Abstract:
BAFFLE Disclosed is a baffle ( 40 ) for locating in a tank ( 16 ) for containing liquid (e.g. an aircraft fuel tank). The baffle ( 40 ) is a tubular member through which a liquid may flow. The baffle ( 40 ) comprises: a first tubular portion ( 42 ) providing a tubular outer wall, and a second tubular portion ( 44 ) located within the first tubular portion ( 42 ) and providing a tubular inner wall. The first and second tubular portions ( 42, 44 ) are substantially parallel. The first and second tubular portions ( 42, 44 ) are spaced apart to define a chamber ( 46 )therebetween. The baffle ( 42, 44 ) further comprises radial side walls between the first and second tubular portions ( 42, 44 ) such that the chamber ( 46 ) is a sealed chamber. The chamber ( 46 ) may be filled with a compressible gas or gaseous mixture.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to baffles for use in liquid storage tanks. 
       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 is a tubular member through which a liquid may flow. The baffle comprises a first tubular portion providing a tubular outer wall, and a second tubular portion located within the first tubular portion and providing a tubular inner wall. The first and second tubular portions are substantially parallel to each other. The first and second tubular portions are spaced apart to define a chamber between the first and second tubular portions (e.g. an annular chamber). The baffle further comprises radial side walls between the first and second tubular portions such that the chamber is a sealed chamber. The radial side walls may space apart the first and second tubular portions 
         [0005]    The chamber may be filled with a compressible gas or gaseous mixture. 
         [0006]    The radial side walls may attach together the first and second tubular portions at or proximate to openings of the first and second tubular portions (i.e. at or proximate to ends of the baffle). 
         [0007]    The baffle may be made of a carbon fibre composite material or a plastic. 
         [0008]    The baffle may have a substantially circular cross section. An external diameter of the baffle may be in the range  10 mm to  100 mm. The baffle may be a substantially straight tubular member. 
         [0009]    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 one or more baffles located within the liquid storage space. Each baffle is in accordance with any of the above aspects. 
         [0010]    The baffles may be substantially straight members. The baffles may be arranged in the liquid storage space such that the baffles are substantially parallel to one another. 
         [0011]    The baffles may substantially fill the liquid storage space within the tank. 
         [0012]    The system may further comprise one or more spacers configured to space apart the ends of the one or more of the baffles from the tank. 
         [0013]    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. 
         [0014]    The tank may be an aircraft fuel tank. The tank may be an aircraft wing fuel tank located in an aircraft wing. The one or more baffles may be aligned along a length of the aircraft wing (e.g. parallel to a lateral axis of the aircraft). Alternatively, the one or more baffles may be substantially perpendicular to a length of the aircraft wing (e.g. parallel to a longitudinal axis of the aircraft). 
         [0015]    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. 
         [0016]    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 being a tubular member through which a liquid within the tank may flow. 
         [0017]    One or more of the baffles may be made of a material selected from the group of materials consisting of carbon fibre composite and plastic. 
         [0018]    One or more of the baffles may have a substantially circular cross section and an external diameter in the range 10 mm to 100 mm. 
         [0019]    One or more of the baffles may be a substantially straight tubular member. The baffles may be arranged in the liquid storage space such that the baffles are substantially parallel to one another. 
         [0020]    The baffles may substantially fill the liquid storage space within the tank. 
         [0021]    The system may further comprise one or more spacers configured to retain one or more of the baffles and the tank in a spaced apart relation. 
         [0022]    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. 
         [0023]    One or more of the baffles may comprise a first tubular member providing an outer wall, and a second tubular member located within the first tubular member and providing an inner wall. The first and second tubular members may be spaced apart to define therebetween at least one chamber. 
         [0024]    The first tubular member and the second tubular member may be sufficiently strong to resist at least the maximum and minimum hydrostatic pressures 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. 
         [0025]    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. 
         [0026]    The tank may be an aircraft fuel tank. 
         [0027]    In a further aspect, the present invention provides a vehicle (for example, an aircraft) comprising a liquid storage tank and baffle system in accordance with the preceding aspect. 
         [0028]    In a further aspect, the present invention provides a baffle for locating in a tank for containing liquid. The baffle comprises a first tubular member providing an outer wall, and a second tubular member located within the first tubular member and providing an inner wall. The first and second tubular members are spaced apart to define therebetween at least one chamber. 
         [0029]    The first tubular member and the second tubular member may be sufficiently strong to resist at least the maximum and minimum hydrostatic pressures 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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      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; 
           [0031]      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; 
           [0032]      FIG. 3  is a schematic illustration (not to scale) showing a further cross section through the fuel tank; 
           [0033]      FIG. 4  is a schematic illustration (not to scale) illustrating effects of a projectile impacting with an external surface of the fuel tank; and 
           [0034]      FIG. 5  is a schematic illustration (not to scale) showing a cross section through a fuel tank in which a further embodiment of a hydrodynamic ram reducing baffle is implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In the following description, like reference numerals refer to like elements. 
         [0036]    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. 
         [0037]    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. 
         [0038]      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. 
         [0039]    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. 
         [0040]    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. 
         [0041]    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 . 
         [0042]    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. 
         [0043]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in the aircraft wing  2  taken parallel to the length of the aircraft wing  2  (i.e. perpendicular to a longitudinal or roll axis of an aircraft to which the wing  2  is attached). 
         [0044]      FIG. 3  is a schematic illustration (not to scale) showing a further cross section through the fuel tank  16  in the aircraft wing  2  taken perpendicular to the cross section of  FIG. 2  (i.e. perpendicular to the length of the aircraft wing  2 ). 
         [0045]    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. 
         [0046]    In this embodiment, the fuel tank  16  comprises hydrodynamic ram reducing baffles  20 . 
         [0047]    In this embodiment, each baffle  20  is a substantially straight tube or pipe. Each baffle  20  is an elongate, hollow cylinder through which the fluid in the fuel tank  16  may flow. 
         [0048]    In this embodiment, the baffles  20  are cylindrical in shape, i.e. have circular cross sections. However, in other embodiments, one or more of the baffles  20  has a different shaped cross section, i.e. other than circular. 
         [0049]    In this embodiment, each baffle  20  has an external diameter in the range 30 mm to 40 mm (compared to a fuel tank depth of, for example, 250 mm). However, in other embodiments, one or more of the baffles  20  may have a different external diameter, for example, an external diameter in the range 10 mm to 30 mm, or alternatively in the range 40 mm to 100 mm. 
         [0050]    In this embodiment, each of the baffles  20  is made of a strong, lightweight material, for example, CFC or a plastic. Preferably, the baffles  20  are sufficiently flexible to allow the aircraft wing  2  to flex during flight. 
         [0051]    In this embodiment, the walls of the baffles  20  are relatively thin, for example a baffle wall may have a thickness in the range 0.25 mm to 1 mm, or in the range 1 mm to 5 mm. Preferably, the thicknesses of the baffle walls 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. 
         [0052]    In this embodiment, the baffles  20  are arranged in the fuel tank  16  as overlapping and touching rows of baffles  20  between the upper skin  10  and the lower skin  12 . Preferably, there is a sufficient number of rows of baffles  20  to provide that the rows of baffles  20  extend substantially from the top of the fuel tank  16  to the bottom of the fuel tank  16  (i.e. from the upper skin  10  to the lower skin  12 ). For reasons of clarity and convenience, the fuel tank  16  is depicted in  FIGS. 2 and 3  as containing three rows of baffles  20  between the aircraft skins  10 ,  12 . However, in practice there may be a different number of rows of baffles  20 . Preferably, there are at least three rows of baffles  20 . More preferably, there are at least five rows of baffles  20 . More preferably, there are at least seven rows of baffles  20 . More preferably, there are at least ten rows of baffles  20 , for example ten or eleven rows. 
         [0053]    As shown in  FIG. 2 , in this embodiment, the rows of baffles  20  extend substantially from one side of the fuel tank  16  to the opposite side of the fuel tank  16  (i.e. from the rib  8  at one side of the fuel tank to the rib  8  at the opposite side of the fuel tank  16 ). Preferably, the baffles  20  are arranged such that adjacent baffles  20  in a row touch. Preferably, there are gaps between the ends of each of the rows of baffles and the ribs  8  such that fluid within the fuel tank  16  is permitted to flow between the baffles  20  and the ribs  8 . 
         [0054]    As shown in  FIG. 3 , in this embodiment, the lengths of the baffles  20  are such that the baffles extend substantially from one side of the fuel tank  16  to the opposite side of the fuel tank  16  (i.e. from the spar  6  at one side of the fuel tank to the spar  6  at the opposite side of the fuel tank  16 ). Preferably, the lengths of the baffles  20  are such that there are gaps between the ends of the baffles  20  and the spars  6  such that fluid within the fuel tank  16  is permitted to flow between the baffles  20  and the ribs  8 , i.e. such that fluid may flow from one baffle  20  to a different baffle  20  as indicated in  FIG. 3  by double headed arrows and the reference numeral  22 . 
         [0055]    In this embodiment, the baffles  20  are arranged in the fuel tank  16  such that the longitudinal axes of the baffles  20  are substantially perpendicular to a direction that points along the length of the aircraft wing  2 . However, in other embodiments, one or more of the baffles  20  has a different orientation. For example, in some embodiments, the baffles  20  are arranged in the fuel tank  16  such that the longitudinal axes of the baffles  20  are parallel with a direction that points along the length of the aircraft wing  2 . In some other embodiments a random orientation may be used for the individual baffles  20 . 
         [0056]    In some embodiments, spacers are used to keep the baffles  20  spaced apart from one or more of the internal walls of the fuel tank  16 . For example, in some embodiments, spacer devices are used at the ends of the baffles  20  so that the baffles  20  remain spaced apart from the spars  6 , i.e. such that a gap is retained between the baffles  20  and the spars  6 . This advantageously maintains a flow path for the fluid within the fuel tank  16  to flow from one baffle  20  to another baffle  20 . This tends to allow for the free movement of fuel within the fuel tank, thereby providing that the fuel is uniformly distributed within the tank. Examples of appropriate spacer devices include, but are not limited to, structures made of open-cell foam which permit the flow of fluid therethrough. In some embodiments, spacer devices may be used to keep the baffles  20  spaced apart from one another, and/or from the aircraft skins  10 ,  12 . This advantageously tends to reduce or eliminate chaffing damage to the baffles  20  and/or the walls of the fuel tank  16 . 
         [0057]    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. 
         [0058]    In this embodiment, the baffles  20  are not fixedly attached to the spars  6 . 
         [0059]    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 walls. In other examples, the projectile  24  may impact with only a single baffle  20 . In other examples, the projectile  24  does not pierce a baffle wall or only pierces a single baffle wall. 
         [0066]    The projectile impacting with a baffle  20  tends to cause that baffle  20  to be 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  experience 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 wall or walls of a baffle  20 , impact energy of the projectile  24  tends to be used to pierce those baffle walls. 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 a wall of a baffle  20 , 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  is generated within the fluid within each baffle  20  that is penetrated by the projectile  24 . The walls 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 experienced in a conventional system due to at least some of the impact energy of the projectile  24  being absorbed by the baffles. In addition, each baffle  20  advantageously tends to limit the distance over which the shock wave 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]    The baffles  20  advantageously tend to decouple the fluid from walls of the fuel tank  16  at least to some extent. 
         [0073]    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 . 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  20  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. 
         [0074]    Advantageously, the baffles  20  are located in the fuel tank  16  so that a shock wave or waves 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 interact with at least one baffle  20  before impinging on the tank external boundary surfaces. 
         [0075]    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. 
         [0076]    The above described baffle advantageously tends to be relative easy and cheap to manufacture. 
         [0077]    The above described baffle tends to be relatively easy to retrofit to existing aircraft fuel tanks. 
         [0078]    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. 
         [0079]    The above described baffle tends to be relatively lightweight so as not to be a significant burden to the aircraft. 
         [0080]    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. 
         [0081]    In the above embodiments, each baffle is a tube made of a relatively thin layer of material. However, in other embodiments, the walls of the baffles are of a different construction, for example, as will now be described. 
         [0082]      FIG. 5  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in which a further embodiment of hydrodynamic reducing baffles (hereinafter referred to as the “further baffles”) is implemented. A further baffle is indicated in  FIG. 5  by the reference numeral  40 . 
         [0083]    In this further embodiment, the further baffle  40  comprises an outer wall  42  and an inner wall  44  which are spaced apart to define therebetween at least one chamber  46 . 
         [0084]    In this further embodiment, the outer wall  42  is a substantially straight tube or pipe having a substantially circular cross section and within which is located the inner wall  44  and the chamber  46 . In other embodiments the cross section may be alternative in shape. 
         [0085]    In this further embodiment, the inner wall  44  is a substantially straight tube or pipe through which the fluid in the fuel tank  16  may flow and having a substantially circular cross section. In other embodiments the cross section may be alternative in shape. The inner wall  44  is located within the outer wall  42 . The outer wall  42  and the inner wall  44  may be connected together, for example, at the ends of the further baffles  40  such that the chamber  46  is a sealed chamber. The outer wall  42  and the inner wall  44  may have spacers located between them at points along their length. 
         [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 dimensions of the further baffles  40  may be the same as those of the baffles  20  which are described in more detail above with reference to  FIGS. 2 to 4 . 
         [0088]    The material from which further baffles  40  are made may be the same as those from which the baffles  20  are made, which are described in more detail above with reference to  FIGS. 2 to 4 . 
         [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 cavity  46  and are spaced apart in each further baffle  40  by an amount sufficient to provide at least one cavity  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 cavity 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 cavity  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 pressures of the liquid in the fuel tank  16 , at least up to the 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 impact 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 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 cavity  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. 
         [0093]    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. 
         [0094]    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. 
         [0095]    In some embodiments, one or more of the baffles comprises one or more rigid or flexible or foam construction barriers along the channel through which the fluid may flow. Such barriers may retard the passage of shockwaves generated within that baffle along the length of that baffle.