Abstract:
Disclosed is a liquid storage system comprising: an outer tank (e.g. an aircraft fuel tank) for containing a liquid (e.g. aircraft fuel), said outer tank enclosing a liquid storage space; an inner tank located within the outer tank and enclosing the liquid storage space; and spacing means associated with the outer tank and the inner tank for maintaining the tanks in a spaced apart relationship; wherein the spacing means are deformable such that, on application of a force to the inner tank, at least part of the inner tank moves relative to the outer tank.

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: an outer tank for containing a liquid, said outer tank enclosing a liquid storage space; an inner tank located within the outer tank and enclosing the liquid storage space; and spacing means associated with the outer tank and the inner tank for maintaining said tanks in a spaced apart relationship. The spacing means are deformable such that, on application of a force to the inner tank, at least part of the inner tank moves relative to the outer tank. The inner tank comprises: a first portion comprising a first base wall and first side walls extending from the first base wall, the first side walls surrounding a first opening opposite to the first base wall; and a second portion comprising a second base wall and second side walls extending from the second base wall, the second side walls surrounding a second opening opposite to the second base wall. The first portion and the second portion are coupled to the outer tank by respective deformable spacing means such that the first and second portions may move independently from each other. 
         [0005]    The second portion may be located within the first portion such that the second opening is located at or proximate to the first base wall and such that the first opening is located at or proximate to the second base wall. 
         [0006]    The inner tank may conform substantially to the shape of the outer tank. The spacing means may maintain the tanks in a spaced apart parallel relationship so as to form an annular space between the tanks. 
         [0007]    The spacing means may be elastic such that, if the inner tank moves relative to the outer tank, the inner tank is returned to its original position with respect to the outer tank. 
         [0008]    The inner tank may be made of one or more materials selected from the group of materials consisting of: carbon fibre composite material, aluminium, plastic, and a material comprising aramid or para-aramid fibres. 
         [0009]    The walls of the inner tank may be rigid. 
         [0010]    One or more walls of the inner tank may comprise a first wall and a second wall. 
         [0011]    The first and second walls of the inner tank may be spaced apart to define therebetween at least one chamber. The first and second walls of the inner tank may be sufficiently strong to resist at least the maximum and minimum hydrostatic pressures of a liquid in the liquid storage system. The at least one chamber may have a volume sufficient to allow a shock wave or waves in the liquid in the liquid storage system resulting from compression of the liquid by impact of a projectile on the liquid storage system to be reduced by expansion of the compressed liquid into the chamber. 
         [0012]    The inner tank may be made of a permeable or semi-permeable material such that a liquid stored in the liquid storage system may flow through walls of the inner tank. 
         [0013]    The inner tank may comprise one or more holes or perforations for allowing the flow of a liquid stored in the liquid storage system therethrough. 
         [0014]    The total cavity volume of the inner tank and the spacing means in the outer tank may be less than or equal to 15% by volume of the outer tank volume. 
         [0015]    The outer tank may be an aircraft fuel tank. 
         [0016]    In a further aspect, the present invention provides a vehicle comprising a liquid storage system for containing a liquid, the liquid storage system being in accordance with any of the above aspects. The vehicle may be an aircraft and the outer tank may be an aircraft fuel tank. The outer tank may be formed, at least in part, by an external skin of the aircraft. 
         [0017]    In a further aspect, the present invention provides a method of providing a liquid storage system. The method comprises: providing an outer tank for containing a liquid, said outer tank enclosing a liquid storage space; positioning an inner tank within the outer tank, the inner tank enclosing the liquid storage space; and positioning spacing means between the outer tank and the inner tank so as to maintain said tanks in a spaced apart relationship. The spacing means are deformable such that, on application of a force to the inner tank, at least part of the inner tank moves relative to the outer tank; the inner tank comprises: a first portion comprising a first base wall and first side walls extending from the first base wall, the first side walls surrounding a first opening opposite to the first base wall; and a second portion comprising a second base wall and second side walls extending from the second base wall, the second side walls surrounding a second opening opposite to the second base wall. The first portion and the second portion are coupled to the outer tank by respective deformable spacing means such that the first and second portions may move independently from each other. 
         [0018]    In a further aspect, the present invention provides a liquid storage system comprising: an outer tank for containing a liquid, said outer tank enclosing a liquid storage space; an inner tank located within the outer tank and enclosing the liquid storage space; spacing means associated with the outer tank and the inner tank for maintaining said tanks in a spaced apart relationship; wherein the spacing means are deformable such that, on application of a force to the inner tank, at least part of the inner tank moves relative to the outer tank. 
         [0019]    The inner tank may conform substantially to the shape of the outer tank. The spacing means may maintain the tanks in a spaced apart parallel relationship so as to form an annular space between the tanks. 
         [0020]    The spacing means may be elastic such that, if the inner tank moves relative to the outer tank, the inner tank is returned to its original position with respect to the outer tank. 
         [0021]    The inner tank may be made of one or more materials selected from the group of materials consisting of: carbon fibre composite material, aluminium, plastic, and a material comprising aramid or para-aramid fibres. 
         [0022]    The inner tank may comprise: a first portion comprising a first base wall and first side walls extending from the first base wall, the first side walls surrounding a first opening opposite to the first base wall; and a second portion comprising a second base wall and second side walls extending from the second base wall, the second side walls surrounding a second opening opposite to the second base wall. The second portion may be located within the first portion such that the second opening is located at or proximate to the first base wall and such that the first opening is located at or proximate to the second base wall. The first portion and the second portion may be coupled to the outer tank by respective deformable spacing means such that the first and second portions may move independently from each other. 
         [0023]    Walls of the inner tank may be rigid. Walls of the outer tank may be rigid. 
         [0024]    One or more walls of the inner tank may comprise a first wall and a second wall, the first and second walls of the inner tank being spaced apart to define therebetween at least one chamber. The first and second walls of the inner tank 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 liquid storage system to be reduced by expansion of the compressed liquid into the chamber. 
         [0025]    The inner tank may be made of a permeable or semi-permeable material such that a liquid stored in the liquid storage system may flow through walls of the inner tank. 
         [0026]    The walls of the inner tank may comprise one or more holes or perforations for allowing the flow of a liquid stored in the liquid storage system therethrough. 
         [0027]    The total cavity volume of the inner tank and the spacing means in the outer tank may be less than or equal to 15% by volume of the outer tank volume. 
         [0028]    The outer tank may be an aircraft fuel tank (e.g. located in the wing of an aircraft). 
         [0029]    In a further aspect, the present invention provides a vehicle comprising a liquid storage system for containing a liquid, the liquid storage system being in accordance with any of the above aspects. 
         [0030]    The vehicle may be an aircraft. The outer tank may be an aircraft fuel tank. The outer tank may be formed, at least in part, by an external skin of the aircraft. 
         [0031]    In a further aspect, the present invention provides a method of providing a liquid storage system, the method comprising: providing an outer tank for containing a liquid, said outer tank enclosing a liquid storage space; positioning an inner tank within the outer tank, the inner tank enclosing the liquid storage space; and positioning spacing means between the outer tank and the inner tank so as to maintain said tanks in a spaced apart relationship. The spacing means are deformable such that, on application of a force to the inner tank, at least part of the inner tank moves relative to the outer tank. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic illustration (not to scale) of an exploded view of an example aircraft wing in which an embodiment of a assembly is implemented; 
           [0033]      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 assembly is implemented; and 
           [0034]      FIG. 3  is a schematic illustration (not to scale) illustrating effects of a projectile impacting with an external surface of the fuel tank of  FIG. 2 . 
       
    
    
     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 identified 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 assembly 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 . 
         [0044]    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. 
         [0045]    In this embodiment, the fuel tank  16  comprises a hydrodynamic ram reducing assembly comprising a first inner tank wall  18   a,  a second inner tank wall  18   b,  a first plurality of coupling elements  20   a,  and a second plurality of coupling elements  20   b.    
         [0046]    The first inner tank wall  18   a  is located within the cavity  14 . The first inner tank wall  18   a  comprises a base wall that is substantially parallel with the upper skin  10  and is coupled to, and spaced apart from, the upper skin  10  by the first plurality of coupling elements  20   a.  The first inner tank wall  18   a  further comprises side walls that extend from the edges of the base wall of the first inner tank wall  18   a  and are substantially parallel to, and spaced part from, the ribs  8 . The first inner tank wall  18   a  further comprises further side walls (not shown in  FIG. 2 ) that extend from the edges of the base wall of the first inner tank wall  18   a  and are substantially parallel to, and spaced part from, the spars  6 . The first inner tank  18   a  comprises an opening opposite the base wall of the first inner tank wall  18   a,  the opening of the first inner tank wall  18   a  being proximate to the lower skin  12 . 
         [0047]    The second inner tank wall  18   b  is located within the cavity  14 . The second inner tank wall  18   b  is located within the first inner tank wall  18   a.  The second inner tank wall  18   b  comprises a base wall that is located within the opening of the first inner tank wall  18   a.  The base wall of the second inner tank wall  18   b  is substantially parallel with the lower skin  12  and is coupled to, and spaced apart from, the lower skin  12  by the second plurality of coupling elements  20   b.  The second inner tank wall  18   b  further comprises side walls that extend from the edges of the base wall of the second inner tank wall  18   b  and are substantially parallel to, and spaced part from, the ribs  8 . The side walls of the second inner tank wall  18   b  are located within the first inner tank wall  18   a  and are substantially parallel with the side walls of the first inner tank  18   a.  The second inner tank wall  18   b  further comprises further side walls (not shown in  FIG. 2 ) that extend from the edges of the base wall of the second inner tank wall  18   b  and are substantially parallel to, and spaced part from, the spars  6 . The further side walls of the second inner tank wall  18   b  are located within the first inner tank wall  18   a  and are substantially parallel with the further side walls of the first inner tank  18   a.  The second inner tank wall  18   b  comprises an opening opposite the base wall of the second inner tank wall  18   b,  the opening of the second inner tank wall  18   b  being proximate to the base wall of the first inner tank wall  18   a  and the upper skin  10 . 
         [0048]    In this embodiment, the first and second inner tank walls  18   a,    18   b  are substantially rigid, i.e. is resistant to flexing or deforming. 
         [0049]    In this embodiment the first and second inner tank walls  18   a,    18   b  are made of the same material from which the spars  6  and ribs  8  are made, i.e. CFC. In other embodiments, one or both of the first and second inner tank walls  18   a,    18   b  are made of one or more different materials instead of or in addition to CFC, for example, aluminium, plastic, or an aramid or para-aramid synthetic fibre-based material such as poly-paraphenylene terephthalamide (which is more common known as Kevlar™) or Twaron™, or UHMWPE fibres (spectra, dyneema). 
         [0050]    In this embodiment, the inner tank walls  18   a,    18   b  have thicknesses less than or equal to those of the aircraft skins  10 ,  12 . In some embodiments, the inner tank walls  18   a,    18   b  are between 2 mm and 5 mm thick. 
         [0051]    In this embodiment, the inner tank walls  18   a,    18   b  are sufficiently permeable to allow the fluid within the fuel tank  16  (i.e. aircraft fuel) to flow through those walls  18   a,    18   b.  In some embodiments, the inner tank walls  18   a,    18   b  may comprise a plurality of small holes, perforations, or cut-outs so as to permit the flow of a liquid therethrough. Such holes or perforations may have diameters of between 10 mm and 30 mm. Thus, flow of the fluid into, within, and out of (e.g. to the aircraft engine) the fuel tank  16  tends not to be detrimentally affected by the presence of the hydrodynamic ram reducing assembly within the fuel tank  16 . In some embodiments, the inner tank walls  18   a,    18   b  may be made of a permeable or semi-permeable material. 
         [0052]    In this embodiment, the coupling elements  20   a,    20   b  space apart the outer tank walls (i.e. the skin  10 ,  12 , spars  6  and ribs  8 ) from the inner tank walls  18   a,    18   b.    
         [0053]    Preferably, the coupling elements  20   a,    20   b  are deformable such that, as described in more detail later below with reference to  FIG. 3 , under application of a force or forces to the inner tank walls  18   a,    18   b,  the coupling elements  20   a,    20   b  permit movement of the inner tank walls  18   a,    18   b  with respect to the outer tank walls. For example, the coupling elements  20   a,    20   b  may be made of crushable foam that is configured to compress upon application of a compressive force. 
         [0054]    More preferably, the coupling elements  20   a,    20   b  are elastically deformable such that, as described in more detail later below with reference to  FIG. 3 , under application of a force or forces to the inner tank walls  18   a,    18   b,  the coupling elements  20   a,    20   b  permit movement of the inner tank walls  18   a,    18   b  with respect to the outer tank walls and then act so as to move the inner tank walls  18   a,    18   b  back to their original positions with respect to each other and the outer tank walls. For example, the coupling elements  20   a,    20   b  may be made of damped resilient springs. 
         [0055]    In this embodiment, the inner tank walls  18   a,    18   b  may move independently from one another. 
         [0056]    Preferably, the size of the components of the hydrodynamic ram reducing assembly is such that the assembly occupies less than 15% of the total internal volume (i.e. capacity) of the fuel tank  16 . In other embodiments, the assembly occupies a different proportion of the fuel tank capacity. 
         [0057]    As will now be described in more detail, the hydrodynamic ram reducing assembly is 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 . 
         [0058]      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  24  through the lower skin  12  is indicated in  FIG. 3  by the reference numeral  26 . 
         [0059]    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. 
         [0060]    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 . 
         [0061]    In this example, after passing through the lower skin  12 , the projectile  24  impacts with the base wall of the second inner tank wall  18   b.  The second inner tank wall  18   b  tends to be deflected and accelerated at least to some extent (e.g. in the direction indicated in  FIG. 3  by solid arrows and the reference numerals  28 ) by the impacting projectile  24 . The projectile  24  impacting with the second inner tank wall  18   b  tends to retard the passage of the projectile  24  into the fuel tank  16 . Furthermore, impact kinetic energy of the projectile  24  tends to be used to deflect and accelerate the second inner tank wall  18   b  through the fluid in the fuel tank  16 , thereby reducing the energy introduced into the fluid directly by the projectile  24 . This movement of the second inner tank wall  18   b  through the fluid is permitted by the second plurality of coupling elements  20   b  being deformable. Advantageously, in embodiments in which the second coupling elements  20   b  are elastic, the second inner tank wall  18   b  is returned to its original position within the fuel tank  16 . This returning of the second inner tank wall  18   b  to its original position uses energy absorbed from the impacting projectile  24 . Thus, elastic coupling elements  20   b  tend to more gradually dissipate the energy of the projectile  24  into the fluid within the fuel tank  16 . 
         [0062]    In this example, the projectile  24  travels through the second inner tank wall  18   b.  The projectile  24  causes high strain rate shear damage to the second inner tank wall  18   b  resulting in a hole in the second inner tank wall  18   b  approximately the size of the projectile  24 . Impact kinetic energy of the projectile  24  tends to be used to penetrate the second inner tank wall  18   b,  thereby reducing the energy introduced into the fluid directly by the projectile  24 . In some examples, the projectile  24  does not fully penetrate the second inner tank wall  18   b  and the projectile  24  is prevented from travelling further into the fuel tank  16 , thereby reducing the energy of shockwaves and/or a cavitation pressure caused by the projectile  24 . 
         [0063]    In this example, after travelling through the second inner tank wall  18   b,  the projectile  24  impacts with the fluid within the inner tank (which is defined by the inner tank walls  18   a,    18   b ). The impacting projectile  24  tends to generate one or more high pressure shock waves  30  within the fluid in the fuel tank  16 . These shock waves  30  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 second inner tank wall  18   b.    
         [0064]    In this example, the shock waves  30  generated by the projectile  24  impinge on the first inner tank wall  18   a.  The first inner tank wall  18   a  tends to be deflected and accelerated at least to some extent (e.g. in the direction indicated in  FIG. 3  by solid arrows and the reference numerals  32 ) by the incident shockwaves  30 . Energy of the shockwaves  30  tends to be used to deflect and accelerate the first inner tank wall  18   a  through the fluid in the fuel tank  16 , thereby reducing the shockwave energy transferred to at least the upper skin  10 . This movement of the first inner tank wall  18   a  through the fluid is permitted by the first plurality of coupling elements  20   a  being deformable. Advantageously, in embodiments in which the first coupling elements  20   a  are elastic, the first inner tank wall  18   a  is returned to its original position within the fuel tank  16 . This returning of the first inner tank wall  18   a  to its original position uses energy absorbed from the shockwaves  30 . Thus, elastic coupling elements  20   a  tend to more gradually dissipate the shockwave energy into the fluid within the fuel tank  16 . 
         [0065]    Furthermore, the first inner tank wall  18   a  may reflect incident shock waves  30  at least to some extent. Also, the first inner tank wall  18   a  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 first inner tank wall  18   a.  The assemblies  18   a,    18   b  advantageously tend to decouple the fluid from walls of the fuel tank  16 . 
         [0066]    The inner tank walls  18   a,    18   b  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 fuels tank  16  (e.g. decoupling of the external skin  10 ,  12  from the spars  6  or ribs  8 ) tends to be reduced. 
         [0067]    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 . Due to the passage of the projectile  24  through the fuel tank  16  being retarded at least to some degree by the second inner tank wall  18   b,  the increased fluid pressure resulting from cavitation caused by the projectile  24  tends to be decreased compared to conventional systems. Thus, pressures resulting from cavitation exerted on the walls of the fuel tank  16  tend to be lower than in conventional systems. 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. 
         [0068]    Additionally, the increased pressure resulting from cavitation caused by the projectile  24  tends to be absorbed by the hydrodynamic ram reducing assembly at least to some extent. In particular, the increased pressure in the fluid tends to cause the inner tank walls  18   a,    18   b  to be deflected and accelerated at least to some extent, e.g., the first inner tank wall  18   a  may be moved in the direction indicated in  FIG. 3  by solid arrows and the reference numerals  32  while the second inner tank wall  18   b  may be moved in the direction indicated in  FIG. 3  by solid arrows and the reference numerals  34 . Energy tends to be used to deflect and accelerate the inner tank walls  18   a,    18   b  in this way, thereby reducing the energy transferred to outer walls of the fuel tank  16 . 
         [0069]    Additionally, were the projectile  24  to continue through the cavity  14  and impact with the first inner tank wall  18   a,  the first inner tank wall  18   a  would tend to cause further retardation of the projectile  24 , thereby further reducing impact energy and reducing the force experienced by at least the upper skin  10 . 
         [0070]    An advantage provided by the above described assembly 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. 
         [0071]    The above described assembly advantageously tends to be relative easy and cheap to manufacture. 
         [0072]    The above described assembly tends to be relatively easy to retrofit to existing aircraft fuel tanks. 
         [0073]    The above described assembly tends to provide protection against hydrodynamic ram damage whilst occupying a relatively small amount of the fuel tank&#39;s capacity. 
         [0074]    In the above embodiments, the hydrodynamic ram reducing assembly is implemented in an aircraft wing fuel tank. However, in other embodiments, the assembly is 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. 
         [0075]    In the above embodiments, the inner tank walls are coupled to the upper and lower aircraft skins. However, in other embodiments an inner tank wall may be coupled to a different surface of the fuel tank instead of or in addition to one or both of the upper and lower aircraft skins. For example, in some embodiments, the inner tank walls may be coupled to the ribs and/or the spars. 
         [0076]    In the above embodiments, the inner tank walls are made of a solid material that may comprise a plurality of holes or perforations. However, in other embodiments, one or both of the inner tank walls is of a different construction. For example, an inner tank wall may comprise two spaced apart walls coupled together to define therebetween at least one chamber. This chamber may, for example, be filled with a gas or may contain a vacuum. Preferably, the two spaced part walls of such an inner tank wall are sufficiently strong to resist at least the maximum and minimum hydrostatic pressures of a liquid in the fuel tank. Also, preferably, the at least one chamber has 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. Also, preferably, the at least one chamber contains a material having a density sufficiently different from the density of a liquid in the tank to provide substantially total reflection within the inner tank wall of the shock wave or waves impinging on the inner tank wall thereby to reduce the hydraulic ram pressure in the liquid.