PRIORITY DOCUMENTS

[0001 ] The present application claims priority from Australian Provisional Patent Application No.

2018901668 titled“PROGRESSIVE PROPELLANT” and filed on 1 1 May 2018, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to a propellant that bums progressively.

BACKGROUND

[0003] Solid propellant charges or propellant grains are used in pyrotechnics, ballistics and rocketry in order to accelerate a piston, projectile, vehicle or missile. Typically, the propellant charge is ignited by a primer, which is a small amount of sensitive energetic material. Once ignition is achieved, it is desirable to have the propellant bum in a controlled manner. In typical gun applications, as the propellant is initially ignited, and propellant combustion gases are being generated, the projectile is either at rest or moving relatively slowly. The generation of propellant gases initially causes the pressure in the gun chamber to increase, resulting in acceleration of the projectile. As the projectile accelerates, the available chamber volume also increases at an accelerating rate. Typically, a transition point will be reached at a point in time when the rate of propellant combustion gas generation is insufficient to provide further increases in chamber pressure as the chamber volume continues to increase. This transition corresponds to the point of maximum gas pressure in the chamber. This pressure level must be controlled so that it will never exceed the strength of the chamber. Thereafter chamber pressure typically decreases as the projectile continues to accelerate, because the rate of chamber volume increase outpaces the rate of propellant gas production. Typically, all or most of the propellant will be burnt prior to the projectile exiting the gun.

[0004] A solid propellant that burns progressively (“progressive propellant”) is defined as a propellant grain or charge which, when combusting at a given pressure, provides successively increasing rates of gas production as it continues to combust (usually with some degressive behaviour at the end of a bum), where gas production rate refers to the mass of gas liberated from the solid propellant per unit of time. By providing an ever-increasing gas production rate, progressive propellants can eliminate or reduce the severity of the post-transition pressure drop in a gun, thereby maintaining higher pressure in the chamber for a longer period, thereby allowing the gun to accelerate the projectile to a higher exit velocity. Thus a progressive propellant can enable higher projectile velocities to be achieved while simultaneously not exceeding the maximum allowable pressure of the chamber. The progressivity of propellants can be controlled by altering the size and shape of the propellant grain so that its total exposed burning surface area increases as it continues to combust.

[0005] Previously, progressive gun propellant grains have usually been produced by extrusion, and cut up into short rods or cylinders, with one, seven, nineteen, thirty seven or more longitudinal cylindrical perforations in the form of through-holes or dead-end holes. Being an extruded product, such propellants are based on geometric fonns of near-constant cross-section. The purpose of the perforations in such propellants is to provide a cylindrical surface area which progressively enlarges as combustion progresses, thereby increasingly exposing more solid propellant surface area to the combustion front. For each cylindrical perforation, the available burning surface area increases in approximate proportion to its radius.

[0006] The progressivity provided by a perforated extruded propellant grain is typically less than that which would ideally achieve the highest possible muzzle velocity for a given gun chamber volume and chamber strength. In addition, multi-perforated extruded propellant grains typically disintegrate into slivers prior to bum completion, at which time the propellant is no longer burning progressively . In addition, such propellant grains often pack inefficiently in the gun chamber, limiting the maximum charge weight for a given chamber volume.

[0007] There is thus a need to provide improved progressivity propellants.

SUMMARY

[0008] The present disclosure arises from research into progressive propellants, in particular, the control of the directionality of a burn in a propellant grain. Control of the directionality of a bum can provide a bum initiation at a defined position within a propellant grain. This can provide a progressive propellant. However, further progressivity can be gained by using that control in a network.

[0009] In a first aspect of the present disclosure, there is provided a solid propellant grain, comprising: a body of at least one first propellant; at least one void comprising a bum initiation region and a bum directing ignition channel extending therefrom, the ignition channel at least partially lined by a retardant layer configured to direct ignition gases along the ignition channel to the bum initiation region from a throat at an opposing end of the ignition channel.

[0010] Accordingly, the retardant layer may direct ignition gases along the ignition channel while preventing or retarding the progress of combustion in a transverse direction relative to a direction of elongation of the ignition channel. [0011] In certain embodiments, the retardant layer lines the ignition channel from the bum initiation region to the throat. In certain embodiments, the retardant layer extends laterally outwardly from the throat. In certain embodiments, the retardant layer lines at least part of an outer surface of the propellant grain.

[0012] In certain embodiments, the retardant layer comprises a bum inhibitor, a bum rate modifier, or a propellant having a slower burn rate than the first propellant.

[0013] In certain embodiments, the at least one void comprises at least one second propellant or other energetic material having a higher burn rate than the first propellant.

[0014] Advantageously, the solid propellant grain may comprise more than one void and a throat of at least one further ignition channel may be positioned at least partially within a regression profile of a bum initiation region of at least one adjacent void. Similarly, when there are further voids, each throat of a plurality of further ignition channels may be at least partially within a regression profile of a burn initiation region of at least one adjacent void.

[0015] In a second aspect of the present disclosure, there is provided a solid propellant grain comprising a void network, each void configured to propagate combustion to a subsequent void in the network, with each void comprising a retardant layer configured to inhibit or deter lateral burning.

[0016] In certain embodiments, each void comprises: at least one burn initiation region and a bum directing ignition channel extending therefrom, the ignition channel at least partially lined by the retardant layer and configured to direct ignition gases along the ignition channel to the burn initiation region from a throat at an opposing end of the ignition channel.

[0017] In certain embodiments, the retardant layer lines the ignition channel from the bum initiation region to the throat. In certain embodiments, the retardant layer extends laterally outwardly from the throat. In certain embodiments, the retardant layer lines at least part of an outer surface of the propellant grain.

[0018] In certain embodiments, a throat of at least one further ignition channel is positioned at least partially within a regression profile of a bum initiation region of at least one adjacent void. In certain embodiments, each throat of a plurality of further ignition channels is positioned at least partially within a regression profile of a burn initiation region of at least one adjacent void. [0019] Advantageously, in certain embodiments, the voids, including the bum initiation regions, ignition channels and throats, are arranged so as to provide cascading stages of initiation which ultimately results in simultaneous or near- simultaneous bum completion across all or most of the initiated burning surfaces.

[0020] In certain embodiments, the void network comprises interconnected voids.

[0021] In certain embodiments, the propellant grain comprises a body of at least one first propellant. In certain embodiments, each void comprises at least one second propellant having a higher bum rate than the first propellant.

[0022] In certain embodiments, the retardant layer comprises a bum inhibitor, a bum rate modifier or a propellant having a slower burn rate than the first propellant.

[0023] In a third aspect, there is provided a complete propelling charge comprising at least one propellant grain of the first or second aspects. The complete propelling charge may comprise a monolithic propellant grain or a plurality of propellant grains.

[0024] In a fourth aspect of the present disclosure, there is provided a method of manufacturing the propellant grain of the first or second aspects or the charge of the third aspect. In certain embodiments, the method of manufacturing comprises additive manufacturing.

BRIEF DESCRIPTION OF DRAWINGS

[0025] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0026] Figure 1 is a drawing of an embodiment of a void of the present disclosure having a substantially spherical burn initiation region;

[0027] Figure 2 is a drawing of an embodiment of a void of the present disclosure having a short ignition channel;

[0028] Figure 3 is a drawing of an embodiment of a void of the present disclosure having a curved ignition channel;

[0029] Figure 4 is a drawing of an embodiment of a void of the present disclosure having a burn initiation region that is a substantially planar surface (eg a flat end) at an end of the bum directing ignition channel; [0030] Figure 5 is a drawing of an embodiment of a void of the present disclosure having a burn initiation region that has a square profile and is of greater lateral width to the burn directing ignition channel;

[0031] Figure 6 is a drawing of an embodiment of a void of the present disclosure where the throat is positioned within the propellant grain;

[0032] Figure 7 is a drawing of an embodiment of a two-dimensional representation of a solid propellant grain of the present disclosure showing a plurality of voids repeated in the form of a network;

[0033] Figure 8 is a drawing of an embodiment of a two-dimensional representation of a solid propellant grain of the present disclosure showing a plurality of voids repeated in the form of a network and the regression profile for each bum initiation region;

[0034] Figure 9 shows a portion of Figure 8 with an embodiment of the position of each bum initiation region, as well as the length and orientation of the ignition channels in the void network that may result in a simultaneous or near-simultaneous completion of bum across all or most of the initiated burning surfaces;

[0035] Figure 10 is a drawing of an embodiment of a three-dimensional representation of a solid propellant grain of the present disclosure showing a ghosted view of the propellant grain with a plurality of voids repeated in the fonn of a network;

[0036] Figure 11 is a drawing of an embodiment of a two-dimensional representation of a solid propellant grain of the present disclosure showing interconnected voids in the fonn of a network; and

[0037] Figure 12 is a graph of interior ballistics modelling results showing gun chamber pressure and projectile muzzle velocity, for an example large-calibre gun and projectile; the baseline result (denoted with a diamond symbol) shows modelled ballistic performance when the propelling charge is comprised of conventional 7-perforated cylindrical propellant grains; other results (denoted by cross, square, and triangle symbols) correspond to modelled ballistic perfonnance for propellant grains of the present disclosure.

[0038] In the following description, like reference characters designate like or corresponding parts throughout the figures DESCRIPTION OF EMBODIMENTS

[0039] The progressivity of propellants has previously been controlled by, for example, the size and shape of the propellant grain, including perforations/ voids, the web thickness or amount of solid propellant between burning surfaces, the chemical composition of the propellant, and the use of burn-rate modifiers, such as deterrents, to change the propellant bum rate. Progressivity refers to the relative rate at which the propellant gas mass production rate increases, as combustion proceeds.

[0040] Some known progressive propellant grains are produced by extrusion and cut up into short rods or cylinders, with one, seven, nineteen, thirty seven or more longitudinal cylindrical perforations in the fonn of through-holes or dead-end holes. If perforations are present, perforations parallel to the length of these propellant grains are usually formed during extrusion using an appropriately shaped extrusion die. Alternatively, perforations may be produced in these propellant grains after extmsion using a die with shaped pins that fonn perforations parallel to the length of the propellant grains or transverse to the length of the propellant grains. However, there are limitations to these types of progressive propellants. One limitation is that such propellants are limited in their achievable progressivity. The radius of each cylindrical perforation increases during combustion, and the corresponding increase in available burning surface area is approximately proportional to that radius. Another limitation is that such propellant grains are often limited to geometries of constant cross-section. Another limitation is that prior to bum completion, multi-perforated extruded propellant grains typically disintegrate into slivers formed by the solid interstitials between perforations, at which time the propellant is no longer burning progressively. Another limitation is that such propellant grains often pack inefficiently in gun chambers, limiting the maximum propelling charge weight for a given chamber volume.

[0041] As outlined above, the present disclosure arises from research into progressive propellants, in particular, the control of the directionality of a bum in a propellant grain. Control of the directionality of a bum can provide a bum initiation at a defined and advantageous position in a propellant grain. This can provide a progressive propellant. However, further progressivity can be gained by using that control in the fonn of a network.

[0042] In the first aspect of the present disclosure, there is provided a solid propellant grain 10, comprising a body 12 of at least one first propellant, and at least one void 18 comprising at least one bum initiation region 14 and a bum-directing ignition channel 16 extending therefrom, the ignition channel 16 at least partially lined by a retardant layer 20 configured to direct ignition gases along the ignition channel 16 to the burn initiation region 14 from a throat 22 at an opposing end of the ignition channel 16. For example, Ihe retardant layer 20 may direct ignition gases along the ignition channel 16 while inhibiting or deterring the progress of combustion in a lateral direction. Accordingly, when, for example, a primer in a gun chamber ignites, combusting material enters the throat 22 and is directed along the ignition channel 16 by the retardant layer 20 to the bum initiation region 14. There, the combusting material contacts and ignites the first propellant to provide an initiated burning surface. Subsequently, most of the propellant gas generated at the burning surface of the first propellant will flow back out to the chamber through the ignition channel 16 and throat 22. This results in an“inside-out” bum of the first propellant. The“inside- out” burn is typically manifested in the form of an approximately spherical burning surface within the first propellant, which increases in radius as combustion progresses. In the case of a non-spherical initiation region, the burning surface will become approximately spherical by the time the bum has progressed a relatively short distance from the initiation region 14. Advantageously, the surface area of the approximately spherical burning surface increases in proportion to the square of its radius. This provides higher propellant progressivity when compared to a corresponding conventional cylindrically- perforated grain. Embodiments of the first aspect are shown in at least Figures 1 through 5. Each of these illustrated embodiments show the void 18 extending inwardly from the outer surface 24 of the propellant grain 10. In Figure 6, the void 18 is positioned entirely within the propellant grain 10.

[0043] In the second aspect of the present disclosure, there is provided a solid propellant grain 10 comprising a void network, each void 18 configured to propagate combustion to a subsequent void 18 in the network, with each void 18 comprising a retardant layer 20 configured to inhibit or deter lateral burning from each void 18. Embodiments of the second aspect arc shown in Figures 7, 8, 9 and 10. Figure 7 shows an embodiment of a two-dimensional representation of a solid propellant grain showing a plurality of voids repeated in the form of a network. Figures 8 and 9 show a similar embodiment and illustrates the regression profile for each bum initiation region 14. Figure 10 is a drawing of an embodiment of a three-dimensional representation of a solid propellant grain of the present disclosure showing a ghosted view of the propellant grain with a plurality of voids repeated in the fonn of a network. A single network is shown in Figure 10 for simplicity, but it is within the scope of the disclosure that the propellant grain may contain a plurality of networks, as required for the particular application.

For example, the propellant grain may contain 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100 or more networks, or as shown in, for example, Figures 7 and 8. In certain embodiments of the second aspect, at least one void in the network comprises a void 18 as defined in the first aspect. That is, a void 18 comprising at least one burn initiation region 14 and a bum-directing ignition channel 16 extending therefrom, the ignition channel 16 at least partially lined by a retardant layer 20 and configured to direct ignition gases along the ignition channel 16 to the burn initiation region 14 from a throat 22 at an opposing end of the ignition channel 16. Alternatively, at least one void 18 in the network may comprise a shape, structure or dimensions as described elsewhere in this disclosure.

[0044] As would be appreciated by the person skilled in the art, the shape, structure and dimensions of a void 18 and the relative spacing and arrangement of a plurality of voids 18 may be varied depending upon the desired progressivity of the propellant grain 10. In certain embodiments, it is advantageous to use arrangements of voids which provide cascading stages of initiation which ultimately result in a simultaneous or near-simultaneous completion of a bum across all or most of the initiated bum initiation regions. In gun applications this allows peak pressure to be maintained at a desired level for an extended duration, followed by a rapid drop in pressure prior to the projectile exiting from the barrel. In certain embodiments, it is advantageous to use an arrangement and staging of bum initiation regions which significantly minimises a mass of slivers produced towards the end of a bum. This results in reduced degressive behaviour at the end of a bum.

[0045] The bum initiation region 14 will typically be positioned at an end of the ignition channel 16 distal from the throat 22. In non-illustrated embodiments, the ignition channel 16 branches and a bum initiation region 14 is positioned at an end of each branch distal from the throat 22. The ignition channel 16 may branch into 2, 3, 4, 5, 6 or more further ignition channels. This may form, for example, a tree-like branching arrangement. The bum initiation region 14 may have any suitable shape/geometry or dimensions as required for, for example, the desired gas production rate or the progressivity of the propellant grain 10. For example, the bum initiation region 14 may be, but is not limited to, a substantially planar surface, an irregular surface, a curved surface, a prism or rounded prism such as a triangular, rectangular or pentagonal or other shaped prism, a sphere, a spheroid, an ellipsoid, a cylinder, a rosette prism, a cube, a cuboid, a cone, or a type of pyramid. In particular embodiments, the bum initiation region 14 is in the fonn of a substantially planar surface, a cube, a sphere or a cylinder. As would be appreciated by the person skilled in the art, in the case of a non-spherical initiation region, the burning surface will become approximately spherical by the time the burn has progressed a relatively short distance from the initiation region 14. The bum initiation region 14 may be of any suitable size. For example, the bum initiation region 14 may have a lateral width that is less than, substantially equal to, or greater than a lateral width of the ignition channel 16. A greater size or lateral width may increase the initial gas production rate by providing an increased burning surface area, but would reduce the amount of the first propellant available to burn. In certain embodiments, the lateral width may be between 0.5 times (x) and 3x the lateral width of the ignition channel 16. For example, 1 , 1 .2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6 or 2.8x the lateral width of the ignition channel 16. In particular embodiments, the lateral width may be greater than 3x the lateral width of the ignition channel 16, such as, 4x, 5x, 6x, 7x, 8x, 9x or lOx. A larger lateral width is particularly suitable for, for example, applications where a high initial gas production rate is required.

[0046] The ignition channel 16 may have any suitable shape or dimensions as required by, for example, the size of the propellant grain 10 region or to provide cascading stages of initiation which ultimately result in a simultaneous or near-simultaneous completion of a bum. For example, a granular propellant comprising multiple propellant grains having an outer diameter of 5 mm will typically have

correspondingly small ignition channels, whereas a monolithic propellant grain that is a solid rocket motor 25 metres in length and 7 metres in diameter will typically have correspondingly larger ignition channels. Typically, an ignition channel in a monolithic propellant grain will be no greater than about 0.5x the length of the gun chamber. Where a void 18 extends inwardly from the outer surface 24 of the propellant grain 10, the length of the ignition channel 16 may be greater than a void 18 that is positioned entirely within the propellant grain 10. This can be seen in Figures 7, 8 and 9, where subsequent voids 18 in the network have progressively shorter ignition channels (or indeed longer ignition channels, as required). Accordingly, the length of the ignition channel 16 can be dictated by the space available within the propellant grain 10 or modified to provide a bum initiation region at a position such that it burns back and meets its neighbours and parent, if present, simultaneously (eg simultaneous or near simultaneous completion of bum). For small and medium calibre guns, a length of the ignition channel 16 will typically be at least 0.5 mm, for example, between about 0.8 mm and about 100 mm, such as between about 1 mm and about 70 mm, or between about 2 mm and about 50 mm. For large calibre guns, a length of the ignition channel 16 will typically be at least 1 mm, for example, between about 1 mm and about 1000 mm, such as between about 2 m and about 700 mm, or between about 3 mm and about 500 mm For a solid rocket motor, these range in size from a hobby rocket to heavy lift boosters for space access, so will typically range in size from about 0.5 mm to about 25 m or more. In certain embodiments, the length of the ignition channel 16 is about 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm,

10 mm, 1 1 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm, 65 mm, 70 mm, 75 mm, 80 m, 85 mm, 90 m ,

95 mm, 100 mm, 110 mm, 120 mm, 130 mm, 140 mm, 150 mm, 160 mm, 170 mm, 180 mm, 190 mm, 200 mm, 220 mm, 240 mm, 260 mm, 280 mm, 300 mm, 350 mm, 400 mm, 450 mm, 500 mm, 550 mm, 600 mm, 650 mm, 700 mm, 750 mm, 800 mm, 850 mm, 900 mm, 1 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, 10 in, 1 1 m, 12 m, 13 m, 14 m, 15 m, 16 m, 17 m, 18 m, 19 m, 20 m, 21 m, 22 m, 23 m, 24 m, 25 m, 26 m, 27 m, 28 m, 29 m, 30 m or any range between these defined lengths.

[0047] A width or diameter of the ignition channel 16 may be determined in view of some of the same considerations as the length of the ignition channel 16 (eg for a void 18 that extends inwardly from the outer surface 24 of the propellant grain 10, the width of the ignition channel 16 may be greater than a void 18 that is positioned entirely within the propellant grain 10). Also, in certain embodiments, the width or diameter of the ignition channel 16 may be determined based upon the estimated amount of propellant gas that will be generated at the burning surface that will have to flow back out to the chamber through the ignition channel 16 and throat 22. The width of an ignition channel 16 may vary or be consistent along its length. For small, medium and large calibre guns, a width of the ignition channel 16 will typically be between about 0.05 mm and about 30 mm, for example, between about 0.1 mm and 10 mm, or about 0.1 mm and 5 mm. For rockets, the width of the ignition channel 16 will typically be less than 0.5x a width of the rocket diameter. The suitable ranges will depend upon the size and application of the rocket motor.

For example, a monolithic propellant grain for a hobby rocket may have an ignition channel 16 with a width between about 0.05 m and about 15 mm, for example, between about 0.1 mm and 10 mm, or about 0.1 mm and 5 mm. A monolithic propellant grain for a heavy lift booster for space access may have an ignition channel 16 with a width between about 1 mm and about 5 m. In certain embodiments, the width of the ignition channel 16 is about 0.05 mm, 0.06 mm, 0.07 mm, 0.08 mm, 0.09 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 m , 0.9 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm, 65 mm, 70 mm, 75 mm,

80 m , 85 mm, 90 m , 95 mm, 100 mm, 150 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm, 700 mm, 800 m , 900 mm, 1 m, 2 m, 3 m, 4 m, 5 in, or any range between these defined widths.

[0048] Similarly to the length of the ignition channel 16, the direction of the ignition channel 16 or the three-dimensional arrangement of the ignition channels 16 within the propellant grain 10 may also vary due to, for example, the size of the propellant grain 10, the application or the desired progressivity. In certain embodiments, four adjacent voids 18 may have ignition channel 16 in a tetrahedral arrangement. This more effectively positions the burn initiation regions 14 in space than a radial arrangement to, for example, maximise progressivity, provide simultaneous or near simultaneous bum completion or reduce slivering. In certain embodiments, the ignition channels 16 may be in any suitable arrangement, including but not limited to, linear, bent, trigonal planar, trigonal pyramidal, t-shaped, tetrahedral, square planar, seesaw, trigonal bipyramidal, square pyramidal, pentagonal planar, octahedral, trigonal prismatic, pentagonal pyramidal, distorted octahedral, pentagonal bipyramidal, square antiprismatic, tricapped trigonal prismatic or capped square antiprismatic. As would be appreciated by the person skilled in the art, a void network could contain one or more of the above arrangements, for example, four tetrahedral, eight bent and one octahedral arrangement.

[0049] The throat 22 will typically be positioned at an end of the ignition channel 16 distal from the bum initiation region 14. The throat 22 may have any suitable shape or dimensions as required for, for example, the desired progressivity of the propellant grain 10 or based upon the estimated amount of propellant gas that will be generated at the burning surface that will have to flow back out to the chamber through the ignition channel 16 and throat 22. In certain embodiments, the throat 22 has a lateral width that is less than, substantially equal to, or greater than a lateral width of the ignition channel 16. A greater size or lateral width may assist in funnelling combusting material into the ignition channel 16 (eg if there is a weak ignition stimulus from an igniting primer), but would reduce the amount of the first propellant available to bum. In certain embodiments, the lateral width may be between about 0.9 times (x) and about 3x the lateral width of the ignition channel 16, for example, between about lx and about 2x, or about 1 and about 1 6x.

[0050] The retardant layer 20 acts to modify burning of the ignition channel 16, such as, the walls of the ignition channel 16. For example, the retardant layer 20 may partially or completely inhibit burning of the ignition channel wall or reduce the rate of burning of the ignition channel wall. In certain embodiments, the retardant layer 20 comprises a bum inhibitor, a bum rate modifier or a propellant having a slower bum rate than the first propellant. The retardant layer 20 may be a coating on the ignition channel 16 or be added or impregnated into the first propellant that fonns the ignition channel 16, as appropriate. The retardant layer 20 may be, for example, a deterrent that deters burning of the ignition channel 16 (eg partially shield the underlying first propellant), so that a bum will initiate at the bum initiation region 14 before the remainder of the ignition channel 16 ignites. The retardant layer 20 will inhibit burning of the ignition channel 16 (eg completely or mostly shield the underlying first propellant), so that a bum will initiate only at the bum initiation region 14 and not elsewhere in the ignition channel 16. The propellant having a slower bum rate than the first propellant (ie retardant layer 20) will ignite when heat and ignition gases enter the ignition channel 16, but will bum at a slower rate than the first propellant at the bum initiation region 14. In certain embodiments, the use of a slow-burning or retardant layer 20 may in some cases be preferred to the use of an inhibitor, in order to allow the ignition channel 16 to slowly radially enlarge with time and handle the ever-increasing combustion gas efflux. Otherwise, the propellant grain may explode. Similarly, in certain embodiments, the use of a slow-burning or retardant layer 20 may in some cases be preferred to the use of an inhibitor to assist/enhance the carriage of the ignition stimulus to the desired bum initiation region 14, especially in the cases of (i) long ignition channels 16 and (ii) voids 18 that do not comprise a fast burning propellant or pyrotechnic (eg second propellant).

[0051] hi certain embodiments, the retardant layer 20 lines the ignition channel 16 from the bum initiation region 14 to the throat 22. In certain embodiments, the retardant layer 20 extends laterally outwardly from the throat 22. This configuration may be useful to inhibit burning of the first propellant around the throat 22 of the ignition channel 16, for embodiments where the retardant layer 20 inhibits or reduces burning of the ignition channel 16. In certain embodiments, the retardant layer 20 lines at least part of an outer surface 24 of the propellant gram 10. In certain embodiments, the retardant layer 20 lines all of the outer surface 24 of the propellant grain 10. These configurations may be useful when the void 18 extends inwardly from the outer surface 24 of the propellant grain 10. A composition and thickness of the retardant layer 20 can be determined by the person skilled in the art, based on the desired level of bum retardation/modification of the first propellant. For example, the composition of the retardant layer 20 may be, but is not limited to, at least one of camphor, dinitrotoluene (DNT), methyl centralite (C2), ethyl centralite (Cl), phthalimido acid dibutyl (DBP) or diphenyl phthalate (DPP). The composition of the retardant layer 20 may be, but is not limited to, at least one of the non-energetic binders defined in this disclosure. The composition of the propellant 20 having a slower burn rate than the first propellant could readily be determined by the person skilled in the art, or, for example, with reference to the publications mentioned in this disclosure. The thickness of the retardant layer 20 will typically be greater than about 0.001 mm, for example, between about 0 002 mm and about 3 mm, or about 0.02 mm and about 2 mm. In certain embodiments, the thickness of the retardant layer 20 is about 0.002 mm, 0.003 mm, 0.004 mm, 0.005 min, 0.006 mm, 0.007 mm, 0.008 mm, 0.009 mm, 0.01 mm, 0.02 mm, 0.03 mm, 0.04 mm, 0.05 mm, 0.06 mm, 0.07 mm, 0.08 mm, 0 09 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, 2 mm, 3 mm or any range between these defined thicknesses.

[0052] Advantageously, the solid propellant grain may comprise more than one void 18 and, a throat 22 of at least one further ignition channel 16 may be positioned at least partially within a regression profile 26 of a burn initiation region 14 of at least one adjacent void 18. As used herein, the term“regression profile” refers to the evolution of the shape, size, position and path of the burning surfaces as the solid propellant (eg the first propellant) is consumed. Similarly, when there are further voids 18, each throat 22 of a plurality of further ignition channels 16 may be positioned at least partially within a regression profile 26 of a burn initiation region 14 of at least one adjacent void 18. For example, there may be 2, 3,

4, 5, 6, 7, 8, 9, 10 or more throats of further ignition channels positioned at least partially within a regression profile of a bum initiation region 14 of at least one adjacent void. See, for example, Figures 7, 8, 9 and 10. Having the throat 22 of a further ignition channel 16 at least partially within the regression profile 26 of the bum initiation region 14 means that, during a bum, combusting first propellant around the bum initiation region 14 will travel down the throat 22 of the further ignition channel 16 and along the further ignition channel 16 to a further bum initiation region 14, where the surrounding first propellant will ignite. In this way, the bum is propagated through the first propellant via the ignition channels 16. This is illustrated in Figures 8 and 9. The illustrated regression profiles 26 show how the burning surface regresses from each bum initiation region 14.

[0053] In alternative embodiments, the at least one void 18 comprises at least one second propellant or other energetic material having a higher bum rate than the first propellant. In certain embodiments, the at least one second propellant or other energetic material lines at least part of the ignition channel 16. For example, the ignition channel 16 may be laced with a pyrotechnic to promote flame-spreading down the ignition channel 16. This may be advantageous in embodiments where the ignition channel 16 is long and narrow. Tn certain embodiments, the retardant layer 20 is positioned between the first and second propellants along at least part of the ignition channel 16. Accordingly, when, for example, a primer ignites, combusting material enters the throat 22 and ignites the second propellant, which burns along the ignition channel 16 to the burn initiation region 14. There, the combusting second propellant contacts and ignites the first propellant. The retardant layer 20 prevents or slows the second propellant from igniting the first propellant elsewhere. This results in an“inside-out” bum of the first propellant. The skilled person would readily understand which propellants or energetic materials would have a higher bum rate than the first propellant, but could also refer to the publications mentioned in this disclosure. When there is more than one void 18 and a throat 22 of at least one or a plurality of further ignition channels 16 is at least partially within a regression profile 26 of the bum initiation region 14, combusting first propellant around the bum initiation region 14 will travel to the throat 22 of the further ignition channel 16 and ignite the second propellant, which bums along the further ignition channel 16 to the further burn initiation region 14, where the first propellant will ignite in this way, the bum is propagated through the first propellant via the ignition channels 16 that comprise the second propellant.

[0054] As illustrated in Figure 11 , in certain embodiments, the void network comprises interconnected voids 18. Accordingly, each void 18 may have a similar structure to that in the second aspect, but each void 18 may be connected. For example, if the embodiment illustrated in Figure 9 were to comprise interconnected voids, then the ignition channels 16 corresponding to“A”,“B” or“C” would be extended through the regions of first propellant designated as“a”,“b” or“c”, respectively, to the adjacent burn initiation region 14. In certain embodiments, each throat 22 may be connected to one or more bum initiation regions 14 Similarly, each burn initiation region 14 may be connected to one or more throats 22 of further ignition channel(s) 16. In certain embodiments, the void network comprises interconnected voids 18 in a tree-like branching arrangement. For example, a single ignition channel 16 that branches into multiple further ignition channels 16 (eg 2, 3, 4, 5, 6 or more ignition channels 16), with a bum initiation region 14 at the end of each of the branching further ignition channels 16.

[0055] As described above, in certain embodiments, it is advantageous to use arrangements which provide cascading stages of initiation which ultimately result in a simultaneous or near-simultaneous completion of bum across all or most of the initiated burning surfaces. This may be achieved, in part, by adjusting the three dimensional position of each bum initiation region, as well as the length and orientation of the ignition channels in the void network. We refer now to Figures 8 and 9, which show a parent ignition channel 30, child ignition channels 32 and grandchild ignition channels 34, with the regression profiles 26 shown in dashed lines, illustrating shape, size, position and path of the burning surfaces as the first propellant is consumed. Figures 8 and 9 are representative of a snap shot in time at which the many burning surfaces of the first propellant meet. These burning smfaces have been initiated at the bum initiation region of each parent ignition channel 30, child ignition channel 32 and grandchild ignition channel 34.

[0056] A simplified example of the three dimensional position of each bum initiation region, as well as the length and orientation of the ignition channels in the void netw ork that may result in a simultaneous or near-simultaneous completion of bum across all or most of the initiated burning surfaces is illustrated in Figure 9 and detailed in Table 1 (below). For this example, a constant propellant bum rate is assumed. If that bum rate were, for example, 400 millimetres per second (mm/ ^' s), then the spherical burning surfaces would meet at 50 milliseconds (ms) post-initiation. Thereafter, remaining small slivers would take a little longer to finally bum out. The example assiunes that: (i) Flame-spread through the ignition channels is effectively instantaneous: A reasonable assumption - it is much faster than the burn rate of the first propellant.

(ii)The first propellant burn rate is constant: The person skilled in the art would appreciate that in reality this is not true and it will vary through time in response to the changing chamber pressure. However, accounting for this is highly dependent on many ballistic parameters, including, for example, the gun chamber volume, projectile weight, shot start resistance, ullage, propellant burn rate exponent and coefficient, propellant energy, as well as other parameters understood by the person skilled in the art. Accordingly, as would be understood by the person skilled in the art, the ignition channel lengths and spatial positioning (defined by a, A, b, B, c, C) can be further adjusted to account for variation of propellant bum rate during the ballistic cycle specific to any gun or rocket motor.

[0057] Table 1 : Simplified example of dimensions which will lead to near-simultaneous completion of burn across the various initiated burning surfaces. This table is to be read in combination with Figure 9, which together show the distances that the various symbols represent (ie a, A, b, B, c, C).

[0058] Different configurations or geometries can be used and these can be modelled to analyse the perfonnance of configuration or geometries. For example, interior ballistic modelling was used to analyse the perfonnance of candidate 3D printed propellant grain geometries in B. Dolman, A. Hart, I. Johnston, C. Prior, Additive Manufacturing of Energetics for Next Generation Munitions, APICAM Conference, December 2017 (http://www.apicam2017. com.au/), which is herein incorporated by reference.

[0059] Accordingly, in certain embodiments, each burning surface of the child ignition channel

32 meets each other and the burning surface of the parent ignition channel 30, simultaneously or near simultaneously. A parent ignition channel 30, child ignition channels 32 and grandchild ignition channels 34 are also shown in Figure 10. As would be appreciated by the person skilled in the art, there may be further generations of ignition channels beyond the parent, child and grandchild ignition channels described above in certain embodiments, there is at least one generation of ignition channels, such as, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more generations.

[0060] As described above, in certain embodiments, it is advantageous to use an arrangement and staging of initiation regions which significantly minimises the mass of slivers produced towards the end of bum. The areas 36 shown in Figures 8 and 9 that are outside of the regression profiles represent the slivers that will be produced towards the end of bum. The bum initiation regions may be positioned to minimise these slivers, for example, by altering the position of bum initiation regions or by introducing additional bum initiation regions with connecting ignition channels extending from within the regression profile of neighbouring bum initiation regions.

[0061 ] The present disclosure may provide a solid propellant grain 10 of any shape, size or composition. For example, the composition of the first or second propellant may be single-based, double-based, triple- based or composite. As would be understood by the person skilled in the art, many different compositions will satisfy the functional requirements that are common for any gun or rocket propellant. Such functional requirements include, for example, mechanical integrity, manufacturability, desired impetus, desired bum rate range/characteristics, desired gas product molecular weight, desired ignitability, desired

sensitiveness, safe life and stability. Many propellants are known in the art and will satisfy the above functional requirements, such as those disclosed in J. Akhaven, The Chemistry of Explosives, The Royal Society of Chemistry, 2004, ISBN 0-85404-640-2 and J.P. Agrawal, High Energy Materials - Propellants, Explosives and Pyrotechnics, WILEY-VCH Vcrlag GmbH & Co, Wcinhcim, 2010, ISBN 978-3-527- 32610-5, the disclosures of which are incorporated herein by reference.

[0062] The first or second propellant may comprise one or more energetic materials or mixtures thereof. Some examples include, cyclo-l,3,5-trimethylene-2,4,6-trinitramine (RDX),

cyclotetramethylenetetranitramine (HMX), 3-nitro-l ,2,4-triazol-5-one (NTO), nitroglycerine (NG), nitrocellulose (13 % N) (NC), 2,4,6,8,10,12-(hexanitro-hexaaza)tetracyclododecane (CL20 or HNIW), 1,3,3-trinitroazetidine (TNAZ), nitroguanidine (NQ), N-Guanyl urea-dinitramide (FOX-12), 1,1 -diamino- 2, 2 -dinitroethene (FOX-7), and ammonium dinitramide (ADN). Tire total amount of energetic material in the propellant grain 10 may vary, but is typically more than 30% by total weight of the propellant grain 10, such as 40-95%, or 45-90%. It is possible that ingredients in the propellant grain 10 perform multiple functions. For example, an energetic material can at the same time be a plasticiser or a binder.

[0063] hi addition to the energetic materials, the propellant grain 10 may further comprise a binder or binders. These binders may or may not be energetic. Suitable non-energetic binders include, but are not limited to, hydroxy tenninated polybutadiene (HTPB), carboxyl terminated polybutadiene (CTPB), hydroxyl terminated polyether (HTPE), polypropylene glycol (PPG), polyphenyl ether (PPE), and hydroxy-terminated caprolactone ether (HTCE) cellulose acetate butyrate (CAB), polyesters, polycaprolactone, polyvinyl acetate, polystyrene, polyethylene, polyisoprene, cellulose acetate and cellulose acetate propionate. Suitable energetic binders include, but are not limited to, nitrocellulose, polyvinylmtrate, polynitropolyphenyle, glycidyl azide polymer (GAP), poly(3-azidomethyl 3-methyl oxetane) (poly AMMO), poly(2-nitratomethyloxirane) (polyGLYN), poly(3-nitratomethyl-3- methyloxetane) (polyNIMMO), copolymer of glycidyl azide polymer and poly(bis(azidomethyl)oxetane (GAP-co-poly(BAMO)) The total amount of binder in the propellant grain 10 can be in the range of 5- 45% by total weight of the propellant grain 10, such as 10-40%, or 15-35%.

[0064] Further ingredients that may be present in the propellant grain 10 include plasticisers (energetic or non-energetic), antioxidants, bonding agents, linear burn rate modifiers, stabilisers, flash suppressants and anti-fouling agents. The total amount of these optional further ingredients may be up to 40% by total weight of the propellant grain 10, such as up to 30%. Plasticisers may be present in an amount of 0-40% by total weight of the propellant grain 10, such as 10-35%, or 15-30%. Antioxidants may be present in an amount of 0-7% by total weight of the propellant grain 10, such as 0-5%. Bonding agents may be present in an amount of 0-7% by total weight of the propellant grain 10, such as 0-5%. Linear bum rate modifiers may be present in an amount of 0-7% by total weight of the propellant grain 10, such as 0-5%. Stabilisers may be present in an amount of 0-7% by total weight of the propellant grain 10, such as 0-5%. Flash suppressants and anti-fouling agents may be present in an amount of 0-5% by total weight of the propellant grain 10. Suitably, at least one of the energetic materials can be dispersed as a solid material in a binder, such as in the form of small crystals. In an embodiment, all of the energetic materials are dispersed as a solid material in a binder.

[0065] The propellant grain 10 of the disclosure can have any desired shape/geometry. Typically useful shapes include a triangular prism or rounded triangular prism, a rectangular prism or rounded rectangular prism, a pentagonal prism or rounded pentagonal prism, a hexagonal prism or rounded hexagonal prism, an octagonal prism or rounded octagonal prism, a sphere, a spheroid, an ellipsoid, a cylinder, a rosette prism, a cube, a cuboid, a cone, a square-based pyramid, a rectangular-based pyramid, a pentagonal-based pyramid, a hexagonal-based pyramid, or an octagonal-based pyramid. In particular embodiments, the propellant grain 10 is in the form of a hexagonal prism, a rosette prism, a sphere or a cylinder.

[0066] A size (ie external dimensions) of the propellant grain 10 will typically be greater than about 0.5 mm in size. However, the exact size will depend on the application, for example, whether the complete propelling charge is a single monolithic propellant grain or a plurality of smaller propellant grains. For example, propellant grains for a small calibre gun may have a size of about 0.5-0.6, 0.6-0.71, 0.71-0.85, 0.85-1 or 1 -2 mm. Alternatively, propellant grains for a 120 mm tank round may be about 1-600 mm in length and will have a width that depends on the type of propellant grain, such as hexagonal prisms (length about 2-50 m and width about 1 -40 mm), cylinders (length about 2-50 and width about 1 -40 mm), cylinders in the fonn of sticks that extend for almost the full length of the cartridge case (length about 500-600mm and width about 5-30mm) or a single monolithic cylinder that fills the cartridge case (length about 500-600mm and width about 90-118mm). Alternatively, there may be a single monolithic propellant grain for a rocket motor that is between about 10 mm and about 25 m in length and about 10 mm and about 7 m in diameter.

[0067] In a third aspect, there is provided a complete propelling charge comprising at least one propellant grain of the first or second aspects. A complete propelling charge may comprise a single monolithic propellant gram of the type described herein, typically with external dimensions and geometry appropriate to the internal dimensions of the gun chamber or rocket motor case. A complete propelling charge may alternatively comprise a plurality of propellant grains of any external geometry as described herein. Advantageously, use of the monolithic charge configuration will allow a greater mass of propellant to be packed into the available gun chamber or rocket motor case volume (ie greater charge density).

[0068] In a fourth aspect, there is provided a method of manufacturing the solid propellant grain 10 of the first or second aspect or the charge of the third aspect. The solid propellant grain 10 of the present disclosure may be produced by any suitable manufacturing technique. The person skilled in the art would understand that additive manufacturing is particularly suitable. In certain embodiments, the method of manufacturing comprises forming the propellant grain using additive manufacturing. Additive manufacturing of propellants is known in the art, for example, in US Patent Application Publication No. 20180044257, B. Dolman, A. Hart, I. Johnston, C. Prior, Additive Manufacturing of Energetics for Next Generation Munitions, AP1CAM Conference, December 2017 (http://www.apicain2017.com.au/) and Whitmore, S. A. et at. 2016 Survey of Selected Additively Manufactured Propellant for Arc-Ignition of Hybrid Rockets, Mechanical and Aerospace Engineering Faculty Publications, Utah State University. The term "additive manufacturing'' as used in this application is meant to refer to a method of making a three- dimensional solid object from a digital model. Additive manufacturing is achieved using an additive process, where successive layers of material are laid down in different shapes. Additive manufacturing is sometimes known as "3D printing", or "additive layer manufacturing" (ALM). Such additive manufacturing suitably comprises layer by layer curing of liquid curable binder material. Suitable techniques for additive manufacturing using thermosets include, but are not limited to, stereolithography (SLA), digital light processing (DLP) and scan, spin and selectively photocure (3SP). A layer of liquid curable binder material is cured to fonn a solid polymer layer, after which a new liquid layer of curable binder layer is cured to fonn a subsequent solid polymer layer that adheres to the previously cured solid polymer layer. By curing the layers imagewise and repeating such curing of layers multiple times, a three- dimensional object can be manufactured. Suitable techniques for additive manufacturing using thermoplastics include, but are not limited to, fused deposition modelling (FDM) or Fused Filament Fabrication (FFF). Other additive manufacturing techniques that may also be suitable including, but not limited to, Selective Laser Sintering (SLS), Selective Laser Melting (SLM) or Binder Jetting (BJ) or Laminated Object Manufacturing (LOM). LOM involves adhering preformed layers together, where each layer may include portions of the ignition channels 16. Alternatively, the portions of the ignition channels 16 may be formed after the fonnation of the layers. The desired areas surrounding those ignition channels 16 may be masked and then the retardant layer 20 applied as a solution or vapour phase, before unmasking and adhering the layers together. These manufacturing techniques may allow manufacturing propellant grains 10 according to the disclosure.

[0069] In certain embodiments, the one or more energetic materials are dispersed in a liquid curable or plastically defonnable binder material. It may also be possible to use one or more liquid curable or plastically defonnable energetic material. In certain embodiments, the liquid curable binder material is cured by radiation (such as ultraviolet or visible radiation) or thermally. In particular embodiments, the liquid curable binder material is cured by ultraviolet radiation.

EXAMPLES

[0070] Example #1: A first propellant comprising the bulk of the propellant grain, with voids lined by a retardant layer (the retardant layer being a burn inhibitor).

[0071] Embodiment 1 ; gun application - may be produced using any suitable additive manufacturing technique

(i) First propellant: Polylactic acid (20% w/w) with RDX (76.7 % w/w), ethylene vinyl acetate (2 % w/w), potassium salts (0.8% w/w); calcium carbonate (0.5% w/w).

(ii)Retardant layer: Polylactic acid (100% w/w).

[0072] Embodiment 2; rocket application - may be produced using any suitable manufacturing technique

(i) First propellant: HTPB (10.3% w/w); A1 (16% w/w); AP (70% w/w); isophorone diisocyanate (1 % w/w); dicotyl adipate (3 % w/w); antioxidant (0.2% w/w).

(ii)Retardant layer: HTPB (14.3 % w/w); Ammonium sulphate (81% w/w); isophorone diisocyanate (2.0 % w/w); dicotyl adipate (2.5 % w/w); antioxidant (0.2% w/w). [0073] Example #2: A first propellant comprising the bulk of the propellant grain, with voids lined by a retardant layer (the retardant layer being a deterred version of the first propellant).

[0074] Embodiment 1; gun application - may be produced using any suitable additive manufacturing technique

(i) First propellant: Nitrocellulose (93.7 % w/w, 13.15% nitration level); potassium salts (1.3% w/w); calcium carbonate (1.0% w/w); diphenylamine (1% ), moisture (2.0% w/w); volatiles (1% w/w).

(ii)Retardant layer: Nitrocellulose (89.7 % w/w, 13.15% nitration level); potassium salts ( 1 % w/w); calcium carbonate (0.5% w/w); diphenylamine ( 0.8% ), 4-(4-Hydroxyphenyl)butan-2-one (5% w/w); moisture (2.0% w/w), volatiles (1% w/w).

[0075] Embodiment 2; gun application - may be produced using any suitable additive manufacturing technique

(i) First propellant: Nitrocellulose (93.7 % w/w, 13.15% nitration level); potassium salts (1.5% w/w); calcium carbonate (0.5% w/w); diphenylamine (1.1% ), moisture (1.5% w/w); volatiles (1.5% w/w).

(ii)Retardant layer: Nitrocellulose (90.9 % w/w, 13.15% nitration level); potassium salts (1.5 % w/w); calcium carbonate (0.5% w/w); diphenylamine ( 1.1% w/w ), dibutyl phthalate (3% w/w); moisture (1.5% w/w), volatiles (1.5% w/w).

[0076] Example #3: A first propellant comprising the bulk of the propellant grain, with voids lined by a retardant layer, and voids comprising with a fast-burning second propellant.

[0077] Embodiment 1 ; gun application - may be produced using any suitable additive manufacturing technique

(i) First propellant: 60% RDX; photosensitive initiator such as benzophenone ( 1 % w/w); pigment such as silica (1 % w/w); reactive monomer polyurethane acrylate (26% w/w); reactive diluent such as hexanediol diacrylate (12% w/w).

(ii)Relardant layer: Photosensitive initiator such as benzophenone (2 % w/w); pigment such as silica (3% w/w); reactive monomer polyurethane acrylate (65% w/w); reactive diluent such as hexanediol di acrylate (30% w/w). (iii) Second propellant: 80% RDX; photosensitive initiator such as benzophenone (0.5 % w/w); pigment such as silica (1% w/w); reactive monomer polyurethane acrylate (11% w/w); reactive diluent such as hexanediol diacrylate (7.5% w/w).

[0078] Embodiment 2; gun application - may be produced using any suitable additive manufacturing technique

(i) First propellant: NC (94% w/w, 12.6% nitration level); potassium sulphate (1% w/w); Bismuth subcarbonate (1 % w/w); Ethyl centralite (2% w/w); acetyl triethyl citrate (2% w/w).

(ii)Retardant layer: Nitrocellulose (28.5% w/w, 12.0% nitration level); diethylene glycol dinitrate ( 10% w/w); nitroglycerine (10% w/w); nitro guanidine (48% w/w); ethyl centralite (1.5% w/w); potassium nitrate (2.0 % w/w)

(iii) Second propellant: NC (63% w/w, 13.25% nitration level); diethylene glycol dinitrate (25% w/w); nitroglycerine (10% w/w); Akardite IT ( 1% w/w); potassium nitrate (1 % w/w).

[0079] Example #4: Interior ballistics modelling results showing gun chamber pressure and projectile muzzle velocity'

[0080] Examples of the performance benefits afforded by the present disclosure are demonstrated in Figure 12. Figure 12 presents interior ballistics modelling results showing gun chamber pressure and projectile muzzle velocity, for an example large-calibre gun and projectile (refer to B. Dolman, A. Hart, I. Johnston, C. Prior, Additive Manufacturing of Energetics for Next Generation Munitions, APICAM Conference, December 2017 for full details). The baseline result (denoted with a diamond symbol) shows modelled ballistic perfonnance when the propelling charge is comprised of conventional 7-perforated cylindrical propellant grains. All other results (denoted by cross, square, and triangle symbols) correspond to modelled ballistic performance for propellant grains of the present disclosure, whereby the bum initiation regions 14 of parent ignition channels 30 have been arranged in a repeating face-centered- cubic pattern within a monolithic propellant block, burn initiation regions 14 of child ignition channels 32 have been arranged interstitially between the parent bum initiation regions 14, and burn initiation region spacing and timing have been differently configured to achieve a variety of perfonnance benefits.

[0081] The“Equal Propellant Mass” propellant grain configuration (cross symbols) affords 10% greater muzzle velocity, while maintaining propellant mass and maximum gun chamber pressure at levels equal to that of the conventional propellant grain. The“Equal Fill Volume” propellant grain configuration (square symbols) affords a 22.5% higher muzzle velocity, by exploiting the higher propelling charge bulk density which allows packing of the gun chamber w ith additional propellant mass, while simultaneously maintaining maximum gun chamber pressure at a level equal to that of the conventional propellant grain. The“Equal Muzzle Velocity” propellant grain configuration (triangle symbols) affords an equal muzzle velocity as that of the conventional propellant, while reducing the maximum gun chamber pressure by 35%, thereby affording the use of cheaper and lighter gun barrels without performance loss. It is noted that the interior ballistics modelling results are not exact, and the modelling neglects or approximates processes of secondary importance such as flame propagation through ignition channel voids 18. Accordingly, the present disclosure provides different propellant grain configurations for achieving different desired performance characteristics.

[0082] The present disclosure finds application in the development of progressive propellants for a variety of applications, such as loads for handguns, rifles, mortars, tanks, artillery, ships, aircraft as well as rocket motors.

[0083] The present disclosure provides particular advantages which are achieved by the particular void structure and placement of the retardant layer to propagate a burn from the throat of an ignition channel to a burn initiation region at opposing distal end of the ignition channel. The repetition of the void structure in a network with the precise positioning of bum initiation regions, ignitions channels and throats, means that it is possible to achieve near-maximum practical progress! vity.

[0084] Alternative or different aspects, embodiments and/or particular features of the present disclosure are not intended to be limited to the context in which they are described, but may be combined as appropriate.

[0085] Throughout the specification and the claims that follow, unless the context requires otherwise, the words“comprise” and“include” and variations such as“comprising” and“including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0086] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art fonns part of the common general knowledge.

[0087] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the disclosure as set forth and defined by the following claims.