Compact and low-volume mechanical igniter and ignition systems for thermal batteries and the like

An inertial igniter including: a body having a base and three or more posts, each post having a hole; a locking ball corresponding to each post, wherein a portion of the locking balls are disposed in the hole; a striker mass movably disposed relative to the posts and having a surface corresponding to the posts, the striker mass further having a concave portion corresponding to the locking balls, wherein a second portion of each locking ball is disposed in a corresponding concave portion for retaining the striker mass relative to the posts; a collar movable relative to the posts; and a biasing element for biasing the collar in a first position which retains the striker mass, the biasing element permitting movement of the collar to a second position to release the striker mass relative to the posts upon a predetermined acceleration profile.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to mechanical igniters, and more particularly to compact, low-volume, reliable and easy to manufacture mechanical igniters and ignition systems for thermal batteries and the like.

2. Prior Art

Thermal batteries represent a class of reserve batteries that operate at high temperature. 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 KClO4. 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)/FeS2or Li(Si)/CoS2couples. 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 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.

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 conventional thermal battery and inertial igniter assembly is shown inFIG. 1. In thermal battery applications, the inertial igniter10(as assembled in a housing) is generally positioned above the thermal battery housing11as shown inFIG. 1. Upon ignition, the igniter initiates the thermal battery pyrotechnics positioned inside the thermal battery through a provided access12. The total volume that the thermal battery assembly16occupies within munitions is determined by the diameter17of the thermal battery housing11(assuming it is cylindrical) and the total height15of the thermal battery assembly16. The height14of the thermal battery for a given battery diameter17is generally determined by the amount of energy that it has to produce over the required period of time. For a given thermal battery height14, the height13of the inertial igniter10would therefore determine the total height15of the thermal battery assembly16. To reduce the total volume that the thermal battery assembly16occupies within a munitions housing, it is therefore important to reduce the height of the inertial igniter10. 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, a schematic of which is shown inFIG. 2, the inertial igniter20may have to be positioned within a housing21as shown inFIG. 3, particularly for relatively small igniters. The housing21and the thermal battery housing11may share a common cap22, with the opening25to allow the ignition fire to reach the pyrotechnic material24within the thermal battery housing. As the inertial igniter is initiated, the sparks can ignite intermediate materials23, which can be in the form of thin sheets to allow for easy ignition, which would in turn ignite the pyrotechnic materials24within the thermal battery through the access hole25.

A schematic of a cross-section of a currently available inertial igniter20is shown inFIG. 2in which the acceleration is in the upward direction (i.e., towards the top of the paper). The igniter has side holes26to allow the ignition fire to reach the intermediate materials23as shown inFIG. 3, which necessitate the need for its packaging in a separate housing, such as in the housing21. The currently available inertial igniter20is constructed with an igniter body60. Attached to the base61of the housing60is a cup62, which contains one part of a two-part pyrotechnic compound63(e.g., potassium chlorate). The housing60is provided with the side holes26to allow the ignition fire to reach the intermediate materials23as shown inFIG. 3. A cylindrical shaped part64, which is free to translate along the length of the housing60, is positioned inside the housing60and is biased to stay in the top portion of the housing as shown inFIG. 2by the compressively preloaded helical spring65(shown schematically as a heavy line). A turned part71is firmly attached to the lower portion of the cylindrical part64. The tip72of the turned part71is provided with cut rings72a, over which is covered with the second part of the two-part pyrotechnic compound73(e.g., red phosphorous).

A safety component66, which is biased to stay in its upper most position as shown inFIG. 2by the safety spring67(shown schematically as a heavy line), is positioned inside the cylinder64, and is free to move up and down (axially) in the cylinder64. As can be observed inFIG. 2, the cylindrical part64is locked to the housing60by setback locking balls68. The setback locking balls68lock the cylindrical part64to the housing60through holes69aprovided on the cylindrical part64and the housing60and corresponding holes69bon the housing60. In the illustrated configuration, the safety component66is pressing the locking balls68against the cylindrical part64via the preloaded safety spring67, and the flat portion70of the safety component66prevents the locking balls68from moving away from their aforementioned locking position. The flat portion70of the safety component66allows a certain amount of downward movement of the safety component66without releasing the locking balls68and thereby allowing downward movement of the cylindrical part64. 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 component66travels up and down without releasing the cylindrical part64. However, once the firing acceleration profiles are experienced, the safety component66travels downward enough to release balls68from the holes69band thereby release the cylindrical part64. Upon the release of the safety component66and appropriate level of acceleration for the cylindrical part64and all other components that ride with it to overcome the resisting force of the spring65and attain enough momentum, then it will cause impact between the two components63and73of 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 height13,FIG. 1. Firstly, the currently available inertial igniters, such as that shown inFIG. 2, are relatively long thereby resulting in relatively long total igniter heights13. 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 housing21, usually with other ignition material23, thereby increasing the height13over the length of the igniter20(seeFIG. 3). In addition, since the pyrotechnic materials of the currently available igniters20are 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 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 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 and smaller in volume 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 housing21shown inFIG. 3), thereby allowing even a smaller total height and volume 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 material23shown inFIG. 3) or a booster;

provide inertia igniters that could be constructed to guide the pyrotechnic flame essentially downward (in the direction opposite to the direction of the firing acceleration—usually for mounting on the top of the thermal battery as shown inFIG. 3), or essentially upward (in the direction opposite of the firing acceleration—usually for mounting at the bottom of the thermal battery), or essentially sidewise (lateral to the direction of the firing);

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, inertial igniters and ignition systems for use with thermal batteries for producing power upon acceleration are provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A schematic of a cross-section of a first embodiment of an inertia igniter is shown inFIG. 4, referred to generally with reference numeral30. The inertial igniter30is constructed with igniter body31, consisting of a base32and at least two posts33, and a housing wall34. The base32and two posts33, which may be integral or may have been constructed as separate pieces and joined together, for example by welding of press fitting or other methods commonly used in the art. In the schematic ofFIG. 4, the igniter body31and the housing wall34are shown to be joined together at the base32; however, the two components may be integrated as one piece and a separate top cap35may then be provided, which is then joined to the top surface of the housing34following assembly of the igniter (in the schematic ofFIG. 4the top cap35is shown as an integral part of the housing34). In addition, the base of the housing32may be extended to form the cap36of the thermal battery37, the top portion of which is shown with dashed lines inFIG. 4.

The inertial igniter30with the thermal battery top cap36is shown in the isometric drawings ofFIGS. 5aand5b. The inertial igniter without its housing34and top cap35is shown in the isometric drawing ofFIG. 5c. The base of the housing32is also provided with at least one opening38(with corresponding openings in the thermal battery top cap36) to allow the ignited sparks and fire to exit the inertial igniter into the thermal battery37upon initiation of the inertial igniter pyrotechnics46and47,FIG. 4, or percussion cap primer when used in place of the pyrotechnics46and47(not shown).

A striker mass39is shown in its locked position inFIGS. 4 and 5c. The striker mass39is provided with vertical recesses40that are used to engage the posts33and serve as guides to allow the striker mass39to ride down along the length of the posts33without rotation with an essentially pure up and down translational motion. In its illustrated position inFIGS. 4 and 5c, the striker mass39is locked in its axial position to the posts33by at least one setback locking ball42. The setback locking ball42locks the striker mass39to the posts33of the inertial igniter body31through the holes41provided in the posts33and a concave portion such as a dimple (or groove)43on the striker mass39as shown inFIG. 4. A setback spring44with essentially dead coil section45, which is preferably in compression, is also provided around but close to the posts as shown inFIGS. 4 and 5c. In the configuration shown inFIG. 4, the locking balls42are prevented from moving away from their aforementioned locking position by the dead coil section45of the setback spring44. The dead coil section45can ride up and down beyond the posts33as shown inFIGS. 4 and 5c, but is biased to stay in its upper most position as shown in the schematic ofFIG. 4by the setback spring44.

In this embodiment, a two-part pyrotechnics compound is shown to be used,FIG. 4. One part of the two-part pyrotechnics compound47(e.g., potassium chlorate) is provided on the interior side of the base32, preferably in a provided recess (not shown) over the exit holes38. The second part of the pyrotechnics compound (e.g., red phosphorous)46is provided on the lower surface of the striker mass surface39facing the first part of the pyrotechnics compound47as shown inFIG. 4. The surfaces to which the pyrotechnic parts46and47are attached are roughened and/or provided with surface cuts, recesses, or the like as commonly used in the art (not shown) to ensure secure attachment of the pyrotechnics materials to the applied surfaces.

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 element46inFIG. 4, and is used with an appropriate binder (such as vinyl alcohol acetate resin or nitrocellulose) to firmly adhere to the bottom surface of the striker mass39. The second component can be potassium chlorate, potassium nitrate, or potassium perchlorate, indicated as element47inFIG. 4, 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 surface of the base32(preferably inside of a recess provided in the base32—not shown) as shown inFIG. 4.

The basic operation of the disclosed inertial igniter30will now be described with reference toFIGS. 4-8. Any non-trivial acceleration in the axial direction48which can cause dead coil section45to overcome the resisting force of the setback spring44will initiate and sustain some downward motion of only the dead coil section45. The force due to the acceleration on the striker mass39is supported at the dimples43by the locking balls42which are constrained inside the holes41in the posts33. If an acceleration time in the axial direction48imparts a sufficient impulse to the dead coil section45(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 balls42are no longer constrained to engage the striker mass39to the posts33of the housing31. If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile provides less impulse than the predetermined threshold), the dead coil section45will return to its start (top) position under the force of the setback spring44. The schematic of the inertial igniter30with the dead coil section45moved down certain distance d1as a result of an acceleration event, which is not sufficient to unlock the striker mass39from the posts33of the housing31, is shown inFIG. 6.

Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the dead coil section45will have translated down full-stroke d2, allowing the striker mass39to accelerate down towards the base32. In such a situation, since the locking balls42are no longer constrained by the dead coil section45, the downward force that the striker mass39has been exerting on the locking balls42will force the locking balls42to move outward in the radial direction. Once the locking balls42are out of the way of the dimples43, the downward motion of the striker mass39is no longer impeded. As a result, the striker mass39moves downward, causing the parts46and47of the two-part pyrotechnic compound to strike with the requisite energy to initiate ignition. The configuration of the inertial igniter30when the balls42are free to move outward in the radial direction, thereby releasing the striker mass39is shown in the schematic ofFIG. 7. The configuration of the inertial igniter30when the part46of the two-part pyrotechnic compound is striking the part47is shown in the schematic ofFIG. 8.

In another embodiment, the dead coil section45may be constructed as a separate collar and positioned similarly over the setback spring44. The collar replacing the dead coil section45may also be attached to the top coil of the setback spring44, e.g., by welding, brazing, or adhesives such as epoxy, or the like. The advantage of attaching the collar to the top of the setback spring44is that it would help prevent it to get struck over the posts33as it is being pushed down by the applied acceleration in the direction of the arrow48,FIGS. 6-8.

Alternatively, the dead coil section45and the setback spring44may be integral, made out of, for example, a cylindrical section with spiral or other type shaped cuts over its lower section to provide the required axial flexibility to serve the function of the setback spring44. The upper portion of this cylinder is preferably left intact to serve the function of the dead coil section45,FIGS. 6-8.

It is appreciated by those skilled in the art that by varying the mass of the striker39, the mass of the dead coil section45, the spring rate of the setback spring44, the distance that the dead coil section45has to travel downward to release the locking balls42and thereby release the striker mass39, and the distance between the parts46and47of the two-part pyrotechnic compound, the designer of the disclosed inertial igniter30can 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 or the like within which it is assembled.

Briefly, the safety system parameters, i.e., the mass of the dead coil section45, the spring rate of the setback spring44and the dwell stroke (the distance that the dead coil section44has to travel downward to release the locking balls42and thereby release the striker mass39) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker39and the separation distance between the parts46and47of the two-part pyrotechnic compound must work together to provide the specified impact energy to initiate the pyrotechnic compound 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 ofFIG. 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 inFIG. 2), in which the safety and striker systems are configured in series. In fact, an initial prototype of the disclosed inertial igniter30has been designed to the fire and no-fire and safety specifications of the currently available inertial igniter shown inFIG. 2and has achieved height and volume reductions of over 60 percent. It is noted that by optimizing the parameters of the disclosed inertial igniter, both height and volume can be further reduced.

In another embodiment, the two-part pyrotechnics46and47,FIG. 4, are replaced by a percussion cap primer49attached to the base32of the inertial igniter60and a striker tip50as shown in the schematic of a cross-section ofFIG. 9. In this illustration, all components are the same as those shown inFIG. 4with the exception of replacing the percussion cap primer49and the striker tip50with striker assembly. The striker tip50is firmly attached to the striker mass39.

The striker mass39and striker tip50may be a monolithic design with the striking tip50being a machined boss protruding from the striker mass, or the striker tip50may 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.

An advantage of using the two component pyrotechnic materials as shown inFIG. 4is 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 mass39has to travel and its required mass is thereby reduced, resulting in a smaller total height (shown as15inFIG. 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.

The disclosed inertial igniters are seen to discharge the ignition fire and sparks directly into the thermal battery,FIGS. 4-9, to ignite the pyrotechnic materials24within the thermal battery11(FIG. 3). As a result, the additional housing21and ignition material23shown inFIG. 3can be eliminated, greatly simplifying the resulting thermal battery design and manufacture. In addition, the total height13and volume of the inertial igniter assembly10and the total height15of the complete thermal battery assembly16are reduced, thereby reducing the total volume that has to be allocated in munitions or the like to house the thermal battery.

The disclosed inertial igniters are shown sealed within their housing, thereby simplifying their storage and increase their shelf life.

FIG. 10shows the schematic of a cross-section of another embodiment80. This embodiment is similar to the embodiment shown inFIGS. 4-8, with the difference that the striker mass39(FIGS. 4-8) is replaced with a striker mass82, with at least one opening passage81to guide the ignition flame up through the igniter80to allow the pyrotechnic materials (or the like) of a thermal battery (or the like) positioned above the igniter80(not shown) to be initiated. In addition, the top cap35(FIG. 4-8) is preferably eliminated or replaced by a cap83with appropriately positioned openings to allow the flames to enter the thermal battery and initiate its pyrotechnic materials. The openings38(FIG. 5b) are obviously no longer necessary.

FIG. 11bshows the schematic of a cross-section of another embodiment90. This embodiment is similar to the embodiment shown inFIGS. 4-8, with the difference that the openings38(FIG. 5b) for the flame to exit the igniter30is replaced with side openings91,FIG. 11a, to allow the flame to exit from the side of the igniter to initiate the pyrotechnic materials (or the like) of a thermal battery or the like (not shown) that is positioned around the body of the igniter90. Alternatively, the igniter housing92may be eliminated, thereby allowing the generated ignition flames to directly flow to the sides of the igniter90and initiate the pyrotechnic materials of the thermal battery or the like.

FIG. 12shows the schematic of a cross-section of another embodiment100. This embodiment is similar to the embodiment shown inFIGS. 4-8, with the difference that the dead coil section45(FIGS. 4-5) is replaced with a solid, preferably relatively very rigid, cylindrical section101. The advantage of using a rigid cylindrical section101is that the balls42(FIGS. 4-5) would not tend to cause the individual coils of the dead coil section45to move away from their cylindrically positioned configuration, thereby increasing the probability that the dead coil section could get stuck by the friction forces due to the pressure exerted by the balls42to the interior of the housing34(FIG. 4) or other similar possible scenarios.

In certain applications, the required reliability levels for initiation of inertial igniters are extremely high. In certain cases, the igniters should be designed and manufactured to perform their function with extremely high reliability of nearly 100 percent. Some cases may even require the use of multiple and redundant inertial igniters to obtain nearly 100 percent reliability.

The cost issue is also another important consideration since in small thermal batteries that have to be initiated by inertial igniters, the cost of inertial igniters may easily be a significant portion of the total cost. However, to significantly reduce the cost, inertial igniters have to be designed with fewer and easy to manufacture parts and be easy to assemble. In addition, the inertial igniters must use mass produced and commercially available parts.

The embodiments of the inertial igniters disclosed below are to provide the aforementioned advantages of the embodiments shown inFIGS. 4-12and in addition: (1) provide inertial igniters that are significantly more reliable and easy to manufacture than currently available inertial igniters for thermal batteries or the like, particularly for relatively small thermal batteries that are used in munitions; (2) provide highly reliable and at the same time very small inertial igniters that do not occupy a significant volumes of small thermal batteries; (3) provide inertial igniters that are easy to manufacture and assemble into thermal batteries; and (4) provide inertial igniters that are readily modified to satisfy a wide range of no-fire and all-fire requirements without requiring costly engineering development and manufacturing equipment changes.

A need exists for novel miniature inertial igniters for thermal batteries used in gun fired munitions, that are extremely reliable, low cost (such as having fewer easy to manufacture parts that are not required to be fabricated to very low tolerances), easy to manufacture and assemble, and easy to assemble into a thermal battery (such as simply “drop-in” component during thermal battery assembly). Such inertial igniters can also be adaptable to a wide range of all-fire and no-fire requirements without requiring a significant amount of engineering development and testing. Such inertial igniters can also be capable of allowing multiple inertial igniters to be readily packed into thermal batteries as redundant initiators to further increase initiation reliability when such extremely high initiation reliability are warranted. Such inertial igniters are particularly needed for small and low power thermal batteries that could be used in fuzing and other similar applications. Such inertial igniters must be safe and in general and in particular they should not initiate if dropped, e.g., from up to 5-7 feet onto a concrete floor for certain applications; should withstand high firing accelerations and do not cause damage to the thermal battery, for example up to 20-50,000 Gs or even more; 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 for very short periods of time, for example accelerations of the order of 1000 Gs when applied for over 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.

An isometric cross-sectional view of a sixth embodiment of an inertia igniter is shown inFIG. 13, referred to generally with reference numeral200. The full isometric view of the inertial igniter200is shown inFIG. 14. The inertial igniter200is constructed with igniter body201, consisting of a base202and at least three posts203. The base202and the at least three posts203, can be integrally formed as a single piece but may also be constructed as separate pieces and joined together, for example by welding or press fitting or other methods commonly used in the art. The base202of the housing can also be provided with at least one opening204(with a corresponding opening(s) in the thermal battery—not shown) to allow ignited sparks and fire to exit the inertial igniter and enter into the thermal battery positioned under the inertial igniter200upon initiation of the inertial igniter pyrotechnics215, or percussion cap primer when used in place of the pyrotechnics, similar to the primer49in the embodiment60shown inFIG. 9. Although illustrated with the opening204in the base, the opening (or openings) can alternatively be formed in a side wall as is shown inFIG. 11aor in the striker mass as is shown inFIG. 10.

The base202of the housing may be extended to form a cap for the thermal battery, similar to the cap36of the thermal battery37shown for the embodiment30inFIGS. 4 and 5.

A striker mass205is shown in its locked position inFIG. 13. The striker mass205is provided with guides for the posts203, such as vertical surfaces206(which may be recessed as shown in the embodiment30inFIGS. 4 and 5), that are used to engage the corresponding (inner) surfaces of the posts203and serve as guides to allow the striker mass205to ride down along the length of the posts203without rotation with an essentially pure up and down translational motion. However, the surfaces206minimize the chances of the striker mass205jamming as compared to the recesses40. Further, manufacturing precision is reduced (for both the posts203and the striker mass205) when the surfaces206are used in place of the recesses40. Consequently, both the striker mass205and the inertial igniter structure (which includes the posts203) is easier to produce and less costly when the surfaces206are used in place of the recesses40.

In its illustrated position inFIGS. 13 and 14, the striker mass205is locked in its axial position to the posts203by at least one setback locking ball207. The setback locking ball207locks the striker mass205to the posts203of the inertial igniter body201through the holes208provided in the posts203and a concave portion such as a dimple (or groove)209on the striker mass205as shown inFIG. 13. A setback spring210, which is preferably in compression, is also provided around but close to the posts203as shown inFIGS. 13 and 14. In the configuration shown inFIG. 13, the locking balls207are prevented from moving away from their aforementioned locking position by the collar211. The setback spring210can be a wave spring with rectangular cross-section. The rectangular cross-section eliminates the need to fix or otherwise retain the striker spring210to the collar211, which is an expensive process; the flat coil spring surfaces minimizes the chances of coils slipping laterally (perpendicular to the direction of acceleration218), which can cause jamming and prevent the release of the striker mass205(preventing the collar to move down enough to release the locking balls). Furthermore, wave springs generate friction between the waves at contact points along the spring wire, thereby reducing the chances for the collar211to rapidly bounce back up and preventing the striker mass205from being released.

The collar211is preferably provided with partial guide212(“pocket”), which are open on the top as indicated by the numeral213. The guide212may be provided only at the location of the locking balls207as shown inFIGS. 13 and 14, or may be provided as an internal surface over the entire inner surface of the collar211(not shown). The advantage of providing local guides212is that it results in a significantly larger surface contact between the collar211and the outer surfaces of the posts203, thereby allowing for smoother movement of the collar211up and down along the length of the posts203. In addition, they prevent the collar211from rotating relative to the inertial igniter body201and makes the collar stronger and more massive. The advantage of providing a continuous inner recess guiding surface for the locking balls207is that it would require fewer machining processes during the collar manufacture. Although only one locking ball207is illustrated inFIG. 13, more than one can be provided, such as a locking ball207associated with each post203. More than one locking ball207can also be associated with each post203.

The collar211rides up and down on the posts203as can be seen inFIGS. 13 and 14, but is biased to stay in its upper most position as shown inFIGS. 13 and 14by the setback spring210. The guides212are provided with bottom ends214, so that when the inertial igniter is assembled as shown inFIGS. 13 and 14, the setback spring210which is biased (preloaded) to push the collar211upward away from the igniter base201, would “lock” the collar211in its uppermost position against the locking balls207. As a result, the assembled inertial igniter200stays in its assembled state and would not require a top can (similar to the top cap35in the embodiment30ofFIG. 4) to prevent the collar211from being pushed up and allowing the locking balls207from moving out and releasing the striker mass205.

In the sixth embodiment, a one part pyrotechnics compound215(such as lead styphnate or other similar compound) can be used as shown inFIG. 13. The surfaces to which the pyrotechnic compound215is attached can be roughened and/or provided with surface cuts, recesses, projections, or the like and/or treated chemically as commonly done in the art (not shown) to ensure secure attachment of the pyrotechnics material to the applied surfaces. The use of one part pyrotechnics compound makes the manufacturing and assembly process much simpler and thereby leads to lower inertial igniter cost. The striker mass can be provided with a relatively sharp tip216and the igniter base surface202is provided with a protruding tip217which is covered with the pyrotechnics compound215, such that as the striker mass is released during an all-fire event and is accelerated down (opposite to the arrow218illustrated inFIG. 13), impact occurs mostly between the surfaces of the tips216and217, thereby pinching the pyrotechnics compound215, thereby providing the means to obtain a reliable initiation of the pyrotechnics compound215.

Alternatively, a two-part pyrotechnics compound as shown and described for the embodiment30ofFIG. 4can be used. One part of the two-part pyrotechnics compound47(FIG. 4), e.g., potassium chlorate, can be provided on the interior side of the base32, such as in a provided recess (not shown) over the exit holes38. The second part of the pyrotechnics compound (e.g., red phosphorous)46can be provided on the lower surface of the striker mass surface39facing the first part of the pyrotechnics compound47, as shown inFIG. 4. In general, various combinations of pyrotechnic materials can be used for this purpose. One commonly used pyrotechnic material consists of red phosphorous or nano-aluminum, indicated as element46inFIG. 4, and is used with an appropriate binder (such as vinyl alcohol acetate resin or nitrocellulose) to firmly adhere to the bottom surface of the striker mass39. The second component can be potassium chlorate, potassium nitrate, or potassium perchlorate, indicated as element47inFIG. 4, and is used with a binder (such as, but not limited to vinyl alcohol acetate resin or nitrocellulose) to firmly attach the compound to the surface of the base32(such as inside of a recess provided in the base32—not shown) as shown inFIG. 4.

Alternatively, instead of using the pyrotechnics compound215,FIG. 13, a percussion cap primer or the like (similar to the percussion cap primer49used in the embodiment60ofFIG. 9) can be used. A striker tip (similar to the striker tip50shown inFIG. 9for the embodiment60) can be provided at the tip216of the striker mass205(not shown) to facilitate initiation upon impact.

The basic operation of the embodiment200of the inertial igniter ofFIGS. 13 and 14is similar to that of embodiment30(FIGS. 4-8) as previously described. Here again, any non-trivial acceleration in the axial direction218which can cause the collar211to overcome the resisting force of the setback spring210will initiate and sustain some downward motion of the collar211. The force due to the acceleration on the striker mass205is supported at the dimples209by the locking balls207which are constrained inside the holes208in the posts203. If an acceleration time in the axial direction218imparts a sufficient impulse to the collar211(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 balls205are no longer constrained to engage the striker mass205to the posts203. If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile provides less impulse than the predetermined threshold), the collar211will return to its start (top) position under the force of the setback spring210.

Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the collar211will have translated down past the locking balls207, allowing the striker mass205to accelerate down towards the base202. In such a situation, since the locking balls207are no longer constrained by the collar211, the downward force that the striker mass205has been exerting on the locking balls207will force the locking balls207to move outward in the radial direction. Once the locking balls207are out of the way of the dimples209, the downward motion of the striker mass205is no longer impeded. As a result, the striker mass205moves downward, causing the tip216of the striker mass205to strike the pyrotechnic compound215on the surface of the protrusion217with the requisite energy to initiate ignition (similar to the configuration shown for the embodiment30inFIG. 8).

In the embodiment200of the inertial igniter shown inFIGS. 13 and 14, the setback spring210is illustrated as a helical wave spring type fabricated with rectangular cross-sectional wires (such as the ones manufactured by Smalley Steel Ring Company of Lake Zurich, Ill.). This is in contrast with the helical springs with circular wire cross-sections used in the embodiments ofFIGS. 4-12. The use of the aforementioned rectangular cross-section wave springs or the like has the following significant advantages over helical springs that are constructed with wires with circular cross-sections. Firstly and most importantly, as the spring is compressed and nears its “solid” length, the flat surfaces of the rectangular cross-section wires come in contact and generate minimal lateral forces that would otherwise tend to force one coil to move laterally relative to the other coils as is usually the case when the wires are circular in cross-section. Lateral movement of the coils can, in general, interfere with the proper operation of the inertial igniter since it could, for example jam a coil to the outer housing of the inertial igniter (not shown inFIGS. 13 and 14), which is usually desired to house the igniter200or the like with minimal clearance to minimize the total volume of the inertial igniter. In addition, the laterally moving coils could also jam against the posts203thereby further interfering with the proper operation of the inertial igniter. The use of the present wave springs with rectangular cross-section eliminates such lateral movement and therefore significantly increases the reliability of the inertial igniter and also significantly increases the repeatability of the initiation for a specified all-fire condition. The second advantage of the use of the aforementioned wave springs with rectangular cross-section, particularly since the wires can and are usually made thin in thickness and relatively wide, the solid length of the resulting wave spring can be made to be significantly less than an equivalent regular helical spring with circular cross-section. As a result, the total height of the resulting inertial igniter can be reduced. Thirdly, since the coil waves are in contact with each other at certain points along their lengths and as the spring is compressed, the length of each wave is slightly increased, therefore during the spring compression the friction forces at these contact points do a certain amount of work and thereby absorb a certain amount of energy. The presence of such friction forces ensures that the firing acceleration and very rapid compression of the spring would to a lesser amount tend to “bounce” the collar211back up and thereby increasing the possibility that it would interfere with the exit of the locking balls from the dimples209of the striker mass205and the release of the striker mass205. The above characteristic of the wave springs with rectangular cross-section therefore also significantly enhances the performance and reliability of the inertial igniter200while at the same time allowing its height (and total volume) to be reduced.

It is appreciated by those skilled in the art that by varying the mass of the striker205, the mass of the collar211, the spring rate of the setback spring210, the distance that the collar211has to travel downward to release the locking balls207and thereby release the striker mass205, and the distance between the tip216of the striker mass205and the pyrotechnic compound215(and the tip of the protrusion217), the designer of the disclosed inertial igniter200can match the all-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 or the like within which it is assembled.

Briefly, the safety system parameters, i.e., the mass of the collar211, the spring rate of the setback spring210and the dwell stroke (the distance that the collar210has to travel downward to release the locking balls207and thereby release the striker mass205) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker205and the aforementioned separation distance between the tip216of the striker mass and the pyrotechnic compound215(and the tip of the protrusion217) must work together to provide the specified impact energy to initiate the pyrotechnic compound when subjected to the remaining portion of the prescribed initiation acceleration profile after the safety system has been actuated.

The striker mass205and striker tip216may be a monolithic design with the striking tip216being formed, as shown inFIG. 13, or as a boss protruding from the striker mass, or the striker tip216may be a separate piece, possibly fabricated from a material that is significantly harder than the striker mass material, and 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.

The use of three or more posts203in the embodiment200ofFIGS. 13 and 14has several significant advantages over the two post designs of the embodiments ofFIGS. 4-5. namely, unlike the embodiment30ofFIGS. 4 and 5in which the striker mass39is provided with vertical recesses40that are used to engage the posts33and serve as guides to allow the striker mass39to ride down along the length of the posts33without rotation, the use of at least three posts203in the embodiment200ofFIGS. 13 and 14eliminates the need for the aforementioned vertical recesses in the striker mass205. As a result, the chances that the striker mass203gets jammed at the interface between the aforementioned vertical recesses (40inFIGS. 4 and 5) and the posts (33inFIGS. 4 and 5) are almost entirely eliminated. As a result, the reliability of the inertial igniter is significantly increased. Furthermore, the design of the striker mass and the igniter posts and their required manufacturing process are significantly simplified and the required manufacturing precision is also reduced. As a result, the manufacturing cost of the striker mass as well as the igniter body is significantly reduced. Still further, the contacting surfaces between the striker mass205and the posts203is increased, thereby allowing for a smoother up and down movement of the striker mass205along the inner surfaces of the posts203.

In the embodiment200ofFIGS. 13 and 14, following ignition of the pyrotechnics compound215, the generated flames and sparks are designed to exit downward through the opening204to initiate the thermal battery below. Alternatively, if the thermal battery is positioned above the inertial igniter200, the opening204can be eliminated and the striker mass could be provided with at least one opening similar to the passage81of the striker mass82of the embodiment80ofFIG. 10to guide the ignition flame and sparks up through the striker mass205to allow the pyrotechnic materials (or the like) of a thermal battery (or the like) positioned above the inertial igniter200(not shown) to be initiated.

Alternatively, in a manner similar to that shown in the embodiment90ofFIGS. 11aand11b, side ports (openings91) may be provided to allow the flame to exit from the side of the igniter to initiate the pyrotechnic materials (or the like) of a thermal battery or the like that is positioned around the body of the inertial igniter. Alternatively, the igniter housing261(FIG. 16) may be eliminated, thereby allowing the generated ignition flames to directly flow to the sides of the igniter200and initiate the pyrotechnic materials of the thermal battery or the like.

InFIGS. 13 and 14, the inertial igniter embodiment200is shown without any outside housing. In many applications, as shown in the schematics ofFIG. 15a(15b), the inertial igniter240(250) is placed securely inside the thermal battery241(251), either on the top (FIG. 15a) or bottom (FIG. 15b) of the thermal battery housing242(252). This is particularly the case for relatively small thermal batteries. In such thermal battery configurations, since the inertial igniter240(250) is inside the hermetically sealed thermal battery241(251), there is no need for a separate housing to be provided for the inertial igniter itself. In this assembly configuration, the thermal battery housing242(252) is provided with a separate compartment243(253) for the inertial igniter. The inertial igniter compartment243(253) is preferably formed by a member244(254) which is fixed to the inner surface of the thermal battery housing242(253), preferably by welding, brazing or very strong adhesives or the like. The separating member244(254) is provided with an opening245(255) to allow the generated flame and sparks following the initiation of the inertial igniter240(250) to enter the thermal battery compartment246(256) to activate the thermal battery241(251). The separating member244(254) and its attachment to the internal surface of the thermal battery housing242(252) must be strong enough to withstand the forces generated by the firing acceleration.

For larger thermal batteries, a separate compartment (similar to the compartment10over or possibly under the thermal battery hosing11as shown inFIG. 1can be provided above, inside or under the thermal battery housing for the inertial igniter. An appropriate opening (similar to the opening12inFIG. 1) can also be provided to allow the flame and sparks generated as a result of inertial igniter initiation to enter the thermal battery compartment (similar to the compartment14inFIG. 1) and activate the thermal battery.

The inertial igniter200,FIGS. 13 and 14may also be provided with a housing260as shown inFIG. 16. The housing260is preferably one piece and fixed to the base202of the inertial igniter structure201, preferably by soldering, laser welding or appropriate epoxy adhesive or any other of the commonly used techniques to achieve a sealed compartment. The housing260may also be crimped to the base202as shown inFIG. 16for the inertial igniter embodiment30. The housing260may also be crimped to the base202at its open end261, in which case the base202is preferably provided with an appropriate recess262to receive the crimped portion261of the housing260. The housing can be sealed at or near the crimped region via one of the commonly used techniques such as those described above.

In addition, as shown inFIG. 17, the base202of the inertial igniter200may be extended to form the cap263, which could be used to form the top cap of the thermal battery as is shown inFIG. 5cand identified with the numeral36for the inertial igniter embodiment30.

The inertial igniter embodiment200ofFIGS. 13 and 14as provided with the aforementioned housing260and shown inFIG. 16may also be hermetically sealed. To this end, and as shown inFIG. 18, the opening204can be covered, preferably with a thin membrane264. The membrane264can be an integral part of the base202and is scorched on its bottom surface (not seen in the view ofFIG. 18) to assist it to break open by the pressure generated by the initiation of the pyrotechnics compound215(FIG. 13) upon initiation of the inertial igniter to allow the generated flame and sparks to enter the thermal battery through the resulting opening.

In another embodiment, more than one inertial igniter, preferably inertial igniters of the embodiment200type are used in a thermal battery to significantly increase the overall reliability of the thermal battery initiation under all-fire condition. As a result, if for any reason one of the inertial igniters fails to initiate or fails to initiate the thermal battery, then there would be one or more (redundant) inertial igniters to significantly reduce the chances that the thermal battery would fail to be activated. The more than one inertial igniters (preferably of embodiment200or any other of the aforementioned embodiments) may in general be assembled in any appropriate configuration in the thermal battery. For the case of small thermal batteries, however and if the thermal battery size allows, the inertial igniters are preferably ganged up together in one location, for example on the top or bottom compartments shown inFIGS. 15aand15bor in the compartment10shown inFIG. 1, to minimize the total volume and size occupied by the inertial igniters. For example, when three inertial igniters of the embodiment200are to be assembled within a thermal battery, for example of the type shown inFIG. 15a, assuming that the amount of space available in the compartment243is appropriate, the three inertial igniters200may be ganged up inside the compartment243as shown in the top view ofFIG. 19(the top cap is removed to show the inertial igniters200inside the compartment243).

When more than one inertial igniter200(or of other embodiment types) are ganged up in a compartment similar to that of243as shown inFIG. 19, the body201of two or more of the inertial igniters200may be integral. For example, the bodies201of the three inertial igniters200shown inFIG. 19may be integral as shown in the top and isometric views ofFIGS. 20aand20b, respectively, and identified with reference numeral265.

In certain applications, it is desired that the inertial igniters ganged up in a compartment such as243as shown inFIG. 19be separated by a wall so that their operations and/or failure (such as flying pieces following initiation or break up of one igniter) would not interfere with the operation of the remaining inertial igniters. In such cases, the inertial igniter bodies (such as the bodies201of the inertial igniters200,FIGS. 13 and 19) and the separation walls206between at least two of the inertial igniters may be integral as shown in the isometric and top views ofFIGS. 21aand21b, respectively, and indicated by reference numeral266. In the drawings ofFIGS. 21aand21b, all three inertial igniters200are intended to be separated from each other by the walls267.

The present inertial igniters are designed such that when ganged up as shown inFIG. 20aorFIG. 21a, their integral bodies201can be readily machined, for example from a solid rod, using commonly used CNC machining centers or the like.