Ammunition and explosives – Shells – With safety means
Reexamination Certificate
2002-06-19
2003-09-09
Johnson, Stephen M. (Department: 3641)
Ammunition and explosives
Shells
With safety means
C102S499000, C102S202100
Reexamination Certificate
active
06615737
ABSTRACT:
The present invention relates to the general field of pyrotechnic munitions and more particularly to that of explosive munitions.
A particular subject-matter of the present invention is a safety igniter for a pyrotechnic munition component comprising a structure in the form of a jacket and a solid pyrotechnic charge present in the structure, the said igniter being intended to bring about the combustion without detonation of the pyrotechnic charge when the munition component is subjected to slow cook off.
BACKGROUND OF THE INVENTION
Stresses of thermal origin, such as kerosene or propellant fires, indirect heatings, can result in the pyrotechnic reaction of the munitions which are subjected to them.
Explosive-comprising munition components, such as missile warheads, bomb casings, penetrators and submarine munitions, can lead to violent blast or detonation reactions because of their high confinement.
To reduce these reactions to an acceptable level, that is to say to a simple combustion without projection of dangerous splinters, the use is known of a composite explosive charge based on inert polymer binders or energetic polymer binders charged with octogen (HMX), hexogen (RDX), nitroguanidine, ammonium perchlorate, triaminotrinitrobenzene (TATB), oxynitrotriazole (ONTA) and/or aluminium, in combination with a system for deconfinement of the structure of the munition.
The deconfinement system can consist of protective caps which can burst at a predefined pressure, which act as safety valve by releasing a discharge surface to the decomposition gases generated by the pyrotechnic reaction. Other techniques exist, such as the use of fusible components, of cutting cords or of incipient fractures.
This safety concept operates perfectly for intense fires of kerosene type. In this case, the very high temperatures are transmitted to the wall of the munition and then to the charge, which reacts by combustion at the structure/explosive interface as soon as the temperature exceeds the self-ignition temperature of the explosive, which is generally between 200° C. and 240° C. The combustion gases subsequently make their way to the discharge surfaces.
The case of less intense and longer lasting stresses is more complex.
The “slow cook off” stress is specified conventionally and consists in subjecting a munition component to heating by a few degrees per hour until it reacts pyrotechnically, which can occur after several tens of hours. These reactions can be very violent as they begin, in some cases, at the core of the pyrotechnic material in a medium which will have the time to decompose by pyrolysis of the binder and beginning of chemical decomposition of the active materials. Core initiations are frequently observed with large-calibre munitions (bombs, penetrators, submarine munitions). They are the consequence of the thermally highly insulating nature of the explosives and of the beginning of exothermic decomposition within the material. The heat given off cannot be discharged towards the outside and leads to an additional internal rise in the temperature which further accelerates the decomposition until the mass reaction. The greater the dimensions, the lower the reaction temperature.
A simple deconfinement system such as those mentioned above is insufficient in this case to limit the overall level of reaction.
It is known, to limit the level of reaction under slow cook off stresses, to insert, in the vicinity of the deconfinement device, a safety igniter which reacts by combustion at a temperature lower than the reaction temperature of the main charge of the pyrotechnic munition, the said combustion of the igniter leading to the combustion without detonation of the main charge.
U.S. Pat. No. 5,786,544 and GB 2 313 653 disclose such safety igniters, composed essentially of a plastic tube comprising an ignition powder or ignition pellets. The igniter is embedded in a ring of foam which separates it from the charge, in the rear part of the munition component, close to deconfinement holes.
The ignition pellets present in the tube are preferably composed of a mixture of boron and of barium chromate. In point of fact, it turns out that barium chromate is particularly toxic and carcinogenic and that it leads to hereditary genetic damage. Furthermore, under thermal stresses, it gives off fumes which are also highly toxic.
Other solutions relating to the nature of the ignition pellets are provided but none is truly satisfactory.
The use of pellets based on nitrocellulose-nitroglycerine double base propellant exhibits, for example, problems of migration of the nitroglycerine on storage, with the pyrotechnic risks which this results in.
There thus exists, for a person skilled in the art, a need for a safety igniter which makes it possible to provide the function described above but which does not exhibit disadvantages, such as those mentioned above.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a solution to this problem and has, as main subject-matter, a novel safety igniter for a pyrotechnic munition component comprising a structure in the form of a jacket and a solid pyrotechnic charge present in the structure, the said igniter being intended to bring about the combustion without detonation of the pyrotechnic charge when the munition component is subjected to slow cook off.
This novel safety igniter according to the invention is characterized in that it comprises a block of solid composition based on pentrite and, preferably, in that it is composed solely of such a block.
Such a safety igniter is particularly simple to manufacture and to insert in the munition component. A single block, it exhibits sufficient intrinsic mechanical properties which do not require an inert housing, such as a plastic tube.
Furthermore, this block of solid composition based on pentrite is a detonatable material contributing additional energy in the context of normal use of the munition, which is not the case with the abovementioned igniters of the state of the art, which are not detonatable. This duality of function, safety igniter in the context of slow cook off and explosive in the context of normal use of the munition, which is specific to the present invention, is particularly advantageous.
It should also be noted that the constituents of the safety igniter according to the invention are neither toxic nor carcinogenic and that the combustion fumes are not particularly toxic.
It is known, in the state of the art, to use solid compositions based on pentrite as priming relay for explosive munitions. Patent Application PCT WO 99/53264 discloses, for example, such a use, which is unrelated to that of safety igniter which is a subject-matter of the present invention. This known priming relay use dissuaded even a person skilled in the art from envisaging the use of these pentrite-based compositions in the function, described above, of safety igniter.
In the context of the present invention, “slow” cook off should be understood as cook off of between 0.5° C./h and 50° C./h, preferably of between 1° C./h and 20° C./h, even better still between 2° C./h and 10° C./h, for example approximately 3° C./h or 4° C./h.
Composition “based” on pentrite should furthermore be understood as a composition having a content by weight of pentrite≧5%, better still≧10% and better still≧25%, the maximum content by weight being approximately 98%.
According to a preferred alternative form of the invention, the block of solid pyrotechnic composition based on pentrite is a composite explosive (cast plastic bonded explosive). Composite explosives are generally well known to a person skilled in the art. They are obtained from explosive compositions with a plastic binder processed by casting and then polymerization and are composed of a charged plastic binder comprising at least one nitrated organic explosive charge, such as hexogen, octagen or pentrite. Other oxidizing charges, such as, for example, ammonium perchlorate, or reducing charges, such as, for example, aluminium, can also be present.
More specifically, to pr
Bonnel Alain
Houdusse Dominique
Nouguez Bruno
Tinet Alain
Johnson Stephen M.
Oliff & Berridg,e PLC
SNPE
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