This invention relates generally to storage vessels. More specifically, although not exclusively, this invention relates to storage vessels for containing liquid propellants, particularly for aircraft and more particularly space craft such as satellites.

The evolution of telecommunications technology has created a growing need for low earth orbiting (LEO) communication satellites. LEO satellites are more cost effective to launch into orbit than geostationary satellites and require a lower signal strength for communication due to their proximity to the ground. LEO satellites also enable the capturing of very high definition imaging of the earth's surface.

Current LEO satellites must retain enough fuel at the end of their life to position themselves for a controlled re-entry trajectory for the ocean to prevent debris from falling into a populated area, which could cause injury or property damage. The fuel required for this manoeuver would otherwise enable the satellite to remain functional for a longer period of time. There is therefore a desire to provide LEO satellites that do not require such controlled re-entry. This has led to efforts in designing satellites of which all but a negligible portion burns up during re-entry. These efforts have focused on replacing components that are fabricated from high melting point materials (e.g. steels and titanium) with lower melting material (e.g. aluminium) to increase the demisability of the satellite during re-entry.

Liquid propellants for satellites are generally cryogenic or hypergolic, both of which present operational challenges. Satellite fuel tanks must therefore be capable of operating at cryogenic temperatures and/or remain chemically stable when exposed to corrosive propellants. It is imperative that the tank materials are compatible with the liquid fuel chemicals, such as hydrazine. Titanium and titanium alloys are generally used for this purpose because of their high chemical compatibility and wettability with hydrazine, other propellants and oxidizers that are typically used to fuel satellite propulsion systems.

Design efforts to increase demisability have led to LEO satellite fuel tanks using aluminium alloys. However, such designs have necessitated the development of novel surface treatment technologies to increase the chemical compatibility and wettability. These surface treatments have proven to be expensive to implement and difficult to inspect on completed components. The Applicants have also observed that delivery of the propellant from the tank to the propulsion systems of the satellite requires a relatively complex system to ensure that it is accurate and controllable.

It is therefore a first non-exclusive aim of the invention to provide a storage vessel that is demisable and which enables effective delivery of a propellant contained therein. It is a further non-exclusive aim of the invention to provide a storage vessel which overcomes or at least mitigates one or more issues with known designs. It is a yet further non-exclusive aim of the invention to provide a storage vessel that provides one or more advantages over known designs.

Accordingly, a first aspect of the invention provides a storage vessel, e.g. for storing a propellant, the vessel comprising an internal surface defining a cavity and a diaphragm dividing the cavity into first and second isolated chambers, wherein the internal surface comprises a fluoroplastics material, e.g. encasing the cavity.

The provision of an internal surface comprising a fluoroplastics material makes the vessel chemically resistant to or compatible with a hypergolic fuel without the need to form the vessel or vessel wall with titanium, aluminium or their alloys. This enhances the demisability of the vessel and reduces weight. The use of a diaphragm enables a gas, such as nitrogen, to be used to expel the propellant from the tank whilst preventing the gas from mixing with the propellant. This enables accurate and controllable delivery of the propellant to the propulsion system. The introduction of pressurised nitrogen to expel a propellant such as hydrazine would, in the absence of such a diaphragm, create a mixture that could cause erratic and inconsistent thrust from the propulsion system. Known systems that use gases to expel the propellant require complex capillary systems and gas separation systems, which require both space and weight.

The diaphragm may be secured or bonded to the internal surface.

A second aspect of the invention provides a storage vessel, e.g. for storing a propellant, the vessel comprising an internal surface defining a cavity and a diaphragm dividing the cavity into first and second isolated chambers, wherein the diaphragm is bonded to the internal surface. The vessel according to the second aspect of the invention may comprise an internal surface defining a cavity. The internal surface may comprise a material and/or layer that is chemically resistant or compatible with a hypergolic fuel. The internal surface may comprise a fluoroplastics material.

The fluoroplastics material preferably comprises fluorinated ethylene propylene (FEP). However, the fluoroplastics material may comprise any fluoropolymeric material. For example, the fluoroplastics material may comprise one or more of fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), Polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF) in any combination.

The diaphragm may be flexible and/or be formed of or comprise a flexible material. The diaphragm may comprise a material and/or layer that is chemically resistant or compatible with a hypergolic fuel. The diaphragm may comprise a chemically compatible plastics material. The diaphragm preferably comprises a fluoroplastics material, but could alternatively comprise another chemically compatible plastics material, which may be resilient or elastomeric.

The diaphragm may comprise a fluoroplastics material that is the same, substantially the same or similar to the fluoroplastics material of the internal surface. The diaphragm preferably comprises fluorinated ethylene propylene (FEP), but may comprise any fluoropolymeric material, such as one or more of fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), Polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF) in any combination.

Thus, at least one or each of the first and/or second chambers is preferably encased in a material and/or layer that is chemically resistant or compatible with a hypergolic fuel, e.g. a fluoroplastics material.

The provision of two chambers, both of which is encased with chemically resistant or compatible material provides a failsafe, such that if a hole or crack is developed in the diaphragm any fuel that passes from the first chamber into the second is still contained by the chemically resistant or compatible material. The internal surface and the diaphragm may both comprise a fluoroplastics material, thereby encasing each of the first and second chambers in fluoroplastics material.

At least one of the first and second chambers may comprise a port, e.g. for the introduction of a fluid therein or expulsion of a fluid therefrom. One of the chambers may comprise a port for the introduction or expulsion of a fuel. One, e.g. the other, of the chambers may comprise a port for the introduction or expulsion of a pressurised fluid, e.g. for expelling fuel from the chamber containing a fuel. Alternatively, one, e.g. the other, of the chambers may be filled with a pressurised fluid and sealed. The ports may be on or in opposing portions of the vessel. For example, where the vessel is spheroidal, spherocylindrical or capsule shaped, the ports may be on opposing spheroidal portions thereof. Alternatively, where the vessel is another shape, for example cylindrical or cuboidal, the ports may be on opposing flat surfaces. The port or ports may be configured to enable the introduction or expulsion of fluid in or along a flow direction that is perpendicular to the diaphragm.

The inner surface or cavity may comprise one or more ribs therein or thereon, which may extend from or away from at least one or each port, e.g. radially or circumferentially therefrom and/or toward the other port. The ribs may be on or in, e.g. recessed in or protrude from, the inner surface or cavity. One or more ribs may be on or in the first chamber and/or one or more ribs may be on or in the second chamber. One or more ribs may be on or in the inner surface or cavity on one side of the diaphragm and/or one or more ribs may be on or in the inner surface or cavity on other side of the diaphragm.

The ribs may be configured to reinforce the vessel or layer or liner. The ribs may be sized or configured to maintain a space between the diaphragm and a facing portion of the inner surface or cavity and/or to inhibit the trapping of fluid between the diaphragm and a facing portion of the inner surface or cavity.

The diaphragm may be flat or planar, but is preferably non-flat or non-planar, for example curved, dome-shaped, concave or convex. The diaphragm may be secured at or about or adjacent its periphery to the internal surface of the vessel, for example such that it spans a section of the cavity. The diaphragm may comprise one or more ribs, which may extend radially or circumferentially, e.g. from the centre or adjacent the centre of the diaphragm to or toward a periphery thereof. The diaphragm may comprise a central portion or region, which may be non-flat or non-planar, for example curved, dome-shaped, concave or convex. The diaphragm may comprise a peripheral portion or region, which may circumscribe the central portion or region. The central portion or region may comprise the one or more ribs, which may extend radially or circumferentially from or toward the peripheral portion or region. The ribs may protrude from one or both major surfaces of the diaphragm. One or more ribs may protrude from one major surface of the diaphragm and/or one or more ribs may protrude from the other major surface of the diaphragm.

The ribs may be configured to reinforce the diaphragm and/or to cause the diaphragm to collapse in a predetermined manner or fashion. Additionally or alternatively, the diaphragm ribs may be sized or configured to maintain a space between the diaphragm and a facing portion of the inner surface or cavity and/or to inhibit the trapping of fluid between the diaphragm and a facing portion of the inner surface or cavity.

The storage vessel may comprise a wall, e.g. a vessel wall. The storage vessel or wall may include a shell, e.g. an outer shell, or a structural or fibre reinforced material, for example a fibre reinforced plastics material. The shell, structural or fibre reinforced material may at least partially surround an internal fluoroplastics layer or liner, which may define the internal surface. The wall may comprise one or more ribs formed thereon or therein, which may extend along an outer surface thereof. The one or more ribs may be aligned with and/or configured to cooperate with the one or more ribs described above in relation to the inner surface or cavity. Additionally or alternatively, the wall may comprise one or more ribs on an external surface thereof in the absence of ribs on the internal surface or cavity.

The fluoroplastics layer or liner may comprise an outer surface, which may be etched, for example chemically etched with a chemical etchant. The chemical etchant may be based on sodium metal, which may be dissolved in a super saturated solution, which may include tetrahydrofuran and naphthalene. Other chemical etchants are also envisaged, as are other forms of etching. However, the Applicants have found that the aforementioned chemical etchant is particularly effective. Where the outer surface of the fluoroplastics layer or liner is etched, the thickness thereof may be 1 mm or less, such as 0.9mm or less, e.g. 0.8mm or less. In fact, the Applicants have observed that the fluoroplastics layer or liner remains effective even at thicknesses of 0.7mm or less, such as 0.6mm or less, for example 0.5mm or even less in some applications. The shell, structural or fibre reinforced material may be bonded to the etched outer surface.

Additionally or alternatively, the fluoroplastics layer or liner or the or an outer surface thereof may comprise a backing, e.g. a fabric or fibre backing such as a fibre (e.g. carbon or glass fibre) knitted cloth. The backing may be bonded and/or at least partially embedded in an outer portion of the fluoroplastics layer or liner. The shell, structural or fibre reinforced material may be bonded to the backing. Where the fluoroplastics layer or liner includes a backing, the thickness thereof is preferably at least 1 mm, for example at least 1.1 mm, such as at least 1.2mm. More preferably, the thickness thereof is at least 1.3mm or at least 1.4mm, for example 1.5mm or more, e.g. 2mm or more.

The shell, structural or fibre reinforced material may be laid up on the fluoroplastics layer or liner. Additionally or alternatively, the fluoroplastics layer or liner may be bonded to preformed shell, structural or fibre reinforced material.

The storage vessel may comprise first and second concave parts. At least one or each part may comprise a fluoroplastics material, which may define a portion of the cavity. At least one or each part may comprise an open side. The open sides of the first and second parts may be secured together. At least one or each or both of the first and/or second parts may be secured with the periphery of the diaphragm. Preferably, the open sides of the first and second parts are secured together and with the periphery of the diaphragm such that the first chamber is defined by the first part and a facing surface of the diaphragm and/or such that the second chamber is defined by the second part and a facing surface of the diaphragm.

In embodiments, each of the first and second parts comprises a flange, for example about the periphery of the open side thereof. The flanges may be secured together, for example with the periphery of the diaphragm sandwiched therebetween. The diaphragm may comprise a flange, which may be flat or planar and/or which may be coextensive or at least aligned with the flanges of the first and second parts. Whilst not necessary, the use of flanges may facilitate manufacturing, e.g. securing or bonding of the parts and/or of the diaphragm, and/or handling of intermediate components and/or completed storage vessels. The flanges may also be used to connect the finished storage vessel to one or more brackets, for example to secure the storage vessel to one or more parts, e.g. a frame, of a space craft and/or to secure one or more components to the storage vessel.

The storage vessel may comprise first and second concave parts each defining a portion of the cavity and each having a flange about the periphery of an open side thereof, wherein the flanges are secured together with the periphery of the diaphragm sandwiched therebetween such that the first chamber is defined by the first part and a facing surface of the diaphragm and the second chamber is defined by the second part and a facing surface of the diaphragm.

In embodiments, the first and second parts may be bonded, e.g. fusion bonded or ultrasonically welded or bonded together by an adhesive, to one another. Additionally or alternatively, the first and/or second parts may be bonded to the periphery of the diaphragm. In embodiments, the first and second parts are secured together and/or to the diaphragm using other means, such as adhesives, fasteners and the like.

The or each of the first and second chambers may comprise a port for the introduction of a fluid therein or expulsion of a fluid therefrom. The vessel may comprise a coupling, connector or nozzle for coupling, connecting, receipt and/or cooperation with the port, for example to enable or facilitate connection thereto. In embodiments, the first chamber includes a port, coupling, connector or nozzle configured for connection with a source of fluid that may be pressurised, for example a source of pressurised gas such as nitrogen. The second chamber may include a port, coupling, connector or nozzle configured for connection with a source of fluid, e.g. a second source of fluid, such as a propellant, e.g. for filling the second chamber with propellant. The port, coupling, connector or nozzle of the second chamber may be configured for connection with a propulsion system, e.g. for supplying propellant thereto. At least one or each of the ports, couplings, connectors or nozzles may comprise a quick release coupling.

The method may comprise forming one or more ribs, e.g. as described above, in or on one or more of the internal surface, diaphragm and/or vessel wall. One or more ribs may be formed during the forming of the layer or liner, the diaphragm or the vessel wall. Additionally or alternatively, one or more ribs may be formed by bonding, welding or otherwise securing additional material on the layer or liner, the diaphragm or the vessel wall. The storage vessel may be demisable. The storage vessel may comprise or be a fuel tank for a space craft. The storage vessel may comprise one or more or only materials that are not radar reflective.

Other aspects of the invention provide a fuel tank comprising a storage vessel as described above, a space craft propulsion system comprising such a fuel tank or a storage vessel as described above and a space craft comprising such a fuel tank or propulsion system or a storage vessel as described above.

Another aspect of the invention provides a kit of parts for assembly into a storage vessel as described above. The kit may comprise any one or more components or intermediate products of the storage vessel. The kit may, for example, comprise one or more of a diaphragm and first and second concave parts, each of which may comprise a fluoroplastics material. Each of the first and second concave parts may define a portion of a cavity and/or have an open side. The open sides of the first and second parts may be configured to be secured together and/or with the periphery of the diaphragm, for example such that a first chamber is defined by the first part and a facing surface of the diaphragm and/or a second chamber is defined by the second part and a facing surface of the diaphragm.

Another aspect of the invention provides a method of making a storage vessel, e.g. for storing a propellant, the method comprising forming the vessel with an internal surface comprising a fluoroplastics material to define a cavity and dividing the cavity into first and second isolated chambers by securing a diaphragm to the internal surface.

The method may comprise securing the periphery of a sheet of material, for example fluoroplastics material, to the internal surface, for example such that it spans the cavity to provide the diaphragm. The method may comprise bonding the diaphragm to the internal surface.

Another aspect of the invention provides a method of making a storage vessel, e.g. for storing a propellant, the method comprising forming a vessel with an internal surface defining a cavity and bonding a diaphragm to the internal surface to divide the cavity into first and second isolated chambers. The diaphragm may be bonded, e.g. fusion bonded or ultrasonically welded or bonded together by an adhesive, to the internal surface.

The method may comprise securing the periphery of a sheet of material to the internal surface such that it spans a section or segment of the cavity to provide the diaphragm. The diaphragm may be flat or planar, but is preferably non-flat or non-planar, for example curved, dome-shaped, concave or convex.

The method may comprise forming the vessel with a wall, e.g. a vessel wall. The vessel or wall may include a shell, structural or fibre reinforced material, for example a fibre reinforced plastics material. The method may comprise forming the shell, structural or fibre reinforced material at least partially surrounding an internal fluoroplastics layer or liner, which may define the internal surface.

The method may comprise etching, e.g. chemically etching, an outer surface of the fluoroplastics layer or liner. The method may comprise bonding the shell, structural or fibre reinforced material to the etched outer surface. The method may comprise chemically etching the outer surface of the fluoroplastics layer or liner with a chemical etchant, which may be based on sodium metal, for example dissolved in a super saturated solution, which may comprise tetrahydrofuran and naphthalene.

The method may comprise bonding and/or at least partially embedding a backing, e.g. a fabric or fibre backing such as a fibre (e.g. carbon or glass fibre) knitted cloth, in an outer portion the fluoroplastics layer or liner. The method may comprise bonding the shell, structural or fibre reinforced material to the backing.

The method may comprise laying up and/or curing the fibre reinforced plastics material on the fluoroplastics layer or liner. Additionally or alternatively, the method may comprise forming and/or bonding one or more, e.g. two or more, preformed fibre reinforced plastics shells to the fluoroplastics layer or liner.

The method may comprise forming first and second concave parts, each of which may comprise a fluoroplastics material. At least one or each of the first and second parts or the fluoroplastics material thereof may define a portion of the cavity and/or may have an open side. The method may comprise securing the open sides together. The method may comprise securing at least one or each or both of the open sides with the periphery of the diaphragm, for example such that the first chamber is defined by the first part and a facing surface of the diaphragm and/or such that the second chamber is defined by the second part and a facing surface of the diaphragm.

The method may comprise forming each of the first and second parts with a flange, e.g. about the periphery of the open side thereof. The method may comprise securing the flanges together. The method may comprise securing at least one or each or both of the flanges with the periphery of the diaphragm sandwiched therebetween.

The method may comprise bonding the first and second parts to one another. The method may comprise bonding at least one or each or both of the parts to the periphery of the diaphragm.

The method may comprise forming at least one or each of the first and second chambers with a port, e.g. for the introduction of a fluid therein and/or expulsion of a fluid therefrom.

For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. For example, the storage vessel and/or kit of parts may comprise any one or more features of the method relevant thereto and/or the method may comprise any one or more features or steps relevant to one or more features of the storage vessel, fuel tank, propulsion system, satellite or kit of parts.

Another aspect of the invention provides a computer program element comprising and/or describing and/or defining a three-dimensional design for use with a simulation means or device or a three-dimensional additive or subtractive manufacturing means or device, e.g. a three-dimensional printer or CNC machine, the three-dimensional design comprising an embodiment of the storage vessel described above or any one or more components or features thereof.

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. For the avoidance of doubt, the terms "may", "and/or", "e.g.", "for example" and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic section view of a storage vessel according to an embodiment of the invention filled with a propellant;

Figure 2 is a view similar to that of Figure 1 illustrating the storage vessel in a partially depleted condition;

Figure 3 is an enlarged view of area A in Figure 2 with a mounting bracket secured to the flange of the vessel; and

Figure 4 is a schematic section view of the liner and diaphragm of Figure 1 with the carbon fibre shell omitted.

Referring now to the Figures, there is shown a demisable storage vessel 1 for storing a propellant for use in the propulsion system of a space craft, such as a satellite (not shown). In this embodiment, the storage vessel 1 is formed of first and second hemispherical parts 2, 3 and a diaphragm 4.

Each part 2, 3 is hollow with an inner liner 20, 30 and an outer shell 21 , 31 bonded to an outer surface of the inner liner 20, 30. The inner liners 20, 30, shown more clearly in Figures 3 and 4, each have a laminate structure formed of a fluorinated ethylene propylene (FEP) layer 22, 32 with a backing 23, 33 embedded therein. The backing 23, 33 is preferably formed of a carbon or glass fibre material, but may comprise any woven fabric material. The purpose of the backing 23, 33 is to enable the outer shell 21 , 31 to bond to the outer surface of the inner liner 20, 30.

However, the use of such a backing 23, 33 requires the FEP layer 22, 32 to have a predetermined minimum thickness, for example at least 1 mm, preferably at least 1.5mm. This thickness is required to ensure the structural integrity of the FEP layer 22, 32 in use. More particularly, should any of the fibres penetrate the FEP layer 22, 32 when they are embedded into the FEP this could create a path for egress of the propellant into the outer shell 21 , 31 , eventually leading to failure of the storage vessel.

The Applicants have discovered that by etching the outer surface of the FEP layer 22, 32 to remove exposed fluorine molecules and expose carbon molecules, the bonding between the outer shell 21 , 31 and the inner liner 20, 30 may be sufficient without the need for a backing 23, 33. This arrangement enables the FEP layer 22, 32 to be reduced significantly, to 1 mm, preferably as low as 0.5mm or even less in some circumstances, thereby reducing the weight of the storage vessel 1 and simplifying the manufacturing process, at least in some respects.

The FEP layer 22, 23 of the inner liner 20, 30 of each part 2, 3 has an inner, exposed surface 24, 34 that is formed of FEP and defines a portion of a cavity 5. Each inner liner 20, 30 also has a flange 25, 35 extending outwardly from and about the periphery of an open side thereof and a port 26, 36 defined by a tubular segment 27, 37 projecting outwardly at its apex. The flanges 25, 35 of the inner liners 20, 30 are coextensive with one another and are bonded together.

The outer shell 21 , 31 is formed of a carbon fibre reinforced plastics composite material in this embodiment. Each outer shell 21 , 31 includes a flange 28, 38 extending outwardly from and about the periphery of an open side thereof, which is coextensive with and bonded to the flange 25, 35 of its respective inner liner 20, 30. Each outer shell 21 , 31 also includes a tubular segment 29, 39 projecting outwardly at its apex which receives and is bonded to the tubular segment 27, 37 of its respective inner liner 20, 30.

The diaphragm 4 is formed of a 0.5mm sheet of FEP with a central region 40 that is deformed into a dome-shape and a peripheral flange 41 extending outwardly therefrom. The peripheral flange 41 is coextensive with and bonded to the flanges 25, 35 of the inner liners 20, 30. The diaphragm 4 separates the cavity 5 into first and second chambers 51 , 52 each being in fluid communication with a respective one of the ports 26, 36.

Thus, the storage vessel 1 is substantially spherical with a circumferential flange 10 about its periphery, formed by the flanges 25, 35, 28, 38, 41 , and a flexible diaphragm 4 bonded to the inner liner 20, 30 at the circumferential flange 10. The storage vessel 1 has a laminated structure with an outer carbon fibre reinforced plastics shell 21 , 31 and an inner liner 20, 30. As a result, the storage vessel 1 is demisable and will burn up on re-entry of a low orbiting satellite within which it is incorporated.

In this embodiment, the circumferential flange 10 of the storage vessel 1 is formed with a circular pattern of holes that are equispaced about the periphery of the vessel for receiving fasteners 6. The fasteners are in the form of bolts 60 and respective nuts 61. As illustrated in Figure 3, the storage vessel 1 may be mounted to a bracket 7 by securing the flange 10 to the bracket via one (or more) of the fasteners 6.

The storage vessel 1 also includes a respective connection pipe 82, 83 extending through the ports 26, 36. Although these are shown to be simple pipes 82, 83, they are preferably formed with a coupling means, for example a quick release coupling or a more permanent fluid coupling. One connection pipe 82 extends through and sealingly engages the tubular segments 27, 29 in the first part 2 and is bonded to a surrounding portion of the inner surface 24 of the inner liner 20. The other connection pipe 83 extends through and sealingly engages the tubular segments 37, 39 in the second part 3 and is bonded to a surrounding portion of the inner surface 34 of the inner liner 30. Filling of the storage vessel 1 with fuel, for example hydrazine, may be achieved by connecting the connection pipe 83 of the second part 3 to a source of fuel and pumping the fuel into the second chamber 52 of the storage vessel 1.

In use, the connection pipe 83 of the second part 3 may be fluidly connected to the propulsion system of a space craft (not shown) and the connection pipe 82 of the first part 2 may be fluidly connected to a source of pressurised fluid, for example nitrogen, via a regulation system (not shown). In order to supply fuel to the propulsion system, pressurised gas G may be controllably supplied to the first chamber 51 via the connection pipe 82 of the first part 2. As a result and as illustrated in Figure 2, the pressurised gas G fills the first chamber 51 and forces the central region 40 of the diaphragm 4 to expel the fuel F from the second chamber 52 via the connection pipe 83 of the second part 3. This is achieved without exposing the fuel to the pressurised gas, thereby obviating the need for complex capillary and/or gas separation systems. The Applicants estimate that over 95% of the fuel contained in the second chamber 52 can be expelled with this arrangement.

One method of manufacturing the storage vessel 1 involves forming each of three sheets of FEP into hemispherical or half-dome shapes 22, 32, 4 to form the FEP layer 22, 32 of each of the first and second parts 2, 3 and the diaphragm 4. This may be done by stretching each sheet over a mould or mandrel (not shown), which may be heated, or by thermoforming the sheets. The sheets can be formed whilst retaining their periphery such that peripheral flanges 25, 35, 41 are provided.

If the embodiment incorporating a backing 23, 33 is required, the backing 23, 33 may be urged against the outer surface of each of two of the sheets. This can be done either after or (preferably) during the forming thereof to embed the backing 23, 33 into the sheet to form the liner 20, 30. If the etched embodiment is required, the outer surface of the formed sheets 22, 32 may be etched after being formed using a chemical etchant based on sodium metal dissolved in a super saturated solution of tetrahydrofuran and naphthalene to form the liner 20, 30.

The outer shells 21 , 31 may be formed by any one of a number of different processes. A first process involves pre-forming the outer shells 21 , 31 by moulding and curing a carbon fibre reinforced composite material over a mould or mandrel (e.g. the same mould or mandrel as the liners 20, 30) and subsequently bonding the pre-formed shells 21 , 31 to the liners 20, 30. A second process involves overwrapping the liners 20, 30 with the carbon fibre reinforced composite material and allowing it to cure, thereby forming an integral laminated structure. A third process involves pre-forming the liners and diaphragm 4 into a spherical assembly, shown in Figure 4, and pressurising the assembly to enable overlaying and curing of the carbon fibre reinforced composite material to form the outer shells 21 , 31.

The liners 20, 30 and diaphragm 4 may be secured together as outlined below, either with the outer shells 21 , 31 already bonded thereto or, in the case of the third process outlined above, in the absence of such outer shells 21 , 31. The liners 20, 30 are brought together with their opposed concave surfaces facing one another and with the diaphragm 4 therebetween. If the liners 20, 30 and diaphragm 4 include flanges 25, 35, 41 , this would involve sandwiching the flange 41 of the diaphragm 4 between the flanges 25, 35 of the liners 20, 30. The flanges 25, 35, 41 can then be fusion bonded or ultrasonically welded together. If no such flanges 25, 35 are formed peripheral portions of the liners 20, 30 and of the diaphragm 4 may be bonded or welded together about their peripheral regions, as will be appreciated by the skilled person.

In the case of the third process for forming the outer shells 21 , 31 , the port 26 of the first liner 20 may be sealed, whilst a pressurised gas, for example air, may be applied to the port 36 of the second liner 30. This produces a relatively rigid spherical structure, which enables a carbon fibre reinforced plastics overwrapping and curing to produce the outer shells 21 , 31.

The holes in the flanges 25, 35, 28, 38, 41 for receiving fasteners 6 may be formed in the sheet materials prior to the aforementioned forming processes, during such forming processes or formed in the finished flange 10 of the vessel 1.

It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. For example, the inner liner 20, 30 and/or diaphragm need not be formed of FEP or even a fluoropolymer, but could alternatively be formed of any other material provide it is chemically resistant or compatible with the fuel in question, which also need not be a hypergolic fuel. Moreover, the diaphragm 4 need not be dome-shaped, curved or convex; it may, for example, be planar. However, given the properties of FEP it is preferable that the diaphragm 4 be non-planar, provided the diaphragm is formed of FEP. The outer shells 21 , 31 need not comprise a carbon fibre material or even a fibre reinforced material; they could be formed of any material, but is preferably formed of a demisable material, such as plastics or aluminium. The inner liner 20, 30 need not comprise a fibre or fabric backing 23, 33 or etching; in some embodiments the liner 20, 30 need not be secured or bonded to the outer shell 21 , 31 , although this is preferable. It is also envisaged that the vessel 1 , or any part thereof, e.g. the liner 20, 30, be formed as a unitary structure, for example using an additive manufacturing technique. It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.