Patent Application
20200116105
This application claims priority to foreign French patent application No. FR 1859505, filed on Oct. 12, 2018, the disclosure of which is incorporated by reference in its entirety.
The present invention lies in the field of the management of pressures. It relates to a common bulkhead for a pressure vessel having two chambers. The invention also relates to a thruster equipped with such a common bulkhead.
In the field of propulsion, the ongoing pursuit of a greater range has led to the development of solid propulsion systems referred to as double-pulse propulsion systems, as shown schematically in
In operation, a first thrust is brought about by initiation of the first solid propellant charge by the first ignition device. During this phase, the gases released are accelerated by the nozzle assembly, like in a conventional single-pulse thruster. It is necessary to protect the second propellant charge from the combustion gases of the first charge, lest it ignite. To provide this protection, a common bulkhead 6 is interposed between the two propellant charges 24, 25. This common bulkhead should withstand the pressure of the gases generated by the first propellant charge with a sufficient margin. Typically, if the maximum operating pressure of the first charge is 15 MPa, the maximum withstand pressure of the common bulkhead should be 15 MPa plus a safety coefficient of around 1.5, i.e. 22.5 MPa. This strength should take into account the heating that arises during the operation of the first propellant charge.
Next, a second thrust is brought about by the initiation of the second solid propellant charge by the second ignition device. The common bulkhead opens. The gases then flow through the common bulkhead and then into the combustion chamber of the first charge, which is now empty, before reaching the nozzle assembly, where the gases are accelerated and converted into thrust.
The common bulkhead has multiple roles: in addition to protecting the second propellant charge during the ignition and combustion of the first charge, it has to open under a low pressure of the second charge in order to minimize the impact upon ignition and to afford a flow cross section for the gases output by the second charge that is as large as possible in order to minimize the erosion of the common bulkhead and of the thermal protection of the first combustion chamber during the phase of homogenizing the resultant flow and to minimize the drop in pressure between the two chambers.
Solutions exist for separating the charges of double-pulse propulsion systems. In particular, the peelable or non-peelable flexible walls may be cited. These solutions have drawbacks. The flexible walls have to be fastened to the structure of the system either by adhesive bonding or by a bulky mechanical system. It is often not possible to test the leaktightness of the wall. The durability of the adhesive bond that is critical for the propulsion function may be a difficulty.
The flexible wall technology generally necessitates a simple charge geometry, which is fairly hard to reconcile with the requirement of a thrust profile that is as constant as possible.
The charging process when a flexible wall is employed is complex. The charges are cast successively, the first being cured twice, the first of which may only be partial. During casting and curing, the flexible wall is subject to significant mechanical and chemical attack.
Moreover, it is absolutely necessary that the flexible wall be chemically compatible with the two propellants of the two charges. The plasticizers contained in the formulations of the propellants might have a negative effect on the durability of the adhesive bonds of the flexible wall.
Finally, in the case of a peelable flexible wall, in order to provide the full performance of a double-pulse thruster, it is necessary to ensure that the entire wall peels after several decades of aging.
There also exist, for example, rigid common bulkheads that have circular perforations for allowing the gases originating from the combustion of the second charge to pass through. Such a prior-art common bulkhead 6 is shown schematically in
The invention aims to remedy all or some of the problems cited above by proposing a rigid common bulkhead that is easy to incorporate into a double-pulse thruster, without necessitating geometric constraints on the charges, allowing a simplified charging process, without any problems of chemical compatibility with the propellant charges. The rigid common bulkhead according to the invention also allows a reduction in its mass, in its volume, while increasing the flow cross section for the gases.
To this end, the subject of the invention is a common bulkhead for a pressure vessel having two chambers, the common bulkhead being intended to be positioned between a first chamber and a second chamber of the pressure vessel and configured to withstand a first predetermined pressure in the first chamber and to allow a fluid from the second chamber to flow above a second predetermined pressure. According to the invention, the common bulkhead comprises:
Advantageously, a section with a polygonal-type pattern has an edge substantially parallel to an edge of a section with a polygonal-type pattern that is adjacent thereto.
According to one embodiment, at least one of the plurality of through-openings comprises a means for reducing the stress concentrations at at least one intersection of two edges of the section, preferably a chamfer or a fillet.
According to another embodiment, at least one of the plurality of through-openings comprises a means for making flow entry easier at the intersection between said through-opening and the second face, preferably a fillet or a chamfer.
The basic structure may be made of ceramic or metallic.
Advantageously, the first face and/or second face and/or lateral faces of at least one of the plurality of through-openings in the metallic basic structure are covered at least partially by a thermal protection.
According to another embodiment, the common bulkhead comprises an external frame at its periphery, the external frame comprises an external face extending on the opposite side from the basic structure and two internal faces intended to be in contact with the first and the second chamber, respectively, and the common bulkhead comprises at least one duct extending between the external face and one of the chambers.
According to another embodiment, the common bulkhead comprises at least one second metallic cap superposed on the first face and on the first cap.
According to another embodiment, the common bulkhead also comprises at least one flexibility element positioned between the first cap and the second cap and/or between two adjacent caps.
According to another embodiment, the basic structure comprises a central through-opening between the first face and the second face and an internal frame delimiting the central opening.
The invention relates to the embodiment of a common bulkhead, the pressure vessel having two chambers being a double-pulse thruster, the double-pulse thruster comprising:
The invention also relates to a thruster comprising at least two combustion chambers, each being intended to receive a propellant charge, and comprising at least one common bulkhead as described in the present application, positioned between the two combustion chambers.
The invention will be understood better and further advantages will become apparent from reading the detailed description of an embodiment given by way of example, said description being illustrated by the appended drawing, in which:
For the sake of clarity, the same elements will bear the same reference signs in the various figures. For a better view and for the sake of greater understanding, the elements are not always shown to scale.
The invention is described in the field of solid propulsion, in relation to a double-pulse thruster. Clearly, the thruster may be a thruster with more than two pulses, with more than two propellant charges and with as many common bulkheads as there are combustion chambers to be separated. Moreover, the invention applies to any other field requiring the management of pressures. In particular, the common bulkhead according to the invention withstands a high-pressure in one direction of stress (from the first chamber 11 to the second chamber 12) and allows a flow of fluid (liquid or gas) to escape in the other direction of stress (from the second chamber 12 to the first chamber 11) as soon as the associated pressure exceeds a low threshold level, typically with a pressure ratio of between 5:1 and 20:1.
Thus, besides the double-pulse thruster, the invention can find application as a diode base or safety element on containers that are intended to withstand high external pressures and release the internal pressure as soon as it goes up, or vice versa. The cylindrical outer shape is not absolutely necessary, it is suitable for the propulsion of tactical missiles (external aerodynamics). For other applications, it could have any other section, for example one that is square, triangular, etc.
In other words, the basic structure 13 comprises a plurality of perforations which have a section in the form of a polygon, in other words a closed broken line.
As explained above, the first charge 24, once ignited, produces a first thrust that corresponds to the first pressure predetermined by the type of propellant charge in question. During this phase, the gases released are accelerated by the nozzle assembly 19. The common bulkhead 10 protects the second propellant charge 25 from the combustion gases of the first charge 24. The metallic cap 51 ensures leaktightness. It is for example welded to the rings of the external frame 41 by a weld bead 52 in order to ensure the leaktightness with regard to the combustion gases output by the first propellant charge 24 and integrity with respect to the pressure forces of the first charge (around 25 MPa). Typically, the thickness of the cap is from 0.05 to 0.2 mm depending on the steel that is used, which preferably needs to be weldable to the rings. Preferably, a steel with a very high deformation outbreak is used to make the cap 51. Advantageously, but not absolutely necessary, use is made of X2CrNi18-09(T651) if the external frame 41 is made of Maraging 300 (stainless steel reinforced with cobalt with addition of 18% nickel). The cap 51 deforms while withstanding the force and ensuring leaktightness with respect to the pressure of the combustion gases from the first chamber 11.
After a period calculated depending on the mission to be carried out (generally from a few seconds to a few tens of seconds), the second charge 25 is ignited. The common bulkhead opens under a pressure that this time comes from the second chamber 12. The expression uncapping is used. Typically, a pressure known as the uncapping pressure for the common bulkhead of around 4 MPa is chosen. The gases then flow through the common bulkhead 10, through the plurality of through-openings 14, and then into the first chamber 1, which is empty, in order to reach the nozzle assembly 19.
Thus, the common bulkhead 10 according to the invention opens under a low pressure originating from the second chamber 12, in order to minimize the impact upon ignition. The uncapping pressure (that is to say the pressure for opening the cap) of the common bulkhead is chosen to be compatible with ignition in accordance with standard practice. The common bulkhead also affords a passage cross section for the gases originating from the second charge that is as large as possible.
Specifically, advantageously, a section with a polygonal-type pattern has an edge 15 substantially parallel to an edge 16 of a section with a polygonal-type pattern adjacent thereto (see the local zoom of the through-openings 14 above
The common bulkhead 10 is optimized in terms of its manufacture. While the production of a bulk head from structural steel by a conventional machining process is difficult, if not impossible, the production by additive manufacturing by melting powder, laser fusion or by electron beam is quite suitable. Re-machining operations are minimal, thereby contributing towards minimizing the overall cost of producing the common bulkhead. Similarly, the thickness of the common bulkhead is minimal on account of the high rigidity and high strength of the geometry, minimizing the space requirement of the common bulkhead.
The invention is based on the repeating pattern having a polygon in section. The section may have a triangular, square, rectangular, pentagonal, hexagonal or octagonal shape or any other polygonal shape or any other combination of polygonal shapes. It should be noted that some sections are preferably, for example hexagonal sections. In the case of pentagonal or octagonal sections, the patterns interlock less optimally with one another, leaving space between the through-openings. Therefore, it is advantageous to position between two spaced-apart openings, an opening with a different section (diamond or the like) and/or with a section of smaller size. Around the perimeter of the structure (outside, and inside if necessary), it is possible for the openings to be incomplete and not then to form a polygonal section. These cells can nonetheless be maintained or, if they have a section that is too small, they can be eliminated.
This results in great rigidity and resistance to bending brought about by the operating pressure of the first propellant charge 24. For example with a hexagonal pattern as shown in
It may be noted that, in the case of the common bulkhead 10 shown in
Finally, on the common bulkhead shown in
With or without a central opening 17, owing to the through-openings 14, the passage cross section for the gases is twice the size compared with a prior-art common bulkhead as shown in
The cap assembly is less stressed by the first propellant charge 24 on account of the smaller size of the polygonal perforations 14 compared with the large circular perforations in a prior-art common bulkhead. Thinner caps can be used, making it possible to all the better meet the requirement of uncapping at low pressure under the effect of the second propellant charge 25.
Finally, on the common bulkhead shown in
It should be noted that, advantageously, the pattern is repetitive. However, the invention also covers the possibility of an alternation of several patterns, for example triangular on one part of the basic structure and hexagonal on another part.
The basic structure may be made of ceramic, for example C-SiC. Since ceramic is not weldable, some other means for fastening the cap(s) than the weld beam has to be considered. Since the mechanical strength of ceramics is not as high as that of steels, the geometry of the basic structure should be adapted. The advantage of a basic structure made of ceramic is that it does not require additional thermal protection.
Alternatively, the basic structure may be metallic.
In another variant, the first face 31 and/or second face 32 and/or lateral faces 35 of at least one of the plurality of through-openings 14 are covered at least partially with a thermal protection 36, in order to protect these parts from the ablation of the flow of combustion gases originating from the second propellant charge. The thermal protection may be a ceramic. Alternatively, it may be thermal protection made of a low-elongation monoplast, a composite made up of around 70% of fibres or powder or both, carbon, glass or silica or Kevlar or a mixture thereof, and around 30% of a thermosetting resin, for example phenolic or cyanate ester or epoxy resin. This resin breaks down under the effect of heat, leaving a significant carbon residue, making it possible to partially evacuate the thermal energy of the gases and to maintain the reinforcement. The thermal protection made of a low-elongation monoplast requires an increase in the thickness of the protection compared with the thickness of a ceramic. Alternatively, a flexible thermal protection can be used, with yet a further increase in the thickness of the protection. The flexible thermal protection is a composite made up of around 40% of fibres or powder or both, carbon, glass, silica or Kevlar or a mixture thereof, and around 60% of a silicone, EPDM (abbreviation of ethylene propylene diene monomer), or polychloroprene.
It may be specified that if the combustion time of the second charge is short or if the ablation of the metallic structure of the common bulkhead is tolerated, no thermal protection is used.
An example of a metallic structure of the common bulkhead is a stainless steel with structural hardening of the type M300, M250, 17-4 PH, 15-5 PH, X12, chromium steel, an alloy of aluminum, of iconel, of titanium, etc.
In the case in which the common bulkhead comprises a through-opening 17 between the first chamber 11 and the second chamber 12 and an internal frame 18 delimiting the central opening 17, the cap 51 is also fastened to the basic structure 13 at the internal frame 18 by a third fastening means 53, for example a weld bead, screw, rivet. Alternatively, the cap(s) can be fastened to the structure of the first combustion chamber.
The common bulkhead may also comprise at least one flexibility element 64 positioned between the first cap 51 and the second cap 61 (and/or between two adjacent caps in the case of more than two caps), making it possible to decouple the uncapping of the two caps 51 and 61 of the system of caps. The flexibility element is advantageously positioned at the peripheries of the caps, preferably at the fastening means. Thus, the fastening means 52 and 53 of the first cap 51 are stressed without any contribution of the cap 61 and its fastening means 54 and 55, and it is the flexibility element 64 that is stressed. Thus, the resistance of the system of caps to the pressure of the first propellant charge 24 is doubled while the uncapping pressure remains the same. In other words, when the common bulkhead is subjected to a pressure originating from the second chamber, it is the first cap 51 that is subjected to this pressure first. The flexibility element 64, for its part, acts as a buffer between the two caps, such that the second cap 61 is not stressed by the pressure originating from the second chamber as long as the first cap 51 is present and intact. The first cap 51 opens at a predetermined pressure (for example 4 MPa). Once open, the second cap 61 is then, only from this moment, exposed to the pressure originating from the second chamber (4 MPa in this example). If the second cap 61 has a thickness and fastening means that are identical to that/those of the first cap 51, it opens likewise at the same predetermined pressure. The two caps 51 and 61 are indeed redundant in this operation and ensure greater safety: the resistance to the pressure originating from the first chamber is double, and despite this, the two caps 51, 61 open at a predetermined pressure, which may be lower, originating from the second chamber. The flexibility elements may be placed between two adjacent caps, depending on the number of caps present.
The invention is also applicable to any pressure vessel comprising more than two juxtaposed chambers, for example three chambers with common bulkheads therebetween, or more than three chambers.
Since the thruster 70 comprises at least two combustion chambers 11, 12, it can thus be a multiple-pulse thruster.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1859505 | Oct 2018 | FR | national |
