Abstract:
Disclosed is a baffle ( 40 ) for locating in a tank ( 16 ) for containing liquid. The baffle ( 40 ) comprises: a baffle wall enclosing an internal cavity ( 48 ); and one or more openings ( 50 ) in the baffle wall configured to permit the flow of a fluid between the internal cavity ( 48 ) of the baffle ( 40 ) and a volume external to the baffle wall. The baffle wall comprises an outer wall ( 42 ), and an inner wall ( 44 ) located within the outer wall ( 42 ). The outer and inner walls ( 42, 44 ) each comprise one or more openings. Each of the openings in the outer wall ( 42 ) is attached to a respective opening in the inner wall ( 44 ) via a respective opening side wall. The outer and inner walls ( 42, 44 ) are spaced apart to define therebetween a chamber ( 46 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to baffles for locating in tanks for containing liquid. 
       BACKGROUND 
       [0002]    A high speed projectile on impact with and penetration into a liquid containing tank generates very high pressure in the liquid. This phenomenon, known as hydrodynamic ram, typically includes the generation of shock waves and subsequent pressure pulses in the liquid. These pressures combined with the penetration damage from the projectile, can cause damage to the tank structure and frequently are the cause of catastrophic failure of the tank. The hydrodynamic ram pressure pulses are intense but of short duration which propagate through the liquid in the tank. 
         [0003]    There is thus a need for means for reducing hydrodynamic ram pressure in the liquid in such a tank and for a generally improved tank which has an improved ability to sustain projectile impact without catastrophic failure. 
       SUMMARY OF THE INVENTION 
       [0004]    In a first aspect, the present invention provides a baffle for locating in a tank for containing liquid. The baffle comprises a baffle wall enclosing an internal cavity, and one or more openings in the baffle wall configured to permit the flow of a fluid between the internal cavity of the baffle and a volume external to the baffle wall. The baffle wall comprises a first portion providing an outer wall, and a second portion located within the first portion and providing an inner wall. The first portion comprises one or more openings. The second portion comprises one or more openings. The number of openings in the second portion are equal to the number of openings in the first portion. Each of the openings in the first portion is attached to a respective opening in the second portion via a respective opening side wall. The first and second portions are spaced apart to define therebetween a chamber. 
         [0005]    The chamber may be a sealed chamber. 
         [0006]    An outer surface defined by the first portion may be substantially spherical in shape. 
         [0007]    The chamber may be filled with a compressible gas or gaseous mixture. 
         [0008]    The baffle may have been produced using an Additive Manufacturing process. 
         [0009]    The baffle may comprise multiple component sections that have been produced and subsequently attached together. 
         [0010]    The baffle may be made of a material selected from the group of materials consisting of carbon fibre composite and plastic. 
         [0011]    The baffle may have an external diameter less than or equal to 10 cm. 
         [0012]    In a further aspect, the present invention provides a liquid storage tank and baffle system comprising a tank for containing a liquid and enclosing a liquid storage space, and one or more baffles located within the liquid storage space. The one or more baffles are in accordance with any of the above aspects. 
         [0013]    The first portion and the second portion may be sufficiently strong to resist at least the maximum &amp; minimum hydrostatic pressure of a liquid in the tank. The chamber may have a volume sufficient to allow a shock wave or waves in the liquid in the tank resulting from compression of the liquid by impact of a projectile on the tank and thus on the liquid to be reduced by expansion of the compressed liquid into the chamber. 
         [0014]    The chamber may contain a material having a density sufficiently different from the density of a liquid in the tank to provide substantially total reflection within the baffle of the shock wave or waves impinging on the baffle thereby to reduce the hydraulic ram pressure in the liquid. 
         [0015]    The one or more baffles may substantially fill the liquid storage space within the tank. 
         [0016]    The total cavity volume of the baffles in the liquid storage space may be less than or equal to 15% by volume of the liquid storage space volume. 
         [0017]    The tank may be an aircraft fuel tank. 
         [0018]    In a further aspect, the present invention provides a vehicle (e.g. an aircraft) comprising a liquid storage tank and baffle system in accordance with any of the above aspects. 
         [0019]    In a further aspect, the present invention provides a liquid storage tank and baffle system comprising a tank for containing a liquid, said tank enclosing a liquid storage space, and a plurality of baffles located within the liquid storage space. Each baffle comprises a baffle wall that encloses a respective internal cavity. Each baffle further comprises a one or more openings in the baffle wall of that baffle such that a fluid may flow between the internal cavity of that baffle and the liquid storage space. 
         [0020]    An outer surface defined by the baffle wall may be substantially spherical in shape (i.e. the baffles may be “baffle balls”). 
         [0021]    A baffle wall may comprise a first portion providing an outer wall, and a second portion located within the first portion and providing an inner wall. The first portion may comprise one or more openings. The second portion may comprise one or more openings, the number of openings in the first portion being equal to the number of openings in the second portion. Each of the openings in the first portion may be attached to a respective opening in the second portion via a respective opening side wall. The first and second portions may be spaced apart to define therebetween at least one sealed chamber. 
         [0022]    The first portion and the second portion may be sufficiently strong to resist at least the maximum &amp; minimum hydrostatic pressure of a liquid in the tank. The at least one chamber may have a volume sufficient to allow a shock wave or waves in the liquid in the tank resulting from compression of the liquid by impact of a projectile on the tank and thus on the liquid to be reduced by expansion of the compressed liquid into the chamber. 
         [0023]    The at least one chamber may contain a material having a density sufficiently different from the density of a liquid in the tank to provide substantially total reflection within the baffle of the shock wave or waves impinging on the baffle thereby to reduce the hydraulic ram pressure in the liquid. 
         [0024]    The chamber may be filled with a compressible gas or gaseous mixture 
         [0025]    The baffles may be objects that have been produced using an Additive Manufacturing process. 
         [0026]    Each baffle may comprise multiple component sections that have been produced and subsequently attached together. 
         [0027]    Each baffle may be made of a material selected from the group of materials consisting of carbon fibre composite and plastic. 
         [0028]    Each baffle may have an external diameter less than or equal to 10 cm. 
         [0029]    The baffles may substantially fill the liquid storage space within the tank. 
         [0030]    The total cavity volume of the baffles in the liquid storage space may be less than or equal to 15% by volume of the liquid storage space volume. 
         [0031]    The tank may be an aircraft fuel tank. 
         [0032]    In a further aspect, the present invention provides a vehicle comprising a liquid storage tank and baffle system in accordance with the preceding aspect. 
         [0033]    In a further aspect, the present invention provides a baffle for locating in a tank for containing liquid. The baffle comprises a baffle wall enclosing an internal cavity, and a one or more openings in the baffle wall configured to permit the flow of a fluid between the internal cavity of the baffle and a volume external to the baffle wall. The baffle wall comprises a first portion providing an outer wall, and a second portion located within the first portion and providing an inner wall. The first portion comprises one or more openings. The second portion comprises one or more openings, the number of openings in the second portion being equal to the number of openings in the first portion. Each of the openings in the first portion is attached to a respective opening in the second portion via a respective opening side wall. The first and second portions are spaced apart to define therebetween at least one sealed chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a schematic illustration (not to scale) of an exploded view of an example aircraft wing in which an embodiment of a baffle is implemented; 
           [0035]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through a fuel tank in which an embodiment of a hydrodynamic ram reducing baffle is implemented; 
           [0036]      FIG. 3  is a schematic illustration (not to scale) showing a hydrodynamic ram reducing baffle; 
           [0037]      FIG. 4  is a schematic illustration (not to scale) illustrating effects of a projectile impacting with an external surface of the fuel tank of  FIG. 2 ; and 
           [0038]      FIG. 5  is a schematic illustration (not to scale) showing a cross section through a further hydrodynamic ram reducing baffle. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    In the following description, like reference numerals refer to like elements. 
         [0040]    The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein. Structural material types and methods of construction provided herein are examples only. 
         [0041]    It will be appreciated that relative terms such as top and bottom, upper and lower, and so on, are used merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented. 
         [0042]      FIG. 1  is a schematic illustration (not to scale) of an exploded view of an example aircraft wing  2  in which an embodiment of a hydrodynamic ram reducing baffle is implemented. 
         [0043]    The aircraft wing  2  comprises a substructure  4  comprising a plurality of spars  6  and ribs  8 . The spars  6  are spaced apart from one another and are aligned along the length of the aircraft wing  2 . The spars  6  are coupled together by the spaced apart ribs  8  which are substantially perpendicular to the spars  6 . The spars  6  and ribs  8  are connected together by fasteners (not shown in the Figures). The spars  6  and ribs  8  are made of carbon fibre composite (CFC) material, i.e. a composite material comprising a polymer matrix reinforced with carbon fibres. In other examples, the spars  6  and ribs  8  are made of a different appropriate material, for example, aluminium. 
         [0044]    The aircraft wing  2  further comprises external skins, namely an upper skin  10  and a lower skin  12 . The upper skin  10  comprises a plurality of panels made of CFC material. The upper skin  10  is attached to an upper surface of the substructure  4  by fasteners (not shown in the Figures). The lower skin  12  comprises a plurality of panels made of CFC material. The lower skin  12  is attached to a lower surface of the substructure  4  by fasteners (not shown in the Figures). The external skin  10 ,  12  may each be, for example, 8 mm thick. 
         [0045]    When the substructure  4  and the external skins  10 ,  12  are attached together (and, for example, bonded with a sealant), a cavity defined by the substructure  4  and skins  10 ,  12  is formed. Such a cavity is used as a fuel tank for storing aircraft fuel and is indicated in  FIG. 1  by the reference numeral  14 . The fuel tank is described in more detail later below with reference to  FIG. 2 . 
         [0046]    The aircraft wing  2  further comprises a leading edge structure, a trailing edge structure and a wing tip structure, which are not shown in  FIG. 1  for reasons of clarity. 
         [0047]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in the aircraft wing  2 . 
         [0048]    In this embodiment, the outer walls of the fuel tank  16  are provided by spars  6 , ribs  8 , and the upper and lower skins  10 ,  12 . Aircraft fuel is stored in the cavity  14  defined by the fuel tank outer walls. 
         [0049]    In this embodiment, the fuel tank  16  comprises hydrodynamic ram reducing baffles  20 . 
         [0050]      FIG. 3  is a schematic illustration (not to scale) showing a perspective view of a baffle  20 . Preferably, the baffles  20  are substantially identical to each other. 
         [0051]    In this embodiment, each baffle  20  is a baffle ball being substantially spherical, although other shapes are within the disclosure of this invention. 
         [0052]    Preferably, the outer diameter of each baffle  20  is less than 10 cm. More preferably, the outer diameter of each baffle  20  is less than 5 cm. More preferably, the outer diameter of each baffle  20  is between 3 cm and 5 cm e.g. 4 cm. 
         [0053]    In this embodiment, each baffle  20  is hollow and comprises an outer skin  20   a  enclosing an internal cavity. 
         [0054]    In this embodiment, the outer skins  20   a  of the baffles  20  are relatively thin. For example, the thickness of the outer skin  20   a  may be less than 3 mm. In other embodiments the thickness of the outer skin  20   a  is a different appropriate value. In some embodiments, the thickness of the outer skin  20   a  is between 0.25 mm and 1 mm. In some embodiments, the thickness of the outer skin  20   a  is between 1 mm and 3 mm. Preferably, the thicknesses of the outer skins  20   a  of the baffles  20  are such that the baffles  20  occupy less than 15% of the total internal volume (i.e. capacity) of the fuel tank  16 . In other embodiments, the baffles  20  occupy a different proportion of the fuel tank capacity. 
         [0055]    Each baffle  20  comprises a plurality of openings  20   b  in its outer skin  20   a  such that the internal cavity of the baffle  20  is in fluid communication with the volume outside the outer skin  20   a  of the baffle  20 . Thus, the liquid in the fuel tank  16  tends to be able to move freely in and out of the baffles  20 . Advantageously, the openings  20   b  tend not to detrimentally affect the structural integrity of the baffles  20  to a significant degree. In some embodiments, each baffle  20  includes eight openings  20   b.  However, in other embodiments, one or more of the baffles  20  include a different number of openings. In some embodiments, the diameter of each opening  20   b  is approximately 10%-20% of the outer diameter of the baffle  20 , however openings having different diameters may be implemented. 
         [0056]    In some embodiments, one or more of the baffles  20  comprise one or more support ribs integral with their internal or external surface to provide additional structural stability. 
         [0057]    In this embodiment, the baffles  20  are made of a strong, tough, non-reactive material, for example CFC or a plastic such as high density polyethylene. Preferably, the baffles  20  are made of a material that is fuel resistant at high temperatures. In this embodiment, each baffle  20  is formed as a single integral unit. The baffles  20  may be produced using any appropriate process, such as moulding or an Additive Manufacturing process. However, in other embodiments, the baffles  20  are formed in multiple sections, e.g. as half-sphere shapes which may be subsequently joined together by any appropriate joining process. 
         [0058]    In this embodiment, the baffles  20  are not fixedly attached together. In other words, the baffles  20  are arranged in the fuel tank  16  such that they may move, at least to some extent, with respect to one another. However, in other embodiments, the baffles  20  are attached to one another so that the relative positions of the baffles  20  are fixed. 
         [0059]    In this embodiment, the baffles  20  are not fixedly attached to the spars  6 . Thus, the baffles  20  are free to move, at least to some extent, within the fuel tank  16  relative to the spars  6 . Also, in this embodiment, the baffles  20  are not fixedly attached to the ribs  8 . Thus, the baffles  20  are free to move, at least to some extent, within the fuel tank  16  relative to the ribs  8 . Also, in this embodiment, the baffles  20  are not fixedly attached to the upper skin  10 . Thus, the baffles  20  are free to move, at least to some extent, within the fuel tank  16  relative to the upper skin  10 . Also, in this embodiment, the baffles  20  are not fixedly attached to the lower skin  12 . Thus, the baffles  20  are free to move, at least to some extent, within the fuel tank  16  relative to the lower skin  12 . 
         [0060]    Preferably, the number and arrangement of the baffles  20  within the fuel tank  16  is such that there is insufficient space in the fuel tank  16  in which to place a further baffle  20 . In other words, preferably the fuel tank  16  is “filled” with baffles  20  so that a further baffle does not fit into the fuel tank  16 . In other words, preferably the baffles  20  fill the entire liquid volume space in the fuel tank  16 . 
         [0061]    As will now be described in more detail, the baffles  20  are operable to reduce hydrodynamic ram pressure in the fuel contained within the fuel tank  16  resulting from impact of a projectile with an external surface of the fuel tank  16 . 
         [0062]      FIG. 4  is a schematic illustration (not to scale) illustrating effects of a projectile  24  impacting with the lower skin  12  of the fuel tank  16 . The path of the projectile through the lower skin  12  is indicated in  FIG. 3  by the reference numeral  26 . 
         [0063]    The projectile  24  may be any appropriate projectile or foreign object such as a bullet, warhead fragment, a vehicle part, a rock, a maintenance tool, hail, ice, a bolt, etc. An example projectile has a weight of approximately 3.5 g, is substantially spherical in shape having a diameter of approximately 9.5 mm, and travels with a velocity of 1500 m/s. A further example projectile is a 44 g 12.5 mm bullet that travels with a velocity of 500 m/s. 
         [0064]    In this example, the projectile  24  initially impacts with an external surface of the lower skin  12  and travels through the lower skin  12 . The projectile  24  causes high strain rate shear damage to the lower skin  12  resulting in a hole in the lower skin  12  approximately the size of the projectile  24 . 
         [0065]    In this example, after passing through the lower skin  12 , the projectile  24  impacts with and travels through (i.e. pierces or penetrates) multiple baffles  20  (i.e. the outer skins  20   a  of multiple baffles  20 ). In other examples, the projectile  24  may impact with only a single baffle  20 . In other examples, the projectile  24  does not pierce an outer skin  20   a  of a baffle  20  or only pierces a single outer skin  20   a  of a baffle  20 . 
         [0066]    The projectile impacting with a baffle  20  tends to cause that baffle  20  to deflect and accelerate within the fluid at least to some extent. Also, the projectile  24  impacting with a baffle  20  tends to cause that baffle  20  to move within the fluid in the fuel tank  16  with respect to the walls of the fuel tank  16 . This in turn tends to cause deflection and/or movement of multiple other baffles  20  within the fuel tank  16 , for example, due to the impinged upon baffle  20  being in contact with multiple other baffles  20 . Thus, impact kinetic energy of the projectile  24  tends to be used to deflect and accelerate the baffles  20  through the fluid in the fuel tank  16 , thereby reducing the energy introduced into the fluid. 
         [0067]    Moving the baffles  20  through the fluid tends to provide that, in effect, the projectile  24  experiences a greater drag force when moving through the fluid in the fuel tank  16  compared to that that would be experienced were the baffles  20  not present. Thus, the passage of the projectile  24  through the fluid in the fuel tank  16  tends to be retarded. The retardation of the passage of the projectile  24  through the fluid tends to decrease the likelihood of the projectile  24  impacting with the upper skin  10 . Thus, the likelihood of a hole being formed in the upper skin  10  tends to be reduced. Furthermore, the increase in drag on the projectile  24  tends to mean that a greater portion of the impact energy is absorbed by the fluid in the fuel tank  16 . Thus, forces exerted on the walls of the fuel tank  16  tend to be reduced. 
         [0068]    Also, in this example, when the projectile  24  travels through the outer skin  20   a  of a baffle  20 , impact energy of the projectile  24  tends to be used to pierce that outer skin  20   a.  Thus, the energy introduced into the fluid by the projectile  24  tends to be reduced, and the passage of the projectile  24  into the fluid is retarded at least to some extent. 
         [0069]    At least some of the impact energy of the projectile  24  tends to be absorbed by the baffles  20  and therefore not transferred to the aircraft substructure  4 . 
         [0070]    In this example, on piercing baffle outer skin  20   a,  the projectile  24  impacts with the fluid within that baffle  20 , thereby generating one or more high pressure shock waves  30  within the fluid within that baffle  20 . In this example, a respective shockwave  30  or set of shockwaves  30  may be generated within the fluid within each baffle  20  that is penetrated by the projectile  24 . The outer skins  20   a  of the baffles  20  tend to reflect incident shock waves  30  at least to some extent. Also, the walls of the baffles  20  tend to be relatively poor transmitters of impinging shock waves  30 . Thus, each baffle  20  tends to restrain or retain shockwaves  30  generated therein at least to some extent. Through multiple shockwave reflections in the fuel tank  16  and the attenuation properties of the liquid, the amplitude of the shock waves  30  tends to be reduced and consequently the pressure experienced by the substructure  4  tends to be diminished by the presence of the baffles  20 . 
         [0071]    Also, the shock waves  30  generated within the baffles  20  tend to be of lower energy than a shock wave or shock waves  30  experienced in a conventional system due to at least some of the impact energy of the projectile  24  being absorbed by the baffles  20 . In addition, each baffle tends to limit the distance over which the shockwave  30  can develop. Furthermore, the baffles  20  tend to disrupt the shockwaves  30  travelling through the fluid in the fuel tank  16  and thereby tend to insulate the upper and lower skins  10 ,  12  at least to some extent. Thus, pressures resulting from the shock waves  30  exerted on the walls of the fuel tank  16  tend to be lower than the shock wave pressures experienced in conventional fuel tanks. Thus, the likelihood of damage to the walls of the fuel tank  16  (e.g. decoupling of the external skin  10 ,  12  from the spars  6  or ribs  8 ) tends to be reduced. 
         [0072]    In this example, as the projectile  24  passes through the fluid in the fuel tank  16 , a cavitation “wake” may form behind the projectile  24 , i.e. a region of low pressure (e.g. a vapour or a vacuum) may form in the wake of the projectile  24 . This causes a fluid displacement and an increase in the pressure of the fluid in the fuel tank  16 . The baffles  20  tend to prevent or oppose the formation of a single large cavity in the wake of the projectile  24 , i.e. the baffles  20  tend to disrupt cavity formation. Instead, multiple smaller cavities may be formed in the fluid within each of the baffles  20  through which the projectile  24  passes. Thus, the increased fluid pressure resulting from cavitation caused by the projectile  24  tends to be constrained within each baffle and decreased compared to conventional systems. This tends to be facilitated by the passage of the projectile  24  through the fuel tank  16  being retarded at least to some degree by the baffles  20 . As a result, pressures resulting from cavitation exerted on the walls of the fuel tank  16  tend to be lower than in conventional systems. Consequently, the likelihood of damage to the walls of the fuels tank  16  (e.g. decoupling of the external skin  10 ,  12  from the spars  6  or ribs  8 ) tends to be reduced. 
         [0073]    Advantageously, the baffles  20  are located in the fuel tank  16  so that a shock wave or waves  30  resulting from compression of the liquid in the tank resulting from impact of the projectile  24  on the external surface of the fuel tank  16  impinges on at least one of the baffles  20  and so that the shock wave or waves  30  interact with at least one baffle  20  before impinging on the tank external boundary surfaces. 
         [0074]    An advantage provided by the above described baffle is that hydrodynamic ram damage to a fuel tank caused by an object impacting with an external surface of the fuel tank tends to be reduced or eliminated. Hydrodynamic pressures and their associated structural responses tend to be reduced or eliminated. Thus, the likelihood of catastrophic failure of the fuel tank structure and corresponding aircraft loss tends to be reduced or eliminated. 
         [0075]    The above described baffle advantageously tends to be relative easy and cheap to manufacture. 
         [0076]    The above described baffle tends to be relatively easy to retrofit to existing aircraft fuel tanks. 
         [0077]    The above described baffle tends to provide protection against hydrodynamic ram damage whilst occupying a relatively small amount of the fuel tank&#39;s capacity. 
         [0078]    The above described baffle tends to be relatively lightweight so as not to be a significant burden to the aircraft. 
         [0079]    In the above embodiments, the baffles are implemented in an aircraft wing fuel tank. However, in other embodiments, the baffles are used in a different type of container for containing fluid. In some embodiment, one or more walls of the container may be made of a different material to that described above. 
         [0080]    In the above embodiments, the outer skins of the baffles are a single relatively thin layer of material. However, in other embodiments, the outer skins of the baffles are of a different construction, for example, as will now be described. 
         [0081]      FIG. 5  is a schematic illustration (not to scale) showing a cross section through a further embodiment of hydrodynamic reducing baffle, hereinafter referred to as the “further baffle”) and indicated by the reference numeral  40 . 
         [0082]    In this further embodiment, the outer skin of the further baffle  40  comprises an outer wall  42  and an inner wall  44  which are spaced apart to define therebetween at least one sealed chamber  46 . The outer skin of the further baffle  40  encloses an internal cavity  48 . The outer skin of the further baffle  40  comprises a plurality of openings  50  therethrough such that the internal cavity  48  of the further baffle  40  is in fluid communication with the volume outside the outer skin of the further baffle  40 . Thus, the liquid in the fuel tank  16  tends to be able to move freely in and out of the baffles  20 . 
         [0083]    In this further embodiment, the outer wall  42  is substantially spherical in shape having a substantially circular cross section and within which is located the inner wall  44  and the chamber  46 . In other alternative embodiments the cross section may be an alternative shape. 
         [0084]    In this further embodiment, the inner wall  44  is substantially spherical in shape having a substantially circular cross section. In other alternative embodiments the cross section may be an alternative shape. 
         [0085]    The inner wall  44  is located within the outer wall  42 . The outer wall  42  and the inner wall  44  may be connected together at the openings  50  by opening walls. 
         [0086]    In this further embodiment, the or each chamber  46  contains a compressible gas or gaseous mixture such as air at reduced, atmospheric, or enhanced pressure. In some embodiments, the or each chamber  46  contains a different material, such as a liquid or a solid instead of or in addition to the compressible gas or gaseous mixture. For example, in some embodiments, the or each chamber  46  contains a compressible or crushable foam. 
         [0087]    The external diameter of the further baffle  40  may be the same as that of the baffle  20  which is described in more detail above with reference to  FIGS. 2 and 3 . 
         [0088]    The material from which the outer and inner walls  42 ,  44  of the further baffle  40  are made may be the same as that from which the outer skin  20   a  of the baffle  20  is made. 
         [0089]    The arrangement of the further baffles  40  within the fuel tank  16  may be the same as that of the baffles  20 , which are described in more detail above with reference to  FIGS. 2 to 4 . 
         [0090]    In this further embodiment, the walls  42 ,  44  of the further baffles  40  are sufficiently strong to withstand the pressure of the gas or gaseous material contained in the chamber  46  and are spaced apart in each further baffle  40  by an amount sufficient to provide at least one chamber  46  with a volume sufficient to allow a shock wave or waves in the liquid in the fuel tank  16 , resulting from compression of the liquid by impact of a projectile on the tank external surface and thus in the liquid, to be reduced by expansion of the compressed liquid into the chamber volume, thereby to reduce the hydraulic ram pressure in the liquid in the fuel tank  16 . Additionally, the gas or gaseous mixture in the or each chamber  46  has a density sufficiently different from the density of the liquid in the fuel tank  16  to provide substantially total reflection within the further baffle  40  of the shock wave or waves impinging on that further baffle  40  thereby to reduce the hydraulic ram pressure in the liquid in the fuel tank  16 . 
         [0091]    Additionally, the walls  42 ,  44  of the further baffles  40  are sufficiently strong to withstand the maximum and minimum hydrostatic pressure of the liquid in the fuel tank  16  at least up to maximum aircraft manoeuvre rate. 
         [0092]    In this further embodiment, the further baffles  40  are placed in the fuel tank  16  such that a shock pulse generated by a projectile impacting the tank walls will impinge on at least one further baffle  40  before impinging upon an opposing tank wall. In defeating the hydraulic ram pressure the further baffles  40  serve at least two functions. Firstly energy from the hydraulic ram shock wave tends to be absorbed by expansion of the liquid into the space created by irreversible or reversible compression of the further baffle  40 , i.e. movement of the outer wall  42  and/or the inner wall  44  of a further baffle  40  into the chamber  46  of that further baffle  40 . Secondly, each further baffle  40  due to the large shock impedance mismatch between the further baffle  40  and the liquid in the fuel tank  16  behaves as a good shock wave reflector and a poor shock wave transmitter. Through multiple shock wave reflections in the fuel tank  16  and the attenuation properties of the liquid, the shock wave amplitude is reduced and consequently the pressure experienced by the substructure  4  is diminished. 
         [0093]    In this embodiment, the further baffle  40  is formed as a single integral unit. The further baffle  20  may be produced using any appropriate process. For example, an Additive Manufacturing (AM) process (also known as Additive Layer Manufacture (ALM), 3D printing, etc.) may be used. Certain AM processes, such as Laser Blown Powder and Laser Wire Feed process, tends to be particularly well suited for the production of relatively complex objects such as the “double-skinned” further baffle  40 . Typically, such processes include providing material (e.g. plastic) in the form of a powder or a wire and using a powerful heat source such as a laser beam, Electron Beam (EB) or an electric or plasma welding arc, to melt an amount of that material and deposit the melted material as a bead (e.g. on a base plate of a work piece). Subsequent layers/beads are then built up upon preceding layers/beads. 
         [0094]    However, in other embodiments, the further baffles  40  are formed in multiple sections, e.g. as half-sphere shapes which may be subsequently joined together by any appropriate joining process. The multiple sections may be produced using any appropriate process. For example, an AM process (such as a Laser Powder Bed process) or a moulding process may be used. 
         [0095]    An advantage provided by the “double-skinned” further baffles  40  is that the further baffles  40  tend to be equally compressible by shock waves impinging on the further baffles  40  from different directions. This tends to be at least partially due to the spherical symmetry of the further baffles  40 . In other words, the further baffles  40  being substantially spherical in shape and exhibiting spherical symmetry tend to provide that the further baffles  40  are equally compressible from all directions. Thus, the further baffles  40  tend to be equally effective irrespective of the shock wave direction, i.e. irrespective of which surface of the fuel tank  16  is impacted by the projectile  24 . 
         [0096]    In the above embodiments, the baffles are implemented in an aircraft wing fuel tank. However, in other embodiments, the baffles are used in a different type of container for containing fluid. In some embodiments, one or more walls of the container may be made of a different material to that described above.