects and gas generation. These explosive materials are ignited by smaller charges, i.e. primer charges, contained within primers, which are in turn ignited by an initiator containing a small initiator charge.

Initiators are typically used to prime a priming composition located within a primer. The priming composition is used to ignite the main charge in an ordinance, shell, rocket, actuator or gas generator. In ammunition, the main charge acts as a propellant charge to propel a projectile or shell. In a gas generator, the main charge is used to generate gas, for example, for use in an airbag or seatbelt tightener. The main charge is typically an energetic material such as an energetic pyrotechnic chemical or black powder. The initiator includes an initiator charge that is ignited by either electricity or percussion, and the initiator charge ignites the primer charge, which ignites the main charge. The initiator, the primer and the main charge are disposed in various housings that are typically connected together so as to be placed in the necessary contacting arrangement for proper functioning of the pyrotechnic device.

Initiators, primers, and main charges are all exposed to both high temperatures and pressures resulting from the ignition of the energetic material contained therein. These high temperatures and pressures can result in flaws within the various components including the primer, initiator and main charge, and of the seals between these various elements, including conventional pressure fit seals. These flaws include potential escape paths for the hot gases generated by the energetic materials in addition to locations for burn throughs. Therefore, an arrangement of initiators and primers is desired that eliminates potential flaws resulting from the ignition of the initiator charges, primer charges, or main charges. Accordingly, Applicant has developed a novel seal arrangement using threads among the components to minimize the damage due to escaping hot gases and burn throughs.

SUMMARY OF THE INVENTION

The present invention is directed to an ignition element for integration into a primer assembly. The ignition element includes an electrode housing containing at least one cavity and an electrode disposed in the electrode housing. The electrode includes a first end disposed in the cavity and a second end opposite the first end and exposed for application of a current to the second end. The electrode housing and electrode are electrically isolated form each other, for example by use of at least one insulator disposed between the electrode and the electrode housing. These insulators are generally constructed from polyetheretherketone.

A resistance element, for example a bridgewire, is located in the cavity between the first end of the electrode and a source of electric ground. Also disposed in the cavity is a pyrotechnic ignition charge, for example, a mixture of lead styphnate and nitrocellulose. The resistance element, upon application of a sufficient amount of current to the second end of the electrode, generates a sufficient amount of heat to ignite the pyrotechnic ignition charge.

A headstock is provided, and the electrode housing is disposed within the headstock such that the second end of the electrode passes through the headstock. An insulator is provided between the headstock and the electrode and, in particular, the insulator may be constructed from polyetheretherketone. The headstock is constructed from a chrome-moly alloy steel. The electrode housing is threaded into the headstock. These same tolerances are applied to the fit among the insulators, electrode and electrode housing. Adhesives and epoxies can also be used to enhance or strengthen the fit among these components.

Two burst disks cover the cavity and separate the ignition charge from a primer charge. In use, the pyrotechnic ignition charge ruptures the burst disks, igniting the primer charge. The burst disk can be constructed from the same material or from different materials and can be used in conjunction with other parts of the ignition element, for example as a source of ground for the resistance element. The primer charge includes about 850 grams of black powder disposed in a single booster holder.

DETAILED DESCRIPTION

Referring toFIG. 1, an exemplary embodiment of an ignition element10in accordance with the present invention is illustrated. The ignition element includes an electrode housing12. Suitable materials for the electrode housing are known and available in the art. In one embodiment, the electrode housing12includes a first part, that is, first threads,32of a two-part threaded fitting so that the electrode housing12can be threaded into a housing or headstock38. The first threads32are located on a surface of the electrode housing12. The second part, that is, second threads,48of the two-part threaded fitting are the corresponding threads situated on a surface of the housing or headstock38. The first part32threadably mates, or engages, with the second part48so that the first part32, or first threads, contact the second part, or second threads48. Accordingly, in an embodiment, the electrode housing12is threaded into a cavity of the headstock38so as to be threadably anchored, or engaged, to the headstock38. Threading of the electrode housing provides a more torturous path for gases that try to escape through the ignition element during actuation of the ignition element. In addition, the threaded fitting can be used to establish a pre-load on the components of the ignition element, for example insulators and sealants, providing added sealing efficiency. The first part32and second part48are each composed of threads with standard dimensions and, in an embodiment; the dimensions include a 0.750 dia-32 UN-2A.

Disposed within the housing12is at least one electrode14. The electrode14is constructed from an electrically conductive material. Suitable electrically conductive materials are known and available in the art. In one embodiment, the electrode14and electrode housing12are arranged as generally concentric cylindrical assemblies with the electrode14running generally through the middle of the electrode housing, although other shapes and arrangements of electrodes and electrode housings are possible. In addition, as illustrated a single electrode is used; however, multiple electrodes can also be used, for example two electrodes, one positive and one negative.

The electrode housing12includes at least one cavity24arranged to hold a pyrotechnic ignition charge or initiating mixture. Suitable ignition charges include any pyrotechnic ignition charge material including black powder, lead styphnate, nitrocellulose or guncotton and mixtures thereof. In an embodiment, the initiating mixture is a mixture of lead styphnate and nitrocellulose. The electrode includes a first end20that is disposed within the cavity24, and a second end16opposite the first end20. The second end16is generally exposed in the assembled ignition element and in any primers or devices into which the ignition element10is integrated to facilitate contact of the second end16with a current source to actuate the ignition element10.

In order to facilitate proper electrical operation of the ignition element and to inhibit uncontrolled actuation, the electrode14is electrically insulated from the electrode housing12. In one embodiment, the ignition element10includes at least one insulator disposed between the electrode14and the electrode housing12. Alternatively, a plurality of insulators is disposed between the electrode14and the electrode housing12. In one exemplary embodiment as illustrated, the insulators include a top insulator30disposed between the electrode14and the electrode housing12and generally adjacent the first end20of the electrode14and a bottom insulator28disposed between the electrode14and the electrode housing12and generally adjacent the second end16of the electrode. In another embodiment, the insulators also include a primer stock insulator26disposed between the electrode14and the electrode housing12or any housing or support structure into which the electrode14and electrode housing12are disposed, for example a headstock. In one embodiment, the bottom insulator28is disposed along a length of the electrode14between the top insulator30and the primer stock insulator26.

Suitable materials for the insulator are capable of withstanding the operating pressures and temperatures to which the ignition element10is exposed. In an embodiment, the insulator is constructed from polyetheretherketone. The insulators26,28,30can be pressure fit against the electrode14or can be attached to the electrode14using an adhesive or epoxy material. In one embodiment, the insulators are attached to the electrode using a two-part epoxy adhesive that provides the desired level of shear and peel adhesion and durability. Suitable two-part epoxy adhesives include Scotch-Weld™ Epoxy Adhesive DP-460, which is commercially available from the 3M Company of St. Paul, Minn. In one embodiment, the ignition element10also includes a sealant34between the top insulator30and the cavity24. Suitable sealants include, but are not limited to, 3M™ Scotchcast™ which is commercially available from the 3M Company of St. Paul, Minn.

In one embodiment, the ignition element10also includes a resistance element18disposed between the first end20of the electrode14and a source of electrical ground. The resistance element18is capable of generating a sufficient amount of heat to ignite the pyrotechnic ignition charge in the cavity24upon application of a current to the second end16of the electrode14. Suitable resistance elements produce a sufficient amount of thermal energy to initiate combustion in the initiating mixture and include filaments, metals films and bridgewires. In one embodiment, the electric resistance element includes a high-resistance wire, for example, a bridgewire, in operative contact with the pyrotechnic ignition charge. For example, the wire can be wrapped in a wisp of guncotton and contained in a mixture of pulverized guncotton and fine black powder in the cavity in the electrode housing. Alternatively, the wire is contained in a mixture of lead styphnate and nitrocellulose. The wire is connected at one end to the first and of the electrode. The opposite end of the wire is connected to ground. Connection to ground can be accomplished through connection to a second electrode, which is connected to ground or through connection to a conducting source, such as, a metallic housing that is connected to ground. In one embodiment, the opposite end of the wire is grounded through the primer stock38and the cartridge case to the metal of a gun.

In one embodiment, the ignition element also includes at least one burst disk22in contact with the electrode housing12and covering the cavity24. In one embodiment, the burst disk22is also in contact with the first end20of the electrode14to function as the resistance element. Alternatively, the burst disk22is in contact with the resistance element18to provide a source to ground. In an embodiment, the ignition element10includes two overlapping burst disks, collectively, referred to as22. In one embodiment, the two burst disks22are constructed from substantially the same material. In another embodiment, the two burst disks22are constructed from substantially different materials. Suitable burst disks are constructed from both electrically conductive and non-conductive materials including plastics, polymers, metals, for example, nickel, metallic foils, graphite and combinations thereof. In one embodiment, the first burst disk22is a conductive material, and the second burst disk22is an insulating material. Since the first disk22is placed in contact with the cavity containing the pyrotechnic initiating mixture, the first end20of the electrode14and the bridgewire, the first burst disk can be placed in contact with the bridgewire to provide a grounding connection, and the second burst disk insulates the first burst disk, which is part of the resistance element, from undesired or unanticipated electrical interference.

The ignition element10can be used in any application to initiate a primer charge, propellant charge, explosive actuator or gas generator, for example in an air bag deployment system. The ignition element in accordance with the present invention and assemblies into which it is integrated provide improved reliability and performance at operating pressures up to about 100,000 psi. Generally, operating pressures are in the range from about 60,000 psi to about 80,000 psi, and in particular, about 65,000 psi or more particularly about 75,000 psi.

Referring toFIGS. 2 and 3, in one exemplary embodiment, the ignition element10is integrated into a primer assembly36. The electrode housing12and electrode are disposed within a housing or headstock38. In general, a headstock refers to a part of an assembly containing or directly supporting the operative parts. Although illustrated as independent of the ignition element10, in an alternative embodiment, the headstock38can be considered as part of the ignition element10. Suitable materials for the headstock include plastics and metals. In an embodiment, the headstock38is made from a chrome-moly alloy steel, that is, a chromium molybdenum based steel. The electrode housing12and electrode14are disposed in the headstock38such that the second end16of the electrode14passes through the headstock38. In one embodiment, the electrode housing12is threaded into the headstock38, and the headstock38includes a complimentary second part to the first part32of the two-part threaded fitting. Alternatively, the electrode housing12is force or press fit into the headstock38. The tolerances between the electrode housing12and the headstock38reduce potential gaps in the assembly. It is also desired to eliminate these gaps between the electrode housing, the electrode and the insulators. In one embodiment, the tolerances between any two of the electrode, electrode housing, headstock or insulators are less than about 0.005 inches. In an embodiment, these tolerances are less than about 0.001 inches.

A booster holder42including a primer charge44is brought into contact with the electrode housing12and the burst disks22. A primer tube40is then either press fit or threaded using a two-part threaded fitting46into the headstock38forming a single primer assembly that can then be integrated into a larger assembly such as a rocket, shell or propelling charge. In one embodiment, the burst disks22separate the pyrotechnic ignition charge from the primer charge44. When the pyrotechnic ignition charge is ignited, it ruptures the burst disks22, igniting the primer charge44. Overall, the arrangement of the ignition element10and the primer assembly36reduces the number of parts required by about half, reducing costs, assembly complexity and the number of joints or gaps that could be points of failure or weakness. In addition, the reduction of the number and complexity of the parts facilitates the use of a larger booster holder42and a single, larger primer charge disposed in a single area. In one embodiment, the primer charge44contains about 850 grams of black powder disposed in the single booster holder42.

In use, the ignition element10and primer assemblies36into which it is integrated are used to initiate a flame for the ultimate purpose of igniting a charge of propellant or gas generation. The initiator or ignition element is used to initiate the flame in the primer assembly. Although ignition elements can be percussion, electric or combination initiators, in an embodiment, the ignition element of the present invention is electric. In electric initiation elements, firing is accomplished by passing a current through an electrode and a resistance element surrounded by a pyrotechnic initiating charge or initiating mixture. The resistance element produces a sufficient amount of thermal energy to initiate combustion in the initiating charge, which causes a burst disk to rupture, exposing the primer charge to the heat and pressure of the ignited charge and igniting the primer charge. The primer charge travels through the primer tube in which is disposed to initiate the main propellant or gas generating charge.

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 at least be construed in light of the number of significant digits and by applying ordinary rounding.