Abstract:
Inertial igniters for initiating a thermal battery and methods of preventing initiation of an inertial igniter are provided. The method including: biasing a mass element from a base element with a spring element connecting the mass element and the spring element; and removably disposing a safety member between the mass element and the base element to prevent relative movement between the mass element and base element to prevent accidental initiation of the inertial igniter. The method can further comprise removing the safety member prior to subjecting the inertial igniter to all-fire conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit to U.S. Provisional Application No. 61/184,787 filed on Jun. 6, 2009, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to inertial igniters for thermal batteries or other pyrotechnic type initiated devices for munitions, such as rockets, with low setback accelerations and equipped with safety pins. 
         [0004]    2. Prior Art 
         [0005]    Thermal batteries represent a class of reserve batteries that operate at high temperatures. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. The most common internal pyrotechnic is a blend of Fe and KClO 4 . Thermal batteries utilize a molten salt to serve as the electrolyte upon activation. The electrolytes are usually mixtures of alkali-halide salts and are used with the Li(Si)/FeS 2  or Li(Si)/CoS 2  couples. Some batteries also employ anodes of Li(Al) in place of the Li(Si) anodes. Insulation and internal heat sinks are used to maintain the electrolyte in its molten and conductive condition during the time of use. Reserve batteries are inactive and inert when manufactured and become active and begin to produce power only when they are activated. 
         [0006]    Thermal batteries have long been used in munitions and other similar applications to provide a relatively large amount of power during a relatively short period of time, mainly during the munitions flight. Thermal batteries have high power density and can provide a large amount of power as long as the electrolyte of the thermal battery stays liquid, thereby conductive. The process of manufacturing thermal batteries is highly labor intensive and requires relatively expensive facilities. Fabrication usually involves costly batch processes, including pressing electrodes and electrolytes into rigid wafers, and assembling batteries by hand. The batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. Thermal batteries, however, have the advantage of very long shelf life of up to 20 years that is required for munitions applications. 
         [0007]    Thermal batteries generally use some type of igniter to provide a controlled pyrotechnic reaction to produce output gas, flame or hot particles to ignite the heating elements of the thermal battery. There are currently two distinct classes of igniters that are available for use in thermal batteries. The first class of igniter operates based on electrical energy. Such electrical igniters, however, require electrical energy, thereby requiring an onboard battery or other power sources with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. The second class of igniters, commonly called “inertial igniters”, operates based on the firing acceleration. The inertial igniters do not require onboard batteries for their operation and are thereby often used in high-G munitions applications such as in gun-fired munitions and mortars. 
         [0008]    In general, the inertial igniters, particularly those that are designed to operate at relatively low impact levels, have to be provided with the means for distinguishing events such as accidental drops or explosions in their vicinity from the firing acceleration levels above which they are designed to be activated. The mechanism or circuitry for distinguishing an accidental drop from an all-fire condition can be complicated and expensive. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, an inertial igniter is provided. The inertial igniter comprising: a mass element; a base element; a spring element connecting the mass element and the base element; and a safety member removably disposed between the mass element and the base element for preventing relative movement between mass element and base element to prevent accidental initiation of the inertial igniter. 
         [0010]    The mass element can comprise one of a two part pyrotechnic and the base element can comprise the other of the two part pyrotechnic. 
         [0011]    The inertial igniter can further comprise a thermal battery connected to the base element, the base element having at least one opening for passing fire and/or sparks resulting from contact between the two part pyrotechnics to the thermal battery. 
         [0012]    A gap can be formed between the mass element and the base element and the safety member can be disposed in the gap. 
         [0013]    The gap can be at least partially annular and the safety member can comprise a semicircular member disposed in the annular gap. 
         [0014]    The spring element, base element and mass element can be circular and the spring member can have a diameter smaller than the mass element and base element. 
         [0015]    The semicircular member can have first and second ends forming a partial arc having a radius such that it provides a positive lock when disposed between the mass element and the base element. 
         [0016]    The safety member can further comprise a tab having a hole for insertion of a wire. 
         [0017]    The spring element can be a bellows. The bellows can be sealed to both the mass element and base element to form a sealed enclosure between the mass element and spring element. 
         [0018]    One or more of the base element and mass element can be disc-shaped. 
         [0019]    Also provided is a method of preventing initiation of an inertial igniter. The method comprising: biasing a mass element from a base element with a spring element connecting the mass element and the spring element; and removably disposing a safety member between the mass element and the base element to prevent relative movement between the mass element and base element to prevent accidental initiation of the inertial igniter. 
         [0020]    The method can further comprise removing the safety member prior to subjecting the inertial igniter to all-fire conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0022]      FIG. 1  illustrates a sectional view of an inertial igniter and thermal battery assembly according to a first embodiment. 
           [0023]      FIG. 2  illustrates the inertial igniter and thermal battery assembly of  FIG. 1  in which a safety member is engaged to prevent accidental initiation of the inertial igniter. 
           [0024]      FIG. 3  illustrates the inertial igniter and thermal battery assembly of  FIG. 2  in which the safety member is disengaged. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    The schematic of a first embodiment is shown in the frontal cross-sectional view of  FIG. 1 . In  FIG. 1 , the inertial igniter  10  is shown to be fixed to the top of the thermal battery  11 . The inertial igniter  10  is shown to consist of a top cap  12 , which also acts as a striker mass for initiating ignition during all-fire conditions. The top cap  12  is attached (such as being welded or brazed) to the base plate  13  by a bellow  14 , which can be stainless steel or other suitable material. The base plate  13  can be the top cap of the thermal battery  20  and is attached (such as being welded or brazed) to the thermal battery housing  19 . The base plate  13  can be fabricated with stainless steel. The top cap  12  can be made out of a one piece stainless steel material. The top cap  12  may also be constructed with two piece elements (not shown in  FIG. 1 ), with an outer shell of stainless steel (to make it easier to weld to the bellow  14 ) and an interior (in the present design disc shaped) element constructed of a denser material, for example tungsten or brass, that is inserted into the shell to make the top cap  12  (i.e., the striker mass) heavier to reduce the overall size and volume of the inertial igniter. 
         [0026]    The above construction provides for a hermetically sealed inertial igniter and thermal battery assembly. The bellow  14  acts as a spring element and allows the top cap (the striker mass) to travel downward when all-fire condition (firing setback acceleration in the direction of the arrow  21  acting over relatively long firing time) is achieved, but provides enough resistance to prevent ignition under no-fire conditions (acceleration profiles with lower than total all-fire impulse levels). 
         [0027]    As is shown in  FIG. 1 , the diameters of the top cap  12  and the base plate  13  are larger than the outside diameter of the bellow  14 . This allows the insertion of a safety member, such as a safety pin in a gap formed between the top cap  12  and the base plate  13  to keep them apart, thus, preventing accidental ignition during transportation of munitions or accidental dropping and the like. 
         [0028]    The overall view of the inertial igniter as assembled with a typical thermal battery together with the safety pin  21  in its engaged and disengaged positions are shown in  FIGS. 2 and 3 , respectively. As can be seen in  FIG. 2 , when the safety pin  21  is engaged, i.e., when it is positioned between the top cap  12  and the base plate  13  as shown in  FIG. 2 , the top cap  12  is prevented from moving towards the base plate  13 , thereby preventing igniter activation. The safety pin  21  can be provided with a positive lock in its engaged position, for example, the safety pin “fork”  23  ( FIG. 3 ) is larger than half the circumference of the bellow  14  and is flexible and preloaded to require certain amount of force in the direction of the arrow  24  to disengage it from the inertial igniter  10 . This provision is made for the sake of safety and to prevent accidental pulling/disengagement of the safety pin  21 . In certain applications, the safety pin  21  may be desired to be “locked” in its engaged position by certain means, for example by a wire passing through a hole  22  and which is wrapped around the inertial igniter and twist tightened to prevent accidental pulling of the safety pin  21 . To disengage the safety pin, the locking wire (or the like) needs to be removed, such as by cutting to allow the release of the safety pin  21 , thereby initiation of the thermal batter  21  as the all-fire condition is encountered. 
         [0029]    When an all fire-condition occurs and if the safety pin  21  has been withdrawn, the setback acceleration in the direction of the arrow  25  ( FIG. 1 ) causes the top cap  12  (striker mass) to be accelerated towards the base plate  13 . The top cap  12  will then impact the base plate  13 , causing the one or two part pyrotechnic (initiation) materials  15  to be initiated. The top cap  12  and the base plate  13  can be provided with at least one protruded portions  16  and  17 , respectively, as shown in  FIG. 1 , to provide “pinching” points within the pyrotechnic (initiation) materials as the top cap  12  impacts the base plate  13  to facilitate the initiation. At least one flame exit port  18  is provided (preferably close to the center of the base plate  13 ) to allow the flames and sparks generated by the ignition of the pyrotechnic (initiation) materials to enter into the thermal battery for its initiation (activation). 
         [0030]    In certain applications, particularly when the no-fire acceleration requirement is relatively high and/or when the no-fire acceleration period is relatively long, the spring rate of the bellow  14  to resist the downward motion of the top cap  12  towards the base plate  13  may not be adequate. In such applications, at least one spring element (not shown) can be provided between the top cap  12  and the base plate  13  to provide for added spring rate. The added spring element can be positioned inside the inertial igniter  10  (i.e., inside the bellow  14 ). The spring element can also be a helical or a similar type of spring element, separate from or integral with the bellow  14 . 
         [0031]    If a spring element, such as a helical coil spring, is used, the safety member can be a comb type element (not shown) having fingers which fit within the helical spaces of the coil spring so as to prevent compression of the coil spring, thus preventing accidental initiation. 
         [0032]    While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.