Abstract:
A mortar system includes a launch unit that comprises a mortar ignition cartridge provided with a flashtube. The flashtube incorporates a ring-like nozzle that is positioned within the flashtube, which acts to restrain the motion of the black powder pellets as well as to choke the flow of the resultant combustion products. The nozzle separates the flashtube into two compartments: a flashtube venting chamber and a pellet combustion chamber. The mortar ignition cartridge combustion chamber acts as the product-gas venting area of the flashtube. The combustion of the black powder pellets essentially takes place in the pellet combustion chamber. The present design modification improves the overall performance of the mortar system.

Description:
GOVERNMENTAL INTEREST 
     The invention described herein may be manufactured and used by, or for the Government of the United States for governmental purposes without the payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of devices for launching mortar projectiles. Particularly, the present invention relates to a flashtube in a launch unit of the mortar. More specifically, a nozzle divides the flashtube into two compartments: a flashtube venting chamber and a pellet combustion chamber, in order to improve the performance of the mortar. 
     BACKGROUND OF THE INVENTION 
     The reliable performance of a propellant driven munition system is dependent on the performance of its propulsion unit. The ignition train has historically been the weakest link in the repeatability of the munition system. Coupling the initiation of the ballistic cycle through various intermediate steps until a smooth transition to full combustion is achieved is the key to reliability. 
     In a mortar system, the mortar propulsion system is initiated by either an electric or a percussion element in the primer head, which in turn ignites a black powder pellet(s) (igniter) in the base of the flashtube. The combustion products (including gases) are then vented through flash holes in the flashtube, igniting the propellant in the mortar ignition cartridge. The combustion products in the mortar ignition cartridge then vent through holes in the mortar boom. Gas pressure from the combustion products starts the motion of the mortar projectile through the launch tube and the pressure and temperature of the products ignites any additional propellant increments attached to the boom. 
     Dynamic and hydrodynamic processes occur during the ignition and combustion phase of the launch. The black powder pellet is seated on a small shoulder at the base of the flashtube. Once the black powder pellet starts to be consumed by the combustion process, its diameter starts to reduce and it becomes free to travel within the flashtube, driven by local pressure gradients. 
     Thus, the black powder pellet becomes free to oscillate within the flashtube, covering and uncovering flash holes in the flashtube and creating pressure oscillations within the mortar ignition cartridge. As combustion is driven by the local pressure that the propellant experiences, the oscillations within the flashtube can generate pressure waves within the mortar ignition cartridge, which can propagate even further into the main combustion chamber. 
     Since pressure is the driving force of the flashtube venting, it can cause the steel pin end of the flashtube to more vigorously initiate local combustion. This can cause further pressure waves within the mortar ignition cartridge, which can be transferred to the combustion in the launch tube. 
     At low zones of propelling charge, which has the lowest loading density of propellant in the mortar system, the effect of the waves may be minimal. At higher zones and loading densities, the pressure waves can cause erratic performance and, in the most severe instances, break fins off the mortar boom. This in turn causes an inaccurate trajectory and at worst a safety problem. 
     As a result, present mortar systems can have severe pressure waves in the flashtube that transfer and grow in the mortar ignition cartridge and the launch tube. 
     What is therefore needed is a nozzle for use in the flashtube of the mortar launch unit, in order to improve the performance of the mortar. The nozzle should create more homogeneous combustion among the black powder pellet(s) in the flashtube. The nozzle should result in a decrease in the variation, in terms of mass flow rate, of venting products from the flashtube vent holes. The nozzle should create a more consistent muzzle velocity of the mortar. The nozzle should result in a decrease in pressure waves within the flashtube and therefore within the mortar ignition cartridge. Prior to the advent of the present invention, the need for such a nozzle has heretofore remained unsatisfied. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies this need, and describes a nozzle in a launch unit of a mortar system, where the launch unit has a propellant charge capable of generating propellant gas for launching the projectile from a barrel of the mortar. 
     It is an object of the present nozzle to create more stable and complete combustion among the black powder pellet(s) in the flashtube over the burn time of these pellet(s). 
     It is another object of the present nozzle to result in a decrease in the variation, in terms of mass flow rate, of venting products from the flashtube vent holes. 
     It is still another object of the present nozzle to result in a decrease in pressure waves within the flashtube and therefore within the mortar ignition cartridge. 
     It is yet another object of the present nozzle to create a more consistent muzzle velocity of the mortar. 
     These and other objects of the present invention are realized by a mortar system that includes a launch unit and a projectile that are threaded together. The launch unit comprises a mortar ignition cartridge that includes a flashtube, which in turn incorporates a nozzle that is made and positioned within the flashtube according to the teaching of the present invention. 
     The nozzle is inserted within the flashtube so that it sits on a shoulder which prevents the nozzle from sliding further into the flashtube. The nozzle separates the flashtube into two compartments: a flashtube venting chamber and a pellet combustion chamber. The flashtube venting chamber acts as the product-gas forming area of the flashtube. 
     The combustion of the black powder pellets takes place in the pellet combustion chamber because the nozzle acts as a hard stop, which keeps the solid phase of the propellant from combusting upstream. The flow of combustion products will be choked by the nozzle, which prevents upstream pressure oscillations from traveling back into the combustion area due to the nature of compressible flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention and the manner of attaining them will become apparent, and the invention itself will be best understood, by reference to the following description and the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view of a mortar bomb, with the cross-hatching removed for clarity of illustration, wherein the mortar bomb includes a launch unit that comprises a mortar ignition cartridge provided with a flashtube, which in turn, incorporates a nozzle according to the present invention; 
         FIG. 2  is a cross-sectional view of the mortar ignition cartridge of  FIG. 1 , with the cross-hatching removed, further illustrating the flashtube of the present invention; 
         FIG. 3  is a cross-sectional, exploded view of the flashtube of  FIGS. 1 and 2 , with the cross-hatching removed, further illustrating the nozzle of the present invention; 
         FIG. 4  is a top view of the flashtube section of  FIG. 3 , further illustrating the position of the nozzle of  FIG. 3  within the flashtube of the present invention; 
         FIG. 5  is a top view of the flashtube section of  FIG. 4  with the cross-hatching removed; 
         FIG. 6  is a another view of the flashtube section of  FIGS. 4 and 5 ; 
         FIG. 7  is a perspective view of the nozzle of  FIG. 3 ; 
         FIG. 8  is a front view of the nozzle of  FIG. 7 ; and 
         FIG. 9  is a side view of the nozzle of  FIGS. 7 and 8 . 
     
    
    
     Similar numerals refer to similar elements in the drawings. It should be understood that the sizes of the different components in the figures are not necessarily in exact proportion or to scale, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , it illustrates a mortar system (also referred to herein as a mortar bomb or a mortar)  10  that includes a launch unit  50  and a projectile  55  that are connected with threads. The projectile  55  may be any suitable projectile that is either known or available in the field, and therefore its construction will not be discussed herein in detail. 
     With further reference to  FIG. 2 , the launch unit  50  comprises a mortar ignition cartridge  65  that includes a flashtube  100 , which in turn incorporates a ring-like, circular nozzle  200  according to the present invention. 
     When the projectile  55  is dropped into a muzzle of a mortar tube  70  ( FIG. 1 ), the ignition train is started through the action of a percussion primer  205  striking a firing pin  210 . The gases and hot solid salts evolved by the percussion primer  205  initiate a couple of centrally perforated black powder pellets  222 . Through shock and solid impact by the percussion primer combustion products  205 , the pellets  222  break up and combust. These black powder pellets  222  are contained within a pellet combustion chamber  305  of the flashtube  100  where the product gases and small solid particles can leave through a series of radial vent holes  230  that are positioned axially relative to the flashtube  100 . The vent holes  230  are evenly spaced about the circumference of the flashtube  100 . 
     When the combustion gases exit the flashtube  100 , they impinge on and ignite a bed of solid granular propellant bed  250  contained within a mortar ignition cartridge combustion chamber  260  of the mortar ignition cartridge  65 , which may in turn ignite several additional charge rounds. The upstream end of the flashtube  100 , is sealed, in this case with a steel pin  255 , so that the combustion gases exit only though the radial vent holes  230 . 
     The uniformity of ignition of the solid granular propellant bed  250 , including optional charge rounds, is critical for the formation of propelling gases that move the mortar system  10  consistently. Critically, the formation of high pressure gradients will have the effect of reducing repeatability and, in turn, accuracy of the weapon system. 
     Pressure gradients in the flashtube  100  are caused by a variety of factors. The combustion of the black powder pellets  222  is subject to non-uniform ignition, mechanical break-up, and resulting movement of the pellets  222 . There is also a complex shock interaction inside the flashtube  100 . The initial burning sets up a pressure wave which travels down the flashtube  100  and eventually reflects back towards the burning grains. There will be a normal shock at some length down the flashtube  100  whose location will be affected by instantaneous flow characteristics, friction, and mass loss down the length of the flashtube  100  through the vent holes  230 . 
     The problem with the combustion of the black powder pellets  222 , in the absence of the nozzle  200 , is that it tends to be a highly unrepeatable event due to its hydroscopic nature, loose tolerances in chemical composition, and brittleness. In addition, such mortar system would suffer from the propagation of pressure waves in the flashtube  100  that transfer to the mortar ignition cartridge  65  and the flashtube venting chamber  270 . 
     With further reference to  FIGS. 3 through 6 , the present invention addresses this problem by integrating the nozzle  200  above the black powder pellets  222 , or any other igniter, in a primer head above the igniter, creating a pellet combustion chamber  305 . The newly created pellet combustion chamber  305  gives more reproducible combustion of the igniter, a more isochronic ignition pulse through the flash holes  230 , a smoother combustion of the propellant bed  250  in the mortar ignition cartridge  65 , and fewer waves traveling from the combustion chamber  305  into the flashtube venting chamber  270 . 
     To this end, the nozzle  200  is inserted within the flashtube  100  so it seats on a shoulder  300  of the flashtube  100 , which acts as a stop, to prevent the nozzle  200  from sliding further within the flashtube  100 . As a result, the nozzle  200  separates the flashtube  100  into two compartments: the flashtube compartment  270  ( FIG. 2 ) and the pellet combustion chamber  305  ( FIG. 3 ). The sizes and lengths of the various components would be defined by the specific mortar system that is in use and its ignition system. The pellet combustion chamber  305  acts as the product-gas evolution area for the flashtube  100 . 
     The combustion of the black powder pellets  222  essentially takes place in the pellet combustion chamber  305  because the nozzle  200  keeps the black powder pellets  222  from moving into the flashtube venting chamber  270 . The present flashtube  100  can be used as a replacement of, or in conjunction with conventional flashtubes with no further modifications to the mortar system  10 . 
     The present flashtube  100  produces even burning and leaves less solid, unburned particulate matter before the combustion products enter the launch tube pellet combustion chamber  305  of the flashtube  100 . In one exemplary embodiment, five black powder pellets  222  were retained held in place by a small shoulder  300 . During pellet combustion, and without the insertion of the nozzle  200 , the black powder pellets  222  break-up and large pieces easily travel down the length of the flashtube  100 , to the launch tube pellet combustion chamber  305 . By placing the nozzle  200  against the shoulder  300 , the combustion is enhanced in several ways. 
     The choked flow of the combustion products at the end of the combustion chamber  305  acts as a quasi-solid boundary to upstream pressure disturbances, in that it acts as a solid boundary to fluids without actually being solid. Pressure waves reflecting off the end of the flashtube  100 , which would otherwise feed back into the pellet combustion chamber  305  are significantly reduced if not entirely eliminated. The choked flow acts to damp out the affect of pressure wave interaction with the combustion of the black powder pellets  222 . 
     The choked flow results in a higher pressure within the pellet combustion chamber  305 , than without the nozzle  200 . This high combustion pressure feeds back to increase the rate of the propellant consumption, thus decreasing the likelihood of uncombusted propellant ( 250 ) fragments escaping the pellet combustion chamber  305 . 
     With further reference to  FIGS. 7 ,  8 , and  9 , the nozzle  200  is generally cylindrically shaped, with a central orifice  700  formed in its center. According to an exemplary embodiment, the nozzle throat diameter is sized according to compressible flow relations, wherein the outer diameter of the nozzle  200  is approximately equal to the inner diameter of the pellet combustion chamber ( 305 ) end of the flashtube  100 . The nozzle  200  is preferably made of an integral piece of metal. It should however be understood that the nozzle can be made from any non-flammable, heat resistant materials, such as metals and ceramics. 
     Also, the physical barrier presented by the central orifice  700  of the nozzle  200  keeps larger propellant fragments constrained in the pellet combustion chamber  305 , and out of the launch tube combustion chamber  270 . The confined pellet combustion chamber  305  creates an essentially isochoric pellet combustion chamber  305  that enhances mixing of hot propellant gases and therefore increases the amount of propellant allowed to go to complete combustion. An additional benefit is that more complete propellant combustion in the pellet combustion chamber  305  will require fewer black powder pellets  222  to transfer the same amount of energy to the propellant bed  250 . 
     Furthermore, burning the black powder pellets  222  at higher pressure and temperature more completely will mitigate the effects of its hygroscopicity. The black powder pellets  222  absorb moisture as a function of processing methods and environmental exposure. The degree of absorption of individual pellets in the mortar system  10  is unknown before firing. Water in the black powder pellets  222  acts to decrease the total energy released during the reaction. Also, more energy is released from the black powder pellets  222  themselves as new reaction pathways open at higher temperatures created in the pellet combustion chamber  305 . 
     The nozzle  200  provides additional advantages, among which are the following: The nozzle  200  creates the separate pellet combustion chamber  305  for the black powder pellets  222  for more reproducible combustion. It alleviates large variations in flashtube  100  performance due to the hygroscopicity of the black powder pellets  222  within the pellet combustion chamber  305 . The nozzle  200  also changes the point of choked flow from the vent holes  230  in the flashtube  100  to the nozzle  200 , yielding more uniform flow through the flash holes  230 . The nozzle  200  further decreases the pressure gradient within the flashtube  100 , leading to a more isochronic ignition of the ignition cartridge propellant bed  250 . 
     Although the present flashtube  100  has been described in connection with an exemplary mortar system  10 , it should be clear that the flashtube  100  is not limited to the particular embodiments described herein. It should be understood that other modifications may be made to the present flashtube  100  without departing from the spirit and scope of the invention.