Patent Publication Number: US-2023159165-A1

Title: Cryogenic tank for an aircraft and aircraft including such a tank

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of the French patent application No. 2112543 filed on Nov. 25, 2021, the entire disclosures of which are incorporated herein by way of reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a cryogenic fluid tank. More specifically, the invention relates to a liquid hydrogen storage tank for storing hydrogen on board an aircraft, and an aircraft carrying such a storage tank. 
     BACKGROUND OF THE INVENTION 
     Liquid hydrogen (or more specifically liquid dihydrogen) can be used as a power source on board an aircraft to power a fuel cell, or to operate a direct combustion. Storing liquid hydrogen used on board an aircraft requires very specific precautions, given the high flammability of hydrogen in the presence of oxygen. The hydrogen distribution network on board the aircraft, as well as the storage tanks, must be designed to prevent all risk of leaks, and to do so must have numerous features that are compiled and described in the manufacturing, testing and certification standards. In consideration of the space constraints in an aircraft, the hydrogen is preferably stored in liquid form, for example at a temperature of −253° C. (20K). Such a storage temperature impacts the distribution and storage installations, and potentially the elements close to these installations. 
     Liquid hydrogen storage tanks are commonly double tanks, notably made of aluminum and/or one or more composite materials. Most commonly, an inner tank is arranged inside an outer tank, also referred to as the outer envelope, and the volume between these two tanks helps to provide thermal insulation. The mechanical properties of the different materials vary as a function of temperature, and the temperature of the components of a hydrogen tank therefore varies between different usage phases (empty, filling, use, etc.). Consequently, depending on the structure of a liquid hydrogen tank, there may be significant mechanical stresses on account of the differences in coefficient of expansion between different materials and/or in expansion between the different elements. 
     Sliding mechanical links (or sliding links) are therefore provided between the inner tank and the outer envelope of a liquid hydrogen tank, for example using a sleeve that slides between two cylindrical necks, of which one cylindrical neck is arranged at the end of the inner tank (and outside the tank) and the other cylindrical neck is arranged at the end of the outer envelope (and inside the outer envelope or about an opening to the outside thereof). There are drawbacks to such a structure enabling a sliding link between the respective ends of the inner tank and the outer envelope positioned on the same side (pole) of a hydrogen tank. It is notably difficult to determine certain load conditions on account of variations in temperature conditions and inertial forces. Furthermore, fatigue may weaken the elements of such a structure as a result of the repeated movements, and there is a risk of jamming resulting from friction wear. Furthermore, the assembly methods may be complex since such structures tolerate little or no fitting tolerances in terms of the coaxiality of an inner tank and an outer envelope of such a tank. 
     The situation can be improved. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to propose a cryogenic tank for an aircraft that overcomes at least some of the drawbacks of the prior art. 
     For this purpose, a storage tank is proposed for a cryogenic fluid comprising an inner tank that is arranged to store the fluid and that is seated in an outer envelope, the inner tank and the outer envelope having a shared longitudinal axis, such that a thermal insulation volume surrounds the inner tank, and the outer envelope surrounds the volume about the inner tank, the tank being such that at least one damping element made of a deformable material (notably a flexible material) is inserted between one end of the inner tank and the outer envelope to wedge the inner tank against the outer envelope. 
     Advantageously, this enables a reliable sliding mechanical link to be formed between the inner tank, or more specifically at least one of the ends thereof, and the outer envelope, thereby increasing resistance to wear and facilitating assembly of the tank. 
     The storage tank for a cryogenic fluid according to the invention may also include the following features, taken individually or in combination:
         The shared longitudinal axis of the inner tank and the outer envelope defines a direction X, and the damping element is fastened firstly to the end of the inner tank and secondly to the outer envelope, and is arranged to create the wedge by positioning the end of the inner tank in relation to the outer envelope when the end of the inner tank moves in translation along the direction X.   First fastening means are arranged to attach the damping element to the end of the inner tank and second fastening means are arranged to attach the damping element to an inner surface of the outer envelope, the attachments of the damping element preferably being removable.   The damping element is ring-shaped, the first fastening means comprise a sleeve fastened to the end of the inner tank and have a first shoulder and a thread that are arranged to fasten the damping element about a portion of the sleeve, against the first shoulder, and to hold the damping element against the first shoulder by tightening an assembly comprising a washer and a nut onto the sleeve portion, and the second fastening means comprise a neck formed about an opening at one end of the outer envelope positioned on the same side of the tank as the end of the inner tank, the neck having a second shoulder, and a locking ring crimped onto the neck and arranged to hold the damping element against the second shoulder.   The damping element is ring-shaped, the first fastening means comprise a sleeve fastened to the end of the inner tank and have a first shoulder and a thread that are arranged to fasten the damping element about a portion of the sleeve, against the first shoulder, and to hold the damping element against the first shoulder by tightening a first assembly comprising a washer and a nut onto the sleeve portion, and the second fastening means comprise a neck formed about an opening at one end of the outer envelope positioned on the same side of the tank as the end of the inner tank, the inner surface of the neck having a slot into which is inserted a ring-shaped metal insert or an internal thread into which a ring-shaped metal insert is screwed, the inner surface of the insert having a second shoulder and a thread that are arranged to hold the damping element against the second shoulder by tightening an assembly comprising a washer and a nut in the ring-shaped metal insert.   The deformable material is an elastomer.   The deformable material has a plurality of metal inserts arranged parallel to the direction X.       

     The invention also relates to an aircraft including at least one storage tank for a cryogenic fluid as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features of the present invention are set out more clearly in the description given below of an example embodiment, the description being provided with reference to the attached drawings, in which: 
         FIG.  1    is a block diagram showing the arrangement of a cryogenic tank according to a first embodiment; 
         FIG.  2    is a perspective view of a fastening assembly between an inner tank and an outer envelope of a tank as shown in  FIG.  1   , before assembly; 
         FIG.  3    is a cross section of the fastening assembly shown in  FIG.  2   , after assembly; 
         FIG.  4    is a perspective view of the fastening assembly between an inner tank and an outer envelope of a cryogenic tank as shown in  FIGS.  2  and  3   , according to a variant embodiment and before assembly; 
         FIG.  5    is a cross section of the fastening assembly shown in  FIG.  4   , after assembly; and, 
         FIG.  6    shows an aircraft including a cryogenic tank as shown in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    is a schematic view of a cryogenic tank  10  arranged to store liquid hydrogen on board an aircraft, according to one embodiment. The term “cryogenic tank” here refers to a tank arranged to store a cryogenic liquid, such as hydrogen in liquid form, for example. In this description, the terms “hydrogen” and “liquid hydrogen” are used interchangeably and refer to hydrogen (H2) in liquid form, potentially combined with an inert gas or hydrogen in gaseous state. The upper part of  FIG.  1    shows the tank  10  containing a quantity of liquid hydrogen  11  and the lower part of  FIG.  1    shows the tank  10  containing only ambient air  1 . The length of the inner tank  14  is therefore increased by a length d by expansion when containing ambient air  1 , compared to the length of the tank containing liquid hydrogen  11 . The temperature of the hydrogen in liquid state causes a contracted form of the inner tank  14  compared to the form of the tank at ambient temperature. The hydrogen tank  10  comprises an inner tank  14  and an outer envelope  12  that are arranged about the same longitudinal axis  100 . The inner tank  14  and the outer envelope  12  both have an overall cylindrical shape and have ends with an overall spherical shape referred to as poles. The volume of the inner tank  14  is less than the volume of the outer envelope  12 , such that a thermal insulation volume  13  is formed between the outer surface of the inner tank  14  and the inner surface of the outer envelope  12 . According to one embodiment, a vacuum can be created in the thermal insulation volume  13  during manufacture of the tank  10  or by using a vacuum pump when using the tank in an installation. According to one variant, an insulating material or several layers of insulating material are used to provide thermal insulation between the inner tank  14  and the outer envelope  12 . 
     According to one embodiment, the inner tank  14  and the outer envelope  12  are held apart from one another at or in the vicinity of one of the poles of the tank  10  by a set of spacers  1214  arranged regularly and fastened between the outer surface of the inner tank  14  and the outer envelope  12 . Furthermore, a sliding link between the end  14   a  of the inner tank  14  and the outer envelope  12  is formed at the opposite pole of the tank  10 , in order to limit the mechanical stresses caused by expansion or contraction of the inner tank  14  during variations of temperature or caused by the pressure of the hydrogen contained in the inner tank  14 , notably during variations in temperature of the inner tank  14  caused by the temperature of the content thereof. According to one embodiment, the spacers  1214  are replaced by equivalent fastening means  1214  that are arranged to minimize the heat bridges between the inner tank  14  and the outer envelope  12 , thereby reducing the effect of the heat bridges. For example, the spacers  1214  are replaced by recessed links enabling the transfer of forces to the inner tank  14  in a direction parallel to the longitudinal axis  100  of the inner tank  14 , and more generally of the tank  10 . The sliding link arranged at the other end of the tank is primarily used to offset the expansion in a direction parallel to the longitudinal axis of the tank  10 . 
     Advantageously, the sliding mechanical link between the end  14   a  of the inner tank  14  and the end of the outer envelope  12  arranged opposite the end  14   a  of the inner tank  14  includes a damping element  16  that is arranged to wedge the end  14   a  against the outer envelope  12 . The term “wedge” here refers to a fitted position to efficiently hold the end  14   a  of the inner tank in relation to the pole of the outer envelope  12  facing the end, thereby holding the inner tank  14  in relation to the outer envelope  12 , while providing a sliding link able to offset the dimensional variations of the elements, notably the inner tank  14 . A wedge created by the damping element  16  between the end  14   a  of the tank  14  and the pole of the outer envelope  12  facing the end therefore means that an element rigidly connected to the end  14   a  is positioned to bear against a first surface of the damping element  16  and that an element rigidly connected to the pole of the outer envelope  12  positioned on the same side of the tank  10  as the end  14   a  of the inner tank  14  bears against at least one second surface of the damping element  16 , the first and second surfaces facing one another overall such that the damping element  16  is held (sandwiched) between the end  14   a  of the inner tank  14  and the pole of the outer envelope  12  facing the end, and creates a wedge at least in a direction X parallel to the shared longitudinal axis  100  of the inner tank  14  and the outer envelope  12 . Advantageously, the damping element  16  is made of a deformable material, which enables the element to hold the inner tank  14  in a stable position while offsetting a variation by a length d of the length of the inner tank  14  as a function of temperature and of internal pressure, notably when there is a cryogenic fluid such as liquid hydrogen in the inner tank  14 . The term “deformable material” here refers to a material having mechanical strength and deformation characteristics similar to an elastomeric material. 
     According to one embodiment, the damping element  16  is made of an elastomeric material or a material having compressibility and elasticity characteristics similar to an elastomer. Advantageously, first fastening means  140  are arranged and configured to fasten the damping element  16  to the end  14   a  of the inner tank  14  and second fastening means  160  are arranged and configured to fasten the damping element  16  to the pole of the outer envelope  12  positioned on the same side of the tank  10  as the end  14   a  (at the same pole). 
     Advantageously, the fastening means  140 ,  160  and the damping element  16  are configured to together form a wedge in three directions orthogonal to one another, of which direction X is parallel to the shared longitudinal axis  100 . Indeed, the forces of inertia and acceleration present during the flight phases and taxiing of an aircraft carrying the tank  10  or a similar tank are such that it is ideally beneficial to create a wedge along the roll, pitch and yaw axes of the aircraft, in addition to offsetting the dimensional variations of the described elements related to the temperature variations in the tank  10 . For this purpose and according to one embodiment, the damping element  16  takes the form of a hoop or ring of square, rectangular or any other section, threaded onto a sleeve fastened to the end  14   a  of the inner tank  14  and seated in a neck or a slot arranged in a polar opening of the outer envelope  12 . Naturally, such a structure is not limiting and other structures performing equivalent wedging functions between the end  14   a  of the inner tank  14  and the outer envelope  12  can be installed about the damping element  16 . 
     Advantageously, the near-incompressibility properties of the elastomer provide a good compromise between the wedge created and the shear strength required in consideration of the deformation of the inner tank  14  (contraction or expansion) and the deformation stresses resulting therefrom on the damping element  16  rigidly connected both to the first fastening means  140  and the second fastening means  160 . 
     According to one embodiment of the invention, metal inserts are provided (inserted during manufacture) in the damping element  16  in order to adjust the properties of incompressibility and shear strength in different directions Therefore, according to one embodiment, metal inserts in the form of plates are arranged parallel to the direction X (and therefore to the longitudinal axis  100 ). 
       FIG.  2    is a perspective view showing implementation details of the first fastening means  140  between the damping element  16  and the inner tank  14  and implementation details of the second fastening means  160  between the damping element  16  and the outer envelope  12 , about a polar opening in the envelope  12  of the tank  10 .  FIG.  2    is an exploded view of an arrangement of the fastening elements, before assembly, that are arranged to be assembled concentrically or near-concentrically about a sleeve  140   a  rigidly connected to the end  14   a  of the inner tank  14 . The damping element  16 , for example in the form of a ring of square section, is configured firstly to be threaded onto the sleeve  140   a  rigidly connected to the end  14   a  of the inner tank  14  (not shown in  FIG.  2   ), and secondly to also be at least partially seated in the neck  160   a  formed about the polar opening in the outer envelope  12 . The damping element  16  is therefore arranged to be positioned between the sleeve  140   a  and the neck  160   a . The sleeve  140   a  has a shoulder  1400  (shown in  FIG.  3   ) and an end thread such that the damping element  16  can be locked in position on the sleeve  140   a  against the shoulder  1400  using a washer  140   b  and a nut  140   c . According to one embodiment, the washer  140   b  and the nut  140   c  are serrated to enable locking in position after tightening. To fasten the damping element  16  in the neck  160   a  arranged about the polar opening in the outer envelope  12 , a locking ring  160   b  is designed to be positioned on the outer surface of the neck  160   a , then crimped onto the neck. According to one embodiment, the locking ring  160   b  is fastened onto the neck  160   a  by mechanical crimping or magnetic crimping, this method using magnetic pulses to deform two parts, one of which is crimped onto the other. A polar cover  170  is provided and configured to close the polar opening in the envelope  12  after assembly of the aforementioned elements comprising some or all of the first fastening means  140  and the second fastening means  160  of the damping element  16 . The polar cover  170  is positioned on the second neck  12   a , formed in the outer envelope  12 , arranged about the polar opening and having a diameter greater than the diameter of the neck  160   a.    
       FIG.  3    is a longitudinal cross section taken along a plane containing the shared longitudinal axis  100  of the inner tank  14  and the outer envelope  12 , showing the elements described in  FIG.  2    following assembly. The sleeve  140   a  onto which the damping element  16  is threaded is held on the end  14   a  of the inner tank  14  by means of a primary sleeve  140   d . Accordingly, the sleeve  140   a  and the primary sleeve  140   d  together form a single sleeve, one portion of which is the sleeve  140   a . Advantageously and according to an example embodiment, the sleeve  140   a  is made of titanium and the sleeve  140   d  is made of a composite material to minimize the thermal transfer towards the inner tank  14 . In this example, the sleeve  140   a  is incorporated into the drape forming of the sleeve  140   d  and slots made in the sleeve  140   a  guarantee good cohesion of the fibers of the sleeve  140   d  on the sleeve  140   a , these latter fitting the shape of the slots during drape forming of the sleeve  140   d .  FIG.  3    shows the positioning of the damping element  16  against the shoulder  1400  arranged on the outer surface of the sleeve  140   a  and the fastening of the damping element  16  on the sleeve  140   a  using a nut/washer assembly. A washer  140   b  is threaded onto the end of the sleeve  140   a  to bear against a face of the damping element  16  on the side of the damping element opposite a face bearing against the shoulder  1400 . A nut  140   c  is then tightened onto a thread arranged on the end sleeve  140   a  so that the damping element  16  is then sandwiched between the washer  140   b  held by the nut  140   c  and the shoulder  1400  of the sleeve  140   a.    
     Furthermore, the neck  160   a  arranged about the polar opening is configured to provide a seat for the damping element  16 , in conjunction with the sleeve  140   a . Once in position in this seat, the damping element  16  is prevented from moving in translation in the direction X by assembling and tightening the locking ring  160   b  onto the neck  160   a.    
     According to the embodiment described here, the first means  140  for fastening the damping element  16  to the inner tank  14  therefore comprise the primary sleeve  140   d  fastened to the end  14   a  of the inner tank  14 , the threaded sleeve  140   a , the washer  140   b , the nut  140   c  and the shoulder  1400 , and the second means  160  for fastening the damping element  16  to the outer envelope  12  therefore comprise the neck  160   a , a shoulder  1600  formed in the neck  160   a , and the locking ring  160   b  crimped onto the neck  160   a . Naturally, other fastening means performing equivalent functions of fastening the damping element  16  to both the inner tank  14  and the outer envelope  12 , thereby wedging the tank against the envelope, can be used. 
     According to a variant embodiment, the sleeve  140   a  and the primary sleeve  140   d  can be replaced by a single sleeve if the material used to manufacture the sleeve has low thermal conductivity (such as titanium). 
       FIG.  3    also shows the assembly of the polar cover  170  on the second neck  12   a  arranged at the end of the outer envelope  12 . 
       FIG.  4    is a perspective view showing a variant of the embodiment illustrated and described in relation to the  FIG.  2   , before assembly of the elements. According to this variant embodiment, only the neck  160   a  is formed about the polar opening of the outer envelope  12  of the tank  10 , i.e., the second neck  12   a  described above is not used. According to this variant, the means  140  for fastening the damping element  16  to the inner tank  14  are unchanged and as described above, but the means  160  for fastening the damping element  16  to the outer envelope  12  are different. According to this variant embodiment, a slot  160   r  is arranged in the inner surface of the neck  160   a  and a metal insert  160   c  is seated in this slot, mounted using liquid nitrogen assembly or inserted by molding, for example. According to another variant, the slot  160   r  is replaced by a threaded hole in the inner surface of the neck  160   a , and the insert  160   c , which then has a thread on the outer surface thereof, is screwed into the threaded hole. The metal insert  160   c  has an overall ring-shape and a thread is formed in the inner surface thereof, as well as the shoulder  1600  machined outside the threaded area. The thread formed in the inner surface of the metal insert  160   c  and the shoulder  1600  are configured so that the damping element  16  can be held between a washer  160   d  and the shoulder  1600  and so that a nut  160   e  can lock the washer  160   d  in position. Thus, in the variant embodiment in which the insert  160   c  is screwed into the neck  160   a  of the outer envelope  12 , the insert has two threads: a first external thread for fastening to the neck  160   a , and a second internal thread for screwing in the nut  160   e . Advantageously, the washer  160   d  also enables the nut  160   e  to be locked in rotation by folding at least one of the tabs thereof into one of the notches of the nut  160   e . Thus, the portion of the damping element  16  furthest away radially from the longitudinal axis  100  is sandwiched between the washer  160   d  and the shoulder  1600  in the ring-shaped metal insert  160   c . According to this variant, the diameter of the cover  170  is reduced to fit onto the neck  160   a  and is arranged to close the polar opening of the outer envelope  12 . 
       FIG.  5    is a longitudinal cross section taken along a plane containing the shared longitudinal axis  100  of the inner tank  14  and the outer envelope  12 , showing the elements described in  FIG.  5    following assembly of the sliding link. 
     Advantageously, the sliding links working according to the described examples using a damping element made of a flexible deformable material, such as an elastomer, possibly including reinforcing metal inserts or more exactly metal inserts adjusting the properties of near-incompressibility and shear strength, provide a sliding link between at least one end of the inner tank and a pole of the outer envelope of a cryogenic tank positioned on the same side as the end, by wedging and with no risk of deterioration related to repeated friction of one part with another part. 
     Naturally, the example embodiments described are not limiting and other variants are possible. For example, depending on the material used to manufacture the neck  160   a , the thread receiving the nut  160   e  and the shoulder  1600  can be formed directly in the inner surface of the neck  160   a . Although the metal insert is required if the neck  160   a  is made of composite material (for example a material based on glass fibers or carbon fibers), and preferable if the neck  160   a  is made of aluminum alloy, it is optional if the neck  160   a  is made of steel. 
       FIG.  6    shows an aircraft  2  comprising at least the cryogenic tank  10  or one or more similar tanks, i.e., having the described features of the tank  10 . This is particularly advantageous on board an aircraft to overcome the stated problems with the cryogenic tank according to the prior art. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.