Technical Field of the Invention

This invention relates to the field of reconfigurable piston apparatus, in particular to piston apparatus suitable for use in interior ballistics measurements.

Background to the Invention

A projectile launched from a gun is propelled as a result of pressure within the gun barrel. This pressure originates from the burning of a propellant and the resulting propellant gases generating a pressure increase behind the projectile, urging it along the barrel. The characteristics of projectile motion within a gun barrel is known as interior ballistics! and is affected by a number of parameters including, but not limited to, propellant composition and mass; projectile diameter and mass; gun barrel length and diameter; internal volume and design layout of ammunition/ammunition cartridge; and forces restricting projectile motion. A number of these parameters vary with the time elapsed after propellant ignition, making interior ballistics a complex science to simulate and test experimentally.

Interior ballistics can be inferred from measurements of exterior ballistics (projectile motion outside of the gun barrel) and theoretical modelling. For instance if a gun barrel design is known, prior art instruments such as high speed cameras can be used to measure 'muzzle velocity' (the velocity at which a project le exits a gun barrel), with interior ballistics of the projectile being estimated theoretically therefrom. However owing to the number and complexity of the parameters affecting interior ballistics, this approach can be of insufficient fidelity to validate interior ballistics models or to provide direct empirical evidence bf the compatibility of a particular gun ba 'rel, with a specific projectile and propellant composition.

Improved apparatus' for use in measuring interior ballistics have been developed in an attempt to provide such direct empirical measurements. An example apparatus is provided in US4147055A. This apparatus comprises a barrel with internal pressure transducers. An elongate projectile is slid into the barrel and fired therefrom. As the elongate projectile emerges from the barrel its length can be recorded over time and correlated with pressure readings from the pressure transducers. However this instrument is limited to evaluating a gun barrel and projectile of fixed dimensions and therefore lacks the configurability to test many parameters of interest.

Therefore it is an aim of the present invention to provide a reconfigurable piston apparatus for use in interior ballistics measurements.

Summary of the Invention

According to a first aspect of the invention there is provided a piston housing for receiving a piston rod, comprising a body defining a tubular region within which a piston rod can slide, and a sleeve retention means for securing a removable sleeve liner within the tubular region, such that an internal diameter of the tubular region can be reconfigured to receive a piston rod having a predetermined rod-diameter. By providing sleeve retention means the diameter of the tubular region can be varied (by using different designs on sleeve) to accommodate different diameter piston rods representing different projectile designs. For instance a projectile of known diameter may need to be represented in an interior ballistics test, and as such a removable sleeve liner having an internal diameter equal to the predetermined diameter of the known projectile, may be used. Furthermore by providing sleeve retention means the geometry inside the tubular region can be varied to represent different gun barrel chamber designs or ammunition cartridge designs. The sleeve retention means prevents a removable sleeve from moving within the tubular region when the piston housing is in use (for instance when a piston rod is being propelled within the sleeve). This provides the piston housing with more degrees of configurability than the prior art. A piston-rod within the tubular region can be ejected therefrom under action of an actuation means. The rate at which the piston-rod exits the tubular region can be measured (for instance using high speed videography) to indicate how a particular projectile type (represented by the piston rod) would behave inside a gun barrel (represented by the tubular region or sleeve liner). The tubular region is intended to be a conduit extending through the body of the housing, such that a removable sleeve liner can be inserted into the tubular region from a first end of the body, and a piston rod be configured to slide within the sleeve liner under action of an actuation means inserted into the tubular region from a second and opposite end of the body. Therefore the tubular region provides a conduit between an actuation means (for instance a particular propellant composition/mass) and a piston rod (representing a projectile design and mass).

In preferred embodiments the sleeve retention means comprises a cover plate received into a first end of the tubular region to abut the removable sleeve liner and being attachable to the body of the housing, the cover plate comprising a piston rod aperture through which a piston-rod having the predetermined rod-diameter can be urged. The cover plate may be attached to the housing using removable fasteners such as alien bolts, hex screws or socket head screws. The piston rod aperture is sized to allow a piston rod of predetermined diameter to propagate through the aperture during use of the piston housing, but to prevent the removable sleeve liner from exiting or moving within the body of the housing. The cover plate may provide a plug fit into the tubular region and may optionally be screwed into the sleeve liner. The piston rod may therefore in some embodiments be fully ejected from the sleeve liner, or alternatively may be trapped partially therein by providing a section of the piston rod (or an attached component such as a piston plate) having a wider diameter than the piston-rod aperture.

In some embodiments the housing comprises a vent port for venting gases from the tubular region. A vent port comprises a conduit extending from the tubular region to an external surface of the body of the housing. Gases under pressure within the tubular region can;thus be vented out of the tubular region, for instance when a particular test has been performed using the piston housing. This is particularly useful for when a piston rod is retained v ithin the tubular region after the piston housing has been used, and therefore when the pressurised gases may also be retained within the tubular region.

In even more preferred embodiments the vent port is sealable by a screw means which may be a thumb screw, such that the port can be operated between a closed position (to keep pressurised gases within the tubular region) and an open position (where the gases can be released controllably). The screw means may itself seal a conduit comprising the vent port directly or may pull/push a sealing means across the vent port as the screw means is operated.

In some embodiments the housing comprises a pressure gauge port for receiving a pressure from the tubular region. The pressure gauge port may comprise a connector for receiving a pressure gauge. The pressure gauge port comprises a conduit from the tubular region to an exterior surface of the housing, such that the pressure within the tubular region can be experienced at the point at which a pressure gauge may be attached. Provision of a pressure gauge port allows a pressure gauge to be attached and used to measure pressure within the tubular region throughout an interior ballistics test. A plurality of pressure gauge ports may be provided each with a respective conduit to a different part of the tubular region, such that pressure can be measured inside the tubular region (and thereby inside the sleeve liner) at different locations along a piston stroke.

Some embodiments of the piston housing further comprise actuation means for urging a piston rod, received into a second end of the tubular region, such that a piston rod can be urged from the tubular region. The second end of the tubular is intended to be the opposite end of the tubular region to that from which a piston rod is urged during use. The actuation means may be a spring means, compressed gas generator, but is more preferably a propellant that when burned generates gases that urge against a piston rod in the piston housing.

In preferred embodiments the actuation means comprises a propellant housing in fluid connection with the tubular region. The propellant housing is used to hold a propellant and adjoins the tubular region such that propellant gases can enter the tubular region and act against a piston rod in the tubular region. This causes an increase in pressure that urges the piston rod causing it to slide. In these embodiments the ability of a propellant to urge a piston rod (representing a projectile design) from the tubular region/sleeve (representing a gun barrel design) can be evaluated (for instance by measuring the rate at which a piston rod departs the piston housing or the pressure generated behind the piston rod as a propellant is burnt).

In even more preferred embodiments the propellant housing seals the second end of the tubular region. This may be achieved by providing a plug fit into the tubular region at the second end. This ensures propellant gases generated in use cannot escape through the second end, and instead must act upon a piston rod. Additional sealing means such as o- rings may be provided to reinforce such a seal. The propellant housing may be attached to the body of the piston housing using removable fasteners such as alien bolts or socket! head screws, such that the propellant housing can be removed for cleaning and re-insertion of a propellant. A plurality of different propellant housings may be attachable to the tubular region and body of the housing, allowing different propellant housing designs to be tested (for instance allowing different sized gun cartridges to be used).

In some embodiments a removable sleeve liner is provided within the tubular region. The sleeve liner may provide an interference fit to the tubular region or may be sealed against the peripheral walls of the tubular region using other means such as o-rings. The sleeve liner itself defines a hollow region within which a piston-rod of predetermined diameter can slide. Therefore a particular gun barrel can be represented by the dimensions of the sleeve liner.

Preferably the removable sleeve liner defines a narrow section having a diameter substantially equal to the predetermined rod diameter, and a wide section having a diameter greater than the predetermined rod diameter. The narrow section may be arranged adjacent the first end of the tubular region. These embodiments may accommodate a piston-rod sliding within the narrow and wide sections, and a separate larger diameter piston plate sliding within the wide section. The narrow section prevents the piston plate from exiting the sleeve liner when the piston housing is in use. Furthermore a piston rod itself may have a section that is of greater diameter than the predetermined rod diameter, or the piston rod may be attached to a piston plate, either of which can be trapped inside the sleeve liner by their abutment with the narrow section when the piston housing is in use. This allows the interior ballistics of captive piston projectiles to be evaluated using the piston housing. The piston plate may also be used to vary the represented projectile diameter.

According to a second aspect of the invention there is provided piston apparatus comprising the piston housing of the first aspect of the invention, and a piston rod received into the tubular region, the piston rod having the predetermined rod diameter. The piston apparatus allows a gun barrel design (represented by the sleeve liner) and a projectile design (represented by the piston rod) to have heir compatibility evaluated experimentally.

Preferably the piston rod comprises a tubular rod-body into which rod ballast can be received. This allows the mass of the piston rod to be adjusted to represent different mass projectiles, without changing the overall diameter of the piston rod (and therefore the projectile diameter it represents in an interior ballistics measurement).

It is preferable that the piston rod further comprises ballast retention means for securing the rod ballast inside the rod-body. This ensures the ballast will not exit the piston rod when the piston apparatus is being used. This also ensures the ballast position can be held fixed in use, preventing the ballast from damaging the piston rod when the piston rod accelerates or decelerates in use.

The ballast retention means may comprise first and second removable end plates. The end plates may provide an interference fit to the inner dimensions of the rod-body. Alternatively the end plates may be threaded to cooperate with threads on the interior surface d,f the tubular rod-body, such that the end plates can be screwed into the rod-body to urge against any contained ballast. The end plates may comprise a recess or aperture into which a tool such as a screwdriver or alien key can be placed to facilitate the screwing and unscrewing.

In some embodiments the piston-rod comprises rod ballast received into the tubular rod- body. The ballast is intended to be removable and may be sand or other particulate, but preferably comprises interlocking weights such that the ballast does not flow or separate and collide during use of the piston rod. The interlocking weights are preferably conformal to the tubular rod-body such that their motion during use is further constrained. Certain embodiments of the piston apparatus comprise sheer pins for temporarily fixing the position of the piston rod in the removable sleeve liner. This fixes the initial position of the piston-rod within the sleeve-liner (for instance if requiring a particular initial low pressure i chamber volume to be set). Furthermore the sheer pins can be selected to sheer when the

¹ pressure acting on the piston-rod increases above a particular trigger pressure. This allows the interior ballistics of a projectile to be evaluated when starting from a predetermined initial chamber pressure.

In some embodiments the piston apparatus further comprises a piston-plate arranged to slide within the removable sleeve liner. The piston-plate may be attached to separate to the piston rod. The piston-plate is arranged to be adjacent to the piston rod so as to urge against the piston rod by action of the actuation means. The piston plate preferably has a larger diameter than the predetermined rod diameter, such that it is prevented from escaping the sleeve liner when the piston apparatus is in use (it cannot escape the piston rod aperture of the sleeve liner, or in some embodiments is retained in a wide section of the j sleeve liner). In embodiments where the piston plate is attached to the piston rod, the piston rod is also prevented from fully exiting the sleeve liner, thereby allowing a captive piston projectile to be represented.

In certain embodiments the piston plate comprises a threaded aperture for receiving a threaded rod. This allows an initial stroke position of the piston-rod to be configured. By passing a threaded rod into the sleeve liner and screwing it into the threaded aperture of the piston plate, the piston-plate (and thereby piston rod) initial stroke position can be adjusted by pulling or pushing the threaded rod. The threaded rod then being removed once the correct position has been established. The piston rod may in some embodiments comprise a threaded protrusion for screwing into the threaded aperture of the piston plate, such they can be attached in order to represent a captive a piston projectile.

It is even more preferable that the piston plate is externally tapered. The piston-plate may in some embodiments initially be positioned proximal to a conduit in the sleeve or tubular region that connects to a pressure gauge port. By providing a tapered piston-plate, blocking of the conduit by the piston plate is mitigated.

According to a third aspect of the invention there is provided a method of manufacturing a piston apparatus, comprising the steps of providing a piston housing comprising a body defining a tubular region within which a piston rod can slide; lining the tubular region with a removable sleeve liner configured to receive a piston rod having a predetermined piston rod diameter; positioning a piston rod having the predetermined piston rod diameter into the removable sleeve liner; and then securing the removable sleeve liner in the tubular region using sleeve retention means, such that when the piston apparatus is in use, the sleeve liner is retained within the tubular region, and the piston rod can slide within the sleeve liner. This allows a piston apparatus for use in interior ballistics measurements to be manufactured having the configurability to test the compatibility of different projectile designs with different gun barrel designs.

According to a fourth aspect of the invention there is provided a method of measuring interior ballistics, comprising the steps of: providing a piston housing for receiving a piston rod, comprising a body defining a tubular region within which a piston rod can slide, a removable sleeve liner located within the tubular region and configured to receive a piston rod of a predetermined rod diameter, and a sleeve retention means securing the removable sleeve liner within the tubular region; arranging a piston rod having the predetermined rod diameter to slide within the removable sleeve liner; configuring the piston housing and the piston rod; propelling the piston rod from the removable sleeve liner; and then measuring an internal ballistics variable from the piston housing or piston rod.

The step of configuring the piston housing and the piston rod may comprise selecting an appropriate removable sleeve according to a required gun barrel diameter and predetermined piston rod diameter (representing a projectile design); selecting an initial stroke position; installing ballast into the piston rod to achieve a piston mass (representing a projectile mass); or selecting and installing an actuation means which may be propellant type/mass. For instance the piston-rod diameter may be preselected by providing a piston- plate attached to the piston-rod of a diameter equivalent to that of a particular projectile. The initial stroke position may be preselected by sliding the piston-rod along the sleeve liner to a start position to achieve a low pressure chamber volume. The propellant rpay be provided with a preselected mass and composition, such that the suitability of the propellant for a particular barrel design and projectile can be evaluated.

The step of measuring an internal ballistics variable may comprise measuring the displacement or rate at which the piston rod exits the tubular region (for instance using high speed videography); or measuring pressure within the tubular region using one or more pressure gauges.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 illustrates in perspective view an embodiment of a piston apparatus arranged with a firing pin mechanism;

Figure 2 illustrates in a different perspective view the piston apparatus of Figure 1; and Figure 3 illustrates in cross sectional view the embodiment of Figure 2.

Detailed Description

Figure 1 illustrates in perspective view an embodiment of a piston housing 11 and piston rod

26, with a firing pin mechanism 12. Both the piston housing 11 and firing pin mechanism 12 are bolted to a base plate 10, itself suitable for mounting to a variety of different surfaces i through bolt holes 13. The base plate 10 is formed from steel to provide an inertial mass that will exhibit limited movement when the piston housing 11 is used. The firing pin mechanism 12 provides a backstop for the piston housing 11 in use. A gasket or other seal is provided between firing pin mechanism 12 and piston housing 11. The piston rod 26 is configured to slide within the tubular region of piston housing 11.

Figure 2 illustrates in perspective view the piston housing 11 of Figure 1. The piston housing 11 comprises a body 20 machined from a metal block to have bevelled edges for safe handling. The body 20 defines a tubular region within which piston rod 26 can slide. A propellant housing 23 is in fluid connection with the tubular region at a first end 2li. The propellant housing 23 comprises a protrusion (not visible) providing a plug fit into the tubular region. The propellant housing 23 is removable from the housing 20 by removing socket head screws 24. Propellant is held by the propellant housing 23 within a gun cartridge 29. The gun cartridge 29 can be removed to vary propellant composition and mass. At a second end 22 of the body 20 there is a cover-plate 25 provided across the tubular region to retain a removable sleeve liner (not visible) within the tubular region. The cover- plate 25 is also attached using socket head screws and comprises a piston-aperture through which piston-rod 26 can be urged. The body 20 further comprises a vent port 27 that extends as a conduit from the exterior surface of the body 20 into the tubular region. The vent port 27 can be opened and closed by turning thumb screw 28. Further provided is a pressure gauge port 30, comprising a conduit extending into the tubular region. The pressure gauge port 30 having a connector for a pressure gauge.

Figure 3 shows an illustration of the piston housing 11 provided in Figure 2 in cross-sectional view. The body 20 is shown defining a tubular region extended between first end 21 and a second end 22. A propellant housing 23 is plugged into the tubular region defined by body 20 at the first end 21, and sealed against the body 20 using an internal o-ring 39. The propellant housing 23 holds a central gur cartridge 29 containing propellant. A cover plate 25 is shown plugged into the tubular region defined by body 20 at the second end 22. The cover plate 25 also comprises a central piston-rod aperture 41. Held within the tubular region of body 20 between the cover plate 25 and propellant housing 23 is a removable sleeve liner 31. The sleeve liner 31 defines a wide diameter region 32 and a narrow diameter region 33. Within the sleeve liner 31 is a piston-rod 26 in the form of a hollow tube containing removable interlocking weights 34. The piston-rod 26 is slidable within the wide diameter region 32 and narrow diameter region 33 and can pass through the piston-rod aperture 41. A piston-plate 35 is adjacent to piston-rod 26 at the propellant facing end of the piston-rod 26. The piston-plate 35 has a wider diameter than the piston-rod 26, such that it can slide within wide diameter region 32 but not narrow diameter region 33. As such piston-plate 35 cannot exit the sleeve liner 31, but piston-rod 26 can be fully ejected through piston-rod aperture 41 . The piston-plate 35 has a diameter of 28mm. A washer 36 is used for impact protection and o-rings 37 are provided to seal piston-plate 35 against the sleeve liner 31. Further o-rings 38 are provided to seal the sleeve liner 31 against the body 20. The pressure gauge port 30 is also shown highlighting the conduit to the tubular region of the body 20. The o-rings 37, 38, 39 and 40, are formed from nitrile and neoprene foam is used for the washers. The cover plate 25 is removably attached to the body 20 using! socket head screws to mitigate screw failure owing to tensile loading when the piston housing 11 is in use. The piston-rod 26 is formed from aluminium whilst the piston-plate 35 is formed from mild steel. The use of mild steel mitigates piston-plate 35 damage when abutting the narrow diameter region 33. The gun cartridge 29 is a .38 inch cartridge.

In use an appropriate sleeve liner 31 is selected for an interior ballistics measurement. The piston-rod 26 is appropriately filled within weights 34 to achieve a simulated projectile mass. The sleeve liner 31 is slid into the tubular region of body 20 and is then retained using cover plate 25. A threaded rod (not shown) is passed into the tubular region and screwed into a threaded aperture of piston plate 35. This is used to move the piston-plate 35 to an initial stroke position along wide diameter region 32. The threaded rod is then rerrioved.

Propellant housing 23 is plugged into the tubular region of body 20 and a suitable propellant composition and mass is loaded into gun cartridge 29. A pressure gauge is attached to pressure gauge port 30. A firing pin mechanism is used to initiate the propellant in gun cartridge 29. Propellant gases flow into the tubular region through a piston plate facing side of gun cartridge 29. The pressure generated by propellant gases causes piston plate 35 to be urged in the direction of the cover plate 25. As the piston plate 35 and piston- rod 26 move in the direction of the cover plate 25 a chamber volume increases between the i piston plate 35 and propellant housing 23. The propellant gases continue to fill this volume and urge piston plate 35 and piston rod 26 further in the direction of the cover plate The piston-rod 26 is urged through the piston-rod aperture 41 and becomes visible externally to the body 20. The piston-rod 26 increasingly emerges from the aperture 41 allowing its length with respect to time to be measured using high speed videography. Simultaneously pressure readings in the chamber volume behind the piston plate 35 are measured using a pressure gauge attached to pressure gauge port 30. When piston plate 35 reaches narrow diameter region 33 it is prevented from further motion along the sleeve liner 31. Therefore the piston-rod 26 is ejected from sleeve liner 31 but piston plate 35 is retained within! the liner 31. The high pressure gases within the tubular region in the chamber volume between the piston plate 35 and propellant housing 23 are now vented through vent port 27 by turning thumb screw 28. The sleeve liner 42 can now be safely removed and a different sleeve liner inserted ready for the next test.

The piston plate 35 may be attached to the piston-rod in alternative embodiments such that captive piston projectiles can be simulated. The sleeve liner 31 may be screwed to cover plate 25. Additional sealant may be provided to the rear of gun cartridge 29 to mitigate leakage of propellant gases around the periphery of the gun cartridge 29. Alternatively a sealing o-ring or gasket could be provided between the gun cartridge 29 and any firing pin mechanism to achieve the same effect. The gun cartridge 29 may contain any one of a number of propellants, for instance nitrocellulose propellant, but other gun cartridges 29 or propellants could be used. The sleeve liner 31 may only partially line the tubular region of housing 20. For instance the sleeve liner 31 may only comprise a narrow diameter region 33 and not the wide diameter region 32 which would allow for a larger diameter piston plate 35 to be used. The tapering of piston plate 35 allows pressure behind the plate 35 'to be measured substantially instantaneously after a propellant in gun cartridge 29 is initiated. However such a tapering is not essential unless such measurements are required in configurations where a piston plate 35 initial stroke position is such that is abuts the gun cartridge 29 or blocks a pressure gauge port. Whilst pressure and piston-rod distance/length measurements may be performed, additionally sound measurements may be taken to establish how interior ballistics parameters can be varied to increase or decrease projectile sound.