Patent Publication Number: US-2023143288-A1

Title: Cryogenic tank

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of European Patent Application No. 21207552, filed 10 Nov. 2021, the contents of which is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates to a cryogenic tank for supplying a propulsion system of a vehicle, and related aspects. 
     BACKGROUND 
     Cryogenic tanks for containing cryogenic fuel have been widely used in space applications for some time, and have attracted some more recent interest in non-space applications such as fixed wing aircraft, eVTOL and marine applications, which have challenging new requirements if satisfactory performance is to be obtained. The present invention builds on this body of work, with an aim to produce an improved cryogenic tank. 
     SUMMARY 
     In general terms, the disclosure provides a cryogenic tank for storing cryogenic fluids. The cryogenic tank is typically configured to be mounted on a vehicle for supplying cryogenic fuel to a propulsion system of the vehicle. The cryogenic tank comprises an inner vessel for containing cryogenic fluids and an outer vessel surrounding the inner vessel to define a vacuum insulating volume therebetween. The outer vessel is configured to transmit static and/or dynamic loads, while the inner vessel is at least partially isolated from such loads. 
     The terms cryogenic fluids and cryogenic fuels used herein have their typical meaning as used in the art. Cryogenic fluids or fuels typically have a boiling point of below 120 Kelvin. Cryogenic fuels include liquified gases such as liquid hydrogen. 
     A first aspect of the disclosure provides a cryogenic tank for supplying a propulsion system of a vehicle, the tank comprising: an inner vessel defining a closed volume configured to carry a cryogenic fuel; an outer vessel enclosing the inner vessel to define an insulating volume therebetween, the insulating volume comprising a vacuum, and the outer vessel comprising one or more mounting members for mounting the tank on a vehicle, the one or more mounting members permitting transfer of static and/or dynamic loads between the vehicle and the outer vessel; and vessel mounting means structurally connecting the inner vessel to the outer vessel, and configured to avoid transfer of said static and/or dynamic loads from the outer vessel to the inner vessel. 
     By transmitting loads through the outer vessel the overall mass and size of the both the vehicle and tank can be minimised since it is not necessary to contain the tank within further structure specifically designed to carry any loads associated with thermal effects, vehicle operation, propulsion etc. This is particularly important in applications where the volume or mass of the tank may be influential on the overall efficiency or viability of a vehicle. For example, in aircraft applications mass must be minimised as a priority, since every extra kilogram has a measurable effect on aircraft efficiency. Moreover, the tank has benefits in applications in which the tank defines the envelope of a vehicle. For example, in aircraft applications the tank must either be accommodated within the aerodynamic envelope of the airframe (e.g. within a wing or fuselage section) or it must be suspended outside of that envelope and therefore be shaped and sized to minimise its impact on aerodynamic drag, including both profile drag and parasitic drag. In particular, the minimised tank volume minimises the surface area exposed to air flow and therefore minimises parasitic drag and enables profile drag to be readily managed. 
     The vehicle may be any vehicle with a propulsion system requiring a supply of a cryogenic fuel. For example, aircraft, marine vehicles, land vehicles, or space vehicles. The disclosed tank is considered to be particularly applicable to applications where weight and space-volume are important design factors. 
     The inner vessel may have any shape appropriate to its function as a receptacle for cryogenic fluids. The inner vessel may be configured to contain cryogenic fluids maintained at an above-atmospheric pressure. The inner vessel may thus be provided as a pressure vessel. That is, the inner vessel may have any shape or configuration appropriate for a pressure vessel. One appropriate shape comprises a central cylindrical portion capped by two domed portions. 
     The vacuum of the insulating volume may be a complete or partial vacuum. For example, a vacuum of 10 mTorr or less may be appropriate. In some embodiments a soft vacuum of 10,000 mTorr or less may be appropriate. In some embodiments the insulating volume is empty, save for structural or systems components of the tank. In other embodiments the insulating volume may contain microbeads (e.g. particles of an insulating material) or aerogel. For example, the insulating volume may contain a plurality of microbeads enclosed within a flexible membrane, or bag; this arrangement may permit some load transfer via the microbeads. 
     The outer vessel may have any shape appropriate to its functions of enclosing the inner vessel and mounting the tank on a vehicle. In particular, the outer vessel may have an outer surface that is configured to provide an outer surface of a vehicle on which the tank is mounted. That is, at least a portion of the outer vessel may have an outer surface that forms at least a portion an outer surface of a vehicle on which the tank is mounted in use. For example, in applications in which the tank is for mounting on an aircraft the outer vessel may have an outer surface that forms an aerodynamic surface of the aircraft. 
     The one or more mounting members may be configured to permit mounting of the tank to a load-carrying member of the vehicle. For example, in aircraft applications the one or more mounting members may be configured to permit mounting of the tank to the airframe, or joining it to the airframe as an integral structural member of the aircraft. 
     The term static and/or dynamic loads is intended to encompass thermal loads, vehicle loads (e.g. flight loads), propulsion loads, or any other forces that are generated within the vehicle, or at the interface between the vehicle and the one or more mounting members. Such loads are transmitted into the outer vessel via the one or more mounting members. Preferably, the one or more mounting members transmit these loads through the outer vessel. 
     Thus, the one or more mounting members may extend longitudinally along the outer vessel, each mounting member may comprise two or more longitudinally-spaced mounting locations for connecting the tank to a vehicle, and/or each mounting member may be configured to transfer of static and/or dynamic loads between the two or more mounting locations. In this way, the mounting members provide a convenient and efficient means of transmitting at least some of the loads transferred between the vehicle and the outer vessel. 
     In preferred embodiments the one or more mounting members include one or more flanges extending outwardly from the outer vessel. The one or more flanges may extend longitudinally along the outer vessel, to permit transfer of loads in a longitudinal direction. For example, in aircraft applications the one or more flanges may extend in a forward-aft direction. Alternatively, the one or more flanges may extend annularly around an outer periphery of the outer vessel. 
     The one or more flanges may each include one or more fastener holes at a mounting location, the one or more fastener holes being configured to accommodate one or more fasteners to connect the tank to a vehicle. The fasteners thus extend through the one or more flanges, but do not penetrate the insulating volume. Such an arrangement has the advantage of enabling ready attachment to a vehicle without providing a potential leak site for the sealed insulating volume. Such an arrangement may also be used to connect aerodynamic fairings and other vehicle components. The one or more fastener holes may be reinforced with a reinforcing component such as a bush. 
     The one or more flanges may have an increased thickness, width, height, volume or surface area in the region of each mounting location, in order to accommodate increased load transfer at the mounting locations. 
     The one or more flanges may be formed integrally with at least one panel of the outer vessel. This arrangement provides a particularly robust, weight-efficient solution. 
     The outer vessel preferably comprises one or more panels and one or more reinforcing members mounted on the one or more panels. The reinforcing members serve to increase stiffness of the panels. They may, for example, comprise stringers and/or components formed from geodesic, geodetic or other typology optimised patterns such as a space frame or three-dimensional truss-like structure constructed from interconnecting struts arranged in a geometric, geodetic or geodesic pattern. 
     In particularly preferred embodiments the one or more reinforcing members project inwardly from the one or more panels of the outer vessel into the insulating volume. This is a particularly efficient use of the available space, and serves to minimise the overall envelope of the tank. It is also an advantageous arrangement in applications in which the outer vessel provides an outer (e.g. aerodynamic) surface of a vehicle on which the tank is mounted or is otherwise exposed in use. 
     In some examples the outer vessel comprises first and second panels, the first panel comprising one or more outwardly-extending flange portions that overlap with one or more corresponding regions of the second panel to thereby join the first and second panels and provide one or more flanges extending outwardly from the outer vessel. In this way, the panels may be joined at the one or more flanges. In particular, mechanical fasteners may be used to effect the join without penetrating the sealed insulating volume. 
     Preferably, the one or more flanges provide the one or more mounting members. Thus, the flanges resulting from the joining approach described above also provide a means for load transfer through the outer vessel. 
     The tank may comprise a plurality of fasteners joining the first and second panels, the plurality of fasteners extending through the one or more flanges and not penetrating the insulating volume. 
     The first and second panels may be joined by one or more of: mechanical fastening means, bonding, or co-curing. In specific embodiments the second panel comprises a pair of opposing walls, the first panel comprises a pair of flange portions, and the first panel is nested within the second panel such that the flange portions overlap with the walls. 
     In some embodiments an elongate sealing channel may be provided in at least one of the one or more outwardly-extending flange portions of the first panel or the overlapping corresponding region of the second panel, the sealing channel comprising (or being configured to comprise) an elastomeric elongate sealing component (e.g. an O-ring type seal) or an elongate bead of curable sealing material to provide a fluid-tight seal between the first and second panels. 
     In some embodiments the elongate sealing channel comprises one or more sealant ports in fluid communication with the sealing channel and an opening configured to enable insertion of the nozzle of a sealant gun. Thus, curable sealant material can be injected into the sealing channel via the one or more sealant ports. In embodiments in which there are a plurality of sealant ports, sealant material injected into a first of the one or more sealant ports may exit via a second (or more) of the one or more of the sealant ports when the sealing channel is sufficiently filled with sealant material. Thus, the sealant ports may provide an indicator that the sealant channel is filled with sealant material. 
     The first and second panels may define a generally tube-shaped (e.g. cylindrical) volume, and the outer vessel may further comprise first and second end caps to seal the generally tube-shaped (e.g. cylindrical) volume, optionally wherein the first and/or second end cap comprises a generally dome-shaped member, and further optionally wherein the second end cap comprises a bulkhead. The tube-shaped volume may have any cross-sectional shape suitable for both maintaining the vacuum in the insulating volume (e.g. by acting as a pressure vessel) and providing an appropriate outer geometry (e.g. an appropriate aerodynamic surface in aircraft applications). Appropriate cross-sectional shapes include circles, distorted circles, ellipses, or multi-lobed circles, for example. 
     In some embodiments an elongate sealing channel may be provided in the first and/or second end caps, the sealing channel comprising (or being configured to comprise) an elastomeric elongate sealing component (e.g. an O-ring type seal) or an elongate bead of curable sealing material to provide a fluid-tight seal between the first end cap and the first and second panels, and/or between the second end cap and the first and second panels. For example, the sealing channel may comprise an annular sealing channel extending around a periphery of the generally tube-shaped volume. 
     In some embodiments the elongate sealing channel comprises one or more sealant ports in fluid communication with the sealing channel and an opening configured to enable insertion of the nozzle of a sealant gun. Thus, curable sealant material can be injected into the sealing channel via the one or more sealant ports. In embodiments in which there are a plurality of sealant ports, sealant material injected into a first of the one or more sealant ports may exit via a second (or more) of the one or more of the sealant ports when the sealing channel is sufficiently filled with sealant material. Thus, the sealant ports may provide an indicator that the sealant channel is filled with sealant material. 
     In some embodiments at least one of the one or more flanges extends longitudinally along the outer vessel. In such embodiments the at least one flange may compriss two or more longitudinally-spaced mounting locations for connecting the tank to a vehicle, and said at least one flange may be configured to transfer of static and/or dynamic loads between the two or more mounting locations. 
     In yet further embodiments at least one of the one or more flanges may extend around a perimeter of the outer vessel. 
     The vessel mounting means is preferably configured to permit relative movement between the inner vessel and the outer vessel. This arrangement serves to reduce, avoid or prevent transfer of loads from the outer vessel to the inner vessel. 
     The vessel mounting means preferably comprises a thermal insulating material for limiting heat transfer between the inner vessel and the outer vessel. This arrangement serves to reduce, avoid or prevent transfer of thermal loads between the outer and inner vessels and to reduce thermal heat transfer or losses into the stored cryogenic fluid. 
     In some embodiments the vessel mounting means comprises a first mounting member connected to the inner vessel and a second mounting member connected to the outer vessel, the first mounting member and second mounting member being interconnected to permit relative linear and/or rotational movement therebetween, and optionally wherein the shaft comprises a thermal insulating material for limiting heat transfer between the first mounting member and the second mounting member. Preferably the second mounting member comprises a shaft and the first mounting member comprises a sleeve that is mounted on the shaft such that it is able to slide along the shaft and rotate relative to the shaft. The first and second mounting members are preferably located generally at a longitudinal axis of the tank. 
     At least one or more panels of the outer vessel may be formed from the same material as the inner vessel, or from a different material. The thermal loads caused by differing thermal expansion coefficients of combinations of different materials may be mitigated by the vessel mounting means described herein. 
     A second aspect of the disclosure provides a vehicle comprising a load-carrying member configured to transmit static and/or dynamic loads, a propulsion system, and a cryogenic tank according to the first aspect, wherein the cryogenic tank is configured to supply the propulsion system, and the one or more mounting members structurally connect the outer vessel of the cryogenic tank to the load-carrying member of the vehicle. 
     A third aspect of the disclosure provides an aircraft comprising an airframe configured to transmit aircraft flight loads, a propulsion system, and a cryogenic tank according to the first aspect, wherein the cryogenic tank is configured to supply the propulsion system and the one or more mounting members structurally connect the outer vessel of the cryogenic tank to the airframe. 
     The airframe optionally comprises a wing spar, and the one or more mounting members optionally structurally connect the outer vessel of the cryogenic tank to the wing spar. 
     The outer vessel of the cryogenic tank optionally comprises an integral structural member of the airframe. For example, the outer vessel may include one or more stiffening members or one or more panels of the airframe. The stiffening members may include fuselage frames, wing spars, wing ribs or engine-mounting pylon structure, for example. The one or more panels may include one or more outer panels of the aircraft, such as wing cover panels or fuselage panels. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and do not exclude other components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    schematically illustrates a number of features of a cryogenic tank; 
         FIG.  2    illustrates appropriate vacuum levels for the insulating volume; 
         FIG.  3    illustrates a cryogenic tank secured to a wing box of an aircraft; 
         FIG.  4    illustrates a cryogenic tank secured to a fuselage of an aircraft; 
         FIGS.  5 A and  5 B  illustrate a cryogenic tank forming a fuselage section of an aircraft; 
         FIG.  6    provides an exploded view of the main components of the outer vessel of a cryogenic tank; 
         FIG.  7    provides a sectional view illustrating the attachment of the longitudinal flange of a cryogenic tank to an airframe of an aircraft; 
         FIG.  8    schematically illustrates a vessel mounting means of a cryogenic tank; 
       and 
         FIGS.  9 A,  9 B and  9 C  schematically illustrate possible sealing configurations. 
     
    
    
     DETAILED DESCRIPTION 
     In general terms, the disclosure provides a cryogenic tank  100  for storage and dispensing of cryogenic fluids. For example, cryogenic fluid in the form of liquid hydrogen, or a mixture of liquid and gaseous hydrogen. In the illustrated embodiments the tank  100  is for mounting on an aircraft to supply fuel (in the form of cryogenic fluid) to an aircraft propulsion system. However, in other embodiments the tank  100  may be applied to marine, land or space vehicles. Moreover, the tank  100  may have utility in any application where weight and space/volume are important design factors. 
     The tank  100  is illustrated schematically in  FIG.  1   , which shows an inner vessel  10  supported within an outer vessel  30 , and an insulating volume  50  therebetween. The insulating volume  50  is maintained at a vacuum, or near vacuum, to provide thermal insulation between the inner  10  and outer  30  vessels. As illustrated in  FIG.  2   , an appropriate target vacuum pressure is considered to be 10-3 mBar (approximately 0.75 mTorr), though a pressure of 10 mTorr or less may be sufficient. 
     The inner vessel  10  comprises a fluid-tight reservoir for containing liquid hydrogen  12  and gaseous hydrogen  14 . The fluid  12 ,  14  within the inner vessel  10  is typically at an above atmospheric pressure, and the inner vessel  10  thus forms a pressure vessel. The inner vessel  10  may have any one of a number of geometries appropriate to pressure vessels suitable for containing cryogenic fluids. In the illustrated embodiments the inner vessel  10  has a generally cylindrical central portion centred on a longitudinal axis  16 , the central portion being capped at each end by two domed or convex end caps. This shape has been demonstrated to provide optimised structural performance for most applications. 
     The outer vessel  30  can have any one of a number of geometries appropriate to the application of the tank  100 . For example, the outer vessel  30  may have an outer geometry optimised for aerodynamic performance in aircraft applications. The outer vessel  30  preferably has a generally cylindrical, or near-cylindrical, inner volume to provide a structurally optimised shape that minimises weight. In some aircraft applications the tank  100  may be designed to be mounted to an aircraft wing, while in others it may be mounted to an aircraft fuselage. In some cases the outer vessel  30  may be incorporated into the aircraft airframe structure, for example by incorporating fuselage frames, wing spars or ribs, or other structural airframe features. 
     Three example configurations for the outer vessel  30  are illustrated in  FIGS.  3 ,  4  and  5 A -B. 
     In  FIG.  3    the tank  100  is mounted underneath a wing box  200  of a wing. As is typical of known wings, the wing box  200  includes a front spar  210 , rear spar  220 , upper cover  230  and lower cover  240 . The wing box may be stiffened by ribs (not shown). The outer vessel  30  of the tank  100  comprises three pairs of mounting locations at which the tank  100  is mounted to the wing box  200 : a pair of front spar mounting locations  60  provide a connection to a lower region of the front spar  210 ; a pair of rear spar mounting locations  62  provide a connection to a lower region of the rear spar  230 ; and a pair of aft mounting locations  64  provide a connection to an upper region of the rear spar  230  via a rear support truss  66 . The aft mounting locations  64  and rear support truss  66  are included to avoid anticipated excitation-vibration issues, particularly in large tanks with a long longitudinal length, but it should be understood that they may be omitted in some embodiments. Of course, yet further mounting locations may be provided in yet further embodiments 
     The forward face of the outer vessel  30  carries a pair of propulsion system mounts  68  to which the propulsion system (not shown) of the aircraft is connected. These pinned fittings transfer a portion of the propulsion system loads through the outer vessel  30  and into the rear spar  220 . The propulsion system mounts  68  may comprise a machined metallic (e.g. aluminium) fitting that is fastened to the front bulkhead  46  of the outer vessel  30 . The fasteners do not penetrate the bulkhead  46  (i.e. do not extend into the insulating volume  50 ) in order to avoid creating a leak path should removal/replacement of the mounts  68  be required. Alternatively, the mounts  68  may be formed integrally with the bulkhead  46 . 
     A fairing (not shown) may conceal forward and rear portions of the tank  100  to improve its aerodynamic performance. In some embodiments there may be fewer than, or more than, two propulsion system mounts  68 . In particular, some embodiments may have no propulsion system mounts. 
     In  FIG.  4    the tank  100  is mounted beneath a fuselage section  300  of an aircraft. As is typical of known fuselages, the fuselage section  300  includes a series of circular frames (not shown) supporting an outer skin  310  that is stiffened by longitudinal stringers (not shown). The outer vessel  30  of the tank  100  comprises two pairs of mounting locations at which the tank  100  is mounted to two respective frames of the fuselage section: a pair of forward mounting locations  70 ; and a pair of rear mounting locations  72 . A fairing  320  conceals forward and rear portions of the tank  100  to improve its aerodynamic performance. 
       FIGS.  5 A and  5 B  shows the tank  100  integrated into a fuselage of an aircraft so that the outer vessel  30  of the tank forms a fuselage section  300 ′ of the aircraft connected between a first mating fuselage section  330  and a second mating fuselage section  332 . In these embodiments annular flanges  33  provide forward and aft mounting locations.In these embodiments the annular flanges  33  extend outwardly at the forward and aft ends of the outer vessel  30  to form a mating joint with the first  330  and second  332  fuselage sections, respectively. Thus, the annular flanges provide an integral butt-strap or lap joint. The joints may be fastened with fasteners, or by any other appropriate fastening means. 
     In the embodiment illustrated in  FIG.  5 A  a longitudinal flange  32  extending outwardly from an upper region of the outer vessel  30  may provide a mounting location for systems and/or control components (e.g. cabling or ducting) passing through the fuselage. A fairing  340  encloses the upper region of the tank  100  to conceal the flange  32  and the systems and/or control components to improve aerodynamic performance. In other embodiments the longitudinal flange  32  and the fairing  340  may be omitted, as shown in  FIG.  5 B . 
     In preferred embodiments the outer vessel  30  will be formed from a material with similar thermal properties to that of the airframe components to which it is connected, in order to avoid thermal loading effects. For example, the outer vessel  30  and airframe components may be formed from a reinforced fibre composite material, such as a carbon fibre reinforced composite material. 
     The outer vessel  30  is distinguished by its integral flanges, including longitudinal flanges  32  and annular flanges  33 . The mounting locations may be provided on the integral flanges, and/or the integral flanges may provide mounting sites for structural, systems or other components. In particular, the integral flanges may be joined to structural airframe components to enable load transfer between the outer vessel  30  and the airframe. 
     The longitudinal flanges  32  extend longitudinally along the outer vessel, and upon which the mounting locations are provided. That is, the outer vessel  30  comprises a pair of outwardly extending longitudinal flanges  32 , each flange providing one of each of the pairs of mounting locations described above. In the  FIG.  3    embodiment each flange carries one of: the pair of front spar mounting locations  60 ; the pair of rear spar mounting locations  62 ; and the (optional) pair of aft mounting locations  64 . In the  FIG.  4    embodiment each flange carries one of: the pair of forward mounting locations  70 ; and the pair of rear mounting locations  72 . Thus, the flanges  32  each provide a mounting member for mounting the tank  100  to an airframe of an aircraft. The flanges  32  may also provide mounting locations for other structural components, or for systems components. 
     The two annular flanges  33  (seen most readily in  FIGS.  5 A to  5 C , but present in all illustrated embodiments) extend around an outer periphery of the outer vessel  30 . As described below, the annular flanges  33  are formed at a join between the upper  38  and lower  34  panels and the end cap  44 , and at a corresponding join between the upper  38  and lower  34  panels and the bulkhead  46 . 
     The integral flanges  32 ,  33  may be shaped such that they have a greater height, thickness, volume and/or surface area in the region of a mounting location. In this way, secondary joining parts such as butt-straps can be eliminated, thus saving weight and design complexity. The flanges  32 ,  33  are also typically shaped to enable sealing at the joint between the flange and airframe structure. For example, where the flange provides a lap joint the flange may be shaped to provide a sufficient lapped area to enable adequate sealing. Similarly, the flange may be shaped to provide a close fit with adjoining structural components, as shown in  FIG.  3   . Thus, additional aerodynamic panels for sealing can also be eliminated. 
     In some embodiments the mounting locations are provided in the form of fasteners, e.g. pins or bolts, that pass through fastener holes in the flanges  32 . One example is illustrated in  FIG.  6   , which shows fasteners in the form of pins  42  that connect the flanges  32  to a structural member of the airframe  400 . In the illustrated arrangement the heads of the pins  42  are countersunk to avoid unwanted steps in the aerodynamic surface that could reduce aerodynamic performance. The pins  42  are located within tapered bushes  43  inserted into fastener holes in the flanges  32 , to create a locating and locking feature for the pins  42 . This design solution is considered particularly advantageous because it will not be subject to fatigue failure. 
     A key advantage of providing the mounting locations on the flanges  32  in this way is that the fasteners do not penetrate the insulating volume  50 . This both avoids the creation of a potential leakage route for vacuum or cryogenic fluids and the possibility of outgassing at a fastener site causing an electro-magnetic hazard, or lightning strike risk. 
     In preferred embodiments the outer vessel  30  is assembled from four main structural components, as illustrated in  FIG.  5   : lower panel  34 , upper panel  38 , end cap  44  and bulkhead  46 . 
     In the illustrated embodiment the lower panel  34  comprises a generally U-shaped member with a curved base  35  and a pair of upstanding opposing walls  36 . The lower panel  34  is formed from a thin sheet, or panel, which in this embodiment is reinforced by longitudinal stringers  37 . The upper panel  38  is generally W-shaped in cross-section, such that it has a curved top portion  39  sandwiched between two upstanding flange portions  40 . Like the lower panel, the upper panel  38  is formed from a thin sheet or panel, which in this embodiment is reinforced by longitudinal stringers  37 . In related embodiments the flange portions  40  (and their cooperating opposing walls  36 ) may be provided so that they are not parallel to one another as shown, but are instead formed at an angle to one another, for example at an acute angle so that they diverge in the manner of the arms of a V-shape. When assembled, the upper panel  38  is slotted into the lower panel  34  such that the flange portions  40  overlap with the upper portions of the upstanding walls  36  to form the longitudinal flanges  32  of the outer vessel  30 . This is best understood by review of  FIG.  6   . 
     In some embodiments a series of fasteners (not shown) connect the flange portions  40  with the upstanding walls  36 , with the benefit that this both avoids the creation of a potential leakage route for vacuum or cryogenic fluids and the possibility of outgassing at a fastener site causing an electro-magnetic hazard, or lightning strike risk. In other embodiments this connection may be provided by co-curing or co-bonding during manufacture of the outer vessel  30 . In mechanically fastened embodiments the joint between the flange portions  40  and upstanding walls  36  is sealed by providing a layer of interfay sealant  48  between the mating faces. An aerodynamic seal  49  (which can be attached to the flange) fills a gap between each flange  32  and the aircraft aerodynamic surface. 
     An end cap  44  encloses one end of the tube-like volume formed by the assembled upper  38  and lower  34  panels, and a reinforced bulkhead  46  encloses the other end. Either or both of these parts may comprise outwardly-extending annular flanges  33  to permit mounting of structural or systems components of the tank  100  or the airframe  400 . These outwardly-extending annular flanges  33  comprise an overlap (or lap joint) between upper  38  and lower  34  panels and the end cap  44  and a corresponding overlap (or lap joint) between upper  38  and lower  34  panels and the bulkhead  46 . These joints can be fastened via mechanical fasteners, bonding or co-bonding. Where mechanical fasteners are used as the joining means, the fasteners do not protrude into the insulating volume  50  of the tank avoiding the creation of a potential leakage route for vacuum or cryogenic fluids and the possibility of outgassing at a fastener site causing an electro-magnetic hazard, or lightning strike risk. 
     The bulkhead  46  is typically the final component to be assembled to enclose the outer vessel  30  around the inner vessel  10  and any systems components to be provided within the insulating volume  50 . 
       FIGS.  9 A-C  illustrate possible configurations for this joint to provide adequate sealing of the insulating volume  50 . The annular flange  33  is formed as a joint between mating annular flanges of the bulkhead  46  and the assembled upper  38  and lower  34  panels. A layer of interfay sealant  84  is provided between the mating surfaces. 
     In the arrangement shown in  FIG.  9 A  an annular sealing channel  80  extends around the mating face of the annular flange of the bulkhead  46 , an O-ring type seal  82  and/or bead of sealant material being provided within the sealing channel  80  during assembly to provide a fluid-tight seal between the bulkhead  46  and the assembled upper  38  and lower  34  panels. 
     In the arrangement shown in  FIGS.  9 B and  9 C  a bead of sealant material  86  is injected into the annular sealing channel  80  by a sealant gun  87  via one of a plurality of sealant ports  88 . The sealant ports  88  comprise channels extending through the full thickness of the assembled upper  38  and lower  34  panels, with an opening configured to enable insertion of the nozzle of a sealant gun  87 . When sealant material exits from another of the sealant ports  88  this indicates that the sealant channel  80  is adequately filled with sealant material. 
     This design in which the outer vessel  30  is assembled from a number of structural components simplifies the tooling required for its manufacture, while simultaneously providing a mounting member in the form of the longitudinal flanges  32  and annular flanges for mounting the tank  100  to the airframe and carrying flight loads, thermal loads and/or propulsion loads. 
     The inner vessel  10  is mounted within the outer vessel  30  by vessel mounting means located on the longitudinal axis  16 , the vessel mounting means including a fixed mount  20  and a floating mount  24 . 
     The fixed mount  20  is located at one axial end of the inner vessel  10  and provides a rigid connection between the two vessels, through which conduits  22  of the fuel systems enter the inner vessel  10 . The conduits  22  in this embodiment include a refuel line  22   a,  liquid fuel outlet line  22   b,  pressure line  22   c,  and vent line  22   d.  The floating mount  24  at the other axial end of the inner vessel  10  provides a floating (or flexible) connection between the inner  10  and outer  30  vessels, that allows relative movement therebetween. Thus, the inner vessel  10  is isolated (either fully or partially) from loads carried by the outer vessel  30 . That is, the inner vessel  10  experiences no or minimal distortion as a result of movement or deformation of the outer vessel  30  caused by flight loads, thermal loads, propulsion loads, or a combination of all three. 
     The floating mount  24  is illustrated in  FIG.  7   . A generally horizontally-orientated shaft  25  with an outer bearing surface is rigidly mounted to the domed end cap of the outer vessel  30  so that it projects inwardly into the insulating volume  50 . A cooperating sleeve  26  with a cooperating inner bearing surface is able to slide along the bearing surface of the shaft  25  and rotate relative to it about the shaft&#39;s longitudinal axis. The sleeve  26  is rigidly mounted to the domed end cap of the inner vessel  30 . The shaft  25  is formed either partially or fully from a thermally-insulating material such as a glass-fibre reinforced composite material (GFRP) in order to isolate the outer vessel  30  from the low temperatures of the cryogenic fluid contained by the inner vessel  10 . In alternative embodiments, the shaft  25  may have an alternative orientation; for example, it may be generally vertically-orientated. 
     In preferred embodiments the outer vessel  30  is formed from a composite material. For example, the lower panel  34 , upper panel  38  and end cap  44  may be moulded from a fibre reinforced composite material such as a carbon fibre reinforced material. The bulkhead  46  may be machine formed from a metal such as aluminium, or alternatively may be moulded from a fibre reinforced composite material. In some embodiments the lower panel  34 , upper panel  38  and end cap  44  may be integrally formed. The inner vessel  10  is preferably formed from a metal such as aluminium, but in some embodiments may also be fibre reinforced material.