Abstract:
An apparatus for generating an impact against a target object comprises a driver reciprocally disposed in a housing. The driver includes a hollow tube having a closed end and a nozzle sealing the other end. A piston is slidably positioned in the tube. Propellant is disposed between the piston and the closed end of the tube and fluid is disposed between the nozzle and the piston. A rupture disc is provided for sealing the nozzle which is adapted to rupture when the pressure in the tube exceeds a predetermined pressure. A striker is also mounted in the housing so that in a retracted position of the striker a head portion is proximate the driver and a portion of a shaft extends outwardly from the housing. The striker member is movable between the retracted position and an extended position. The propellant is ignited so that combustion gases build pressure in the tube between the piston and the closed end of the tube causing the pressure in the tube to exceed the predetermined pressure for rupturing the disc. This causes the piston to move toward the nozzle and fluid to be expelled through the nozzle for moving the driver against the head portion of the striker. The driver transfers energy to the striker for moving the striker to the extended position at high velocity for driving the end of the striker with great force against the target object. Recoil action is cushioned by the fluid exiting the nozzle.

Description:
CROSS-REFERENCES  
       [0001]     This application is a continuation-in-part of application Ser. No. 09/710,073, filed Nov. 10, 2000, the contents of which are hereby incorporated by reference. 
     
    
     GOVERNMENT RIGHTS  
       [0002]     The inventions described herein may be manufactured and used by or for the U.S. Government for U.S. Government purposes. 
     
    
     BACKGROUND  
       [0003]     This invention relates generally to hand-held impact devices, and more particularly to hand-held impact devices for gaining entry to locked or barricaded structures.  
         [0004]     There is often a need for authorized personnel to rapidly gain access to locked, barricaded or otherwise secured buildings and to damaged structures, particularly in response to illegal activity or an emergency. Portable, hand-held forcible entry devices have been developed that enable law enforcement and emergency personnel to forcibly open a locked or fortified door, barricaded passage, damaged structure, or any other barrier that requires the use of force to gain access to a building or structure.  
         [0005]     A typical forcible entry device comprises a piston-driven striker housed within a generally cylindrical case. The end of the striker extends from the front end of the case. A modified, conventional firearm is secured to the other end of the case for discharging the forcible entry device. The modified firearm fires a blank cartridge or other explosive charge which generates a combustion gas for driving the piston-driven striker outwardly of the housing to produce an extreme percussive force. In use, the striker is placed against a target object, such as a locked or barricaded door or damaged structure, and the firearm is fired. The striker extends from the front end of the case with great force and impacts the target object for breaking through the door or structure.  
         [0006]     A problem with conventional forcible entry devices is the recoil generated when the device is fired due to the large force necessary to drive the striker. The recoil makes the device difficult for the user to hold and to control in use. Another problem with using forcible entry devices occurs when the target object offers little resistance to the striker. The force generated by the high velocity extension of the striker results in “forward” recoil wherein the device jerks forward in the user&#39;s hands. Forward recoil is also a problem when the devices are “dry fired”, that is, fired when the striker does not impact a target object.  
         [0007]     For the foregoing reasons, there is a need for a new impact generating device for use in forcible entry of locked or damaged structure which is recoilless. The new device should be recoilless in the traditional sense and minimize forward recoil in the case of soft target objects or dry firing. Ideally, the new impact device should also be compact and lightweight, and thus portable enough to be rapidly positioned and deployed to gain access to a structure without the need for an external power source.  
       SUMMARY  
       [0008]     Therefore, it is an object of the present invention to provide an impact generating device which is recoilless.  
         [0009]     Another object of the present invention is to provide an impact generating device which minimizes forward recoil, even when impacting soft target objects or when dry fired.  
         [0010]     A further object of the present invention is to provide a recoilless impact generating device which is useful in forcible entry of a locked or damaged structure.  
         [0011]     According to the present invention, an apparatus for generating an impact against a target object comprises a housing defining an interior chamber and having a closed first end and an open second end. A drive member is reciprocally disposed in the interior chamber adjacent the second end of the housing for movement relative to the housing from a first firing position to a second driven position. The drive member includes a hollow tube member having a first closed end and a second open end. A nozzle member having a plurality of openings is sealably mounted in the second end of the tube. A piston is disposed in the tube for movement relative to the tube and propellant is disposed between the piston and the closed end of the tube. Fluid is also in the tube between the nozzle member and the piston. Means are provided for sealing the openings in the nozzle member, wherein the nozzle opening sealing means is adapted to rupture when the pressure in the tube exceeds a predetermined pressure. A striker member having a head portion and a shaft portion is mounted within the interior chamber so that in a first retracted position of the striker member the head portion of the striker member is proximate the first end of the drive member and a portion of the shaft portion extends outwardly from the interior chamber through a passage formed in the closed end of the housing. The striker member is movable relative to the housing between the first position and a second extended position where the head portion is adjacent the first end of the housing. Means are provided for igniting the propellant so that combustion gases build pressure in the tube member between the piston and the closed end of the tube member causing the pressure in the tube member to exceed the predetermined pressure for rupturing the nozzle sealing means. This causes the piston to move toward the nozzle member and fluid to be expelled through the nozzle member for moving the drive member against the head portion of the striker member and to the driven position. The drive member transfers energy to the striker member for moving the striker member to the second position at high velocity for driving the end of the striker with great force against the target object Recoil action in the apparatus is cushioned by the fluid exiting the tube member through the nozzle member as the piston moves toward the nozzle member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings:  
         [0013]      FIG. 1  is a perspective view of an embodiment of a recoilless impact device according to the present invention.  
         [0014]      FIG. 2  is a front elevation view of the recoilless impact device shown in  FIG. 1 .  
         [0015]      FIG. 3  is a rear elevation view of the recoilless impact device shown in  FIG. 1 .  
         [0016]      FIG. 4  is a side elevation view of the recoilless impact device shown in  FIG. 1 .  
         [0017]      FIG. 5  is a side cross-section view of the recoilless impact device shown in  FIG. 1   
         [0018]      FIG. 6  is a side cross-section view of the recoilless impact device as shown in  FIG. 5  with the striker assembly forced together.  
         [0019]      FIG. 7  is a side cross-section view of the recoilless impact device shown in  FIG. 6  after firing of the device.  
         [0020]      FIG. 8  is an exploded cross-section view of the recoilless impact device shown in  FIG. 5 .  
         [0021]      FIG. 9  is a longitudinal cross-section view of a driver assembly for use with the recoilless impact device of  FIG. 1 .  
         [0022]      FIG. 10  is an exploded cross-section view of the driver assembly shown in  FIG. 9 .  
         [0023]      FIG. 11A  is a flat plan view of a nozzle for use with the recoilless impact device shown in  FIG. 1 .  
         [0024]      FIGS. 11B-11D  are cross-sectional views of the nozzle shown in  FIG. 11A  taken along lines  11 A- 11 A,  11 B- 11 B, and  11 C- 11 C, respectively.  
         [0025]      FIG. 12  is a side elevation view of a key block assembly for use with the recoilless impact device shown in  FIG. 1 .  
         [0026]      FIG. 13  is an embodiment of the recoilless impact device as shown in  FIG. 1  including a handle assembly.  
         [0027]      FIG. 14  is a side cross-section view of a removable shaft tip for use with the recoilless impact device shown in  FIG. 1 .  
         [0028]      FIG. 15  an end view of a shaft for use with the recoilless impact device shown in  FIG. 1 .  
         [0029]      FIG. 16  is a close-up side cross-section view of a primer and primer block for use with the driver assembly shown in  FIG. 9 .  
         [0030]      FIG. 17  is a flat plan view of another embodiment of a nozzle for use with the recoilless impact device shown in  FIG. 1 .  
         [0031]     FIGS.  18  is a cross-sectional view of the nozzle shown in  FIG. 17  taken along line  17 - 17 .  
         [0032]      FIG. 19  is another embodiment of a striker assembly for use with the recoilless impact device shown in  FIG. 1 . 
     
    
     DESCRIPTION  
       [0033]     The impact generating device according to the present invention is similar to the forcible entry device shown and described in U.S. patent application Ser. No. 09/065,746, the contents of which are hereby incorporated by reference.  
         [0034]     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” and “downward” merely describe the configuration shown in the Figures. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise.  
         [0035]     Referring now to the drawings, wherein like reference numerals designate corresponding or similar elements throughout the several views, an embodiment of the impact device according to the present invention for use, for example, in forcible entry of locked or barricaded structures or doors is shown in FIGS.  14  and designated generally at  20 . The impact device  20  includes a housing  22 , a front cap  24  and an elongated striker shaft  26  extending through the cap  24  outwardly of the housing. At the end of the striker shaft  26  is a tip  28 . The tip  28  may be any useful shape, depending upon the structure to be opened, removed or cut. For example, a chisel type tip  28  is shown in  FIG. 1 . The tip  28  may be made removable, as in the present device  20 , to ease application of the device to different situations. An outer channel weldment  30  extends from the front end of the housing  22  to a key block assembly  32  at the rear of the housing. The channel weldment  30  is held in place by straps  34  which are secured around the housing  20  by threaded fasteners  36 . Unless otherwise specified, all of the parts of the impact device  20  are aluminum except the striker shaft  26  and tip  28  which are steel.  
         [0036]     Referring now to  FIGS. 5 and 8 , the housing  22  defines a generally cylindrical interior chamber  38 . The front end of the housing  22  is sealed by the front cap  24  which is threaded into the housing  22 , and the rear end of the housing is open.  
         [0037]     A striker assembly  40  and a driver assembly  42  are reciprocally disposed within the chamber  38  at the front and rear of the housing  22 , respectively. The striker assembly  40  comprises the striker shaft  26  and a striker head  44 . One end of the striker shaft  26  extends outwardly of the housing  22  from the interior chamber  38  through a central opening  46  formed in the front cap  24 . A brass bushing  48  fits in the cap opening  46  between the cap  24  and striker shaft  26  to permit the striker shaft to reciprocate freely relative to the front cap. Optionally, the cap  46  may be provided with an annular groove  48  for receiving an o-ring  50  which fits snugly around the striker shaft  26  to seal the space between the cap  24  and striker shaft. However, if the bushing  48  is machined to sufficiently close tolerance with the shaft  26 , the o-ring  50  is not necessary. The striker head  44  includes two generally cylindrical pieces, an outer striker head  52  and an inner striker head  54 . The outer striker head  52  has three spaced circumferential grooves: a forward groove  56  which holds a rubber wiper ring  58 , a middle groove  60  which holds a polymer guide ring  62  and a rear groove  64  which holds a copper contact ring  66  which is insulated from the outer striker head  52 .The inner striker head  54  is steel and includes four spaced guide pins  68 , only two of which are shown in  FIGS. 5 and 8 . The guide pins  68  are movably received in corresponding openings  70  in the rear of the outer striker head  52  so that the inner striker head  54  and outer striker head  52  fit reciprocally together. The outer striker head  52  also has an axial pass through opening  72  for receiving a reduced diameter portion of the inner end of the striker shaft  26 . The inner end of the striker shaft  26  has an internally threaded axial opening  73  for receiving a shaft screw  74  which passes through the axial opening  72  in the outer striker head  52  thus securing the outer striker head to the striker shaft. A small coil spring  76  is interposed between the inner striker head  54  and outer striker head  52  for biasing the heads apart.  
         [0038]     A large coil spring  78  is disposed around the striker shaft  26  within the housing  22 . One end of the spring  78  is positioned against the outer striker head  52  and the other end of the spring is against the front cap  24 . The spring  78  biases the striker assembly  40  inwardly of the housing  22 . As best seen in  FIG. 8 , the inner diameter of the interior chamber  38  of the housing  22  is decreased intermediate the ends of the housing forming a seat  80  against which the inner striker head  54  is biased proximate to a front end of the driver assembly  42  ( FIG. 5 ). The size of the coil spring  78  is selected so that the space between the inner striker head  54  and outer striker head  52 is maintained by the striker head spring  76 .  
         [0039]     Referring to  FIG. 14 , the removable tip  28  is shown in more detail. The tip  28  has an axial bore  164  for slidably receiving a pin  166  which is held in the tip  28  by a hollow, peripherally-threaded plug  168  which journals the end of the pin  166 . A key retainer  170  is secured to the end of the pin  166  by a cap  172  and screw  174  which is received in an axial threaded bore in the end of the pin  166 . A spring  175  is disposed around the pin  166  in the tip  28 . One end of the spring  175  is against the pin  166  head and the other end of the spring is against the plug  168  to bias the pin  166  and attached key retainer  170  inwardly of the tip  28 . The outer end of the striker shaft  26  is shown in  FIG. 15  to include a blind channel  176  defined along an axial bore  178 . A transverse slot  180  is formed at the inner end of the channel  176 . To attach the tip  28  to the end of the striker shaft  26 , the key retainer  170  is aligned with the channel  176  in the end of the shaft  26  and tip  28  pushed into the shaft along the depth of the channel. A counterclockwise turn (as seen in  FIG. 15 ) of the tip  28  will cause the key retainer  170  to move along the slot  180  thus locking the tip  28  in place in the shaft  26 . Removal of the tip  28  is the reverse of attachment.  
         [0040]     The driver assembly  42  is shown in  FIGS. 9 and 10 . The driver assembly  42  is similar to the recoilless propulsion unit shown and described in U.S. Pat. No. 5,099,764, the contents of which are hereby incorporated by reference, which expels a pressurized fluid from the unit through a nozzle using a closed-breech piston activated by a propellant charge.  
         [0041]     The driver assembly  42  according to the present invention comprises a generally cylindrical hollow tube  82 , a piston assembly  84  and a nozzle assembly  86 . The tube  82  has a closed inner end  88  and an open rear end  90  and defines an interior chamber  92 . The closed end  88  of the tube  82  has an axial passage  94  of stepped diameter opening outwardly of the end of the tube. The open end  90  of the tube  82  is internally threaded and is slightly thicker, which strengthens this portion of the tube.  
         [0042]     The piston assembly  84  includes a cup-shaped piston  96  slidably disposed in the interior chamber  92  adjacent the closed inner end  88  of the tube  82 . The piston  96  may be nylon for most fluids, but is preferably metal when gas permeability of the fluid is a consideration. The outer surface of the metal piston  96  is sealed against the walls of the interior chamber  92  by two spaced o-rings  98  with metal backing rings which fit in spaced circumferential grooves  100  in the piston. The o-rings  98  also serve as a guide for movement of the piston  96  in the tube  82 . Alternatively, the o-rings  98  and backing rings may be replaced by T-seals typically used in high-pressure dynamic sealing applications.  
         [0043]     A frustoconical ring seal  102  fits between chamfered surfaces  101 ,  103  at the front of the end of the tube  82  and the piston  96 . The piston  96  separates the interior chamber  92  of the tube  82  into front and rear variable volume chambers. The ring seal  102  prevents fluid, particular permeable gases, in the rear variable volume chamber from entering the front variable volume chamber. Preferably, the ring seal  102  comprises a polymer material, but could be a soft metal. Alternatively, the periphery of the front of the piston can be grooved and coated with a soft metal, such as copper or silver, for sealing the space between the piston  96  and tube  82 . In any case, the pressure of the fluid in the chamber  92  forces the piston  96  forward thereby compressing the ring seal  102  against the chamfered surface at the inner end of the tube  82  for sealingly separating the front and rear variable volume portions of the tube chamber  92 .  
         [0044]     The piston  96  has a central recess  104  for retaining a propellant charge  106 . It is understood that the present invention is not limited to the type of propellant used. For example, a suitable propellant is Winchester  231  smokeless powder. Adhesive paper  108  seals the propellant  106  in the recess  104  which centralizes the propellant in a contained target area. Although not shown in the FIGs., the rear portion of the piston  96  may include a protrusion of slightly less diameter than the body of the piston  96 . As will be described below, when the impact device  20  is fired, the piston  96  is driven rearward with great force into the nozzle assembly  86 . The protrusion on the rear portion of the piston  96  strengthens the surface of the piston  96  that impacts the nozzle assembly  86  thereby minimizing the potential for deformation of the piston  96  edges.  
         [0045]     A primer  110  is disposed in the axial passage  94  in the closed end of the tube  82  and held in place by a threaded plug  112 . Suitable primers  110  include M52A3B1 or PA520 military grade electrically-initiated primers available from Lake City (Ohio) Army Ammunition Plant. A small amount of electrical energy, approximately 1 mJ, will form an arc within these primers which ignites a very small amount of propellant. The passage  94  serves to communicate the primer  110  with the propellant charge  106  in the piston  96  and directs gases from the primer into the front variable volume chamber.  
         [0046]     Another embodiment of the driver assembly  42  according to the present invention is shown in  FIG. 16 . In this embodiment, the primer  110  is disposed in a peripherally threaded cylindrical primer block  182  which is received in a larger diameter portion of the opening  94  in the inner end of the tube  82 . The primer  110  fits in an opening in the primer block and is held in place by a hollow, peripherally threaded retainer  184 . The retainer  184  defines an opening  186  in the primer block  186  that allows access to the inner end of the primer  110 . The large diameter primer block  182  provides a contact point for completing an electrical firing circuit as will be described below.  
         [0047]     The nozzle assembly  86  includes a peripherally-threaded cylindrical nozzle  114  which is threaded into the open end of the tube  82 . An o-ring  115  seals the inner surface of the nozzle  114  against a shoulder  119  in the open end of the tube  82 . When CO 2  is the fluid, the o-ring is preferably polyurethane which is less susceptible to gas permeability. The inner surface of the nozzle  114  has a plurality of blind bores  116  ( FIGS. 11A-11D ) of stepped diameter. A small vent hole  117  leads from the end of each bore  116  outwardly of the nozzle  114 . A plurality of angled passages  118  branch from a point intermediate along the length of the bores  116  and open outwardly of the outer surface of the nozzle  114  forming elliptical openings in the bores and the outer surface of the nozzle. The hole pattern formed by the passage  118  openings in the outer surface of the nozzle  114  is selected so as to disperse the fluid in as many jets as possible without adversely affecting the flow characteristics of the fluid and to optimize the safety of the exit area of the nozzle  114 . The greater the exit area the more optimal the propulsion of the impact device  20 .  FIG. 3  shows another multiple hole pattern in the outer surface of the nozzle  114 . This pattern results from seven spaced bores  116  and four angled passages  118  from each bore  116 . The thickness of the nozzle  114  is determined by the structural integrity of the hole pattern and the flow characteristics of the fluid  18  through the passages  118 .  
         [0048]     Fluid  124  contained within the second variable volume chamber is preferably a liquid and, more preferably, the fluid is liquid CO 2 . Liquid CO 2  is stored in the tube  82  as a high pressure liquid/gas mixture wherein liquid CO 2  fills from about 50% to about  95 % of the volume of the chamber  92 . At CO 2  liquid levels below about 50% there is typically not enough power delivered for propelling the driver assembly  42  forward with sufficient force when the device  20  is fired. CO 2  liquid levels above 95% become too volatile since the CO 2  pressure will change due to temperature. Thus, the upper limit to the liquid level is determined based on an expected storage temperature range. A preferred CO 2  liquid level is about 75% at which the interior chamber  92  pressure will range from about 600 psi at 0° F. to about 3000 psi at 145° F. It is understood that other fluids may be used which have different preferred fill levels. For example, if water is the chosen fluid, the water preferably fills substantially 100% of the volume of the second variable volume chamber of the tube  82 .  
         [0049]     A brass burst disc  126  is disposed in each bore  114  against the shoulder  128  formed where the bore changes diameter ( FIG. 11C ). The burst disc  126  is formed from a brass shim stock with a protective coating. Each burst disc  126  is sealed in place with a hollow hex head retainer screw  130  for sealing the interior chamber  92  of the tube  82 . When liquid CO 2  is used as the fluid in the driver assembly  42 , the burst disc is designed to withstand 3700 psi.  
         [0050]     A simplified nozzle  114  design according to the present invention is shown in  FIGS. 17 and 18 . This nozzle  114  has seven straight passages  188  for fluid ejection. Each passage  188  is sealed by a burst disc  126  held in place by a peripherally threaded cylindrical retainer sleeve  190 . This nozzle  114  design is possible with the use of stainless steel non-fragmenting burst discs  126  available from BS&amp;B Safety System of Tulsa, Okla.  
         [0051]     As best seen in  FIG. 18 , the nozzle  114  also has a central fill hole  120  which opens into the interior chamber  92  of the tube  82 . A threaded plug  122  is provided for sealing the fill hole  120 . The plug  122  is a hollow modified set screw with an opening  123  that feeds into the interior  92  of the tube  82  when the plug  122  is slightly backed out of the hole  120 . An appropriate adapter (not shown) is provided on the plug  122  for coupling to a fluid feed line for loading the second variable volume portion of the interior chamber  92  of the tube  82  between the piston  96  and the nozzle  114 .  
         [0052]     Means for retaining the driver assembly  42  in the housing  22  are provided. The driver assembly retention means comprises the key block assembly  32  mounted on the rear of the housing  22 . As best shown in  FIG. 12 , the key block assembly  32  includes a block  132 , a stop hammer  136  and a plunger  138  reciprocally disposed in the channel weldment  30 . The stop hammer  136  is a flat piece having an opening  137  therethrough which is reciprocally received in a slot in the block  132 . The key block assembly  132  is positioned over a peripheral slot  140  in the housing  22  ( FIG. 5 ) which opens through to the interior chamber and allows the stop hammer  136  to extend into the housing  22 . The plunger  138  has a forward end  141  and a conically-shaped rear end  142  and is slidably disposed in the block  132 . The plunger  138  passes through the opening  137  in the stop hammer  136 . Movement of the plunger  138  in the key block assembly  32  relative to the stop hammer  136 .mioves the stop hammer  136  between a first position where a portion of the stop hammer extends into the housing  22  and a second position where the stop hammer is out of the housing. In the first position, the stop hammer  136  extends through the slot  140  in the housing and engages the rear of the tube  82  for securing the driver assembly  42  in the housing  22 . In the second position, the stop hammer  136  is in a non-blocking position with respect to the tube  82  so that the driver assembly  42  may be removed from the housing  22 . The stop hammer  136  is biased into the first, blocking position by one or more springs in the block  132 . A yoke  144  is shown connected to the front end of the plunger  138  for attachment to an appropriate release mechanism operable by the user.  
         [0053]     In keeping with the present invention a firing mechanism is provided. It is understood that there are many ways to fire the primer  110 , including mechanical and electrical means. Preferably, the firing mechanism is electrical since electrical means are less prone to accidental actuation. The specifics of the electrical circuitry for firing the device  20  can be easily developed by those skilled in the art and will not be addressed A preferred approach for carrying an electrical charge from a power source through the housing  22  and to the driver assembly  42  will be described. This approach includes first and second electrical contact plungers  146 ,  148  schematically shown in  FIG. 5 . The plungers  146 ,  148  are spring-biased through respective openings in the housing  22  to a position adjacent the striker head  44 . The first plunger  146  is biased into an open area in the housing  22  between the outer striker head  52  and inner striker head  54  when the impact device  20  is in a non-firing condition. An electrical wire  150  (not shown) connected to the copper contact ring  66  passes through a transverse hole (not shown) in the outer strike head  52  and into the axial opening in the striker head  44 . The wire leads to an electrical plunger  152  ( FIG. 9 ) disposed on the inner end of the driver assembly  42  and contacting the primer  110  for delivering electric current for firing the primer  110 . The ground connection is through the primer  110  skirt which is in close contact with the primer plug  112 . A plurality of electrical contact plungers  200 , two of which are shown in  FIG. 8 , nested in the rear end of the inner striker head  54  contact the primer plug  112 . The second plunger  148  is biased through the housing  22  and connects the inner striker head  54  to the electrical power source when the striker assembly  40  is in the firing position.  
         [0054]     When preparing to fire the device  20 , the housing  22  is loaded with a driver assembly  42  through the open end of the housing. The inside diameter of the housing  22  is larger than the closed end of the tube  82  to facilitate loading. The closed end of the driver assembly  42  engages the stop hammer  136  which has a ramped surface  139  for allowing the advancing driver assembly  42  to force the stop hammer up into the block  132 . This movement is possible because the hole  137  in the stop hammer  136  is larger than the diameter of the plunger  138 . The driver assembly  42  is advanced until the rear of the tube  82  is clear of the stop hammer  136  which is biased into the housing to hold the driver assembly  42  in the housing  22 .  
         [0055]     Referring now to  FIG. 6 , the tip  28  of the striker shaft  26  is then positioned against an object such as a locked door, damaged structure or other barrier to be opened and manual force applied to the device  20  toward the object This moves the striker shaft  26  inwardly of the housing  22  pushing the outer striker head  52  against the inner striker head  54  against the force of the interposed spring  76 . When the outer striker head  52  is moved rearward, the first electrical contact plunger  146  engages the copper contact ring  66  on the outer striker head to complete the electrical circuit. Thus, the preferred firing mechanism requires the user to physically engage the target object with the striker shaft tip  28  and manually force the striker shaft  26  into the housing a predetermined distance to enable the firing mechanism. This is a safe arrangement which prevents accidental “dry” firing of the device  20 .  
         [0056]     Another embodiment of the striker head  44  according to the present invention is shown in  FIG. 19 . In this arrangement, the outer striker head  52  and inner striker head  54  fit slidably together. A contact assembly  192  is positioned in axial openings across the striker heads  52 ,  54  for movement with the outer striker head  52  relative to the inner striker head  54 . The contact assembly  192  comprises a nylon contact holder  194 , a housing  196 , a probe contact  198  and a ground contact  200 . The contact holder  194  is fixed to a reduced inner end of the housing  196  which is formed from an electrically conductive material such as, for example, brass. The housing  196  has an axial bore which receives the electrically conductive probe contact  198 . The probe contact  198  is held in the housing  196  by a retaining ring  202 . A spring  204  is disposed in the housing  196  for biasing the probe contact  198  outwardly of the housing  196 . The housing  196  is slidably received in an insulator sleeve  206  positioned in the inner striker head  54 . The insulator sleeve  206  separates the ground contact  200  from the housing  196 . A wave spring  208  is disposed between the ground contact  200  and the inner striker head  54  for biasing the ground contact outwardly of the housing  196  and against the primer block  182 . A circular retainer disc  210  is fixed to the rear end of the inner striker head  54  to hold the contact assembly  192  elements in the inner striker head  54 .  
         [0057]     In this embodiment of the striker assembly  40 , the periphery of the inner striker head  54  includes two peripheral grooves which hold electrically conductive contact rings  212 . The spring-loaded contact pins  146 ,  148  are positioned in the housing  22  to engage the rings  212  in the both the non-firing condition and the firing position of the impact device  20 . Spring-biased contact pin assemblies  214 ,  216  disposed in transverse passages in the inner striker head  54  electrically connect the contact bands  212  with the housing  196  and ground contact  200 , respectively. This provides the electrical path from the exterior of the housing  22  to the probe contact  198  and ground contact  200 . When the inner and outer striker heads  52 ,  54  are brought together in the firing position of the impact device  20 , the probe contact  198  is extended from the rear end of the inner striker head  54  and engages the primer  110 . Since the ground contact  200  is against the primer block  182  the firing circuit is completed.  
         [0058]     In either embodiment of the striker assembly  40 , a cup  218  may be secured to the front end of the outer striker head  52 . The cup  218  serves as a witness panel for a proximity sensor (not shown) positioned in the outer cylinder of the housing. The proximity sensor senses when the inner and outer heads  52 ,  54  of the striker assembly  40  are compressed in the firing position of the impact device  20 . This is a redundant arming feature. When the impact device  20  is in firing position, the operator fires the device  20  by actuating the firing mechanism which delivers an electrical charge to the primer  110 . The primer cap  110  is discharged by the electrical charge. When the primer  110  fires, hot flame and gases generated by the primer pass into the first variable volume chamber through the passage  94  in the end of the tube  82 . The gases are directed by the passage  94  at a target area on the paper  108  retaining the propellant  106 . The primer gases penetrate the paper  108  and ignite the propellant  106  while simultaneously blowing the propellant around the first variable volume chamber.  
         [0059]     Expansion of the propellant gases builds up pressure in the first variable volume chamber between the piston  96  and the front end of the tube  82 . The pressure increase generates a force on the piston  96  which is transferred to the fluid  124 . The propellant gases continue to expand causing fluid pressure to rise until the burst discs  126  are ruptured. In the embodiment of the nozzle assembly  86  employing fragmenting burst discs  126 , the vent holes  117  allow pieces of the burst discs  126  to be driven safely into the blind end of the nozzle bores  116 . The vent holes  117  are too small to let pieces of the discs  126  escape. Alternatively, spikes (not shown) extending from the blind end of the bores  116  for capturing the burst discs  126  could replace the vent holes  117 . The inner elliptical openings of the secondary nozzle passages  118  are small enough to prevent pieces of the burst disc from exiting the nozzle  114 .  
         [0060]     The propellant gases continue to expand causing fluid  124  to be expelled through the nozzle  114  and into the atmosphere away from the user. Referring to  FIG. 7 , the momentum and the pressure generated by the fluid  124  expelled into the atmosphere force the driver assembly  42  forward against the striker head  44  which moves the striker assembly  40  towards the front end of the housing  22  with great force. The striker shaft tip  28  impacts against the locked door, damaged structure or other barrier so that the user, such as law enforcement or emergency personnel, may gain access to the building or structure. The recoilless feature of the device  20  is due to the Davis Gun Principle which holds that when a mass is expelled from a body there is an equal and opposite reaction generated propelling that body. In the present invention, the expelled fluid  124  generates a driving force. Since this reaction takes place within the housing  22  which is not rigidly attached to the propelling body, the result is no recoil transferred to the housing.  
         [0061]     Ideally, the burning propellant generates a pressure in the first variable volume chamber acting on the piston which, after an initial increase, is relatively constant over time as the piston travels toward the nozzle. Eliminating an initial pressure spike when the propellant is ignited allows a less robust tube to be manufactured. This goal is realized in the present invention due to a number of factors related to interior ballistics principals for pyrotechnically driven devices. First, the ratio of propellant charge to the initial available volume of the first variable volume chamber contributes to the desired propellant ignition and initial burn cycle. Maintaining the proper ratio controls the explosive nature of the burning propellant and the rate of the initial pressure increase upon firing of the device. Too much propellant or too little volume can lead to too high of an initial pressure spike. The cup shape of the piston is also a factor in the chamber configuration to optimize the burning of the propellant. The initial location of the piston  96  sets the chamber volume which matches an optimum burning solution for the propellant. The position of the recess  104  and the retaining paper  108  fixes the propellant conditions and minimizes the initial area exposed to the primer flame and gases for slowing the initial propellant burning rate. Blowing the propellant around the chamber helps produce a consistent repeatable burn.  
         [0062]     The pressure in the first variable volume chamber increases until the burst discs  126  rupture and fluid  124  is expelled from the nozzle. The burst discs  126  are designed to burst at a predetermined pressure in order to insure proper propellant burn pressure and temperature. As the piston  96  moves down the tube  82 , the first chamber volume ahead of the piston  96  increases proportionally to the amount of fluid  124  displaced. This increase in the first chamber volume directly affects the burning characteristics of the propellant charge  106 . The rate at which fluid  124  is expelled from the tube  82  is directly proportional to the number and total cross-sectional area of holes  118  in the nozzle  114  which determine the amount of resistant force, or back pressure, acting on the piston  96  as the piston moves down the tube and causes propellant to burn to a relatively steady rate. Thus, with a known initial volume of the first variable volume chamber and a specific nozzle design, a propellant charge  106  can be selected by those skilled in the art so as to generate a controlled propellant burn cycle and provide a desired pressure curve for the system.  
         [0063]     In a preferred embodiment, the propellant charge is 4.1 g which occupies about 0.1496 cubic inches. The empty volume of the first variable volume chamber is about 1.988 cubic inches. Thus, the ratio of the propellant charge to the initial chamber volume is 0.075. The driver assembly  42  is loaded with approximately 0.42 lbs. of liquid CO 2 . The burst discs retain at least an additional 1000-1200 psi of pressure before the discs break to properly initiate propellant buring. This configuration produces about 7000 psi of pressure within the propellant chamber and produces relatively constant pressure over time during firing. The impact force of the device  20  having these characteristics is designed to be 65,000 lbs. of peak force at 20 lb-sec impulse at ambient temperatures against a rigid surface. The liquid CO 2  turns into solid flakes, like snow, as it passes through the nozzle  114 . The driver assembly  42  is recessed into the housing  22  to create a cavity for the expanding CO 2  liquid-to-gas effect to increase impulse from the pressure generated by the phase change of the fluid.  
         [0064]     The striker assembly  40  compresses the spring  78  between the striker head  44  and front cap  24  as the striker shaft  26  extends from the housing  22 . The spring  78  and air compressed between the front cap  24  and striker head  44  serve as a pneumatic damping mechanism for slowing the striker assembly  40  to a stop and minimizing forward recoil. A small vent hole  156  is provided in the housing  22  near the front end. Air is forced through the vent hole  156  only if pressure in the housing reaches a predetermined pressure, for example about 250 psi, which happens only if the striker is over-accelerated. This feature is particularly advantageous when the device  20  is dry-fired or a target object is easily penetrated when fired. The tube  82  is slightly tapered at the nozzle end  90  to allow propellant gases to vent between the piston assembly  84  and the tube wall to relieve the pressure in the driver assembly  42  as the piston  96  is nearing the nozzle  114 . The compression spring  78  returns the striker assembly  40  and driver assembly  42  into the housing to the pre-firing position shown in  FIG. 5 .  
         [0065]     After firing, the device is reloaded by advancing the plunger  138  which raises the stop hammer  136  away from the rear of the driver assembly  42 . The spent driver assembly  42  is slipped out of the housing  22  and replaced with a fresh driver assembly. The spent driver assembly is reusable.  
         [0066]     An embodiment of the device  20  including a handle assembly  157  is shown in  FIG. 13 . The handle assembly  157  is preferably formed from a fiber reinforced composite material which is both strong and light, and comprises two hand grips  159  extending transversely to the housing  22 . The handle assembly  157  accommodates a power source, such as a 9-volt battery. When the user holds the device  20 , the user&#39;s thumbs are over a forward safety button  158  and a rear firing switch  160  positioned on the outside of the device which is easily accessible to the user holding the device. In a preferred firing sequence, when the user pre-loads the device  20  by pressing the tip  28  against a rigid object, an LED under the safety button  158  lights-signaling the user the device is pre-loaded. The user then presses the safety button  158  which powers up the device  20 . When the device  20  has enough energy to fire, an LED under the firing switch  160  lights and the user knows the device is ready to fire.  
         [0067]     A pivoting release lever  162  on the rear of the handle assembly  157  is pressed downward to raise the stop hammer  136  and allow a spent driver assembly to be removed and replaced.  
         [0068]     The previously described versions of the present invention have many advantages, including delivery of a large impact to a target object, such as a locked or damaged structure, while generating no recoil, even when impacting soft target objects or accidental dry firing. The device is a great improvement over existing forcible entry devices for gaining entry to locked or damages structures through doors or other barriers. The impact device of the present invention is also compact and lightweight. This reduces the amount of time required to gain access to the building or damaged structure. Further, the impact device is versatile enough to be utilized in the many different situations in addition to those noted above, including for forcibly cutting materials and the dispatching of animals to be processed for nutritional purposes.  
         [0069]     Although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the invention to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the impact device of the present invention has numerous other applications including delivering destructive blows to objects or dispatching animals. The significant advantage of the device is the forceful impact delivered with no recoil. Accordingly, we intend to cover all such modifications, omissions, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a crew may be equivalent structures.