Abstract:
A liquid storage system comprising: a tank for containing a liquid, the tank enclosing a liquid storage space; multiple layers of a fabric material; and attachment means attaching the multiple layers of the fabric material to an internal surface of the tank. The attachment means may comprise a non-permeable envelope attached to the internal surface of the tank. The multiple layers of the fabric material may be enclosed in the envelope such that the multiple layers of a fabric are isolated from a fluid in the tank. The envelope may contain a fluid (e.g. air) in addition to the multiple layers of the fabric material.

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; multiple layers of a fabric material; and attachment means attaching the multiple layers of the fabric material to an internal surface of the tank. 
         [0005]    The multiple layers of the fabric material may comprise a plurality of fabric sheets arranged as a stack of sheets. The fabric material may comprise aramid or para-aramid fibres. The aramid or para-aramid fibres may be poly-paraphenylene terephthalamide. Each of the layers of the fabric material may have a thickness of less than 0.5 mm. The multiple layers of the fabric material may include at least twenty layers. 
         [0006]    The attachment means may include a non-permeable envelope. The envelope may be attached to the internal surface of the tank. The multiple layers of the fabric material may be enclosed in the envelope such that the multiple layers of a fabric are isolated from a fluid in the tank. The envelope may contain a fluid (for example, air) in addition to the multiple layers of the fabric material. 
         [0007]    The attachment means may comprise one or more pins. Each pin may comprise a base portion for attachment to an internal surface of a wall of the tank, and a threaded elongate member passing through the multiple layers of the fabric material. 
         [0008]    The total cavity volume in the tank of the multiple layers of the fabric material and the attachment means may be less than or equal to 15% by volume of the tank volume. 
         [0009]    The multiple layers of the fabric material may be proximate to and substantially parallel with an internal surface of a wall of the tank. 
         [0010]    A penetration force required to penetrate a layer of the fabric material may be greater than a force with which the attachment means attaches the multiple layers of the fabric material to the internal surface of the tank. 
         [0011]    The multiple layers of the fabric material may cover the entirety of the internal surface of the tank. 
         [0012]    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. 
         [0013]    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 multiple layers of a fabric material; and attaching the multiple layers of the fabric material to an internal surface of the tank. 
         [0014]    The method may further comprise: providing a non-permeable envelope; placing the multiple layers of a fabric material into the envelope; sealing the envelope with the multiple layers of a fabric material located therein; and attaching the sealed envelope to the internal surface of the tank. 
         [0015]    In a further aspect, the present invention provides a liquid storage system comprising a tank for containing a liquid, said tank enclosing a liquid storage space, and an assembly located within the tank and configured to reduce the effects of hydrodynamic ram within the tank. The assembly comprises a plurality of flexible sheets of a material arranged as a stack of sheets, and attachment means for releasably attaching the stack of sheets to an internal surface of the tank. 
         [0016]    One or more of the flexible sheets, for example all of the flexible sheets, may be made of a material comprising aramid or para-aramid fibres. The aramid or para-aramid fibres may be poly-paraphenylene terephthalamide. 
         [0017]    Each of the flexible sheets may have a thickness of less than 1 mm, for example, less than 0.5 mm. 
         [0018]    The assembly may include at least 20 flexible sheets. 
         [0019]    The attachment means may comprise one or more pins, each pin comprising a base portion for attachment to an internal surface of a wall of the tank, and a threaded elongate member passing through the plurality of flexible sheets. 
         [0020]    The total cavity volume of the assembly in the tank may be less than or equal to 15% by volume of the tank volume. 
         [0021]    The assembly may be arranged in the tank such that the plurality of flexible sheets is proximate to and substantially parallel with an internal surface of a wall of the tank. 
         [0022]    The assembly may be arranged in the tank such that the stack of flexible sheets is substantially equidistant from two opposite walls of the tank. 
         [0023]    The assembly may be configured such that at least part of a sheet becomes detached from the wall of the tank in response to the application of a load force to that sheet. 
         [0024]    The assembly may be configured such that the sheets are free to move at least to some extent within the tank relative to each other and with respect to the walls of the tank without becoming detached from the walls of the tank. 
         [0025]    The tank may be an aircraft fuel tank. 
         [0026]    In a further aspect, the present invention provides a vehicle (e.g. an aircraft) comprising a liquid storage system for containing a liquid, the liquid storage system being in accordance with the preceding aspect. 
         [0027]    In a further aspect, the present invention provides an assembly for reducing the effects of hydrodynamic ram in a liquid in a tank in which it is located. The assembly comprises a plurality of flexible sheets of a material arranged as a stack of sheets, and attachment means for releasably attaching the stack of sheets to an internal surface of the tank. The flexible sheets are made of a material comprising aramid or para-aramid fibres. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      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; 
           [0029]      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; 
           [0030]      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 ; 
           [0031]      FIG. 4  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 assembly is implemented; 
           [0032]      FIG. 5  is a schematic illustration (not to scale) illustrating effects of a projectile impacting with an external surface of the fuel tank of  FIG. 4 ; and 
           [0033]      FIG. 6  is a schematic illustration (not to scale) showing a cross section through a fuel tank in which a second further embodiment of a hydrodynamic ram reducing assembly is implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In the following description, like reference numerals refer to like elements. 
         [0035]    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. 
         [0036]    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. 
         [0037]      FIG. 1  is a schematic illustration (not to scale) of an exploded view of an example aircraft wing  2  in which embodiments of a hydrodynamic ram reducing assembly are implemented. 
         [0038]    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. 
         [0039]    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. 
         [0040]    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 . 
         [0041]    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. 
         [0042]      FIG. 2  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in the aircraft wing  2  in which a first embodiment of a hydrodynamic ram reducing assembly is implemented. 
         [0043]    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. 
         [0044]    In this embodiment, the fuel tank  16  comprises two hydrodynamic ram reducing assemblies, hereinafter referred to as “the first assembly” and the “second assembly” and indicated in  FIG. 2  by the reference numerals  18   a  and  18   b  respectively. The first assembly  18   a  is attached to an internal surface of the upper skin  10 , i.e. the surface of the upper skin  10  that is inside the fuel tank  16 . The second assembly  18   b  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 . 
         [0045]    In this embodiment, the first assembly  18   a  comprises a plurality of threaded pins, hereinafter the “first pins”  20   a , and a plurality of sheets of material, hereinafter the “first sheets”  22   a . Similarly, the second assembly  18   b  comprises a plurality of threaded pins, hereinafter the “second pins”  20   b , and a plurality of sheets of material, hereinafter the “second sheets”  22   b.    
         [0046]    Each of the pins  20   a ,  20   b  comprises a base portion and a threaded elongate member attached to the base portion at one end and extending perpendicularly away from the base portion to a free pointed end. The base portion of each of the first pins  20   a  is attached to the upper skin  10  by a suitable attachment means, for example an adhesive, such that the elongate members of the first pins  20   a  are substantially perpendicular to the upper skin  10 . Likewise, the base portion of each of the second pins  20   b  is attached to the lower skin  12  by a suitable attachment means, for example an adhesive, such that the elongate members of the second pins  20   b  are substantially perpendicular to the lower skin  12 . 
         [0047]    In this embodiment, the first pins  20   a  are located at or proximate to the edges of the internal surface of the portion of the upper skin  10  that forms a wall of the fuel tank  16 , i.e. at or proximate to the spars  6  and ribs  8 . The first sheets  22   a  are attached to the first pins  20   a  so as to form a stack of sheets proximate to and substantially parallel with the internal surface of the upper skin  10 . Preferably, the first sheets  22   a  cover substantially the entirety of the portion of the internal surface of the upper skin  10  that defines the fuel tank  16 . The first sheets  22   a  may be attached to the first pins  20   a  by pushing the first sheets  22   a  onto the elongate members of the first pins  20   a  so that the elongate members of the first pins  20   a  pass through each of the first sheets  22   a . The threaded portions of the elongate members of the first pins  20   a  tend to loosely retain the first sheets  22   a  on the first pins  20   a , i.e. such that the first sheets  22   a  may be released or detached from the first pins  20   a  e.g. as described in more detail later below with reference to  FIG. 3 . Preferably, the first sheets  22   a  are not pulled taught between the first pins  20   a , and instead the first sheets  22   a  are relatively loose, i.e. the intermediate portions of the first sheets  22   a  between the first pins  20   a  are free to move (e.g. towards/away from the upper skin  10 ) at least to some degree. Preferably, there are at least 5 first sheets  22   a . More preferably, there are at least 10 first sheets  22   a . More preferably, there are at least 20 first sheets  22   a , e.g. between 20 and 30 first sheets  22   a . In some embodiments, there are more than 30 first sheets  22   a.    
         [0048]    In this embodiment, the second pins  20   b  are located at or proximate to the edges of the internal surface of the portion of the lower skin  12  that forms a wall of the fuel tank  16 , i.e. at or proximate to the spars  6  and ribs  8 . The second sheets  22   b  are attached to the second pins  20   b  so as to form a stack of sheets proximate to and substantially parallel with the internal surface of the lower skin  12 . Preferably, the second sheets  22   b  cover substantially the entirety of the portion of the internal surface of the lower skin  12  that defines the fuel tank  16 . The second sheets  22   b  may be attached to the second pins  20   b  by pushing the second sheets  22   b  onto the elongate members of the second pins  20   b  so that the elongate members of the second pins  20   b  pass through each of the second sheets  22   b . The threaded portions of the elongate members of the second pins  20   b  tends to loosely retain the second sheets  22   b  on the second pins  20   b , i.e. such that the second sheets  22   b  may be released or detached from the second pins  20   b  e.g. as described in more detail later below with reference to  FIG. 3 . Preferably, the second sheets  22   b  are not pulled taught between the second pins  20   b , and instead the second sheets  22   b  are relatively loose, i.e. the intermediate portions of the second sheets  22   b  between the second pins  20   b  are free to move (e.g. towards/away from the lower skin  12 ) at least to some degree. Preferably, there are at least 5 second sheets  22   b . More preferably, there are at least 10 second sheets  22   b . More preferably, there are at least 20 second sheets  22   b , e.g. between 20 and 30 second sheets  22   b . In some embodiments, there are more than 30 second sheets  22   b.    
         [0049]    In this embodiment, the first and second sheets  22   a ,  22   b  are flexible sheets made of a fibre-based material, for example woven, crimped/stitched or a mat of fibres. In this embodiment, the sheets  22   a ,  22   b  are fabric. The fabric may include ballistic fibres. Each of the sheets  22   a ,  22   b  is thin, for example, each sheet may have a thickness of between 0.1 mm and 0.5 mm, for example 0.25 mm. Preferably, the sheets  22   a ,  22   b  are less than 1 mm thick. More preferably, the sheets  22   a ,  22   b  are less than 0.5 mm thick. Each of the sheets  22   a ,  22   b  is made of a tough and strong material such as 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 sheets  22   a ,  22   b  are substantially continuous. However, in other embodiments, one or more of the sheets is not continuous, for example, one or more of the sheets may include a plurality of perforations, e.g. a sheet may be made of a mesh or net-like material. 
         [0051]    Preferably, the size of the components of the assemblies  18  are such that the assemblies  18  occupy less than 15% of the total internal volume (i.e. capacity) of the fuel tank  16 . In other embodiments, the assemblies  18  occupy a different proportion of the fuel tank capacity. 
         [0052]    As will now be described in more detail, the assemblies  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]    In this example, after passing through the lower skin  12 , the projectile  24  impinges upon one or more of the second sheets  22   b . The second sheet or sheets  22   b  impinged upon by the projectile  24  tend to be deflected and accelerated at least to some extent. The projectile  24  impacting with one or more of the second sheets  22   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 at least one of the second sheets  22   b  through the fluid in the fuel tank  16 , thereby reducing the energy introduced into the fluid directly by the projectile  24 . 
         [0057]    Deflection of the second sheets  22   b  by the projectile tends to be facilitated by the second sheets not being taught, i.e. being relatively “loose” and able to move to some degree within the fuel tank  16 . 
         [0058]    In this example, when travelling through the fuel, the projectile  24  in combination with the second sheets  22   b  moved by the projectile  24  tends to experience a greater overall drag force from the fluid in the fuel tank  16  compared to that that would be experienced by the projectile  24  if the second sheets  22   b  were not present. This tends to be at least in part due to the increased surface area of the combination of the projectile  24  and second sheets  20   b  compared to projectile  24  alone. Thus, the passage of the projectile  24  through the fluid in the fuel tank  16  tends to be retarded. 
         [0059]    In some situations, the projectile  24  may travel through (i.e. pierce or penetrate) one or more of the second sheets  22   b . In such cases, impact energy of the projectile  24  is used to pierce those second sheets  22   b , thereby reducing the energy introduced into the fluid by the projectile  24  and retarding at least to some extent the passage of the projectile  24  into the fluid. The likelihood of the projectile  24  piercing the second sheets  22   b  may be reduced by making the second sheets  22   b  from a strong, tough material such as Kevlar™. 
         [0060]    In some situations, the projectile  24  does not travel through (i.e. does not pierce or penetrate) one or more of the second sheets  22   b.    
         [0061]    In some cases where the projectile  24  does not pierce one or more of the second sheets  22   b , one or more of the second sheets  22   b  may be detached from one or more of the second pins  20   b . In other words, the projectile  24  may “pull” one or more of the second sheets  22   b  from one or more of the second pins  20   b  so that those sheets are free to move with the projectile  24 . Such detachment of the second sheets  22   b  from the second pins  20   b  is facilitated by the second sheets  22   b  being only loosely retained by the second pins  20   b . In other words, in some embodiments the sheets are releasably attached to fuel tank walls, e.g., by threaded pins. In some embodiments, releasable attachment of the sheets to the walls of the fuel tank  16  is provided by a force required for the projectile  24  to penetrate a sheet (i.e. a penetration force) being greater than a force required to detach that sheet from the wall of the fuel tank  16  (i.e. a force that retains that sheet against the wall of the fuel tank  16 ). In some embodiments, releasable attachment of the sheets to the walls of the fuel tank  16  is provided by a tensile and/or compressive strength of a sheet being greater than a force required to detach that sheet from the wall of the fuel tank  16  (i.e. a force that retains that sheet against the wall of the fuel tank  16 ). 
         [0062]    The second sheets  22   b  that are detached from the second pins  20   b  by the projectile  24  advantageously tend to “wrap around” the projectile  24  at least to some extent, for example, due to the movement of the projectile  24  through the fluid in the fuel tank  16 . The projectile  24  with one or more of the second sheets coupled thereto tends to have a much larger surface area than the projectile  24  alone. Thus, the projectile  24  with one or more of the second sheets coupled thereto tends to experience a greater drag force when moving through the fluid in the fuel tank  16  compared to that that would be experienced by the projectile  24  alone. 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. 
         [0063]    In some cases where the projectile  24  does not pierce one or more of the second sheets  22   b , one or more of the second sheets  22   b  are not detached from the second pins  20   b . Thus, the projectile  24  may be prevented from travelling further into the fuel tank  16 . At least some of the impact energy of the projectile  24  tends to be absorbed by the second sheets  22   b  and the second pins  20   b  and therefore not transferred to the aircraft substructure  4 . 
         [0064]    In this example, on impact of the projectile  24  with the fuel tank  16 , one or more high pressure shock waves  30  tend to be generated. 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 assembly  18   b . Furthermore, the assemblies tend to disrupt the shockwaves 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. 
         [0065]    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 sheets  22   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. 
         [0066]    Additionally, in this example, the first assembly  18   a  (which is disposed on the upper skin  10 ) is located within the fuel tank  16  such that the shock waves  30  resulting from compression of the fluid in the fuel tank  16  resulting from impact of the projectile  24  with the lower skin  12  impinge on the first assembly  18   a  so that the shock waves  30  interact with the first assembly  18   a  before impinging on the upper skin  10 . The first assembly  18   a  may reflect incident shock waves at least to some extent. Also, the first assembly  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 assembly  18   a . The assemblies  18   a ,  18   b  advantageously tend to decouple the fluid from walls of the fuel tank  16 . 
         [0067]    Furthermore, were the projectile  24  to continue through the cavity  14  and impact with the first assembly  18   a , the first assembly  18   a  would tend to cause further retardation of the projectile  24 , thereby further reducing impact energy and reducing force experienced by at least the upper skin  10 . 
         [0068]      FIG. 4  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in the aircraft wing  2  in which a second embodiment of a hydrodynamic ram reducing assembly is implemented. 
         [0069]    In this embodiment, the fuel tank  16  comprises a hydrodynamic ram reducing assembly, hereinafter referred to as “the third assembly” and indicated in  FIG. 4  by the reference numerals  18   c . The third assembly  18   c  is attached to an internal surface of the lower skin  12 . 
         [0070]    The third assembly  18   c  comprises a plurality of sheets of material, hereinafter referred to as the “third sheets”  22   c , and an envelope  34 . 
         [0071]    In this embodiment, the third sheets  22   c  are arranged as a stack of sheets. The stack of third sheets  22   c  are encased, i.e. wholly contained, within the envelope  34 . The envelope  34  is a non-permeable membrane such that the third sheets  22   c  are isolated from fluid (e.g. aircraft fuel) in the fuel tank  16 . 
         [0072]    An example method of producing the third assembly  18   c  is as follows. Firstly, the third sheets  22   c  are stacked together, i.e. arranged as a stack of sheets atop one another. Secondly the stack of third sheets  22   c  is placed into the envelope  34 . Thirdly, air is drawn out of the envelope  34 . Lastly, the envelope  34  is sealed. The seal of the envelope  34  is an air-tight seal. 
         [0073]    In some embodiments, substantially all of the air is drawn out of the envelope  34  such that the third sheets  22   c  are held in a vacuum within the envelope  34 . This tends to provide that the third sheets  22   c  are forced together and provides for relatively high friction between the third sheets  22   c.    
         [0074]    In some embodiments, some air, or another fluid, is retained in the envelope  34 . For example, air may be trapped between the third sheets  22   c  within the envelope  34 . Thus, the envelope  34  may include a cavity 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 . Preferably, the fluid (e.g. air) in the cavity in the envelope  34  has a density sufficiently different from the density of the liquid in the fuel tank  16  to provide that the cavity is crushable. Preferably, the fluid (e.g. air) in the cavity in the envelope  34  has a density sufficiently different from the density of the liquid in the fuel tank  16  to provide substantially total reflection within the third assembly  18   c  of the shock wave or waves impinging on the third assembly  18   c  thereby to reduce the hydraulic ram pressure in the liquid in the fuel tank  16 . 
         [0075]    In this embodiment, the envelope  34  is fixedly attached, using an adhesive, to the internal surface of the lower skin  12  that defines the fuel tank  16 . Preferably, substantially the entirety of the underside of the envelope  34  is fixed to the internal surface of the lower skin  12 , for example, by a layer of adhesive that has been applied to the entirety of the lower surface of the envelope  34 . The terminology “layer of adhesive” very broadly refers to any type of coating layer which is adhesive/tacky towards an arbitrary kind of surface, such as in particular towards an aircraft skin surface. 
         [0076]    In other embodiments, only part of the underside of the envelope  34  is fixed to the internal surface of the lower skin  12 , for example, in some embodiments the envelope  34  is attached to the fuel tank  16  along one or more edges of the envelope  34 , and an intermediate portion of the envelope  34  is not directly adhered to the tank walls. 
         [0077]    In this embodiment, the third assembly  18   c  is fixedly attached to the walls of the fuel tank  16 . In some embodiments, fixed attachment of the envelope  34  to the walls of the fuel tank  16  is provided by a force required to detach the envelope  34  from the wall of the fuel tank  16  (i.e. a force with which the layer of adhesive retains the envelope  34  against the wall of the fuel tank  16 ) being greater than a force required for the projectile  24  to penetrate the envelope  34  and/or a sheet (i.e. a penetration force). In some embodiments, fixed attachment of the envelope  34  to the walls of the fuel tank  16  is provided by a force required to detach the envelope  34  from the wall of the fuel tank  16  (i.e. a force with which the layer of adhesive retains the envelope  34  against the wall of the fuel tank  16 ) being greater than tensile and/or compressive strength of that sheet. 
         [0078]    In other embodiments, the third assembly  18   c  may be releasably attached to fuel tank walls, e.g., by threaded pins or other releasable attachment means. 
         [0079]    In this embodiment, in the fuel tank  16 , internal static pressure of fuel in the fuel tank  16  tends to push the third sheets  22   c  of the third assembly  18   c  together, and against the internal surface of the fuel tank  16 . This tends to result in increased friction between the third sheets  22   c . An increased level of impact energy of the projectile  24  tends to be absorbed by the movement and deformation of the third sheets  22   c , for example, due to having to overcome the increased friction between individual third sheets  22   c  as they move relative to each other during penetration. 
         [0080]    Preferably, the third sheets  22   c  cover substantially the entirety of the portion of the internal surface of the lower skin  12  that defines the fuel tank  16 . 
         [0081]    Preferably, there are at least  5  third sheets  22   c . More preferably, there are at least  10  third sheets  22   c . More preferably, there are at least  20  third sheets  22   c . Surprisingly, having air trapped within the envelope  34  of the third assembly tends to provide a hydrodynamic ram damage reduction equivalent to having a greater number of sheets in the assembly. Thus, by having air trapped within the envelope  34 , an improved solution, with regard to minimising assembly mass and quantity of fluid displaced from the fuel tank due to the presence of the assembly, tends to be provided. 
         [0082]    In this embodiment, the third sheets  22   c  are flexible sheets made of a fibre-based material, for example woven, crimped/stitched or a random matting of fibres. In this embodiment, the third sheets  22   c  are fabric. The fabric may include ballistic fibres. Each of the third sheets  22   c  is thin, for example, each sheet may have a thickness of between 0.1 mm and 0.5 mm, for example 0.25 mm. Preferably, the third sheets  22   c  are less than 1 mm thick. More preferably, the third sheets  22   c  are less than 0.5 mm thick. Each of the third sheets  22   c  is made of a tough and strong material such as 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). 
         [0083]    In this embodiment, the third sheets  22   c  are substantially continuous. However, in other embodiments, one or more of the third sheets  22   c  is not continuous, for example, one or more of the sheets may include a plurality of perforations, e.g., a sheet may be made of a mesh or net-like material. 
         [0084]    Preferably, the third assembly  18   c  occupies less than 15% of the total internal volume (i.e. capacity) of the fuel tank  16 . 
         [0085]    In this embodiment, the fuel tank  16  includes a single third assembly  18   c . However, in other embodiments, the fuel tank  16  includes one or more further assemblies, such as a further third assembly  18   c , or a different type of assembly such as a first or second assembly  18   a ,  18   b . For example, in some embodiments, the fuel tank  16  includes a further third assembly that is attached to the portion of the internal surface of the upper skin  10  that defines the fuel tank  16 . 
         [0086]    As will now be described in more detail, the third assembly  18   c  reduces 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 . 
         [0087]      FIG. 5  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. 5  by the reference numeral  26 . 
         [0088]    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. 
         [0089]    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 . After passing through the lower skin  12 , the projectile  24  impacts with the third assembly  18   c . On impact of the projectile  24  with the third assembly  18   c , the third assembly  18   c  tends to be deflected and accelerated at least to some extent. The projectile  24  impacting with the third assembly  18   c  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 one or more of the third sheets  22   c , thereby reducing the energy introduced into the fluid directly by the projectile  24 . 
         [0090]    Deflection of the third sheets  22   c  by the projectile tends to be facilitated by the third sheets  22   c  not being relatively flexible and able to move to some degree within the fuel tank  16 . 
         [0091]    In this example, the projectile  24  travels through (i.e. pierces or penetrates) the third sheets  22   c . Impact energy of the projectile  24  is used to pierce the third sheets  22   c , thereby reducing the energy introduced into the fluid by the projectile  24  and retarding at least to some extent the passage of the projectile  24  into the fluid. 
         [0092]    In some situations, the projectile  24  does not travel through (i.e. does not pierce or penetrate) one or more of the third sheets  22   c . In this example, when travelling within the cavity  14  of the fuel tank  16 , the projectile  24  in combination with the third assembly sheets  22   c  moved by the projectile  24  tends to experience a greater overall drag force from the fluid in the fuel tank  16  compared to that that would be experienced by the projectile  24  if the third sheets  22   c  were not present. This tends to be at least in part due to the increased surface area of the combination of the projectile  24  and second sheets  20   b  compared to projectile  24  alone. Thus, the passage of the projectile  24  through the fluid in the fuel tank  16  tends to be retarded. 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. 
         [0093]    In this example, on impact of the projectile  24  with the fuel tank  16 , one or more high pressure shock waves  30  tend to be generated. 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 third assembly  18   c . Furthermore, the third assembly  18   c  tends to disrupt the shockwaves  30  travelling through the fluid in the fuel tank  16  and thereby tends 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. 
         [0094]    The projectile  24  travelling through the third assembly  18   c  tends to generate shockwaves  30  within the fuel tank  16  that travel in directions outwards and along the lower aircraft skin  12 . Advantageously, the arrangement of the third sheets  22   c , for example the relatively high friction between the third sheets  22   c , tends to provide that at least part of the kinetic energy causing such shockwaves  30  is absorbed by the third assembly  18   c . In addition, the air or fluid trapped within the third assembly envelope  34  substantially reflects and/or reduces the shock pressures moving across the lower skin  12 . Further, shockwaves  30  travelling across the surface of the third assembly  18   c  to the edges of the fuel tank  16  where the lower external skin  12  is coupled to the spars  6  and ribs  8  tend to be reduced. Thus, the amplitudes of the shockwaves  30  that impinge upon the external skins  10 ,  12 , the spars  6 , and the ribs  8  tend to be diminished. 
         [0095]    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 third assembly  18   c , 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. 
         [0096]    In some cases, the projectile  24  does not pierce one or more of the third sheets  22   c . In such cases, one or more of the third sheets  22   c  move with the projectile  24  away from other third sheets  22   c  in the envelope  34 . The projectile  24  with one or more of the third sheets  22   c  coupled thereto tends to have a much larger surface area than the projectile  24  alone. Thus, the projectile  24  with one or more of the third sheets  22   c  coupled thereto tends to experience a greater drag force when moving through the fluid in the fuel tank  16  compared to that that would be experienced by the projectile  24  alone. Thus, the passage of the projectile  24  through the fluid in the fuel tank  16  tends to be retarded. 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. 
         [0097]    In some cases where the projectile  24  does not pierce one or more of the third sheets  22   c , those third sheets  22   c  may remain fixedly attached to the wall of the fuel tank  16  by the envelope  34 . Thus, the projectile  24  may be prevented from travelling further into the fuel tank  16 . At least some of the impact energy of the projectile  24  tends to be absorbed by the third sheets  22   c  and therefore not transferred to the aircraft substructure  4 . Also, the fluid-proof envelope  34  tends to prevent fluid exiting the fuel tank  16  via the hole pierced in the lower skin  12  by the projectile  24 . 
         [0098]    An advantage provided by the above described assemblies 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. 
         [0099]    The above described assemblies advantageously tend to be relative easy and cheap to manufacture. 
         [0100]    The above described assemblies tend to be relatively easy to retrofit to existing aircraft fuel tanks. 
         [0101]    The above described assemblies tend to provide protection against hydrodynamic ram damage whilst occupying a relatively small amount of the fuel tank&#39;s capacity. 
         [0102]    The above described assemblies tend to be relatively lightweight so as not to be a significant burden to the aircraft. 
         [0103]    In some embodiment, the stack of sheets of an assembly is enclosed in an envelope, i.e. a container, such as a sealed bag which may be made of a liquid impermeable material such as a plastic. This advantageously tends to facilitate fitting of the assembly into the fuel tank. Furthermore, this advantageously tends to prevent or oppose contamination of the fuel within the fuel tank with contaminants that may be present in or on the sheets (e.g. water or loose sheet fibres). Furthermore, this advantageously tends to prevent or oppose the sheets of the assembly becoming saturated with fuel in the fuel tank. 
         [0104]    In the above embodiments, the assemblies are implemented in an aircraft wing fuel tank. However, in other embodiments, the assemblies 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. 
         [0105]    In the above embodiments, assemblies are disposed on the internal surfaces of the upper and/or lower aircraft skins. However, in other embodiments an assembly may be disposed on a different surface of the fuel tank instead of or in addition to 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 have one or more assemblies attached thereto. In some embodiments, an assembly is only disposed on a single surface of the fuel tank, for example, on only the internal surfaces of the lower aircraft skin. 
         [0106]    In the above embodiments, the sheets of the assemblies are attached to the walls of the fuel tank by threaded pins, or by being contained in an envelope that is adhered to the fuel tank wall. However, in other embodiments, one or more of the sheets of one or more of the assemblies may be attached to a wall of the fuel tank using a different appropriate attachment means. For example, a pin having one or more barbs arranged to permit the sheets to be pushed onto the pin, but oppose removal of a sheet, may be used. In some embodiments, a fastener that is configured to release the sheets in response to a projectile impact is used. In some embodiments, the sheets may be bonded or adhered to a substructure component (i.e. a spar, rib, or skin) in such a way that the sheet “peels” away and detaches from the substructure component when impact loaded. In other embodiments, sheets have one or more further attachment points spaced across them. 
         [0107]    In the above embodiments, each assembly is attached to an internal surface of the fuel tank such that the sheets lie across and proximate to that internal surface. However, in other embodiments, one or more assemblies may be located at a different position within the fuel tank. For example, in some embodiments, an assembly may be located in a “mid-tank” position, for example, such that the sheets of the assembly are remote from the upper and lower skins, e.g. substantially equidistant from and parallel to the upper and lower skins.  FIG. 6  is a schematic illustration (not to scale) showing a cross section through the fuel tank  16  in which a further embodiment of an assembly, hereinafter referred to as the “fourth assembly”  18   d  is implemented. The fourth assembly  18   d  comprises a plurality of sheets of material, hereinafter “fourth sheets”  22   d , which are arranged as a stack and are located in the fuel tank substantially equidistant from and parallel to the upper and lower skins  10 ,  12 . The fourth sheets  22   d  are loosely retained in position by threaded rods  32  extending between the upper and lower skin  10 ,  12 . In some embodiments, the fourth sheets  22   d  may be directly attached to the spars  6  or ribs  8  e.g. using an adhesive. 
         [0108]    In the above embodiments, an assembly includes a plurality of sheets of material. The sheets are fabric layers made of a ballistic fibre-based material, for example woven, crimped/stitched or a mat of fibres. However, in other embodiments, the sheets of material are made of different material to those described above and/or may have different dimensions to those provided above. In some embodiments, the term “sheet” refers to an article having a thickness that is much less than its length and width, for example a thickness that is at least 10 times smaller than its length and width, or a thickness that is at least 100 times smaller than its length and width, or a thickness that is at least 1000 times smaller than its length and width. The terminology “sheet” or “sheet of material” broadly refers any type of fabric, cloth, film, layer or sheet material and includes, but is not limited to, layers or material including elastic fabric materials (knitted, woven or non-woven), apertured plastic or non-plastic films, sheets of polymeric foam with open or closed cells, non-woven materials in general, breathable elastic materials in general, perforated or non-perforated breathable polyurethane sheet materials, extruded materials such as extruded films, and the like. 
         [0109]    In some embodiments, one or more of the assemblies includes only a single sheet of material. 
         [0110]    In the above embodiments, the sheets of material of an assembly are arranged as a stack, i.e. the sheets are layered one on top of another. In some embodiments, the stacked sheets may be substantially parallel or aligned. In some embodiments, for each sheet in a stack of sheets, an upper or lower surface of that sheet may be in contact with an upper or lower surface of an adjacent sheet in the stack. Thus, in some embodiments, if a stack of sheets includes three or more sheets, at least one sheet is sandwiched between two other sheets. However, in other embodiments, a sheet of material of an assembly is not in a stack of sheets. For example, in some embodiments, an assembly includes a sheet that has been folded one or more times to provide multiple (parallel) layers of the material. The sheets may be oriented within the fuel tank in any appropriate direction. 
         [0111]    In the above embodiments, the sheets of material are flexible. For example, a sheet of material may be sufficiently flexible (i.e. have low stiffness) such that, when impacted by a projectile, that sheet of material deforms or bends to wrap around, at least to some extent, or wholly envelop, that projectile. However, in other embodiments, one or more of the sheet are not flexible, i.e. one or more of the sheets is rigid such that, when impacted by a projectile, that sheet does not wrap around the projectile to any extent. In some embodiments, the sheets of material are sufficiently flexible to inhibit undesirable transfer of structural loads into them, ensuring the aircraft structure operates as desired in terms of providing designed structural load paths.