Method for manufacturing a composite material tank with simplified molding tooling

Abstract

Method for manufacturing a composite material tank with simplified molding tooling. The present invention relates to a method for manufacturing a composite material tank in which an assembly comprising base precursor domes with thermoset reinforcement is implemented so as to simplify the molding tooling. Figure for the abstract: Fig. 1.

Description

Title of the invention: Method for manufacturing a composite material tank with simplified molding tools Technical field

[0001] The present disclosure relates to a method for manufacturing a composite material tank using simplified molding tools to produce the ends of the tank. The invention is of particular interest for the manufacture of tanks intended to equip space launchers without the invention being limited to this application. Prior art

[0002] Composite materials provide a weight saving compared to metallic materials, which is of particular interest in aerospace and aeronautical applications with a view to improving performance.

[0003] The recent development of reusable launchers is accompanied by the desire to continue developing lighter structures to the extent that it is necessary to retain a certain quantity of propellant for the return of the stage to Earth, which leads to the embarkation of an additional mass.

[0004] In current techniques for manufacturing composite material tanks, the part is draped over a rotating mandrel, by winding or by automatic fiber placement technique ("Automated Fiber Placement"; "AFP"). The implementation of these techniques can nevertheless prove relatively complex.

[0005] The mandrels must, in fact, be removable and extractable through small openings at the tank bottoms located at the ends. Disassembly can be a delicate operation, with the associated risks of damage to the composite structure (impacts that can cause damage to the structure). This will generate significant costs because complex specific tools will have to be designed for disassembly through these small openings. In addition, it is, in certain cases, necessary to position a vacuum cover around the part draped over the mandrel and to carry out vacuum polymerization. It is then necessary to ensure a seal between the different parts constituting the removable mandrel, so as not to affect the material health.

[0006] It is also desirable to reduce the manufacturing cycle time since the draping tool also serves as polymerization tooling, which requires waiting for the end of this step and the disassembly of the structure before being able to drape again.

[0007] The limits of current techniques are even more marked when it comes to to integrate reinforcement elements into the tank, such as stiffeners, or interface parts.

[0008] It is therefore desirable to have a method for manufacturing a composite material tank using simplified tooling and which addresses all or part of the aforementioned drawbacks. Statement of the invention

[0009] The present disclosure relates to a method of manufacturing a composite material tank, comprising: - the positioning of a precursor assembly of the tank to be manufactured in a vacuum molding tool, the assembly comprising (i) a cylinder, precursor of the body of the tank, made of thermosetting pre-impregnated fibrous material extending along a longitudinal axis, and (ii) two domes, precursors of the bottom of the tank, each of these domes being located on the side of an opposite longitudinal end of the cylinder, the domes each having an internal surface which delimits with the cylinder an internal volume of the tank to be obtained and an external surface located outside the internal volume, each of the domes comprising a frame made of thermosetting composite material, defining a bottom of the tank, covered by at least one thermosetting pre-impregnated fibrous skin defining a cylindrical junction zone positioned opposite the cylinder and inside the latter, the frame being configured to allow a conformation of said at least one skin against the cylinder on this cylindrical junction zone, the tooling comprising an internal positioning part coated with an internal vacuum cover, located inside the internal volume, on which the assembly is positioned, and an external molding part, located outside the internal volume, comprising a molding portion of the body of the tank having a cylindrical shape located opposite the precursor cylinder, each external surface of the domes being covered in a sealed manner by an external vacuum cover, and - vacuum curing of the assembly in the tool during which the internal vacuum tank applies pressure to conform the assembly to the molding portion, with the junction zones held in contact with the precursor cylinder and co-curing of the domes and the cylinder to secure the bases to the body and obtain the composite material tank.

[0010] Co-firing makes it possible to polymerize the thermosetting resin impregnating the cylinder with that impregnating said at least one skin of the domes, and to create covalent bonds between the polymer chains present. This joint polymerization leads to a bonding of the bottoms to the body of the tank made of material composite, without requiring the addition of a third-party adhesive compound. However, it does not go beyond the scope of the invention if a third-party adhesive compound is used on the interfaces of the elements constituting the bottoms and the body or on the interfaces between the bottoms and the body, so as to provide additional mechanical strength. Examples of adhesives include: FM® adhesive films marketed by SOLVAY, for example: FM300, FM309-1, FM377, FM209-1, HexBond® adhesive films marketed by HEXCEL, for example: HexBond 322, HexBond 319, HexBond 340, HexBond ST 1480, or adhesive films marketed by 3M, for example AF191 or AF3109-2.

[0011] The frame is a thermoset composite material, i.e. one whose matrix has been completely polymerized, so as to give the domes sufficient rigidity to maintain the desired geometry for the bottoms during the manufacture of the tank, and in particular during vacuum cooking. The frame also makes it possible to dispense with the presence of molding tooling on the domes during the manufacture of the tank, thus making it possible to maintain a relatively simple tooling structure. On the other hand, each dome remains sufficiently deformable on its respective cylindrical junction zone to allow the expansion or deformation of said at least one skin, thus ensuring good attachment of the bottom to the cylinder. As will be described below, the frame can have various structures making it possible to fulfill this function.

[0012] In the invention, the molding part is external to the precursor assembly, and therefore to the composite tank obtained. This external positioning makes it easier to disassemble compared to the internal molding mandrel of the prior art and to more easily guarantee the sealing required for vacuum drawing. Unlike the mandrel of the prior art, the internal part in the invention only has a positioning function and not a molding function, and therefore has a much simpler design and is more easily disassembled.

[0013] In the following and unless otherwise stated, the word “skin(s)” designates said at least one thermosetting pre-impregnated fibrous skin which is present within each of the domes.

[0014] In an exemplary embodiment, the cylinder extends beyond each of the two domes so as to define skirt precursors, and in which each dome comprises a first skin covering a first face of the frame and defining a first part of the cylindrical junction zone which extends in the direction of the other dome, and a second skin covering a second face of the frame, opposite the first face, and defining a second part of the cylindrical junction zone which extends in a direction opposite to said other dome, the second part of the cylindrical junction zone of each dome being secured to a respective skirt precursor during sous vide cooking.

[0015] Such a feature makes it possible to further improve the conformation of the bottoms on the cylinder and therefore the quality of the connection to the body, as well as to reduce the risk of misalignment. The extension of the cylinder so as to form the skirts is of particular interest in the context of a tank intended to equip an aerospace launcher, by allowing integration of the skirts in a single piece with the tank, thus making it possible to form a complete launcher stage in a simplified manner.

[0016] In particular, each dome may further comprise a filling material between the first and second parts of the cylindrical junction zone.

[0017] Such a characteristic makes it possible to further improve the conformation of the bottoms on the cylinder and therefore the quality of the connection to the body, as well as to reduce the risk of misalignment.

[0018] In an exemplary embodiment, the cylindrical junction zone of each dome is at least partly formed by an extension of said at least one skin beyond the frame.

[0019] Such a characteristic corresponds to a possibility for conferring flexibility on the cylindrical junction zone by implementing a reinforcement which stops sufficiently early upstream of it.

[0020] In an exemplary embodiment, the frame of each dome defines a plurality of flexible tabs allowing the conformation of said at least one skin against the cylinder on the cylindrical junction zone.

[0021] These tabs are sufficiently flexible so as not to hinder the conformation during vacuum cooking. This arrangement is functional even if the frame is extended compared to the example previously described due to the presence of a flexible circumferential zone on the side of the cylindrical junction zone.

[0022] In an exemplary embodiment, said at least one skin of each dome has a thinning of thickness on the cylindrical junction zone in the direction of a circumferential edge thereof.

[0023] Such a characteristic advantageously makes it possible to avoid any risk of bridging the tank on a misalignment during vacuum cooking, and to further improve the material health of the skin, the cylinder and the skin / cylinder interface. By misalignment we mean a local projection on the surface, that is to say that the surface is not smooth, and that the ends of the assembled parts form a step. In addition, this characteristic makes it possible to minimize stress concentrations (at the stop of the bottom) when the tank is subjected to mechanical stresses (filling, thermal stresses, pressurization, forces applied during the takeoff phases and potentially return to Earth, etc.).

[0024] In an exemplary embodiment, each skin comprises several juxtaposed sectors. around the longitudinal axis.

[0025] According to this example, each dome is sectorized and comprises several skin sectors distributed in a circumferential direction. The sectorization makes it possible to further improve the conformation to the cylinder by allowing a conformation of the sectors by relative sliding between them and an assembly with the cylinder without constraint.

[0026] In particular, the neighboring sectors may have a thinning of thickness on their overlap zone in the direction of their longitudinal edges.

[0027] The thinning of thickness makes it possible to locally avoid thickness jumps and to further minimize misalignments and stress concentrations.

[0028] In an exemplary embodiment, the cylinder and said at least one skin of each dome are assembled in the half-baked state to form the precursor assembly.

[0029] A semi-cured thermosetting material has a partially cured resin that has a degree of progress of polymerization of between 15% and 70%, for example between 25% and 50%. For a given resin, the degree of progress of polymerization can be determined by differential scanning calorimetry (“Differential Scanning Calorimetry”; “DSC”).

[0030] A semi-cooked material has a certain rigidity at room temperature (20°C) which facilitates its handling, and allows for a simplification of the tools. Its use also allows for less constraints on the material's ambient life because the polymerization of a semi-cooked material changes little at room temperature. The resin of a semi-cooked material regains fluidity when the material is raised in temperature during vacuum cooking, which allows the elements to soften and conform to the molding portion by relative sliding.

[0031] In an exemplary embodiment, the method further comprises, before positioning the precursor assembly in the molding tool, forming the cylinder and said at least one skin of each of the domes by automatic placement of fibers on a separate shape of the internal positioning part.

[0032] This technique allows access to a wide variety of geometries, in particular compared to winding which does not allow draping in the direction of the longitudinal axis, or automated draping of local overthicknesses or reinforcements. The automatic placement of fibers also makes it possible to obtain elements of low permeability in comparison with parts obtained by winding, suitable for example for the storage of cryogenic propellants. In addition, the draping tooling is dissociated from the polymerization tooling, which makes it possible to reduce the duration of the manufacturing cycle.

[0033] In one exemplary embodiment, the cylinder and domes comprise carbon fibers, glass fibers, aramid fibers, or a mixture of such fibers.

[0034] These fibers are particularly suitable for space launcher applications and cryogenic environments.

[0035] In an exemplary embodiment, the cylinder and the domes are pre-impregnated with an epoxy resin, for example a class 180 epoxy resin which polymerizes at a temperature between 175°C and 185°C, for example substantially at 180°C.

[0036] This material is particularly suitable for space launcher applications and cryogenic environments, and can be easily reworked after partial polymerization.

[0037] In an exemplary embodiment, the assembly further comprises (iii) a common bottom precursor located inside the cylinder and between the domes so as to define with each of them a compartment distinct from the internal volume, said common bottom precursor comprising a common bottom reinforcement made of thermoset composite material, defining the common bottom, covered by at least one common bottom fibrous skin which is pre-impregnated and thermosettable and which defines an intermediate cylindrical junction zone positioned opposite the cylinder, the common bottom reinforcement being configured to allow conformation of said at least one common bottom skin against the cylinder on this intermediate cylindrical junction zone, and in which there is co-firing of the common bottom precursor with the cylinder to secure the common bottom to the body during vacuum firing.

[0038] In an exemplary embodiment, the cylinder, precursor of the body of the tank, is sectorized and is formed by panels juxtaposed around the longitudinal axis with overlap between the neighboring panels. If necessary, the molding portion of the body can be stiffened on its face opposite the assembly in order to retain its shape despite a significant length, without affecting the quality of the molding or the disassembly nature.

[0039] Forming the cylinder from a plurality of panels makes it possible to simplify its manufacture when a very long cylinder is envisaged.

[0040] In particular, the neighboring panels may have a thinning in thickness over their overlapping zone towards their longitudinal edges.

[0041] Such a characteristic helps to further minimize the misalignments and stress concentrations in the reservoir.

[0042] In one exemplary embodiment, the panels and domes each comprise a plurality of stiffeners on a face opposite the molding portion.

[0043] The external nature of the molding tool does not interfere with the presence of the stiffeners. The invention is therefore of particular interest for the manufacture of a stiffened tank which can be relatively difficult to obtain in the techniques of the prior art using an internal molding mandrel. Brief description of the drawings [Fig.l] [Fig.l] represents, schematically in perspective, an example of a panel usable within the framework of the invention to form the cylinder. [Fig.2] [Fig.2] represents, schematically and partially, a cross-section of the panel of [Fig.l] in relation to its longitudinal axis. [Fig.2A] [Fig.2A] schematically and partially represents the bevel angle of the panel of [Fig.2]. [Fig.3] [Fig.3] represents, schematically in perspective, the overlapping of two neighboring panels according to an exemplary implementation of the invention. [Fig.4] [Fig.4] represents, schematically and partially, a variant of cylinder. [Fig.5] [Fig.5] represents, schematically and partially, another variant of cylinder. [Fig.6] [Fig.6] represents, schematically and partially, another variant of cylinder. [Fig.7] [Fig.7] represents, schematically and partially, another variant of cylinder. [Fig.8] [Fig.8] represents, schematically and partially, a sectional view of an example of a dome in a plane containing its axis of revolution. [Fig.9] [Fig.9] represents a sector of a first skin intended to form part of the dome of [Fig.8]. [Fig. 10] [Fig. 10] represents the juxtaposition of several sectors according to [Fig.9] in the context of the manufacture of the dome of [Fig.8]. [Fig.l 1] [Fig.l 1] represents, schematically and partially, a sectional view of a variant of dome in a plane containing its axis of revolution. [Fig. 12] [Fig. 12] schematically and partially represents a sectional view of another variant of dome in a plane containing its axis of revolution. [Fig. 13] [Fig. 13] schematically and partially represents a sectional view of another variant of dome in a plane containing its axis of revolution. [Fig. 14] [Fig. 14] shows, schematically and partially, a sectional view of another variant of dome in a plane containing its axis of revolution. [Fig. 15] [Fig. 15] schematically and partially represents a precursor assembly of the tank to be manufactured positioned in a vacuum molding tool as part of an example of implementation of the invention. [Fig. 16] [Fig. 16] schematically represents the sous vide cooking of the assembly illustrated in [Fig. 15]. Description of the embodiments

[0044] The invention is now described by means of figures, presented for descriptive purposes for illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0045] The examples which will be described below, in connection with the figures, concern an example where a compartmentalized tank is manufactured comprising a common bottom, but the person skilled in the art will recognize that the invention is not limited to this case and that it can in particular be applied to the manufacture of a non-compartmentalized tank, without a common bottom.

[0046] The description below addresses, firstly, the structure and production of the precursor cylinder of the body, then the precursor domes of the bottoms. The details of the vacuum molding tool, inside which the precursor assembly is positioned, as well as the vacuum cooking phase will be detailed in a second step.

[0047] Precursor cylinder of the body

[0048] The example described here relates to a sectored cylinder obtained by juxtaposition of thermosetting pre-impregnated fiber panels, but it does not depart from the scope of the invention when the cylinder is formed in a single piece. In the latter case, the cylinder does not contain a continuous circumferential fiber which hinders the increase in diameter or comprises sectored circumferential fibers. The circumferential direction corresponds to the direction around the axis of the cylinder, designated by "axis X" in the following.

[0049] The precursor cylinder of the body is here obtained by juxtaposition of thermosetting pre-impregnated fiber panels. Generally speaking, it may comprise at least two fiber panels, or even at least three fiber panels. The example described here concerns the case of a cylinder with four panels.

[0050] The panels are advantageously produced by automatic placement of fibers, which constitutes a technique known per se. The panels 10 each comprise a fiber reinforcement pre-impregnated with a thermosetting resin. The choice of the reinforcing material and the resin depends on the intended application. For example, the fiber reinforcement comprises carbon fibers, glass fibers, aramid fibers, or a mixture of such fibers. For example, the resin is an epoxy resin, for example based on a monomer DGEBA (diglycidyl ether of bisphenol A), TGPAP (triglycidyl paraaminophenol), or TGMDA (tetraglycidyl methylenedianiline), to which an amine-type hardener, for example DDS (diaminodiphenyl sulfone), may optionally be added. The same resin is advantageously used in the different panels, or, failing that, compatible resins.

[0051] The panels 10 have an elongated shape along a longitudinal axis X and have a curved shape in cross-section relative to the axis X. The panels 10 define two longitudinal edges 10a, 10b which are intended to be su perposed with a neighboring panel, as will be described hereinafter. [Fig.l] illustrates an example of a possible structure for the panels 10 which are draped in a female tool, the person skilled in the art will recognize that the panels can alternatively be draped on a male tool without departing from the scope of the present invention. The draping form is not an integral part of the vacuum molding tooling so as to allow parallel operation between draping and curing, and thus reduce the manufacturing cycle time.

[0052] The panels 10 are, in the example of [Fig. 1], provided with longitudinal stiffeners 12 made of thermosetting composite. These stiffeners 12 are formed by conventional means known to those skilled in the art, for example by manual or automated draping, draping in a shaped mold or press stamping of raw flat plates, production of dry preforms and injection of resin into a mold by a resin transfer molding technique ("Resin Transfer Molding"; "RTM"). The stiffeners 12 may have any cross-section, for example Omega, T, J, among other possible shapes. The stiffeners 12 may or may not have a local variation in shape or cross-section. As illustrated, the stiffeners 12 may be present over at least the majority (more than 50%) of the length LO 10 of the panels 10. The panels 10 nevertheless have areas devoid of stiffening elements which will be described below.

[0053] The illustrated panels 10 are also provided with circumferential frame sectors 14 made of thermosetting composite. As for the stiffeners 12, the frame sectors 14 are formed in a manner known per se. The frame sectors 14 may have any cross-section, for example Z-shaped, C-shaped, F-shaped, T-shaped, Omega-shaped or J-shaped.

[0054] The stiffeners 12 and frame sectors 14 may be half-baked, or have complete polymerization, or little or no advanced polymerization.

[0055] The panels 10 have circumferential zones 16, intended for docking with the domes, which are devoid of stiffening elements. The zones 16 are each located on the side of an opposite longitudinal end of the panel 10 in question. As will be detailed below, the domes will be secured to the body of the tank on these zones 16. The panels 10 also have an intermediate circumferential zone 16a, intended for docking with the common bottom in the example considered here, which is also devoid of stiffening elements. In the case where the manufactured tank does not include a common bottom, there is no zone 16a but the stiffeners 12 extend into this zone and the frame sectors 14 are also present. The zone 16a is located between the zones 16 and interrupts the zone 18 provided with stiffeners 12.The panels 10 also have a longitudinal zone 19, located on the side of the longitudinal edge 10b and extending over their entire length LO10, which is devoid of any stiffening element. It will also be noted that, in the example . illustrated, the frame sectors 14 protrude from the panel 10 on the side opposite the zone 19 (on the side of the edge 10a), and form an extension 15 of the frame sectors 14. The presence of the zone 19 as well as the extension of the frames 14 are the result of the arrangement envisaged for the overlap between neighboring panels 10.

[0056] [Fig. 2] shows the evolution of the thickness elO of the panels 10 over their width (dimension between the edges 10a and 10b). The panels 10 have a middle zone 101 of their width where the thickness elO is substantially constant, and two lateral zones 103a and 103b each located on the side of a respective longitudinal edge 10a, 10b which have an evolving thickness. The zone 101 is located between the zones 103a, 103b. The zone 103a connects the zone 101 to the edge 10a, and the zone 103b connects the zone 101 to the edge 10b. More precisely, the zones 103a, 103b have a thinning of thickness in the direction of the associated longitudinal edge 10a, 10b. The thickness elO may be strictly decreasing in the direction of the longitudinal edge 10a, 10b. The thickness elO is minimal on the edges 10a, 10b and maximal on the median zone 101. The panels 10 have a bevel on their edges 10a, 10b.The panels 10 have, in cross-section relative to the X axis, a tapered shape towards their edges 10a, 10b. The zones 103a and 103b jointly occupy at least 5%, for example at least 50%, of the width LA10 of the panel 10. Unless otherwise stated, the width LA10 corresponds to the length of the arc of the curve connecting the edge 10a to the edge 10b. The thinning of the thickness leads to a reduction in the thickness el0 of at least 50%, for example at least 90%. This reduction in the thickness el0 is for example between 50% and 95%, or even between 90% and 95%. [Fig.2A] schematically represents a thinning of the thickness of the panel 10 in the direction of a longitudinal edge 10b linked to the presence of a decreasing quantity of superimposed folds PS in the direction of the edge 10b.The bevel angle a, corresponding to the angle taken locally on the edge 10b, can advantageously be less than or equal to 1.5°, so as to further improve the smoothing of the forces.

[0057] The panels 10 which have just been described include stiffening elements but it is not outside the scope of the invention if they are devoid of them. The following describes, in connection with Figures 3 to 7, the juxtaposition of these panels 10 to form the cylinder 100 which is, in the example considered, a precursor of the body of the tank but also of front and rear skirts which extend beyond the bottoms formed by the domes.

[0058] [Fig. 3] illustrates the juxtaposition of two neighboring panels 10 around the X axis. The two panels 10 illustrated have the same structure and the same references are retained with respect to what has just been described. The panels 10 partially overlap here. The panels have a thinning of thickness on their overlap zone, that is to say that the zone 103b of the first panel 10 is covered by the area 103a of the second neighboring panel 10. The edge 10b of the first panel 10 is superimposed with the second neighboring panel 10, and the edge 10a of the second panel 10 is superimposed with the first panel. The stiffeners 12 and frame sectors 14 of the second panel 10 cover the area 19 of the first panel 10. The extension 15 of the frame sectors 14 of the second panel 10 extends over the area 101 of the first panel and is positioned adjacent and in line with the frame sectors 14 of the first panel. The extension 15 of the second panel 10 is assembled with the frame sectors 14 of the first panel 10 by techniques known per se, for example by overlapping, joisting or splicing. Furthermore, the zones 16 of the first panel 10 come in the extension of the zones 16 of the second panel 10, and the zone 16a of the first panel 10 in the extension of the zone 16a of the second panel 10.

[0059] The arrangement which has just been described can be applied to each pair of neighboring panels 10 juxtaposed to form the sectored cylinder. In general, the overlap zones between neighboring panels can occupy at least 5%, for example at least 30% of the perimeter of the sectored cylinder. Having spread overlap zones makes it possible to further improve the mechanical properties of the resulting tank. In particular when all the panels are assembled to form the sectored cylinder, the joining of the zones 16 forms a 360° circumferential zone devoid of any stiffening element. Furthermore, the joining of the frame sectors 14 of each of the panels defines a plurality of 360° circumferential frames distributed along the length of the cylinder.

[0060] Figures 4 to 7 show different variants of juxtaposition of the panels 10 to form the sectored cylinder 100. The cylinder 100 extends along the X axis with panels juxtaposed around this axis.

[0061] In the variant of [Fig. 4], the cylinder 100 is formed by tiling all the panels 10, that is to say that each of the panels has a first longitudinal edge 10b covering (above) a first neighboring panel, and a second longitudinal edge 10a, opposite the first edge 10b, covered (below) by a second neighboring panel, opposite the first neighboring panel. In particular, the thinning of the thickness of the neighboring panels 10 over their overlapping zone ZR10 in the direction of their longitudinal edges 10a, 10b is noted.

[0062] In the variant of [Fig.5], there is tiling except for panel 10 at the bottom right of the figure which covers its two neighboring panels.

[0063] The variants of Figures 6 and 7 show cases where there is no tiling which are also covered by the present invention.

[0064] The part just described concerns the panels and their juxtaposition to form the precursor cylinder of the body. The following concerns the precursor domes of funds which are intended to be secured to the cylinder to delimit the internal volume of the composite material tank to be obtained.

[0065] Background precursor domes

[0066] [Fig. 8] illustrates an example of a dome 300, precursor of a tank bottom, intended to be positioned at a longitudinal end of the cylinder as will be illustrated in connection with [Fig. 15]. The dome 300 comprises a frame 302 made of thermoset composite material. The frame 302 thus comprises a fibrous reinforcement densified by an organic matrix obtained after polymerization of a thermoset resin. The resin of the frame 302 is completely polymerized.

[0067] The example of dome 300 considered comprises a thermosetting pre-impregnated fibrous skin on each of the faces of the frame 302. Each of the skins is here formed by a juxtaposition of sectors 304, 306 around the X axis. The skins or sectors 304, 306 can be produced by automatic placement of fibers. The skins or sectors 304, 306 each comprise a fibrous reinforcement pre-impregnated with a thermosetting resin. The example considered comprises first sectors 304 which cover a first face 3024 of the frame 302, and second sectors 306 which cover a second face 3026 of the frame 302 opposite the first face 3024. The first sectors 304 are juxtaposed circumferentially, like the second sectors 306. The frame 302 is housed between the first 304 and second 306 sectors.The frame 302 comprises an internal circumferential edge 3021 capable of cooperating with the tooling to position the precursor 300, and an external circumferential edge 3023 intended to be positioned on the side of the cylinder 100 in the assembly. The cooperation between the internal circumferential edge of the frame and the tooling will be described below.

[0068] The following details a draping possibility to obtain dome 300.

[0069] The sectors 304, 306 can be draped onto male or female tooling, and onto a draping form that is not an integral part of the vacuum molding tooling.

[0070] In the example considered, it is first possible to drape the second sectors 306, which have a more complex shape, then position the frame 302 on the second sectors 306 thus draped and then drape the first sectors 304 on the assembly obtained in order to obtain the dome 300. The frame 302 has been completely polymerized beforehand, before its assembly with the sectors 304, 306. An adhesive film can be positioned at the interfaces between the frame 302 and the sectors 304, 306. The first face 3024 of the frame 302 is covered over 360° by the first sectors 304 and the second face 3026 of the frame 302 is covered over 360° by the second sectors 306. Figures 9 and 10 which will be described below detail the draping of the first sectors 304 being understood that the draping of the second sectors 306 is carried out in a similar manner.

[0071] The sectors 304 each define two edges 304al, 304bl each extending along a longitude, called longitudinal edges, which are intended to be superimposed with a neighboring sector. The edges 304al, 304bl are intended to extend along the longitudinal axis X of the cylinder 100 in the precursor assembly which will be described below. The sectors 304 define two edges 304cl, 304dl each extending along a latitude (or circumferential) which are transverse to the edges 304a 1, 304b 1. Each of the edges 304c 1, 304dl connects the edge 304a 1 to the edge 304b 1. The edge 304c 1 has a first curvilinear length, and the edge 304dl has a second curvilinear length which is greater than the first curvilinear length.

[0072] The sectors 304 have a thickness that varies between the edges 304al and 304b 1. The sectors 304 thus have a median zone 3041 where the thickness is substantially constant, and two lateral zones 3043a and 3043b each located on the side of a respective edge 304al, 304b 1 which have a thickness that varies. Area 3043a is bounded by edge 304al and longitude 305 and area 3043b by edge 304bl and longitude 307. Area 3041 is located between areas 3043a and 3043b, or between longitudes 305 and 307. The description provided above relating to areas 103a, 101 and 103b applies respectively to areas 3043a, 3041 and 3043b mutatis mutandis.

[0073] The sectors 304 define a sector 3043d of a part 311 of the intermediate cylindrical junction zone 310 which is intended to come opposite the precursor cylinder of the body, and more particularly opposite a docking zone 16 which has been described above. The sector 3043d corresponds to a circumferential zone delimited by the edge 304dl and by a latitude 309. The sectors 304 may have a thinning of thickness on the sector 3043d in the direction of the edge 304dl.

[0074] The sectors 304 are juxtaposed at their edges 304al, 304b 1 with overlap between neighboring sectors. Thus, [Fig. 10] represents the positioning of an additional sector 304 in partial overlap with the first initial sector 304, it being understood that two other sectors are juxtaposed to form the first skin. The sectors 304 are thus juxtaposed circumferentially.

[0075] As indicated above, the draping of the second sectors 306 is done in a similar manner to that of the first sectors 304. It will nevertheless be noted that, in the illustrated example, the second sectors 306 are folded back on the side of the edge 3023 of the frame 302 so as to form a second part 312 of the zone 310. The second part 312 extends in a direction opposite to the direction of extension of the first part 311. The part 312 is also intended to come opposite the precursor cylinder of the body, opposite one of the docking zones 16.

[0076] Generally speaking, the sectors can be draped onto the reinforcement in the raw state with a raw prepreg, or in the semi-cooked state with a prepreg previously semi-cooked before draping. In the case where they are draped in the semi-cooked state, we can favor a single-layer architecture of sectors which will be draped directly on the reinforcement or a two-layer architecture with the two layers joining beyond the reinforcement.

[0077] In the example of [Fig. 8], the skins each extend beyond the frame 302 (beyond the end 3023) so as to define the first 311 and second 312 parts of the cylindrical junction zone 310. More precisely, the skins are directly assembled beyond the end 3023 on an assembly region 308 up to a separation edge 305 from which they separate to define the first 311 and second 312 parts. This separation defines a space which can be filled by a filling material 320 distinct from the skins. The material 320 is present between the first 311 and second 312 parts. The material 320 can be chosen from: cellular materials including filling foams (“foam core” in English) as well as honeycombs (or NIDA or “honeycomb” in English).Foams can be foams from the PU (Polyurethane), PVC (Polyvinyl Chloride), PMI (PolyMethacrylate Imide), PEI (PolyEther Imide), PAI (Polyamide Imide) family, foams from the PAEK (PolyArenylEtherKetone) family such as PEEK (PolyEtherEtherKetone), PEKK (PolyEtherKetoneKetone), or metal foams, or carbon foams, etc. For honeycombs we can have for example Nomex® honeycombs (based on aramid fibers and phenolic resin), aluminum honeycombs, Kevlar® honeycombs, carbon honeycombs, etc. The 320 material may or may not be loaded with fibers.

[0078] The extension of the skins beyond the reinforcement ensures that they can conform against the cylinder during the vacuum curing step. Thus, the possibility of conformation is here ensured by the absence of reinforcement, that is to say of completely rigid polymerized material, at the level of zone 310. Nevertheless, other reinforcement configurations are conceivable to allow the desired conformation of the skins as will be described below.

[0079] The choice of the reinforcing material and the resin depends on the intended application. For the skins, a resin identical to that of the panels 10 is chosen, or failing that, compatible with it. The reinforcement and the resin of the skins may be as described above for the panels. Although this is not illustrated, it will be noted that the dome 300 may or may not be provided with stiffeners or supports.

[0080] Figures 11 to 14 represent dome variants that can be used in the context of the invention.

[0081] [Fig. 11] represents a dome 300a of structure similar to that of [Fig. 8], comprising a frame 302a interposed between first 304a and second 306a skin sectors. The first 304a and second 306a sectors extend beyond the frame 302a and are in particular assembled directly on an assembly region 308a. Unlike [Fig.8], this assembly region 308a extends to the cylindrical junction zone 310a. There is therefore no filling material in this example between the first 311a and second 312a parts of the zone 310a.

[0082] [Fig. 12] shows a dome 300b comprising a frame 302b interposed between first 304b and second 306b skin sectors. The first 304b and second 306b sectors extend beyond the frame 302b and are in particular assembled directly on an assembly region 308b. Unlike the domes of FIGS. 8 and 11, the first 304b and second 306b sectors extend on the same side to form the cylindrical junction zone 310b. It will be noted that a similar structure can be had with a single layer of sectors, or a one-piece cap-shaped skin rather than a sectored structure. In the latter case, the skin does not contain a continuous circumferential fiber that hinders the increase in diameter or comprises sectored circumferential fibers.

[0083] [Fig. 13] shows a dome 300c of a structure similar to that of [Fig. 12], except that it has a reinforcement extension. A reinforcement 302c 1 is interposed between first 304c and second 306c skin sectors. The dome 300c comprises a reinforcement extension 302c2 which extends beyond the first reinforcement 302c 1 so as to avoid the formation of a sudden variation in radius during the shaping of the skins during the vacuum cooking step. The extension 302c2 is local and only covers a fraction of the first reinforcement 302cl in the vicinity of the end 302cla of the latter. This guarantees good pressurization of the skin sectors 306c located between the two reinforcements 302c 1 and 302c2 during cooking. The 302c 1 frame and its 302c2 extension stop early enough to allow the skins to be joined together.In a variant not illustrated and preferably, the frame 302c2 can also be covered with a skin going in a direction opposite to that along which the skins formed from the sectors 304c, 306c extend.

[0084] [Fig. 14] illustrates a variant of dome in which the frame is extended compared to the examples which have just been described.

[0085] [Fig. 14] illustrates a dome 300d comprising at least one skin 306d assembled to a “spider” type frame 302d which comprises a rigid portion 3025d located on the side of the circumferential edge 302 Id and a plurality of flexible tabs 3027d extending from the portion 3025d to the circumferential edge 3023d. The flexibility of the tabs 3027d allows the skin to be secured to the cylinder despite an increased length of the frame. The tabs 3027d are not in direct contact with the panels of the cylinder during the final polymerization, the skin 306d coming to the interface between the frame and the cylinder. The tabs 3027d may stop before the circumferential edge 306dl of the skin 306d as shown, or alternatively extend to this edge on the cylindrical joining area.

[0086] The various constituent elements of the assembly have just been described. An example of the kinematics of mounting the vacuum molding tool 30 and positioning the precursor assembly will now be detailed in connection with [Fig. 15]. In this figure, as well as in [Fig. 16], the relative thicknesses as well as the spacings between the various elements have not been strictly respected for reasons of readability.

[0087] Mounting the vacuum casting tool and positioning the precursor assembly in this tool

[0088] A first internal vacuum cover 341 is initially placed on a first internal positioning portion 321. The first portion 321 is of a much simpler design than the mandrel implemented in the techniques of the prior art insofar as it is not, in the context of the invention, used as a molding surface and does not need to be sealed. It is, therefore, more easily removable. The first portion 321 is supported by a shaft 36 which extends along a longitudinal axis X. The axis X corresponds to the longitudinal axis of the panels 10 and the cylinder 100 which was described previously. The first portion 321 may have an aerated structure, for example a lattice structure, or comprise a plurality of retractable positioning elements fixed to the shaft. The first portion 321 may, as illustrated, generally have the shape of a first compartment C1 of the tank to be obtained. The first tarpaulin 341 covers the first portion 321.The first tarpaulin 341 may be made of elastomeric material, reinforced or not, and constitutes an element known per se.

[0089] A first dome 300a can then be positioned on the portion 321 at a first longitudinal end 3211. The bottom of the dome 300a defines an orifice 206, here in the general shape of a disc, through which the shaft 36 extends. The dome 300a can have a general shape of revolution around the axis X. The illustrated example implements the embodiment of dome 300a according to [Fig.l 1] but domes having a different structure can, of course, be used. The dome 300a extends circumferentially around the axis X. The dome 300a may have a general shape of revolution around the axis X. The dome 300a extends with a non-zero component along a radial direction R, perpendicular to the axis X, between a first internal end 3051a located on the side of the axis X and the shaft 36, and a second external end 3053a opposite the first end located on the side of the cylinder 100.The first 31a and second 312a portions extend from the second end 3053a so as to form the cylindrical junction area 310a. intended to allow the dome 300a to be secured to the cylinder 100 during vacuum cooking. More precisely, the first part 311a of the dome 300a extends towards the other dome which will be positioned on the opposite end of the cylinder, and the second part 312a extends in a direction opposite to said other dome.

[0090] The frame 302a is present on the first end 3051a and, more generally, on the bottom zone. Thus, the frame 302a makes it possible to dispense with the presence of a molding tool on the bottom, which thus simplifies the structure used for molding, and gives the dome 300a sufficient rigidity so that it substantially retains its shape during vacuum cooking.

[0091] The frame 302a may extend beyond the skins at the end 3051a in a manner similar to that illustrated in [Fig. 8], so as to cooperate with the shaft 36 to position the dome 300a in the tooling. In an exemplary embodiment, the end 3051a is housed in a circumferential support tooling 362 attached around the shaft 36, for example in the form of a ring. The cover 341, as well as the cover 35 discussed below, are fixed in a sealed manner to the tooling 362. The tooling 362 may, as illustrated, have beveled axial ends facilitating the creation of seals. Those skilled in the art will recognize that other solutions are possible for positioning the dome 300a, the illustrated possibility being only one example among different possible embodiments.In the example illustrated, the frame 302a stops upstream of the second end 3053a to provide sufficient flexibility to the parts 311a, 312a and thus allow them to be shaped and secured to the cylinder 100 during vacuum cooking.

[0092] In the illustrated example, a common bottom precursor 400a is then positioned at a second longitudinal end 3213 of the portion 321 which is opposite the first end 3211. In the illustrated example, the precursor 400a has the same structure as the dome 300a and the elements having a similar function bear an identical reference except that it begins with the number 4. The precursor 400a is intended to form a common bottom separating the compartments C1 and C2 in the resulting tank. The precursor 400a extends with a non-zero component along a radial direction R, perpendicular to the axis X, between a first internal end 4051a located on the side of the axis X and the shaft 36, and a second external end 4053a opposite the first end located on the side of the cylinder 100.The first 411a and second 412a parts extend from the second end 4053a so as to form an intermediate cylindrical junction zone 410a intended to allow the precursor 400a to be secured to the cylinder during vacuum cooking. The end 4051a is housed in a circumferential support tool 362 attached around the shaft 36 in a manner similar to that which has just been described for the dome 300a.

[0093] The second portion 322 covered with the second internal vacuum cover 342 is then positioned in the extension of the first portion 321 along the X axis. The second portion 322 is of a much simpler design than the mandrel implemented in the techniques of the prior art insofar as it is not used as a molding surface and does not need to be sealed. It is, therefore, more easily removable. The second portion 322 is supported by the shaft 36. The second portion 322 may have an aerated structure, for example a lattice structure, or comprise a plurality of retractable positioning elements fixed to the shaft. The second portion 322 may, as illustrated, generally have the shape of a second compartment C2 of the tank to be obtained. The second cover 342 may have the same characteristics as the first cover 341 described above.

[0094] A second dome 300a is then positioned which has the same structure and attachment to the shaft as that which has just been described. The illustrated example shows two domes 300a of symmetrical structure but it is, of course, not outside the scope of the invention if this is not the case.

[0095] In the example considered, a third and a fourth external vacuum tarpaulins 35 are then placed respectively for the formation of the front and rear skirts which will be described below. The third and fourth tarpaulins 35 may have the same characteristics as the first and second tarpaulins 341, 342.

[0096] Positioning rings 40 are then positioned at the ends 361, 363 of the shaft 36 which define a cylindrical positioning surface 42 whose function will be described below.

[0097] The panels 10 can then be positioned and juxtaposed, in the manner described above, so as to form the precursor cylinder 100 of the tank body. The cylinder 100 is located around the domes 300a and the precursor 400a. The cylinder 100 is positioned on the first 321 and second 322 portions. The cylinder extends between the end 3223 and the end 3211, possibly extending beyond them to form the front and rear skirts as illustrated. The domes 300a are located inside the cylinder 100. The precursor 400a is located inside the cylinder 100. The cylinder 100 extends from one dome 300a to another. In the illustrated example, the cylinder 100 extends beyond the domes 300a to form front 110 and rear 120 skirt precursors positioned respectively opposite the third and fourth tarpaulins 35.However, it does not go beyond the scope of the invention if the cylinder does not form such skirts by stopping at the level of the domes 300a. The domes 300a each have an internal surface SI which delimits with the cylinder 100 an internal volume of the tank to be obtained, and an external surface SE located outside the internal volume and which is covered in a sealed manner by the third or fourth tarpaulin 35 depending on the dome considered. The tarpaulins 35 are located outside. of the internal volume while the tarpaulins 341, 342 are located inside the internal volume.

[0098] Thus, after positioning the panels 10, a precursor assembly of the tank to be manufactured is obtained, which comprises the cylinder 100, the domes 300a each located on the side of an opposite longitudinal end of the cylinder 100 and the common bottom precursor 400a located between the domes 300a in the illustrated example. The precursor 400a delimits with the first dome 300a a first compartment C1, and the precursor 400a delimits with the second dome 300a a second compartment C2 distinct from the first compartment. The domes 300a close the cylinder 100 on the side of each of its longitudinal ends. The cylinder 100, the skins of the domes 300a and the skins of the precursor 400a are each made of thermosetting pre-impregnated fibrous material.

[0099] The panels 10 are positioned opposite the zones 310a and 410a so as to allow the bases to be secured to the body during the vacuum cooking which will be described below. In particular, the zones 310a are opposite the zones 16 of the panels 10, and the zone 410a is opposite the zone 16a. The zones 16 and 16a are devoid of any stiffening element and have been described above.

[0100] The molding portion 60 of the tank body is then positioned, which is in the shape of the tank body to be obtained and which surrounds the cylinder 100 over its entire length. The cylinder 100 is located inside the portion 60. The portion 60 is located around the cylinder 100. The portion 60 makes it possible to hold the panels 10 in place. It is possible to envisage indexing the panels 10 on the portion 60. Like the panels 10, the portion 60 can be sectorized and formed by several sectors assembled together to form a 360° molding portion. As illustrated, the portion 60 extends from one ring 40 to the other. It is positioned on the surface 42, which avoids having the weight of this external tooling supported on the panels 10.The portion 60 may include stiffeners (not shown) on its face opposite the panels 10 in order to have significant rigidity and thus retain the same shape, without flexing, despite a significant length if a very long tank is desired. An internal surface SI of the precursor assembly is located on the side of the tanks 341 and 342 (and delimits the internal volume V of the tank), and an external surface S2 of the assembly is located on the portion 60.

[0101] The seals 52 are then produced.

[0102] The precursor assembly is thus positioned in the vacuum molding tool 30. In particular, the cylinder 100 is interposed between the tarpaulins 341, 342 and the molding portion 60, and each dome 300a is interposed between the tarpaulin 341 or 342 and a respective tarpaulin 35. The parts 321, 322 are located inside the precursor assembly, that is to say located inside the internal volume V of the tank to be obtained. Portion 60 is located outside this internal volume and forms an external part of the assembly, intended for molding the tank.

[0103] Vacuum curing of the precursor assembly in the tooling and disassembly

[0104] The sous vide cooking phase is illustrated in [Fig. 16]. This cooking phase can be carried out with or without additional pressure.

[0105] The heating applied leads to a fluidification of the resin(s) present. The vacuuming of the tool 30 causes the tarpaulins 341, 342 to be pressed against the cylinder 100 so as to press the latter against the portion 60 and to conform it to the desired shape. The domes 300a are pressed against the panels 10 at the zones 310a, and the precursor 400a is pressed at the zone 410a. The parts 311a and 411a, 411b thus conform to the desired shape without any general deformation of the shape since the frame is completely polymerized. The tarpaulins 35 apply pressure to the skirt precursors 110 and 120 as well as to the parts 312a to press them against the skirts 110 and 120. The tarpaulins 35 also apply pressure to the surface SE of each of the domes 300a during vacuum cooking.

[0106] Once all the parts are in place, heating allows the resin to finish polymerizing with the creation of bonds at the level of the polymer chains of the resins (creation of a three-dimensional network), freezing the interfaces and the overall shape of the reservoir structure. The polymerization of thermosetting materials is complete after the vacuum curing step.

[0107] Vacuum curing can be carried out in a heating chamber, such as an oven or an autoclave. The oven will allow polymerization under vacuum, the autoclave will allow additional pressure to be provided in addition to the vacuum draw. Alternatively or in combination, molding tools equipped with heating elements (not shown) can be used to carry out this curing. The temperature imposed during vacuum curing depends on the resin used and may, for example, be greater than or equal to 150°C, and for example be between 175°C and 185°C, for example close to 180°C.

[0108] Tooling 30 made of composite material may be used to reduce the phenomenon of differential expansion between the tooling and the tank. Alternatively, the tooling 30 may be metallic, for example made of Invar, or steel. In the latter case, it may be advantageous to minimize the stresses during cooling, for example by breaking the vacuum in the vacuum tank at the end of the polymerization stage, or by providing a half-opening of the portion 60.

[0109] The tooling 30 is then dismantled, which is facilitated by the positioning of the molding portion outside the tank, and by the simplified design of the internal part. The tank 1 is demolded, which has in in particular frames made of thermoset composite material, covered by one or more thermoset composite skins, at each of its longitudinal ends defining the bottoms 3000 of the tank 1.

[0110] The tank 1 obtained which has just been described has a front skirt 1100 and a rear skirt 1200 extending the body 1000 beyond the bottoms 3000. In the example illustrated, it also has a common bottom 4000 which separates the tank into two separate compartments C1 and C2, the presence of this common bottom 4000 being however optional as indicated above. The skirts 1100 and 1200 are monolithic with the rest of the tank 1.

[0111] The invention which has just been described is suitable for the manufacture of tanks for the main stage of space launchers (lower stage in English). The invention also applies to tanks for upper stages. In operation, the composite tank can be filled, for example and in a non-limiting manner, with liquid methane, liquid hydrogen or refined kerosene (such as RP-1 for example) or a combination of these compounds in a first compartment, and liquid oxygen in a second compartment. The tank can be used in a cryogenic environment.

[0112] In a variant not illustrated, the reservoir is not compartmentalized and the precursor assembly is positioned on a single internal positioning part, going from one dome to another, covered with an internal vacuum cover. In the latter case, the reservoir can be used in a powder acceleration stage in addition to the applications just mentioned.

[0113] The field of the invention is however not limited to a tank for integration into an aerospace launcher but can find an application in the aeronautical field or, more generally, in any application requiring a tank made of composite material.

[0114] The expression “between ... and ...” must be understood as including the limits.

Claims

Claims

1. Method of manufacturing a tank (1) made of composite material, comprising: - positioning a precursor assembly of the tank to be manufactured in a vacuum molding tool (30), the assembly comprising (i) a cylinder (100), precursor of the body (1000) of the tank, made of thermosetting pre-impregnated fibrous material extending along a longitudinal axis (X), and (ii) two domes (300a), precursors of the bottom of the tank, each of these domes being located on the side of an opposite longitudinal end of the cylinder, the domes each having an internal surface (SI) which delimits with the cylinder an internal volume of the tank to be obtained and an external surface (SE) located outside the internal volume, each of the domes comprising a frame (302a) of thermosetting composite material, defining a bottom of the tank, covered by at least one skin (304a;306a) thermosetting pre-impregnated fiber defining a cylindrical junction zone (310a) positioned opposite the cylinder and inside the latter, the reinforcement being configured to allow a conformation of said at least one skin against the cylinder on this cylindrical junction zone, the tooling comprising an internal positioning part (321; 322) coated with an internal vacuum cover (341; 342), located inside the internal volume, on which the assembly is positioned, and an external molding part, located outside the internal volume, comprising a molding portion (60) of the body of the tank having a cylindrical shape located opposite the precursor cylinder, each external surface of the domes being covered in a sealed manner by an external vacuum cover (35), and; - vacuum curing of the assembly in the tool during which the internal vacuum tank applies pressure to conform the assembly to the molding portion, with the junction zones held in contact with the precursor cylinder and co-curing of the domes and the cylinder to secure the bases to the body and obtain the composite material tank.

2. A method according to claim 1, wherein the cylinder (100) extends beyond each of the two domes (300a) so as to define skirt precursors (110; 120), and wherein each dome comprises a first skin (304a) covering a first face of the frame and defining a first portion (311a) of the cylindrical junction zone which extends towards the other dome, and a second skin (306a) covering a second face of the frame, opposite the first face, and defining a second portion (312a) of the cylindrical junction zone which extends in a direction opposite to said other dome, the second portion of the cylindrical junction zone of each dome being secured to a respective skirt precursor during vacuum cooking.

3. The method of claim 2, wherein each dome (300) further comprises a filler material (320) between the first (311) and second (312) portions of the cylindrical joining area (310).

4. Method according to any one of claims 1 to 3, in which the cylindrical junction zone (310a) of each dome is at least partly formed by an extension of said at least one skin (304a; 306a) beyond the frame (302a).

5. Method according to any one of claims 1 to 4, in which the frame (302d) of each dome (300d) defines a plurality of flexible tabs (3027d) allowing the conformation of said at least one skin (306d) against the cylinder on the cylindrical junction zone.

6. Method according to any one of claims 1 to 5, in which said at least one skin (304a; 306a) of each dome (300a) has a thinning of thickness on the cylindrical junction zone (310a) in the direction of a circumferential edge thereof.

7. Method according to any one of claims 1 to 6, in which each skin comprises several sectors (304) juxtaposed around the longitudinal axis (X).

8. Method according to claim 7, in which the neighboring sectors have a thinning of thickness on their overlap zone in the direction of their longitudinal edges (304a 1; 304b 1).

9. A method according to any one of claims 1 to 8, wherein the cylinder (100) and said at least one skin (304a; 306a) of each dome (300a) are assembled in the half-baked state to form the precursor assembly.

10. A method according to any one of claims 1 to 9, wherein the method further comprises, before positioning the precursor assembly in the molding tooling, forming the cylinder (100) and said at least one skin (304a; 306a) of each of the domes (300a) by automatic placement of fibers on a separate shape of the internal positioning part (321; 322).

11. The method of any one of claims 1 to 10, wherein the cylinder (100) and domes (300a) comprise carbon fibers, glass fibers, aramid fibers, or a mixture of such fibers.

12. A method according to any one of claims 1 to 11, wherein the cylinder (100) and the domes (300a) are pre-impregnated with an epoxy resin.

13. A method according to any one of claims 1 to 12, wherein the assembly further comprises (iii) a common bottom precursor (400a) located inside the cylinder and between the domes so as to define with each of them a separate compartment (C1; C2) of the internal volume, said common bottom precursor comprising a common bottom reinforcement made of thermoset composite material, defining the common bottom, covered by at least one common bottom fibrous skin which is pre-impregnated and thermosettable and which defines an intermediate cylindrical junction zone (410a) positioned opposite the cylinder, the common bottom reinforcement being configured to allow conformation of said at least one common bottom skin against the cylinder on this intermediate cylindrical junction zone, and wherein there is co-firing of the common bottom precursor with the cylinder to secure the common bottom to the body during vacuum firing.

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