Patent Publication Number: US-2015083010-A1

Title: Practice Projectile

Description:
FIELD OF INVENTION 
     The present invention relates to practice ammunition rounds. More specifically the invention relates to practice mortar bombs, a method of explosively releasing compressed fluid from a pressure vessel via an outlet valve system, a method of firing a practice mortar bomb, a method of propelling a practice mortar bomb and a kit of parts. 
     BACKGROUND OF THE INVENTION 
     A mortar is a weapon system typically used by infantrymen to provide indirect fire. Mortars are based around a mortar tube (the orientation of which is adjustable for aiming) down which is dropped a mortar bomb. Mortar bombs have a tail portion containing explosive propellant which is initiated by a firing pin at the bottom of the mortar tube. The resulting explosion combined with the tight fit of the bomb within the tube propels the bomb from the mortar barrel. 
     As will be appreciated it is highly desirable that operators should be given the opportunity of practicing with mortars before they are called upon to use them in combat. There are many skills to learn, including transporting, assembling, aiming and firing the mortar. Beyond this there are more specialised proficiencies such as firing patterns and use of different bomb types (e.g. smoke, illumination and high explosive). 
     In practicing these skills it is in many cases desirable to actually fire the weapon or at least to re-create actual firing of the weapon as accurately as possible. As will be appreciated there are however several draw-backs to practicing with a real mortar and live ammunition. A live mortar bomb is very expensive and is not re-usable. Firing live rounds also requires a large area (which may be quickly damaged if sufficient practice is undertaken). Further the use of live ammunition gives rise to the need for a great deal of attention to be given to safety, not only in use but also in procurement, storage and transport. This is time consuming and expensive. 
     As a consequence of the above it is normal for mortar teams to have very limited practice using live ammunition. In substituting for this it is known to practice by ‘going through the motions’ without actually firing a round. This is very limited in terms of the experience that it can provide and further is often considered boring and ineffective by mortar teams. Computer simulations are also used which can provide a more useful substitute for certain aspects such as aiming. Nonetheless there is still a need for a cheaper, safer method of more realistically practicing firing a mortar. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention a practice mortar bomb is provided comprising optionally a pressure vessel and optionally an outlet valve system, the pressure vessel being arranged in use to optionally contain compressed fluid and the outlet valve system being arranged in use to optionally allow explosive release of the compressed fluid from the pressure vessel optionally upon activation of the valve system by a firing pin in a mortar tube. This practice mortar bomb may provide a relatively safe, inexpensive and yet realistic way of practicing the firing of a mortar. 
     In some embodiments the practice mortar bomb further comprises an inlet valve system arranged in use to allow charging of the pressure vessel with fluid such that the fluid is compressed within the pressure vessel. The inlet valve system may provide a convenient way of filling the pressure vessel and may also allow re-use of the mortar bomb by re-filling the pressure vessel via the inlet valve system. This may make repeated practice considerably less expensive. 
     In some embodiments the inlet valve system comprises a one-way valve allowing storage of the compressed fluid in the pressure vessel once charged. The one-way valve may offer a convenient way of allowing filling of the pressure vessel and the retention of compressed fluid for an extended period thereafter. 
     In some embodiments the inlet valve system is positioned at a head end of the pressure vessel. Positioning of the inlet valve system in this way may allow for a reduced impact on the appearance and performance of the practice mortar bomb so as it better resembles a standard mortar bomb in appearance and performance. 
     In some embodiments the outlet valve system is positioned at a base end of the pressure vessel. This may lead to a more direct release of compressed fluid in the desired direction for firing of the bomb. 
     In some embodiments the outlet valve mechanism comprises a valve seat and valve body, the valve body being mobile such that in use a valve seat end of the valve body is engageable with the valve seat to block the flow of fluid from the pressure vessel, and is disengageable from the valve seat to allow the flow of fluid from the pressure vessel. 
     In some embodiments the outlet valve system is a pressure balance valve, being arranged such that in use, compressed fluid from the interior of the pressure vessel impinges on at least two opposed surfaces of the valve body, the resulting net force being towards the valve seat until the outlet valve system is activated, whereupon the net force is away from the valve seat. The pressure balance valve may be a particularly effective way of allowing explosive release of compressed fluid, as the compressed fluid itself may be used to move the valve body away from the valve seat and so open the outlet valve system. This may lead to a very rapid opening of the outlet valve system. Further the pressure balance valve may allow re-setting of the outlet valve system after explosive fluid discharge. 
     In some embodiments a first of the opposed surfaces is a head end of the valve body and a second of the opposed surfaces, opposed to the first is a biasing surface proximate the valve seat end of the valve body. 
     In some embodiments the outlet valve system further comprises a chamber arranged such that in use, compressed fluid in the chamber impinges on the head end of the valve body with sufficient force to cause engagement of the valve seat end of the valve body with the valve seat. The chamber may provide a convenient method of allowing compressed fluid to impinge on the head end, while also isolating (at least to an extent) the fluid in the chamber from fluid in the pressure vessel. This may mean that through discharging the chamber, at least some of the force biasing the valve body towards the valve seat may be removed. 
     In some embodiments the practice mortar bomb is arranged such that in use, activation of the outlet valve system leads to the release of compressed fluid from the chamber at a faster rate than it is replenished, such that the force biasing the valve body towards the valve seat is reduced and the net force on the valve body is reversed so as it is away from the valve seat. This may allow for rapid opening of the outlet valve system to permit explosive release of compressed fluid from the pressure vessel. 
     In some embodiments release of the compressed fluid from the chamber is controlled by a firing valve, operated in use directly or indirectly by a firing pin in a mortar tube. This may allow the firing pin in a standard mortar tube to initiate firing of the practice mortar bomb. In this way a specialist mortar tube may not be required, potentially increasing the realism of practice. 
     In some embodiments the outlet valve system is arranged such that in use it re-sets before all of the compressed fluid has been explosively released from the pressure vessel. In this way a positive pressure may be maintained in the pressure vessel, helping to prevent the ingress of foreign material such as dust, sand and/or water into the practice mortar bomb. This may be particularly advantageous in view of the fact that the practice mortar bomb may dig into the earth on landing (at least to an extent). 
     In some embodiments the outlet valve mechanism further comprises a cylinder in which the valve body moves and is contained in a close fit. The cylinder may be an effective way of guiding movement of the valve body and of creating the chamber adjacent the head end of the valve body. 
     In some embodiments the chamber is defined by the cylinder and the head end of the valve body. 
     In some embodiments the cylinder has one or more cylinder through bores arranged so that the interior of the pressure vessel is in fluid communication with the chamber, such that in use pressurised fluid passes from the pressure vessel into the chamber. The cylinder through bores may therefore allow charging of the chamber from compressed fluid in the pressure vessel, in order that the valve body may be biased towards the valve seat. 
     In some embodiments a region proximate the valve seat end of the valve body is provided with one or more rebated portions, each creating a cavity between the cylinder and the valve body. The cavities may assist in creating one or more surfaces for implementation of the pressure balance valve. 
     In some embodiments the rebated portions create the biasing surface of the valve body, arranged such that at least a component of a force applied towards the biasing surface would act to bias the valve body away from the valve seat. 
     In some embodiments the rebated portions comprise chamfering of the valve seat end of the valve body that extends beyond the valve seat and into the cylinder when the valve seat end of the valve body is engaged with the valve seat. The chamfering may assist in locating of the valve seat end of the valve body within the valve seat, and it may conveniently also provide the biasing surface. 
     In some embodiments one or more passages is provided passing through the cylinder wall allowing fluid communication between the interior of the pressure vessel and the cavities. This may allow fluid pressure to be applied to the biasing surfaces from the pressure vessel for implementing the fluid balance valve. 
     In some embodiments fluid gaps are provided intermediate the passages and cavities, created by an increase in the diameter of the cylinder between the passages and the cavities. Where these fluid gaps have a relatively narrow diameter in comparison to the passages, the fluid gaps may be sufficient to cause the valve body to rapidly disengage the valve seat. Thereafter as the valve body clears the passages fluid may be explosively released via the passages which may be of a relatively large diameter. 
     In some embodiments the head end of the valve body has a sufficient surface area such that if the same pressure were applied to the head end of the valve body and the biasing surface, the net force on the valve body would cause the valve seat end of the valve body to engage with the valve seat. This may ensure that the default condition of the outlet valve system is closed so as to prevent discharge of fluid from the pressure vessel until compressed fluid is discharged from the chamber. 
     In some embodiments the valve body has a valve body through bore passing from the head end of the valve body to the valve seat end of the valve body. This may provide a convenient path for discharging compressed fluid from the chamber when the outlet valve system is activated. 
     In some embodiments the head end of the valve body through bore is provided with a firing valve seat. 
     In some embodiments a firing valve body is provided which, when there is compressed fluid in the chamber, is biased by the compressed fluid into engagement with the firing valve seat. This may ensure that the default condition of the firing valve is closed, preventing the discharge of fluid from the chamber (and so the opening of the outlet valve system) until the valve is activated. 
     As will be appreciated in some embodiments a spring may be provided to increase the force biasing the firing valve body into engagement with the valve seat. 
     In some embodiments a pin of smaller diameter than the valve body through bore extends from the firing valve body through the valve body through bore. This pin may provide a convenient method of activating the outlet valve system, in that pushing on the pin may open the firing valve, by raising the firing valve body from the firing valve seat. 
     In some embodiments the pin is arranged to force the firing valve away from the firing valve seat when an end of the pin is impacted by a firing pin in a mortar tube. The pin may therefore act as an intermediary between the firing pin of the mortar tube and the firing valve body. It this way it may be possible to use a conventional mortar tube. 
     In some embodiments the cylinder through bore and valve body through bore are arranged such that greater quantities of fluid at a given pressure are passable through the valve body through bore than through the cylinder through bore in a given time. This may ensure that for as long as the firing pin of the mortar tube is keeping the firing valve open, the chamber cannot re-charge with pressurised fluid. This may mean that the outlet valve system will open and remain open, at least until the bomb is fired. 
     In some embodiments the practice mortar bomb further comprises a dummy fuse nose portion. The nose portion may mean that the practice mortar bomb is more like a standard mortar bomb, i.e. in terms of appearance and/or aerodynamic profile and/or shape and/or size and/or weight, thus potentially making its use in practice more realistic. 
     In some embodiments the nose portion simulates a known mortar bomb fuse type. This may make the practice mortar bomb more like a standard mortar bomb, i.e. in terms of appearance and/or aerodynamic profile and/or shape and/or size and/or weight, thus potentially making its use in practice more realistic. The fuse may for example simulate a point detonation fuse, a proximity fuse, a mechanical time fuse, a multi-option fuse or a practice fuse. 
     In some embodiments the nose portion is releasably engageable with the bomb. This may allow the nose portion to be interchangeable with alternatives simulating different dummy fuses, depending on the practice to be undertaken. 
     In some embodiments the nose portion provides a cap over the inlet valve system. This may serve to protect the nose portion especially during impact of the practice mortar bomb on landing. 
     In some embodiments the bomb further comprises a hollow tail portion, the interior of the tail portion being in fluid communication with the pressure vessel when the outlet valve system is open. The tail portion may allow more accurate simulation of the appearance of a standard mortar bomb. Further the tail portion may receive the explosively released fluid from the outlet valve system, allowing it to be directed. 
     In some embodiments the tail portion comprises one or more ports allowing fluid communication between its interior and exterior. The ports may be used to direct explosively released fluid into a mortar tube below the level of the pressure vessel. 
     In some embodiments the tail portion has a tail portion through bore arranged in use to receive at least one of the pin and, in use the firing pin of a mortar tube. The tail portion through bore may help to ensure that the firing pin of the mortar tube is properly aligned and received by the practice mortar bomb in order that it may displace the pin. 
     In some embodiments the pin is of sufficient length and aligned with the tail portion through bore so as a firing pin of a mortar entering the tail portion through bore will impact and push against the pin so as to push the firing valve body away from the firing valve seat. This may help to ensure a proper contact between the pin and firing pin of a mortar tube. 
     In some embodiments the practice mortar bomb has an overfill pressure release valve. This may be an effective safety feature against over charging of the pressure vessel. 
     According to a second aspect of the invention there is provided a kit of parts comprising optionally one or more pressure vessels for practice mortar bombs in accordance with the first aspect and optionally one or more of the following:
         one or more nose portions optionally of various designs to simulate different known mortar bomb fuse types,   one or more tail portions,   one or more pumps.
 
Such kits may be convenient especially when practice is being undertaken. For example a mortar team may select different fuses when practicing different fire missions and may re-charge the pressure vessel on the practice range using the pump.
       

     According to a third aspect of the invention a method of explosively releasing compressed fluid from a pressure vessel via an outlet valve system is provided, the pressure vessel and outlet valve system being provided in a practice mortar bomb, comprising the steps of:
         exposing a valve body of the valve system to pressure from the compressed fluid acting in at least two different locations so as to bias the valve body both towards and away from a valve seat, with the net force being towards the valve seat,   activating the valve to release compressed fluid from at least one of the location to reduce the force acting to bias the valve body towards the valve seat so that the net force is away from the valve seat.
 
This method may allow rapid release of the compressed fluid in a manner sufficient to propel the practice mortar bomb a desired distance.
       

     In some embodiments the method further comprises re-charging the location from which compressed fluid has been released so as the net force is again towards the valve seat. This may allow the practice mortar bomb to be re-used by re-charging the pressure vessel. 
     According to a fourth aspect of the invention a method of firing a practice mortar bomb is provided, the practice mortar bomb comprising optionally a pressure vessel and optionally an outlet valve system optionally arranged upon activation to explosively release at least a portion of a compressed air reservoir in the pressure vessel, the method comprising the steps of:
         optionally charging the pressure vessel with compressed fluid, and   optionally dropping the mortar bomb down a mortar tube.
 
This method may be more realistic for those practicing the skill of mortaring than going through the motions without actually firing a mortar bomb. Further the method may be safer and relatively inexpensive in comparison to using a real mortar bomb.
       

     In some embodiments the method further comprises collecting the practice mortar bomb for optional re-use. This may improve cost effectiveness of mortar practice. 
     In some embodiment the method further comprises selecting a head portion for attachment to the practice mortar bomb before it is fired. This may allow a mortar user to more accurately simulate performing different fire missions by selecting different dummy fuse head portions. 
     According to a fifth aspect of the invention a method of propelling a practice mortar bomb is provided comprising explosively releasing compressed fluid contained within the practice mortar bomb via an outlet valve system. This system may eliminate the need for using an explosive charge (which may be more expensive and dangerous) during practicing the skill of mortaring. 
     The skilled person will appreciate that a feature described in relation to any one of the above aspects of the invention may be applied mutatis mutandis to any other aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
       Embodiments of the invention will now be described by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1  is a cut-away view showing a practice mortar bomb in accordance with an embodiment of the invention; 
         FIG. 2  is an enlargement of part A of  FIG. 1  showing a cross-section of an outlet valve system according to an embodiment of the invention; 
         FIG. 3  is a cut-away view showing an alternative practice mortar bomb in accordance with an embodiment of the invention; 
         FIG. 4  is an enlargement of part A of  FIG. 3  showing a cross-section of an outlet valve system according to an embodiment of the invention; 
         FIG. 5  is a cut-away view showing an alternative practice mortar bomb in accordance with an embodiment of the invention; and 
         FIG. 6  is an enlargement of part A of  FIG. 5  showing a cross-section of an outlet valve system according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Referring first to  FIG. 1  a practice mortar bomb is generally provided at  100 . Such a practice mortar may be provided in various calibres to facilitate firing from standard mortar tubes (e.g. 51, 60, 81 or 120 mm). The practice mortar bomb  100  is 60 mm, but as will be appreciated this is not intended to be limiting. 
     The bomb  100  comprises a main body portion  104  positioned intermediate a nose portion  102  and a tail portion  106 . 
     The main body portion  104  is substantially ellipsoid in shape and is provided by head element  108  and base element  110 . The head and base elements  108 ,  110  are releasably engageable using cooperating main body screw threaded regions  112 . The base element is provided with an annular rebate  114  on its exterior surface which may be used to receive an obturating ring (not shown). 
     The main body portion  104  defines a pressure vessel  116  in the form of a void surrounded by the main body portion  104  walls. 
     At a head end of the pressure vessel  116  the head element  108  is shaped to provide an inlet valve system housing  118  in the form of a cylindrical portion, coaxial with the main body  104  and having a reduced diameter. In the inlet valve system housing  118  is positioned an inlet valve system (not shown here for clarity) comprising a one way valve allowing charging of the pressure vessel  116 . 
     At a base end of the pressure vessel  116  the base element  110  is shaped to provide a valve seat  120  for an outlet valve system  122  (described in greater detail later with reference to  FIG. 2 ). 
     The nose portion  102  (which in some embodiments may be a dummy fuse) is substantially conical in shape and has a hollow interior portion  124  to accommodate protruding portions of the inlet valve system. The nose portion  102  is releasably engageable with the head element  108  using cooperating nose screw threaded regions  126 . 
     The tail portion  106  is substantially cylindrical in shape and has a hollow interior portion  128  in fluid communication with the valve seat  120 . The tail portion  106  is provided with a plurality of ports  130  through its sidewall  132 , the ports  130  providing fluid communication between the hollow interior portion  128  and the exterior of the tail portion  106 . The tail portion further comprises a plurality of stabilising fins  134  projecting outwards from the tail portion  106 . The tail portion  106  has a tail portion through bore  136  which receives a pin  138  passing from the outlet valve system  122 , through the hollow interior portion  128  and through the tail portion through bore  136 . The tail portion  106  is releasably engageable with the base element  110  using cooperating tail screw threaded regions  140 . 
     Referring now to  FIG. 2  the parts of the outlet valve system  122  are described. The outlet valve system  122  has a valve body  142  engageable with the valve seat  120 . In  FIG. 2  the valve body  142  is shown in an engaged position with the valve seat  120 . It will be appreciated however that the valve body  142  is mobile and consequently may also be disengaged from the valve seat  120  by raising the valve body  142 . 
     The valve body  142  is substantially cylindrical and has a head end  144  and a valve seat end  146 . 
     The head end  144  is provided with a firing valve seat  148  comprising a countersunk region of a valve body through bore  150  which passes through the valve body  142  from the head end  144  to the valve seat end  146 . Engaged with the firing valve seat  148  is a firing valve body  152 . In  FIG. 2  the firing valve body  152  is shown in an engaged position with the firing valve seat  148 . It will be appreciated however that the firing valve body  152  is mobile and consequently may also be disengaged from the firing valve seat  148  by raising the firing valve body  152 . Extending from the firing valve body  152 , through the valve body through bore  150  is the pin  138 . The pin  138  has a smaller diameter than the valve body through bore  150 . The pin  138  is guided and supported proximate the valve seat end  146  by a pin guide plate  154 . 
     The valve seat end  146  has a chamfered peripheral (and is therefore rebated) that extends beyond the valve seat and into a cylinder  156  when the valve seat end  146  is engaged with the valve seat  120 . The chamfering provides a biasing surface  158  opposed to the head end  144  of the valve body  142 . 
     The cylinder  156  surrounds the valve body  142  in a close fit, supporting movement of the valve body  142 . The cylinder and the head end  144  define a chamber  160 . The chamber  160  is in fluid communication with the pressure vessel  116  via a cylinder through bore  162 . 
     The cylinder  156  and biasing surface  158  define a cavity  164 . As a consequence of fluid gaps  165  and passages  166 , the cavity  164  is in fluid communication with the interior of the pressure vessel  116 . The passages  166 , which pass through the cylinder  156 , are further defined by the cylinder  156  and base element  110  walls. The fluid gaps  165  have a total cross-sectional area that is considerably less than the total cross-sectional area of the passages  166 . The fluid gaps  165  are formed by an increase in the diameter of the cylinder  156 , between the passages  166  and the cavity  164 . 
     In use the practice mortar bomb  100  may be used by someone wishing to practice the skill of firing a mortar and or associated skills. Exemplary use and operation of the practice mortar bomb  100  is now provided with reference to  FIGS. 1 and 2 . 
     The user first disengages the nose portion  102  using nose screw threaded regions  126 , giving access to the inlet valve system. The user then charges the pressure vessel  116  with compressed fluid via the inlet valve system  116 . This may for example be achieved via use of a pump or compressor. Any suitable compressed fluid may be used, for example air or nitrogen. The pressure vessel  116  is charged to a pressure in accordance with a desired range and the calibre of the practice mortar bomb. By way of example the pressure vessel of a 60 mm practice mortar bomb  100  might be charged to between 20 and 100 bar, preferably between 35 and 60 bar and more preferably to between 45 and 50 bar. 
     Compressed fluid in the pressure vessel  116  is free to pass into chamber  160 , where it impinges on the head end  144  of the valve body  142  with sufficient force to engage the valve seat end  146  of the valve body  142  with the valve seat  120 . This prevents the flow of compressed fluid from the pressure vessel  116  through the outlet valve system  122 . 
     Compressed fluid in the pressure vessel  116  is also free to pass through passages  166  and fluid gaps  165  into cavities  164 , where it impinges on the biasing surface  158 . This pressure on the biasing surface  158  biases the valve seat end  146  to disengage from the valve seat  120 . However the net force on the valve body  142  is such that the valve seat end  146  remains engaged with the valve seat  120 . This is due to the relative surface areas of the opposed surfaces (the head end  144  and biasing surface  158 ). 
     Compressed fluid in the chamber  160  also impinges on the firing valve body  152  forcing it to engage the firing valve seat  148 . This prevents compressed fluid being discharged from the chamber  160  via the valve body through bore  150 . 
     The user then selects a nose portion  102  corresponding to the appearance of a particular fuse type, (e.g. detonation fuse, proximity fuse, a mechanical time fuse, a multi-option fuse or a practice fuse) corresponding to a simulated fire mission to be undertaken. The selected nose portion  102  is then engaged using the nose screw threaded regions  126 . 
     The user then drops the practice mortar bomb  100  down a mortar tube (not shown) in which the practice mortar bomb  100  is a tight fit. When the practice mortar bomb  100  reaches the bottom of the mortar tube it engages a firing pin in the bottom of the tube. The firing pin is received in the tail portion through bore  136  and displaces the pin  138  upwards. The pin  138 , guided and supported by the tail portion through bore  136  and pin guide plate  154 , disengages the firing valve body  152  from the firing valve seat  148 , moving it upwards. This allows compressed fluid in the chamber  160  to rapidly discharge from the chamber  160  via the fluid body through bore  136 , hollow interior portion  128  and plurality of ports  130 . Further due to the relative diameters of the cylinder through bore  162  (smaller) and fluid body through bore  136  (larger), the cavity  160  cannot be re-pressurised while the firing valve body  152  is disengaged from the firing valve seat  148 . 
     The practice mortar bomb  100  is also suitable for use in mortar tubes that have an externally triggerable firing pin which is activated after the practice mortar bomb  100  has been inserted into the tube. 
     As a consequence of the drop in pressure in the chamber  160 , compressed fluid impinging on the biasing surface  158  causes a reversal in the net force on the valve body  142 , forcing the valve body  142  upwards so as the valve seat end  146  disengages with the valve seat  120 . As the valve body  142  moves upwards the valve seat end  146  passes the passages  166 , which due to their large diameter allow rapid discharge of the compressed fluid from the pressure vessel  116  via the hollow interior portion  128  and plurality of ports  130 . This explosive release of compressed fluid rapidly fills the area of the mortar tube between the practice mortar bomb and the tube. Due to the tight fit of the practice mortar bomb  100  within the mortar tube, and action of an obturating ring (not shown) to create an enhanced seal, the released compressed air propels the practice mortar bomb  100  from the mortar tube. 
     As the practice mortar bomb  100  is fired the firing pin in the mortar tube is removed from the tail portion through bore  136 . This leaves the pin  138  free to return to its original position and the firing valve body  144  to engage with the firing valve seat  148  under the influence of remaining compressed gas in the chamber  160 . In this condition the chamber  160  is allowed to re-pressurise via the cylinder through bore  162  and the valve body  142  is forced to engage the valve seat  120 . This ensures that as the practice mortar bomb  100  is fired, compressed fluid remaining in the pressure vessel  116  is retained, meaning that a positive pressure is maintained within the pressure vessel  116 . This may help to prevent the ingress of foreign material into the practice mortar bomb  100 . Further it means that the valve is re-set and ready for re-use of the practice mortar bomb. 
     In view of the influence on the position of the valve body  142  exerted by the compressed fluid acting on the opposed surfaces (head end  144  and biasing surface  158 ) the outlet valve system  122  may be referred to as a pressure balance valve. Further the opposed surfaces may be considered to be locations on which pressure from the compressed fluid may act. 
     Referring now to  FIGS. 3 and 4  an alternative practice mortar bomb is generally provided at  200 . The practice mortar bomb  200  is similar to the practice mortar bomb  100  except that an alternative outlet valve system  222  is provided. 
     The outlet valve system  222  is similar to the outlet valve system  122 , but several modifications have been made. 
     The outlet valve system  222  has a valve body  242  engageable with the valve seat  220 . In  FIG. 4  the valve body  242  is shown in an engaged position with the valve seat  220 . It will be appreciated however that the valve body  242  is mobile and consequently may also be disengaged from the valve seat  220  by raising the valve body  242 . 
     The valve body  242  is substantially cylindrical and has a head end  244  and a valve seat end  246 . 
     The head end  244  is provided with a firing valve seat  248  comprising a countersunk region of a valve body through bore  250 , passing through the valve body  242  from the head end  244  to the valve seat end  246 . Engaged with the firing valve seat  248  is a firing valve body  252 . In  FIG. 4  the firing valve body  252  is shown in an engaged position with the firing valve seat  248 . It will be appreciated however that the firing valve body  252  is mobile and consequently may also be disengaged from the firing valve seat  248  by raising the firing valve body  252 . Extending from the firing valve body  252 , through the valve body through bore  250  is a pin  238 . The pin  238  has a smaller diameter than the valve body through bore  250 . The pin  238  is guided and supported proximate the valve seat end  246  by a pin guide plate  254 . 
     The valve seat end  246  has a chamfered peripheral and a rebated portion  255  formed by a reduction in the diameter of the valve body  242 . The rebated portion  255  provides a biasing surface  258  opposed to the head end  244  of the valve body  242 . 
     A cylinder  256  surrounds the valve body  242  in a close fit, supporting movement of the valve body  242 . The cylinder and the head end  244  define a chamber  260  and a spring guide  261 . The chamber  260  is in fluid communication with the pressure vessel  216  via a cylinder through bore  262  and a flat  262   a  in the firing valve body  252 . A spring  263  provided within the spring guide is in contact with the firing valve body  252  and biases it towards the firing valve seat  248 . 
     The cylinder  256  and biasing surface  258  define a cavity  264 . As a consequence of passages  266 , the cavity  264  is in fluid communication with the interior of the pressure vessel  216 . The passages  266 , which pass through the cylinder  256 , are further defined by the cylinder  256  and base element  210  walls. 
     In the embodiment of  FIGS. 3 and 4  the spring  263  helps to prevent leakage of fluid through the valve body through bore  250  before firing of the practice mortar bomb. The spring also assists with re-engaging the firing valve body  252  with the firing valve seat  248  once explosive release of compressed fluid has occurred, in order that the outlet valve system  222  is re-set and a positive pressure is maintained in the pressure vessel  216 . Further the arrangement of the rebated portion  255 , which increases the size of the cavity  264 , means that compressed fluid may be released more quickly than with the arrangement of  FIG. 2 . 
     Referring now to  FIGS. 5 and 6  an alternative practice mortar bomb is generally provided at  300 . The practice mortar bomb  300  is similar to the practice mortar bomb  200  except that for a number of features. 
     The exterior wall of a spring guide  361  is provided with a spanner/wrench formation  370  (in this case hexagonal) to facilitate screwing and unscrewing of a cylinder  356  to a base element  310  during assembly/disassembly of the practice mortar bomb  300 . 
     A further modification of the practice mortar bomb  300  is that a valve body  342  is provided with a seal  372  (in this case an O-ring) to improve the seal between the valve body  342  and the cylinder  356 . Further a countersunk region of a valve body through bore  350  is curved so as a firing valve seat  348  in a head end  344  of the valve body  342  cooperates in a close fit with a curved engagement surface  374  of a firing valve body  352 . This curved arrangement (which may be hemispherical) improved seating of the firing valve body. 
     It will be further noted that in the  FIGS. 5 and 6  embodiments no pin guide plate is provided, which highlights that this feature is optional. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the various concepts described herein. Any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein in any form of practice mortar bomb.