Abstract:
A rupturable bag assembly comprises an inflatable bag which parts at a target internal pressure to produce an acoustic shock wave having a minimum target noise level at a prescribed distance. The inflatable bag comprises inner and outer walls. The inner wall has greater elasticity than the outer wall. Both inner and outer walls are constructed from first and second disk shaped sections with the first and second sections being sealed along outside perimeters. The second section of the inner wall carriers a heat resistant shield on one face. An inflation port assembly provides a chamber for a fuel source and flame abatement elements between the fuel source and an inlet port to the rupturable bag.

Description:
BACKGROUND 
     1. Technical Field 
     The field relates to inflatable devices such as air bags and more particularly to an air bag having a burst point control envelope with particular application to stun grenades. 
     2. Description of the Technical Field 
     U.S. Pat. No. 8,117,966 taught a non-pyrotechnic stun grenade for generating loud, explosive sound by inflation to rupture of an inflatable bag. To make the point of rupture consistent from bag to bag and to achieve target noise levels within a limited time period the &#39;966 patent proposed to construct a single layer inflatable bag with a rupture seam. Upon inflation the rupture seam parted abruptly at a particular and predetermined degree of tension on the seam. The rupture seam parted at a design volume of the bag and pressure within the bag to produce an N-wave. The explosive sound produced consistently met a minimum target volume level. Although the &#39;966 patent provided for a non-pyrotechnic, compressed air, inflation source the patent suggests that pyrotechnic gas generation more readily produced high gas flow rates than compressed gas sources. 
     The use of chemical reactions to generate gas generators for inflation of automotive air bags is known. One issue addressed during the development of such air bags was the type of gas generator to use. Among the concerns was the byproducts produced by the chemical reactions or combustion of the fuel source used to generate the gas. 
     A popular contemporary gas generator for automotive applications is a mixture of sodium azide (NaN 3 ), potassium nitrate (KNO B ) and silicon dioxide (SiO 2 ). An exothermic (heat producing) decomposition of sodium azide into nitrogen gas and sodium can be initiated by exposure of the compound to 300° C. The free nitrogen gas inflates the bag while the potassium nitrate reacts with the sodium in a second reaction to produce potassium oxide (K 2 O), sodium oxide (Na 2 O) and more free nitrogen (N 2 ). A final reaction translates the reactive potassium oxide and sodium oxide compounds into more stable byproducts by a reaction with the silicon dioxide to produce potassium silacate and sodium silicate (K 2 O 3 Si and Na 2 O 3 Si). These are chemically stable compounds which pose no known environmental and health threat. See  Gas Laws Save Lives: The Chemistry Behind Airbags , Casiday, R. and Frey, R. (2000). In addition, the initiating materials are not hygroscopic as water absorption can slow or stop gas generating reactions limiting the shelf life of units. Alternative pytrotechnic formulations for a gas generator may make use of potassium nitrite (KNO 2 ). Such fuel sources result in reactions which are highly exothermic and can produce higher temperatures than the reaction based on sodium azide. 
     Construction of an inflatable bag which ruptures at a consistent degree of inflation to produce predictable noise levels using an exothermic chemical reaction to produce the inflation gas poses issues not present when a compressed air source is used. In contrast, where a compressed gas source is used for inflation the temperature of compressed gas falls upon expansion. 
     SUMMARY 
     A rupturable bag assembly for a stun grenade comprises an inflatable bag which parts at a target internal pressure to produce an acoustic shock wave having a minimum target noise level at a prescribed distance. The inflatable bag comprises inner and outer walls with the inner wall having greater elasticity than the outer wall. Both inner and outer walls are constructed from first and second disk shaped sections with the first and second sections being sealed along an outside perimeters. The second section of the inner wall carriers a heat resistant shield on its relatively inner face. 
     An inflation port is provided from outside into the rupturable bag through the first sections of the outer and inner walls to deliver gas into the rupturable bag and against the heat reistant shield. 
     An inflation gas generater and flow arrester assembly is fitted to the inflation port outside of the rupturable bag. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a rupturable bag assembly for a stun grenade. 
         FIG. 2  is a perspective view of a rupturable bag. 
         FIG. 3  is a side elevation of the rupturable bag of  FIG. 2 . 
         FIG. 4  is a cross section view of the rupturable bag of  FIGS. 2 and 3 . 
         FIG. 5  is an exploded perspective view of the rupturable bag. 
         FIG. 6  is an exploded side view of the rupturable bag. 
         FIG. 7A  is an cross sectional view of mating of an air inlet with the rupturable bag. 
         FIG. 7B  is a detail view of clamping the bag with the air inlet. 
         FIG. 7C  is detail of the rupturable bag upper wall. 
         FIGS. 8A and 8B  are exploded perspective and side views of the rupturable bag assembly. 
         FIG. 9  is a perspective view of the pressurization gas arrester for the rupturable bag assembly. 
         FIG. 10  is a top view of the gas arrester. 
         FIG. 11  is a cross-sectional view of the gas arrester taken along section lines  11 - 11  of  FIG. 10 . 
         FIGS. 12A , B and C are detail views of a assembly washer for the gas arrester. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, example sizes/models/values/ranges may be given with respect to specific embodiments but are not to be considered generally limiting. 
     Referring now to the figures and in particular to  FIG. 1 , a self-inflating rupturable bag assembly  10  is shown. Rupturable bag assembly  10  may conceptually be divided into two sections, a rupturable bag  12  and an inflation gas generator assembly  14  which is mounted to bag inflation port  16 . Rupturable bag  12  parts along a perimeter seam  18  upon inflation to a minimum pressure and tension on the seam. The rupturable bag assembly  10  may be used with a variety of stun grenades to generate an explosive sound. A pair of electrical studs  52  allow connection to an electrical circuit which may be used to ignite a fuel source located in the inflation gas generator assembly  14 . 
     In  FIG. 2  the rupturable bag  12  is shown with inflation gas generator assembly  14  detached to better show inflation port  16 . The upper portion of inflation port  16  is threaded for attachment to the inflation gas generator assembly  14  and provides an inlet  20  disposed through its center. Inflation gas is introduced to rupturable bag  12  via inlet  20 . 
     The details of construction of rupturable bag  12  are shown in  FIGS. 3-6 . Inflation port  16  is a multiple element assembly extending through an upper wall of rupturable bag  12 . The inflation port  16  incorporates a conduit  34  which is flattened and thickened at one end to form an inner bulkhead  26 . Conduit  34  extends through a first of two walls  13 ,  15  of rupturable bag  12  which places inner bulkhead between the two walls, inside an assembled an assembled rupturable bag  12 . 
     Located between the inner bulkhead  26  and the first wall  13  is an inner collar  30 . Outside of first wall  13  is an outer collar  28 . Adjacent the outer collar  28  moving along conduit  34  is a washer  42 . The collars  30 ,  28 , clamp washer  22  and washer  42  are held in place by a nut  32  which is threaded onto the conduit  34 . 
     The rupturable bag  12  comprises first and second walls  13 ,  15 . The rupturable bag  12  also comprises an inner elastic balloon  38  and an outer reinforced envelope  36 . The material of the outer envelope  36  is less elastic than the material used to construct the inner balloon  38 . A nylon weave fabric would be suitable. Both the inner elastic ballon  38  and the outer reinforced envelop  36  are constructed from first and second layers, in the case of the inner elastic balloon, first and second layers  38 A and  38 B, and in the case of the outer reinforced envelope  36 , first and second layers  36 A and  36 B. The halves of inner elastic ballon  38  are closed along seam  19 . The halves of outer reinforced envelope  36  are closed along seam  18 . Seam  18  is constructed to part upon application of pressure from within. Failure of seam  18  results in a cascade failure of inner elastic balloon  38 . Seam  18  may be constructed in a number of ways. Where closed mesh, rip stop (a type of weave) nylon is used as a fabric from which outer reinforced envelop  36  is constructed. The seam  18  may be formed using braided nylon or polyester with a typical strength range of 20 to 50 lbs. tensile strength stitching the two halves together. A zig-zag stitch allows the use of lower tensile strength materials for the burst envelope and the seam than a straight stitch allows. The inner elastic balloon may be made with vinyl with the halves welded together. Welding may be done a number of ways, for example, sonically, chemically or radio frequency welded. Adhesives and heat bonding are also possible. In this way a volumetrically small envelope can be constructed which can be inflated to a target burst pressure of 375 psi. A bag having a diameter of 5 inches on inflation producing a 180 dB peak over pressure shock wave on rupture can be built. Such a bag can be inflated to rupture in 20 to 30 milliseconds using a sodium azide or similar gas source. 
     Applied to the inner face of second layer  38 B of inner elastic balloon  38  is a heat shield layer  40 , which may be constructed of aluminum foil of mylar. Heat shield layer  40  is used to prevent premature failure of rupturable bag  12  due to ejection of hot gas from inlet  20 . 
       FIGS. 7A-C  illustrate of the juncture between inlet port assembly  16  and the first wall  13  of rupturable bag  12  and of the second wall  15  of the rupturable bag. The clamp washer  22  carries an annular dimple  44  on one face displaced outwardly from the conduit  24 . Annular dimple  44  aligns on and is shaped to conform to an annular depression  46  on the adjacent face of inner bulkhead  26 . The first wall  13  of the rupturable bag  12  is pinched between the inner bulkhead  26  and the clamp washer  22 . Adhesive layers may be used between wall elements in the area of the clamp washer  22  to improve sealing. 
       FIGS. 8-12  illustrate construction of the inflation gas generator assembly  14 . Gas arrester assembly  14  includes a housing/body  50  which is essentially a tube which is open an one end, closed at the other. The open end of the body  50  is mated with a connector  48  fitted between the inflation gas generator assembly  14  and the inflation port  16 . Connecter  48  is fitted to conduit  24  outside nut  32  on the exposed end of the conduit relative to the rupture bag  12 . The remaining elements of the inflation gas generator assembly  14 , excluding a pair of electrical studs  52 , are located in the housing  50 . The electrical studs  52  pass through the housing to allow application of an electrical trigger signal from outside the housing to a fuel source  54  located in the housing  50 . 
     Combustion of fuel source  54 , which may be a dry, packed blend of sodium azide, silicon dioxide and potassium nitrate, results in a jet of high temperature gas being ejected from the open end of the inflation gas generator assembly  14  into a connector  48  between the assembly  14  and the inlet  20  of the inflation port  16 . Fuel source  54  is shaped an a ring with a plurality of radial connecting rods  64  aimed inwardly on the ring for connection to the electrical studs  52  by wires (not shown). As an alternative to a fuel source including sodium azide, more conventional pyrotechnic fuel sources may be used, typcially incorporating potassium nitrite. To protect the elastomeric and fabric layers of the rupturable bag  12  from the full force and heat of gas ejected from the gas generator assembly  14  the path from fuel source  54  to connector  48 , while axial, is not direct. A variety of trigger mechanisms may be used, particularly where an electronic trigger signal is provided. 
     Upon assembly of inflation gas generator assembly  14  the fuel source  54  is located deepest in the housing  50 , proximate to the closed end of the housing and distal to its open end. Moving toward the open end of housing  50  a lower washer  58 B is located having a central annular opening through which gas is ejected. Next in line is a lower spacing washer  56 B which defines openings between its perimeter edge and the inner wall of the housing  58 B. Spacing elements are constructed into the lower spacing washer  56 B so that gas can pass from the central annular opening of washer  58 B to the perimeter openings. This cycle is repeated once with an upper washer  58 A and an upper spacing washer  56 A. The lower and upper spacing washers  56 B and  56 A are illustrated in detail in  FIGS. 12A-C  generally at reference numeral  56 . Washers  58 A,  58 B,  56 A and  56 B, along with top cap  60 , provide a flame arresting function the fuel source  54  and the inlet port  20 . A more extensive flame arresting system incorporating additional washers of alternating types may be employed for pyrotechnic devices as the target temperature range in the rupture envelope is below  100  to  125  degrees Celsius. 
     Gas is ejected from housing  50  through a perforated top cap  60 . Top cap  60  is retained in housing  50  using a spring spacing ring  62  which fits in an annular slot  66  in the inner wall of the housing proximate to the open end of the housing.