Abstract:
An inertial igniter including: a first member having a wall and internal cavity; a second member slidable in the internal cavity, a striker disposed thereon and a first concave portion; a third member slidable on an exterior surface of the wall, a second concave portion; biasing springs for biasing the first and second members in a direction opposite an acceleration; locking balls in the first and second concave portions for preventing movement of the second and third members when the acceleration time profile is below a predetermined threshold; and a percussion cap primer on the first member; wherein when the acceleration time profile is greater than the predetermined threshold the locking balls are released from the concave portions to first permit relative movement of the third member with the first member and after a time delay to permit relative movement of the second member with the first member.

Description:
GOVERNMENT RIGHTS 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of SBIR Grant No. DAAE30-03-C-1077 awarded by the Department of Defense on Jul. 17, 2006. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to mechanical igniters, and more particularly to axially compact mechanical igniters for thermal batteries and the like. 
   2. Prior Art 
   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. 
   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. 
   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 igniters operate 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 non-spinning gun-fired munitions and mortars. 
   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. This means that safety in terms of prevention of accidental ignition is one of the main concerns in inertial igniters. 
   In recent years, new improved chemistries and manufacturing processes have been developed that promise the development of lower cost and higher performance thermal batteries that could be produced in various shapes and sizes, including their small and miniaturized versions. However, the existing inertial igniters are relatively large and not suitable for small and low power thermal batteries, particularly those that are being developed for use in miniaturized fuzing, future smart munitions, and other similar applications. 
   The need to differentiate accidental and initiation accelerations by the resulting impulse level of the event necessitates the employment of a safety system which is capable of allowing initiation of the igniter only during high total impulse levels. The safety mechanism can be thought of as a mechanical delay mechanism, after which a separate initiation system is actuated or released to provide ignition of the pyrotechnics. An inertial igniter that combines such a safety system with an impact based initiation system and its alternative embodiments are described herein together with alternative methods of initiation pyrotechnics. 
   Inertia-based igniters must therefore comprise two components so that together they provide the aforementioned mechanical safety (delay mechanism) and to provide the required striking action to achieve ignition of the pyrotechnic elements. The function of the safety system is to fix the striker in position until a specified acceleration time profile actuates the safety system and releases the striker, allowing it to accelerate toward its target under the influence of the remaining portion of the specified acceleration time profile. The ignition itself may take place as a result of striker impact, or simply contact or proximity. For example, the striker may be akin to a firing pin and the target akin to a standard percussion cap primer. Alternately, the striker-target pair may bring together one or more chemical compounds whose combination with or without impact will set off a reaction resulting in the desired ignition. 
   In addition to having a required acceleration time profile which will actuate the device, requirements also commonly exist for non-actuation and survivability. For example, the design requirements for actuation for one application are summarized as: 
   1. The device must fire when given a [square] pulse acceleration of 900 G±150 G for 15 ms in the setback direction. 
   2. The device must not fire when given a [square] pulse acceleration of 2000 G for 0.5 ms in any direction. 
   3. The device must not actuate when given a ½-sine pulse acceleration of 490 G (peak) with a maximum duration of 4 ms. 
   4. The device must be able to survive an acceleration of 16,000 G, and preferably be able to survive an acceleration of 50,000 G. 
   A schematic of a cross-section of a thermal battery and inertial igniter assembly is shown in  FIG. 1 . In thermal battery applications, the inertial igniter  10  (as assembled in a housing) is generally positioned above the thermal battery housing  11  as shown in  FIG. 1 . Upon ignition, the igniter initiates the thermal battery pyrotechnics positioned inside the thermal battery through a provided access  12 . The total volume that the thermal battery assembly  16  occupies within munitions is determined by the diameter  17  of the thermal battery housing  11  (assuming it is cylindrical) and the total height  15  of the thermal battery assembly  16 . The height  14  of the thermal battery for a given battery diameter  17  is generally determined by the amount of energy that it has to produce over the required period of time. For a given thermal battery height  14 , the height  13  of the inertial igniter  10  would therefore determine the total height  15  of the thermal battery assembly  16 . To reduce the total volume that the thermal battery assembly  16  occupies within a munitions housing, it is therefore important to reduce the height of the inertial igniter  10 . This is particularly important for small thermal batteries since in such cases the inertial igniter height with currently available inertial igniters can be almost the same order of magnitude as the thermal battery height. 
   With currently available inertial igniters (produced by Eagle Pitcher Technologies, LLC), a schematic of which is shown in  FIG. 2 , the inertial igniter  20  has to be positioned within a housing  21  as shown in  FIG. 3 . The housing  21  and the thermal battery housing  11  may share a common cap  22 , with the opening  25  to allow the ignition fire to reach the pyrotechnic material  24  within the thermal battery housing. As the inertial igniter is initiated, the sparks can ignite intermediate materials  23 , which can be in the form of thin sheets to allow for easy ignition, which would in turn ignite the pyrotechnic materials  24  within the thermal battery through the access hole  25 . 
   A schematic of a cross-section of a currently available inertial igniter  20  is shown in  FIG. 2  in which the acceleration is in the upward direction (i.e., towards the top of the paper). The igniter has side holes  26  to allow the ignition fire to reach the intermediate materials  23  as shown in  FIG. 3 , which necessitate the need for its packaging in a separate housing, such as in the housing  21 . The currently available inertial igniter  20  is constructed with an igniter body  60 . Attached to the base  61  of the housing  60  is a cup  62 , which contains one part of a two-part pyrotechnic compound  63  (e.g., potassium chlorate). The housing  60  is provided with the side holes  26  to allow the ignition fire to reach the intermediate materials  23  as shown in  FIG. 3 . A cylindrical shaped part  64 , which is free to translate along the length of the housing  60 , is positioned inside the housing  60  and is biased to stay in the top portion of the housing as shown in  FIG. 2  by the compressively preloaded helical spring  65  (shown schematically as a heavy line). A turned part  71  is firmly attached to the lower portion of the cylindrical part  64 . The tip  72  of the turned part  71  is provided with cut rings  72   a , over which is covered with the second part of the two-part pyrotechnic compound  73  (e.g., red phosphorous). 
   A safety component  66 , which is biased to stay in its upper most position as shown in  FIG. 2  by the safety spring  67  (shown schematically as a heavy line), is positioned inside the cylinder  64 , and is free to move up and down (axially) in the cylinder  64 . As can be observed in  FIG. 2 , the cylindrical part  64  is locked to the housing  60  by setback locking balls  68 . The setback locking balls  68  lock the cylindrical part  64  to the housing  60  through holes  69   a  provided on the cylindrical part  64  and the housing  60  and corresponding holes  69   b  on the housing  60 . In the illustrated configuration, the safety component  66  is pressing the locking balls  68  against the cylindrical part  64  via the preloaded safety spring  67 , and the flat portion  70  of the safety component  66  prevents the locking balls  68  from moving away from their aforementioned locking position. The flat portion  70  of the safety component  66  allows a certain amount of downward movement of the safety component  66  without releasing the locking balls  68  and thereby allowing downward movement of the cylindrical part  64 . For relatively low axial acceleration levels or higher acceleration levels that last a very short amount of time, corresponding to accidental drops and other similar situations that cause safety concerns, the safety component  66  travels up and down without releasing the cylindrical part  64 . However, once the firing acceleration profiles are experienced, the safety component  66  travels downward enough to release balls  68  from the holes  69   b  and thereby release the cylindrical part  64 . Upon the release of the safety component  66  and appropriate level of acceleration for the cylindrical part  64  and all other components that ride with it to overcome the resisting force of the spring  65  and attain enough momentum, then it will cause impact between the two components  63  and  73  of the two-part pyrotechnic compound with enough strength to cause ignition of the pyrotechnic compound. 
   The aforementioned currently available inertial igniters have a number of shortcomings for use in thermal batteries, specifically, they are not useful for relatively small thermal batteries for munitions with the aim of occupying relatively small volumes, i.e., to achieve relatively small height total igniter compartment height  13 ,  FIG. 1 . Firstly, the currently available inertial igniters, such as that shown in  FIG. 2  are relatively long thereby resulting in relatively long total igniter heights  13 . Secondly, since the currently available igniters are not sealed and exhaust the ignition fire out from the sides, they have to be packaged in a housing  21 , usually with other ignition material  23 , thereby increasing the height  13  over the length of the igniter  20  (see  FIG. 3 ). In addition, since the pyrotechnic materials of the currently available igniters  20  are not sealed inside the igniter, they are prone to damage by the elements and cannot usually be stored for long periods of time before assembly into the thermal batteries unless they are stored in a controlled environment. 
   SUMMARY OF THE INVENTION 
   A need therefore exists for the development of novel miniature inertial igniters for thermal batteries used in gun fired munitions, particularly for small and low power thermal batteries that could be used in fuzing and other similar applications, thereby eliminating the need for external power sources. The development of such novel miniature inertial ignition mechanism concepts also requires the identification or design of appropriate pyrotechnics. The innovative inertial igniters can be scalable to thermal batteries of various sizes, in particular to miniaturized igniters for small size thermal batteries. Such inertial igniters must be safe and in general and in particular they should not initiate if dropped, e.g., from up to 7 feet onto a concrete floor for certain applications; should withstand high firing accelerations, for example up to 20-50,000 Gs; and should be able to be designed to ignite at specified acceleration levels when subjected to such accelerations for a specified amount of time to match the firing acceleration experienced in a gun barrel as compared to high G accelerations experienced during accidental falls which last over very short periods of time, for example accelerations of the order of 1000 Gs when applied for 5 msec as experienced in a gun as compared to for example 2000 G acceleration levels experienced during accidental fall over a concrete floor but which may last only 0.5 msec. Reliability is also of much concern since the rounds should have a shelf life of up to 20 years and could generally be stored at temperatures of sometimes in the range of −65 to 165 degrees F. This requirement is usually satisfied best if the igniter pyrotechnic is in a sealed compartment. The inertial igniters must also consider the manufacturing costs and simplicity in design to make them cost effective for munitions applications. 
   To ensure safety and reliability, inertial igniters should not initiate during acceleration events which may occur during manufacture, assembly, handling, transport, accidental drops, etc. Additionally, once under the influence of an acceleration profile particular to the firing of ordinance from a gun, the device should initiate with high reliability. In many applications, these two requirements often compete with respect to acceleration magnitude, but differ greatly in impulse. For example, an accidental drop may well cause very high acceleration levels—even in some cases higher than the firing of a shell from a gun. However, the duration of this accidental acceleration will be short, thereby subjecting the inertial igniter to significantly lower resulting impulse levels. It is also conceivable that the igniter will experience incidental low but long-duration accelerations, whether accidental or as part of normal handling, which must be guarded against initiation. Again, the impulse given to the miniature inertial igniter will have a great disparity with that given by the initiation acceleration profile because the magnitude of the incidental long-duration acceleration will be quite low. 
   Those skilled in the art will appreciate that the inertial igniters disclosed herein may provide one or more of the following advantages over prior art inertial igniters: 
   provide inertial igniters that are significantly shorter than currently available inertial igniters for thermal batteries or the like, particularly relatively small thermal batteries to be used in munitions without occupying very large volumes; 
   provide inertial igniters that can be mounted directly onto the thermal batteries without a housing (such as housing  21  shown in  FIG. 3 ), thereby allowing even a smaller total height for the inertial igniter assembly; 
   provide inertial igniters that can directly initiate the pyrotechnics materials inside the thermal battery without the need for intermediate ignition material (such as the additional material  23  shown in  FIG. 3 ) or a booster; 
   provide inertial igniters that allow the use of standard off-the-shelf percussion cap primers instead of specially designed pyrotechnic components; and 
   provide inertial igniters that can be sealed to simplify storage and increase their shelf life. 
   Accordingly, an inertial igniter for use with a thermal battery for producing power upon acceleration is provided. The inertial igniter comprising: a first member having a wall defining an internal cavity; a second member disposed on one of the internal cavity and an exterior surface of the wall, the second member being slidable relative to the first member and having one of a striker and a first pyrotechnic material disposed thereon, the second member further having a first concave portion; a third member disposed on the other of the internal cavity and an exterior surface of the wall, the third member being slidable relative to the first member, the third member having a second concave portion; a first biasing means for biasing the second member in a direction opposite the acceleration; a second biasing means for biasing the third member in the direction opposite the acceleration; one or more locking balls each having a portion thereof disposed in the first concave portion and another portion thereof disposed in the second concave portion for preventing relative movement of the second and third members with the first member when an acceleration time profile is below a predetermined threshold; and one of a percussion cap primer and a second pyrotechnic material disposed on the first member; wherein when the acceleration time profile is greater than the predetermined threshold the one or more locking balls are released from one or more of the first concave portion and the second concave portion to first permit relative movement of the third member with the first member and after a time delay to permit relative movement of the second member with the first member such that the one of the striker and the first pyrotechnic material strikes the one of the percussion cap primer and the second pyrotechnic material. 
   The first member can further have an opening for allowing an ignition fire resulting from the striking of the one of the striker and the first pyrotechnic material with the one of the percussion cap primer and the second pyrotechnic material to exit therethrough. 
   The inertial igniter can further comprise a housing portion connected to the first member for enclosing the second and third members. The housing can comprise a wall portion and a top portion. The top portion can be sealed to the wall portion to seal the internal cavity from an exterior of the housing. 
   The wall, second member and third member can be circular in cross-section. 
   The first concave portion can be one of a groove or dimple formed on an outer surface of the second member. The second concave portion can be a shoulder formed on an inner surface of the third member. 
   The first member can include a cup for holding the one of the percussion cap primer and the second pyrotechnic material. The second member can comprise a bore and the striker can be disposed in the bore. The second member can comprise a central element for holding the first pyrotechnic material. The central element can further have one or more cut ring portions for facilitating the holding of the first pyrotechnic material. 
   Also provided is an inertial igniter and thermal battery assembly for producing power upon acceleration. The assembly comprising: an inertial igniter comprising: a first member having a wall defining an internal cavity; a second member disposed on one of the internal cavity and an exterior surface of the wall, the second member being slidable relative to the first member and having one of a striker and a first pyrotechnic material disposed thereon, the second member further having a first concave portion; a third member disposed on the other of the internal cavity and an exterior surface of the wall, the third member being slidable relative to the first member, the third member having a second concave portion; a first biasing means for biasing the second member in a direction opposite the acceleration; a second biasing means for biasing the third member in the direction opposite the acceleration; one or more locking balls each having a portion thereof disposed in the first concave portion and another portion thereof disposed in the second concave portion for preventing relative movement of the second and third members with the first member when an acceleration time profile is below a predetermined threshold; and one of a percussion cap primer and a second pyrotechnic material disposed on the first member; wherein when the acceleration time profile is greater than the predetermined threshold the one or more locking balls are released from one or more of the first concave portion and the second concave portion to first permit relative movement of the third member with the first member and after a time delay to permit relative movement of the second member with the first member such that the one of the striker and the first pyrotechnic material strikes the one of the percussion cap primer and the second pyrotechnic material; and a thermal battery having a third pyrotechnic material and operatively connected to the inertial igniter such that an ignition fire resulting from the striking of the one of the striker and the first pyrotechnic material with the one of the percussion cap primer and the second pyrotechnic material ignites the third pyrotechnic material. 
   The inertial igniter and the thermal battery can each have an opening such that the ignition fire can communicate with the third pyrotechnic material through the openings. 
   The inertial igniter can further comprise a housing portion connected to the first member for enclosing the second and third members. The housing can comprise a wall portion and a top portion. The top portion can be sealed to the wall portion to seal the internal cavity from an exterior of the housing. 
   The wall, second member and third member can be circular in cross-section. 
   The first concave portion can have one of a groove or dimple formed on an outer surface of the second member. The second concave portion can have a shoulder formed on an inner surface of the third member. The first member can include a cup for holding the one of the percussion cap primer and the second pyrotechnic material. The second member can comprises a bore and the striker can be disposed in the bore. The second member can comprises a central element for holding the first pyrotechnic material. The central element can further have one or more cut ring portions for facilitating the holding of the first pyrotechnic material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the apparatus of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  illustrates a schematic of a cross-section of a thermal battery and inertial igniter assembly. 
       FIG. 2  illustrates a schematic of a cross-section of a conventional inertial igniter assembly known in the art. 
       FIG. 3  illustrates a schematic of a cross-section of a conventional inertial igniter assembly known in the art positioned within a housing and having intermediate materials for ignition. 
       FIG. 4  illustrates a schematic of a cross-section of a first embodiment of a inertial igniter in a locked position. 
       FIG. 5  illustrates the schematic of the inertial igniter of  FIG. 4  upon a non-firing accidental acceleration. 
       FIG. 6  illustrates the schematic of the inertial igniter of  FIG. 4  upon a firing acceleration. 
       FIG. 7  illustrates a schematic of a cross-section of a second embodiment of an inertial igniter in a locked position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A schematic of a cross-section of a first embodiment  30  of an inertia igniter is shown in  FIG. 4 , referred to generally with reference numeral  30 . The inertial igniter  30  is constructed with an igniter body  31  and a housing wall  32 . In the schematic of  FIG. 4 , the igniter body  31  and the housing wall  32  are joined together at one end; however, the two components may be integrated as one piece. In addition, the base of the housing  31  may be extended to form the cap  33  of the thermal battery  34 , the top portion of which is shown with dashed lines in  FIG. 4 . The base of the housing  31  is provided with a recess  35  to receive the percussion cap primer  37 . The base of the housing  31  is also provided with the opening  36  within the recess  35  to allow the ignited sparks and fire to exit the primer  37  into the thermal battery  34  upon initiation of the percussion cap primer  37 . The internal components of the inertial igniter  30  are sealed by a cap  42  which can be brazed or welded at seam  42   a  or applied with a suitable adhesive. 
   Integral to the igniter housing  31  is a cylindrical part  38  (or bodies with other cross-sectional shapes) having a wall defining a cavity, within which a striker mass  39  can travel up and down. The striker mass  39  is however biased to stay in its upper most position as shown in  FIG. 4  by a striker spring  41 . In its illustrated position, the striker mass  39  is locked in its axial position to the cylindrical part  38  of the housing  31  of the inertial igniter  30  by at least one locking ball  43 . The setback locking ball  43  locks the striker mass  39  to the cylindrical part  38  of the housing  31  through the holes  45  provided on the cylindrical part  38  of the housing  31  and a concave portion such as a groove (or dimple)  44  on the striker mass  39  as shown in  FIG. 4 . In the configuration shown in  FIG. 4 , the locking balls  43  are prevented from moving away from their aforementioned locking position by the cylindrical setback collar  46 . The cylindrical setback collar  46  can ride on the outer surface of the cylindrical part  38  of the housing  31 , but is biased to stay in its upper most position as shown in the schematic of  FIG. 4  by the setback spring  48 . The cylindrical setback collar  46  has a concave portion such as an upper enlarged shoulder portion  47 , within which the locking balls  43  loosely fit and are kept in their aforementioned position locking the striker mass  39  to the cylindrical part  38  of the housing  31 . The striker mass  39  has a tip  40 , which upon release of the striker mass and appropriate level of acceleration for the striker mass  39  to overcome the resisting force of the striker spring  41  and strike the percussion cap primer  37  with enough momentum, would initiate the percussion cap primer  37 . This tip  40  will have a form appropriate to reliably initiate the percussion cap primer, such as being spherical or hemispherical in shape. 
   The basic operation of the disclosed inertial igniter  30  will now be described with reference to  FIGS. 4-6 . Any non-trivial acceleration in the axial direction  49  which can cause the cylindrical setback collar  46  to overcome the resisting force of the setback spring  48  will initiate and sustain some downward motion of only the setback collar  46 . The force due to the acceleration on the striker mass  39  is supported by the locking balls  43  which are constrained by the shoulder  47  of the setback collar  46  to engage the striker mass. 
   If an acceleration time in the axial direction  49  imparts a sufficient impulse to the setback collar  46  (i.e., if an acceleration time profile is greater than a predetermined threshold), it will translate down along the axis of the assembly until the setback locking balls  43  are no longer constrained to engage the striker mass  39  to the cylindrical part  38  of the housing  31 . If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile is less than the predetermined threshold), the setback collar will return to its start position under the force of the setback spring. The schematic of the inertial igniter  30  with the setback collar  46  moved down certain distance as a result of an acceleration event, which is not sufficient to unlock the striker mass  39  from the cylindrical part  38  of the housing  31 , is shown in  FIG. 5 . 
   Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the setback collar  46  will have translated down full-stroke, allowing the striker mass  39  to accelerate down towards the percussion cap primer  37 . In such a situation, since the locking balls  43  are no longer constrained by the shoulder  42  of the setback collar  46 , the downward force that the striker mass  39  has been exerting on the locking balls  43  will force the locking balls  43  to move in the radial direction toward the housing wall  32 . Once the locking balls  43  are tangent to the outermost surface of the striker mass  39 , the downward motion of the striker mass  39  is impeded only by the elastic force of the striker spring  41 , which is easily overcome by the impulse provided to the striker mass  39 . As a result, the striker mass  39  moves downward, causing the tip  40  of the striker mass  39  to strike the target percussion cap primer  37  with the requisite energy to initiate ignition. The latter configuration of the inertial igniter  30  when the sharp tip  40  of the striker mass  39  is striking the primer  37  is shown in the schematic of  FIG. 6 . 
   The striker mass  39  and tip  40  may be a monolithic design with the striking tip  40  being a machined boss protruding from the striker mass, or the tip may be a separate piece pressed or otherwise permanently fixed to the striker mass. A two-piece design would be favorable to the need for a striker whose density is different than steel, but whose tip would remain hard and tough by attaching a steel ball, hemisphere, or other shape to the striker mass. A monolithic design, however, would be generally favorable to manufacturing because of the reduction of part quantity and assembly operations. 
   In another embodiment, the striker mass  39  is pre-loaded downwards by a tensile force in the striker spring  41 , in which case must be fixed at both ends to the striker mass  39  and the base of the inertial igniter body  31 , to force the striker mass  39  towards the percussion cap primer  37  upon its release. Alternatively, an elastic element such as a spring (not shown), which is preloaded in compression, can be positioned between the striker mass  39  and the top cap  42  of the inertial igniter  30 . As a result, the striker mass  39 , upon its release, is forced down towards the percussion cap primer  37 , thereby requiring a shorter travel distance to achieve a desired velocity, i.e., momentum. As a result, an inertial igniter that is shorter, lighter and more compact than the one shown in  FIGS. 2 and 3  is obtained. This embodiment therefore allows the construction of an inertial igniter with a lighter striking mass  39  and a closer striker tip  40  to the percussion cap primer  37 , thereby a smaller required height  13  (see  FIG. 1 ). In addition, inertial igniters may even be constructed without a striker spring  41 . 
   In yet another embodiment, the setback collar  46  may be constructed with an integrated elastic element, e.g., as part of its lower sliding section extending to the base of the igniter body  31  (not shown) to function as the setback spring  48 . This would simplify the manufacture and assembly of the inertial igniter and reduce the number of required parts. 
   In the schematic of  FIG. 4 , the percussion cap is shown to be fitted from the top of the assembly before the striker spring  41  and the striker mass  39  is installed and secured to the safety system. The intent is to guard against the possibility of the percussion cap being pushed out of the assembly during acceleration or initiation striking. The percussion cap may, however, be assembled from the bottom side of the inertial igniter assembly as the final assembly operation, thereby reducing the possibility of accidental ignition. In addition, inertial igniter assemblies without percussion caps could then be stored indefinitely, having the desired percussion cap applied as usage becomes imminent. 
   It is appreciated by those familiar with the art that by varying the mass of the striker  39 , the mass of the setback collar  46 , the spring rates of the striker spring  41  and setback spring  48 , the distance that the setback collar  46  has to travel downward to release setback locking balls and thereby release the striker mass  39 , and the distance between the striker tip  40  and the percussion cap primer, the designer of the disclosed inertial igniter  30  can match the fire and no-fire impulse level requirements for various applications as well as the safety (delay or dwell action) protection against accidental dropping of the inertial igniter and/or the munitions within which it is assembled. 
   Briefly, the safety system parameters, i.e., the mass of the setback collar  46 , the spring rate of the setback spring  48  and the dwell stroke (the distance that the setback collar  46  has to travel downward to release the setback locking balls and thereby release the striker mass  39 ) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker  39 , the spring rate of the striker spring  41 , the radius of striker tip  40 , and the separation distance from the striker tip  40  to the percussion cap primer  37  must work together to provide the specified impact energy to the primer when subjected to the remaining portion of the prescribed initiation acceleration profile after the safety system has been actuated. 
   In addition, since the safety and striker systems each require a certain actuation distance to achieve the necessary performance, the most axially compact design is realized by nesting the two systems in parallel as it is done in the embodiment of  FIG. 4 . It is this nesting of the two safety and striker systems that allows the height of the disclosed inertial igniter to be significantly shorter than the currently available inertial igniter design (as shown in  FIG. 2 ), in which the safety and striker systems are configured in series. In fact, an initial prototype of the disclosed inertial igniter  30  has been designed to the fire and no-fire and safety specifications of the currently available inertial igniter shown in  FIG. 2  and has achieved a height reduction of about  44  percent. It is noted that by optimizing the parameters of the disclosed inertial igniter, its height can be further reduced. 
   In another embodiment, the percussion cap primer is replaced by a two-part pyrotechnic material combination that ignite upon the coincident impact of the two components (fuel and oxidizer). A schematic of a cross-section of the resulting inertial igniter  50  is shown in  FIG. 7 . In this illustration, all components are the same as those shown in  FIG. 4  with the exception of the replaced percussion cap primer  37  with the cup  51  and the striker tip  40  with striker assembly  52 . The cup  51  is provided with a bottom hole  53  and contains one component of the pyrotechnic material  54 , preferably leaving a center opening  58  to allow the ignition spark and fire to easily exit and enter the thermal battery. The striker assembly  52  consists of a central element  55 , which is firmly attached to the striker mass  39 . The extended portion of the element  55  is provided with cuts  57  (shown as rings in  FIG. 7 ), to which the second part of the pyrotechnic materials  56  is firmly adhered. The striker assembly  52  is also provided with a cap portion, which is intended to loosely cover the top portion of the cup  51  as the striker portion of the pyrotechnic material  56  impacts the cup portion  54 . 
   In general, various combinations of pyrotechnic materials may be used for this purpose. One commonly used pyrotechnic material consists of red phosphorous or nano-aluminum, indicated as element  56  in  FIG. 7 , and is used with an appropriate binder (such as vinyl alcohol acetate resin or nitrocellulose) to firmly adhere to the striker surface  57 . The second component can be potassium chlorate, potassium nitrate, or potassium perchlorate, indicated as element  54  in  FIG. 7 , and is used with a binder (preferably but not limited to with such as vinyl alcohol acetate resin or nitrocellulose) to firmly attach the compound to the inside of the cup  51  as shown in  FIG. 7 . 
   An advantage of using the two component pyrotechnic materials as shown in  FIG. 7  is that these materials can be selected such that ignition is provided at significantly lower impact forces than are required for commonly used percussion cap primers. As a result, the amount of distance that the striker mass  39  has to travel and its required mass is thereby reduced, resulting in a smaller total height (shown as  15  in  FIG. 1 ) of the thermal battery assembly. This choice, however, has the disadvantage of not using standard and off-the-shelf percussion cap primers, thereby increasing the component and assembly cost of the inertial igniter. Another advantage of this embodiment is directing the pyrotechnic output via a bottom hole  53  to reduce the cup size or volume. 
   The disclosed inertial igniters are seen to discharge the ignition fire and sparks directly into the thermal battery,  FIGS. 4-6 , to ignite the pyrotechnic materials  24  within the thermal battery  34  ( FIG. 4 ). As a result, the additional housing  21  and ignition material  23  shown in  FIG. 3  can be eliminated, greatly simplifying the resulting thermal battery design and manufacture. In addition, the total height  13  of the inertial igniter assembly  10  and the total height  15  of the complete thermal battery assembly  16  are reduced, thereby reducing the total volume that has to be allocated in munitions to house the thermal battery. 
   The base of the disclosed igniter body  31  may be extended to form the cap  33  of the thermal battery  34 . As a result, the total height of the inertial igniter and thermal battery assembly  15  ( FIG. 1 ) can be further reduced. 
   The disclosed inertial igniters are shown sealed within their housing, thereby simplifying their storage and increase their shelf life. 
   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.