Abstract:
A liquid storage system comprising: a tank for containing a liquid, the tank enclosing a liquid storage space; and a tank liner fixedly attached to an internal surface of the tank ( 16 ). The tank liner comprises: a plurality of elements, each element having a hardness value of 2 GPa or above; and a binder material in which the plurality of elements are embedded. The elements have a higher hardness value than the binder material. A distance between a first element and the internal surface of the tank in a direction normal to the internal surface of the tank is different to a distance between a second element and the internal surface of the tank in the direction normal to the internal surface of the tank, the first element being different to the second element.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to liquid storage systems. 
       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 liquid storage system comprising: a tank for containing a liquid, said tank enclosing a liquid storage space; and a tank liner fixedly attached to an internal surface of the tank. The tank liner comprises: a plurality of elements, each element having a hardness value of 2 GPa or above; and a binder material in which the plurality of elements are embedded. The elements have a higher hardness value than the binder material. A distance between a first element and the internal surface of the tank in a direction normal to the internal surface of the tank is different to a distance between a second element and the internal surface of the tank in the direction normal to the internal surface of the tank, the first element being different to the second element. Thus, the hard elements are distributed within the binder matrix at different depths of the tank liner. A distance between a first element and the internal surface of the tank may be different to a distance between a second element and the internal surface of the tank, i.e. multiple different elements may have different respective depths with the tank liner. The depth of the tank liner may be a dimension that points from a proximal surface of the liner to a distal surface of the liner. 
         [0005]    The elements may be substantially uniformly distributed throughout the binder material. The elements may be distributed throughout the entire bulk of the binder material, i.e. throughout the entirety of the binder material. 
         [0006]    The binder material may be more flexible than walls of the tank, for example, such that loads carried in use by the binder material are very small (preferably zero, or insignificant) compared to loads carried by the structure of the tank. 
         [0007]    The fuel tank liner may comprises a proximal surface fixedly attached to an internal surface of the tank, and a distal surface opposite to the proximal surface. The elements may be arranged as multiple layers of elements between the proximal surface and the distal surface. A distance between the proximal surface and the distal surface (i.e. a thickness of the tank liner) may be between 10 mm and 25 mm. The tank liner may have a uniform thickness. 
         [0008]    A diameter of each of the elements may be in the range 2 mm to 6 mm. Each element may have a hardness value of 20 GPa or above. The elements may be made of ceramic, metal, or a metal alloy. The elements may be substantially spherical in shape. The binder material may be a rubber or a polymer. The tank may be an aircraft fuel tank. 
         [0009]    In a further aspect, the present invention provides a vehicle comprising a liquid storage system in accordance with any of the above aspects. 
         [0010]    In a further aspect, the present invention provides a method of producing a liquid storage system. The method comprises: providing a tank for containing a liquid, said tank enclosing a liquid storage space; providing, in a liquid or semiliquid form, a binder material; adding a plurality of elements to the liquid or semiliquid binder material, each element having a hardness value of 2 GPa or above; solidifying the binder material so as to embed the plurality of elements in the binder material; and fixedly attaching, to an internal surface of the tank, the solidified binder material with the elements embedded therein, thereby proving a tank liner for the tank. The tank liner is such that a distance between a first element and the internal surface of the tank in a direction normal to the internal surface of the tank is different to a distance between a second element and the internal surface of the tank in the direction normal to the internal surface of the tank, the first element being different to the second element. 
         [0011]    The method may further comprise: applying the mixture of the liquid or semiliquid binder material and the plurality of elements to the internal surface of the tank (while the binder material is in its liquid or semiliquid form); and, thereafter, solidifying the mixture of the liquid or semiliquid binder material and the plurality of elements that has been applied to the internal surface of the tank, thereby bonding the binder material with the elements embedded therein to the internal surface of the tank. 
         [0012]    The method may further comprise: providing a mould, the mould having a shape complementary to the internal surface of the tank; applying the mixture of the liquid or semiliquid binder material and the plurality of elements to the mould (while the binder material is in its liquid or semiliquid form); thereafter, solidifying the mixture of the liquid or semiliquid binder material and the plurality of elements that has been applied to the mould; and fixedly attaching, to the internal surface of the tank, the solidified binder material with the elements embedded therein. 
         [0013]    In a further aspect, the present invention provides at least part of a wall of a tank for containing a fluid, the at least part of the wall comprising a plurality of elements, each element having a hardness value of  2  GPa or above, and a binder material in which the plurality of elements are embedded. The elements have a higher hardness value than the binder material. 
         [0014]    Each element may have a hardness value of 5 GPa or above. Each element may have a hardness value of 10 GPa or above. Each element may have a hardness value of 15 GPa or above. Each element may have a hardness value of 20 GPa or above. 
         [0015]    The elements may be arranged in the binder material such that, when the at least part of the wall of the tank is impacted by a projectile having sufficient kinetic energy for at least part of the projectile to fully penetrate the at least part of the wall, the projectile impacts with at least one of the elements. 
         [0016]    The elements may be made of ceramic, metal, or a metal alloy. 
         [0017]    The elements may be substantially spherical in shape. 
         [0018]    A diameter of each of the elements may be in the range 2 mm to 6 mm 
         [0019]    The binder material may be a rubber or a polymer. For example, the binder material may be a polymer matrix in which is also embedded a fibre-based such as carbon fibres. Thus, the at least part of a wall may be a carbon fibre composite (CFC) panel (e.g. an outer skin of an aircraft), in which is embedded the relatively hard elements. 
         [0020]    The at least part of a wall of a tank may be a liner for a tank which is configured to be applied to an internal surface of a tank. 
         [0021]    In a further aspect, the present invention provides a tank for containing a liquid, wherein at least part of a wall of the tank is in accordance with the preceding aspect. 
         [0022]    The at least part of the tank wall may be a liner for the tank which is applied to an internal surface of the tank. The total cavity volume of the tank liner in the tank may be less than or equal to 15% by volume of the tank volume. 
         [0023]    The tank may be an aircraft fuel tank. 
         [0024]    In a further aspect, the present invention provides a vehicle (e.g. an aircraft) comprising a tank for containing a liquid in accordance with the preceding aspect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic illustration (not to scale) of an exploded view of an example aircraft wing in which an embodiment of a fuel tank liner is implemented; 
           [0026]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through a fuel tank located in the aircraft wing; and 
           [0027]      FIG. 3  is a schematic illustration (not to scale) illustrating effects of a projectile impacting with an external surface of the fuel tank. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In the following description, like reference numerals refer to like elements. 
         [0029]    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 identified are examples only. 
         [0030]    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. 
         [0031]      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 fuel tank liner is implemented. 
         [0032]    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. 
         [0033]    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. 
         [0034]    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 . 
         [0035]    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. 
         [0036]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in the aircraft wing  2 . 
         [0037]    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. 
         [0038]    In this embodiment, the fuel tank  16  comprises two fuel tank liners  18 . A first of the fuel tank liners  18  is disposed on an internal surface of the upper skin  10 , i.e. the surface of the upper skin  10  that is inside the fuel tank  16 . A second of the fuel tank liners  18  is disposed on an internal surface of the lower skin  12 , i.e. the surface of the lower skin  12  that is inside the fuel tank  16 . 
         [0039]    Preferably, the fuel tank liners  18  cover the entirety of the internal surfaces of the external skins  10 ,  12  that define the fuel tank  16 . Further, the fuel tank liners  18  may also cover the surfaces of the ribs  8  or spars  6 . 
         [0040]    In this embodiment, each fuel tank liner  18  is a layer of composite material that comprises a binder matrix  20  in which is embedded a plurality of objects  22 . 
         [0041]    The binder matrix  20  may be any appropriate material that may be used to bind together or retain the objects  22 . For example, the binder material  20  may be rubber or a polymer. 
         [0042]    Preferably, the binder matrix  20  is relatively flexible compared to the structure of the aircraft wing  2  to which it is to be attached. In other words, preferably, the binder matrix  20  is more flexible that the external skins  10 ,  12 , the spars  6 , and the ribs  8 . The binder matrix  20  being more flexible than the aircraft structure advantageously tends to provide that, when the tank liner  18  is fixed into the fuel tank  16 , the fuel tank liner  18  tends not to carry any load, or only an insignificant load, compared to the structure of the aircraft (i.e. compared to the spars  6 , the ribs  8 , and/or the external skins  10 ,  12 ). Thus, the tank liner  18  tends not to change, to any significant extent, the loading distribution of the aircraft wing  2 . Thus, the design of the wing  2 , and the structural design process performed in designing the aircraft wing  2 , tend not to be undermined or invalidated by the application of the tank liner  18  to the wing  2 . Thus, application of the tank liner  18  to a wing  2  (e.g. retrofitting to an existing wing) tends to be facilitated. 
         [0043]    In some embodiments, the objects  22  may be included (i.e. embedded) directly within the composite layup of the tank structure, for example, within the skins  10 ,  12 . 
         [0044]    In this embodiment, the objects  22  are substantially spherical in shape, each having a diameter of approximately 2 mm to 6 mm. However, in other embodiments, one or more of the objects  22  may have a different shape and/or size, for example, an object  22  may be rod-shaped (i.e. elongate) or a cube (e.g. a rounded cube). In some embodiments, the objects  22  are larger than 6 mm in diameter, for example the objects  22  may have diameters between 6 mm and 22 mm. In this embodiment, the objects  22  are harder and/or denser than the binder matrix  18 . In this embodiment, the objects  22  are harder and/or denser than the CFC material that forms the external skins  10 ,  12 . In this embodiment, the objects  22  are made of ceramic, e.g. zirconia, silicon nitride, alumina, or silicon carbide. Preferably the objects  22  have a hardness value (e.g. a Vickers hardness or a Knoop hardness) of 2 GPa or above. More preferably the objects  22  have a hardness value (e.g. a Vickers hardness or a Knoop hardness) of 5 GPa or above. More preferably the objects  22  have a hardness value (e.g. a Vickers hardness or a Knoop hardness) of 10 GPa or above, for example the objects  22  may be made of Zirconias and Aluminium Nitrides having a Knoop hardness in the range of 10 GPa to 14 GPa. More preferably the objects  22  have a hardness value (e.g. a Vickers hardness or a Knoop hardness) of 15 GPa or above, for example the objects  22  may be made of Aluminas and Silicon Nitrides which typically have a Vickers hardness of 15 GPa to 20 GPa. More preferably the objects  22  have a hardness value (e.g. a Vickers hardness or a Knoop hardness) of 20 GPa or above, for example the objects  22  may be made of a carbide having a Knoop hardness of over 20 GPa, e.g. Silicon Carbides and Boron Carbides which typically have a Vickers hardness of about 20 GPa to 30 GPa. 
         [0045]    In this embodiment, the concentration and arrangement of the objects  22  within the fuel tank liners  18  are such that a projectile passing through the thickness of a fuel tank liner  18  will tend to impact with at least one of the objects  22 , and preferably multiple objects  22 . In some embodiments, the objects  22  are arranged in the binder matrix  20  in layers e.g. between 3 and 5 layers. More preferably there are more than 5 layers of objects  22 . The objects  22  within a fuel tank liner  18  may be more closely packed together closer to the external skin  10 ,  12  to which that fuel tank liner  18  is attached. 
         [0046]    In this embodiment, the objects  22  are distributed substantially uniformly within a fuel tank liner  18 . In other words, the objects  22  are evenly spread throughout the fuel tank liners  18 . This advantageously tends to increase the likelihood of a projectile that impacts with and penetrates a fuel tank liner  18  impacting with at least one of the objects  22 . 
         [0047]    In this embodiment, a distance between an object  22  and the internal surface of the tank  16  in a direction normal to the internal surface of the tank  16  is different to a distance between at least one other object  22  and the internal surface of the tank  16  in the direction normal to the internal surface of the tank  16 . In other words, multiple different objects  22  each have different respective depths in the fuel tank liner  18 . The depth of a fuel tank liner  18  is understood to be the dimension of that fuel tank liner  18  along a direction that points from a proximal surface of the tank liner  18  (i.e., a surface of the tank liner  18  that is attached to the internal surface of the fuel tank  16 ), and a distal surface of the tank liner  18  (i.e. the surface opposite to the proximal surface), for example, in a direction normal to the proximal and/or distal surfaces. This may be due at least in part to the uniform distribution of the objects  22 , and the sizes of the objects  22  relative to the thickness of a fuel tank liner  18 . Thus, in effect, objects  22  may be arranged in multiple layers within a fuel tank liner  18 . This advantageously tends to increase the likelihood of a projectile that impacts with and penetrates a fuel tank liner  18  impacting with multiple objects  22  as it passes through the liner  18 . 
         [0048]    Each fuel tank liner  18  may have any appropriate depth (i.e. thickness), for example, 10 mm to 20 mm, or 15 mm to 19 mm. Preferably, the thicknesses of the fuel tank liners  18  are such that the fuel tank liners  18  occupy less than or equal to 15% (e.g. approximately 10%) of the fuel tank capacity. In other embodiments, the fuel tank liners  18  are a different thickness that provides that the fuel tank liners  18  occupy a different proportion of the fuel tank capacity. Preferably, the fuel tank liners  18  are of uniform thickness. 
         [0049]    The fuel tank liners  18  may be attached to the externals skins  10 ,  12  by any appropriate means for example using an adhesive or by performing a bonding process. 
         [0050]    In some embodiments, a fuel tank liner  18  is formed and attached to an internal surface of the fuel tank as follows. Firstly, the binder material  20  is provided in a liquid (e.g. molten) or semiliquid state. Secondly, the objects  22  are added to, and thoroughly mixed with, the liquid binder material  20 . The mixing of the objects  22  into the liquid binder material  20  tends to substantially uniformly distribute the objects  22  within the binder material  20 . Thirdly, the liquid binder material  20  with the object  22  therein is spread across a surface of the aircraft component to which the fuel tank liner  18  is to line. For example, the mixture of the binder material  20  and the objects  22  may be poured over a surface of an aircraft panel  10 ,  12  that is to form an internal surface of the fuel tank  16 . The mixture of the binder material  20  and the objects  22  may be applied to an aircraft component before, during, or after assembly of the aircraft wing  2 . Lastly, the mixture of the binder material  20  and the objects  22  is solidified, for example, the liquid binder material  20  may be allowed to harden (e.g. by cooling), or may be cured. This solidification of the mixture of the binder material  20  and the objects  22  tends to bond the binder material  20  to the aircraft component to which the mixture has been applied. Thus, a solid fuel tank liner  18  is formed on an aircraft component. In some embodiments, a cast or mould may be used to shape the fuel tank liner  18  on the aircraft component. This cast or mould may be removed after solidification of the mixture of the binder material  20  and the objects  22 . 
         [0051]    In some embodiments, a fuel tank liner  18  is formed and attached to an internal surface of the fuel tank as follows. Firstly, the binder material  20  is provided in a liquid (e.g. molten) or semiliquid state. Secondly, the objects  22  are added to, and thoroughly mixed with, the liquid binder material  20 . The mixing of the objects  22  into the liquid binder material  20  tends to substantially uniformly distribute the objects  22  within the binder material  20 . Thirdly, the liquid binder material  20  with the object  22  therein is applied to a mould tool or cast in a desired shape (i.e. a shape that conforms to the shape of the surface to which the fuel tank liner  18  is to be applied). The mixture of the binder material  20  and the objects  22  is then solidified, for example, by cooling or curing. Thus, a solid fuel tank liner  18  is formed. The fuel tank liner may be removed from the cast or mould after solidification. The solidified fuel tank liner  18  may be attached to an internal surface of the fuel tank  16 , for example, using a layer of adhesive. 
         [0052]    As will now be described in more detail, the fuel tank liners  18  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 . 
         [0053]      FIG. 3  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 . 
         [0054]    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. 
         [0055]    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 . 
         [0056]    After passing through the lower skin  12 , the projectile  24  impacts with the fuel tank liner  18  disposed on the lower skin  12 . 
         [0057]    The projectile  24  impacting with one or more of the objects  22  in the fuel tank liner  18  tends to retard the passage of the projectile  24  into the fuel tank  16 . Impact kinetic energy of the projectile  24  tends to be absorbed at least to some extent by the fuel tank liner  18  and also transferred to the objects  22  which may be ejected into the fluid volume. 
         [0058]    Also, projectile  24  impacting with one or more of the objects  22  in the fuel tank liner  18  may cause the projectile  24  to break up, or fragment, or be eroded into smaller parts prior to it entering the cavity  14 . To facilitate this, preferably the objects  22  are harder and/or denser than the projectile  24 . In some embodiments, the size of the objects  22 , the hardness size of the objects  22 , the density size of the objects  22 , the concentration size of the objects  22  within the binder matrix  20 , and or any other attribute of the objects  22  and/or binder matrix  20  may be determined (e.g. by experimentation or modelling) so as to maximise the likelihood of fragmentation of the projectile  24 . The impact of the projectile  24  with objects  22  in the fuel tank liner  18  may also cause fragmentation of a portion of the fuel tank liner  18  disposed on the lower skin  12 . The fragments into which the projectile  24  is broken up, and the fragments of the fuel tank liner  18 , are hereinafter collectively referred to as “fragments” and are indicated in  FIG. 3  by the reference numerals  28 . Thus, impact energy of the projectile  24  is absorbed at least to some extent by the fuel tank liner  18  disposed on the lower skin in the fragmentation of the projectile and/or the fuel tank liner  18 . 
         [0059]    Impact with one or more of the objects  22  may cause the projectile  24  to have increased tumble when travelling through the fluid. 
         [0060]    In this example, on impact of the fragments  28  with the fuel, the fragments tend to generate respective high pressure shock waves  30 . Each of these shock waves  30  tend to be of significantly lower energy than a shock wave or shock waves that would have been generated if the projectile  24  had not fragmented. 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 that would have been exerted on the walls of the fuel tank  16  if the projectile  24  had not fragmented. Thus, 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. 
         [0061]    In this example, as a fragment  28  passes through the fuel, a cavitation “wake” may form behind that fragment  28 , i.e. a region of low pressure (e.g. a vapour or a vacuum) may form in the wake of a fragment  28 . This causes a fluid displacement and an increase in the pressure of the fluid in the fuel tank  16 . 
         [0062]    The increased fuel pressure resulting from cavitation caused by the fragments  28  tends to be significantly lower than the pressure increase that would have been caused by cavitation if the projectile  24  had not fragmented. Thus, pressures resulting from cavitation exerted on the walls of the fuel tank  16  when the projectile  24  is fragmented tend to be lower than the pressures that would have been exerted on the walls of the fuel tank  16  if the projectile  24  had not been fragmented. Thus, 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. 
         [0063]    In this example, when travelling through the fuel, the fragments  28  tend to experience a greater overall drag force compared to that that would be experienced by the projectile  24  if the projectile  24  had not fragmented. This tends to be at least in part due to the increased surface area of the fragments  28  compared to the non-fragmented projectile  24 . Thus, the passage of the projectile/fragments through the fluid in the fuel tank  16  tends to be retarded. The retardation of the passage of the projectile/fragments through the fluid tends to decrease the likelihood of the projectile/fragments impacting with the upper skin  10 . Thus, the likelihood of a hole being formed in the upper skin tends to be reduced. Furthermore, the increase in drag on the projectile/fragments 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. 
         [0064]    Additionally, in this example, the fuel tank liner  18  disposed on the upper skin  10  is located within the fuel tank  16  such that the shock waves  30  resulting from compression of the fuel in the fuel tank  16  resulting from impact of the projectile  24  with the lower skin  12  impinge on the fuel tank liner  18  disposed on the upper skin  10  and so that the shock waves  30  interact with the fuel tank liner  18  disposed on the upper skin  10  before impinging on the upper skin  10 . The fuel tank liner  18  disposed on the upper skin  10  tends to reflect incident shock waves  30  at least to some extent. Also, the fuel tank liner  18  disposed on the upper skin  10  tends to be a relatively poor transmitter of impinging shock waves  30 . Thus, the amplitude of the shock waves  30  impinging upon the upper skin  10  tends to be reduced and consequently the pressure experienced by the upper skin  10  tends to be diminished by the presence of the fuel tank liner  18  disposed on the upper skin  10 . The fuel tank liners  18  advantageously tend to decouple the fuel from walls of the fuel tank  16 . 
         [0065]    Furthermore, were any of the fragments  28  (or even the non-fragmented projectile  24 ) to continue through the cavity  14  and impact with the fuel tank liner  18  disposed on the upper skin  10 , the fuel tank liner  18  disposed on the upper skin  10  would tend to cause further break-up or fragmentation of the impinging fragment  28  (or the non-fragmented projectile  24 ), thereby further reducing impact energy and reducing force experienced by at least the upper skin  10 . 
         [0066]    An advantage provided by the above described fuel tank liner 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 and corresponding aircraft loss tends to be reduced or eliminated. 
         [0067]    The above described fuel tank liner advantageously tends to be relative easy and cheap to manufacture. 
         [0068]    The above described fuel tank liner tends to be relatively easy to retrofit to existing aircraft fuel tanks. 
         [0069]    The above described fuel tank liner tends to provide protection against hydrodynamic ram damage whilst occupying a relatively small amount of the fuel tank&#39;s capacity. 
         [0070]    In the above embodiments, the fuel tank liners are used to line the surfaces of an aircraft wing fuel tank. However, in other embodiments, the fuel tank liners are a different type of liner and may be used to line an internal or external surface of 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. 
         [0071]    In the above embodiments, fuel tank liners are disposed on the internal surfaces of the upper and lower aircraft skins. However, in other embodiments a fuel tank liner may be disposed on a different surface of the fuel tank instead of or in addition one or both of the internal surfaces of the upper and lower aircraft skins. For example, in some embodiments, all internal surfaces of the fuel tank are covered by fuel tank liners. In some embodiments, a fuel tank liner is only disposed on a single surface of the fuel tank, for example, on only the internal surfaces of the lower aircraft skin. 
         [0072]    In the above embodiments, the fuel tank liners include objects embedded in the binder material. These objects are substantially spherical in shape, are made of ceramic, and have a diameter of 4 mm to 5 mm. However, in other embodiments, one or more of the objects may be a different shape, e.g. an irregular shape. In other embodiments, one or more of the objects may be made of a different appropriate material, e.g. metal or alloy such as hardened steel, for example the objects may be made of stainless steel which typically has a 
         [0073]    Vickers hardness of about 5 GPa or cemented carbide which typically has a Vickers hardness of above 15 GPa. Also, in other embodiments one or more of the objects may have a different size, for example, in some embodiments, some or all of the objects are larger than 5 mm in diameter. Similarly, in other embodiments, some or all of the objects are smaller than 4 mm in diameter. 
         [0074]    Preferably, the objects have diameters between 3 mm and 5 mm. 
         [0075]    In the above embodiments, the liner material is a layer of material which may be applied to an internal surface of the fuel tank. However, in other embodiments, the fuel tank liner may be integral to one or more of the walls of the fuel tank. For example, in some embodiment, the objects may be directly embedded in one or more of the walls of the fuel tank. For example, in some embodiment, the objects are embedded in the portion of the lower skin that forms a wall of the fuel tank, i.e. the objects are directly embedded in the CFC material that forms the lower skin.