Abstract:
The invention relates to the field of propulsion nozzles, and in particular to a device ( 105 ) for connecting together first and second segments ( 103   a,    103   b ) of a propulsion nozzle that are made of thermally dissimilar materials. The device ( 105 ) comprises at least one pin ( 106 ) and an eccentric bushing ( 107 ). The pin ( 106 ) presents both a first axisymmetric surface ( 106   a ) that is to be housed in a radial orifice ( 108 ) of the first nozzle segment ( 103   a ) and also a second axisymmetric surface ( 106   b ) that is eccentric relative to said first axisymmetric surface ( 106   a ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the field of propulsion nozzles, and in particular the field of rocket engine nozzles. More specifically, the present invention relates to assembling a propulsion nozzle comprising first and second segments, said first and second segments being made of materials that are thermally dissimilar. 
         [0002]    The term “propulsion nozzle” is used to mean a nozzle of a shape that is appropriate for producing thrust by accelerating a propulsive fluid in a direction opposite from the thrust direction. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the propulsive fluid through the nozzle, and the terms “inside” and “outside” indicate respectively the regions inside and outside the nozzle. 
         [0003]    Propulsion nozzles may in particular be convergents, for fluids that are not compressible or that reach only subsonic speeds, or they may be convergent-divergent for propulsive fluids that are compressible and that reach supersonic speeds. Rocket engines normally have convergent-divergent propulsion nozzles located directly downstream from combustion chambers. The expansion of the hot combustion gas leaving the combustion chamber through the propulsion nozzle serves to convert the thermal energy of the gas into kinetic energy. Consequently, the propulsion nozzles of rocket engines are typically subjected to extreme thermal stresses, since they come directly into contact with such combustion gas. 
         [0004]    Furthermore, in order to be able to increase the propelled payload, it is appropriate to lighten the nozzle as much as possible. To do this, one possibility is to use segments made of materials that differ as a function of the thermal and mechanical stresses acting on each segment. Thus, by way of example, an upstream segment of the nozzle may be made at least in part out of metal in order to better remove the heat that is transmitted to the walls of the nozzle by the combustion gas, while a downstream segment, and in particular a divergent segment of the nozzle, where the combustion gas is significantly less hot after expanding and accelerating beyond the speed of sound, may be made of a composite material that is lighter in weight for comparable mechanical strength. 
         [0005]    The different thermal characteristics of such materials can nevertheless raise major drawbacks. In particular, the physical connection between the segments may be subjected to large thermal and mechanical stresses as a result of the dissimilar thermal properties of the materials of the two segments. 
         [0006]    Thus, the different coefficients of thermal expansion may lead to major mechanical stresses on the connection between the two segments. Also, the difference between the thermal conductivities of the two materials can also give rise to large temperature differences in the proximity of the junction between the two segments. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0007]    In a first aspect, the present disclosure seeks to propose a device for connecting together a first segment and a second segment of a propulsion nozzle that are made of thermally dissimilar materials, which provides a mechanical connection that is very reliable between said nozzle segments even under high thermal stresses. 
         [0008]    This object is achieved by the fact that the connection device includes at least one pin with a first axisymmetric surface that is to be housed in a radial orifice of the first nozzle segment and a second axisymmetric surface that is eccentric relative to said first axisymmetric surface, and at least one eccentric bushing presenting an inside axisymmetric surface complementary to the second axisymmetric surface of the pin and an outside axisymmetric surface, that is eccentric relative to said inside axisymmetric surface and that is to be housed in a radial orifice of the second nozzle segment. The radial orientation of the pin when housed in the orifices of the two nozzle segments in order to connect them together may avoid large temperature gradients even when the temperatures of the inside walls of the two nozzle segments are very different in the proximity of their junction. Furthermore, the eccentricity between the two axisymmetric surfaces of the pin, and also between the two axisymmetric surfaces of the bushing, make it possible to adjust the position of the first axisymmetric surface of the pin in a plane perpendicular to the pin relative to the outside position of the axisymmetric surface of the bushing, in order to connect together the two segments even if their radial orifices are not accurately in alignment, e.g. as a result of axial prestress that needs to be maintained between the two nozzle segments in order to ensure a constant mechanical connection between the nozzle segments. 
         [0009]    In particular, the axes of symmetry of the inside and outside axisymmetric surfaces of the eccentric bushing may present substantially the same offset between them as between the axes of symmetry of the first and second axisymmetric surfaces of the pin. Thus, the eccentric bushing and the pin turning jointly enables the relative position of the radial orifices of the two segments to be adjusted only in a direction parallel to a central axis of the nozzle, without necessarily giving rise to a corresponding relative movement in a tangential direction. 
         [0010]    In order to retain the pin after it has been put into place between the two nozzle segments, the connection device may further include at least one axial retention member for axially retaining said pin, possibly associated with members for fastening said axial retention member to one of said nozzle segments. 
         [0011]    At least some of said axisymmetric surfaces may in particular be cylindrical, thereby facilitating fabrication and facilitating installation of the bushing and of the pin. Nevertheless, it is also possible to envisage using other axisymmetric shapes, e.g. frustoconical shapes. 
         [0012]    The present disclosure also relates to a propulsion nozzle including first and second nozzle segments made of thermally dissimilar materials, each having a radial shoulder bearing against a corresponding radial shoulder of the other one of said segments, together with a plurality of radial orifices facing corresponding orifices in the other one of said segments, and a plurality of the above-mentioned connection devices, with the first axisymmetric surface of the pin of each of them being housed in one of said radial orifices of the first segment, and the respective eccentric bushing is housed in the corresponding radial orifice of the second segment, the second axisymmetric surface of the pin co-operating with the inside axisymmetric surface of the eccentric bushing. The connection devices may thus maintain axial prestress between the two segments so as to maintain a strong mechanical connection between the segments, even under high levels of vibration. 
         [0013]    In order to retain the eccentric bushings inside the radial orifices of the second nozzle segment after the two segments have been assembled together, each eccentric bushing may be retained between an outer surface of the first nozzle segment and a shoulder in the radial orifice of the second nozzle segment in which the eccentric bushing is housed. 
         [0014]    The present disclosure also relates to a rocket engine with such a propulsion nozzle. 
         [0015]    A second aspect of the present disclosure relates to a method of connecting together a first segment and a second segment of a propulsion nozzle that are made of thermally dissimilar materials, each of said segments including a plurality of radial orifices. The method includes at least the following steps: 
         [0016]    Firstly inserting eccentric bushings in the radial orifices of the second nozzle segment, each bushing presenting an inside axisymmetric surface, and an outside axisymmetric surface that is eccentric relative to said inside axisymmetric surface. 
         [0017]    Thereafter, causing a radial shoulder of the first segment to press against a radial shoulder of the second segment, said radial orifices of the first segment being put into register with corresponding orifices among the radial orifices of the second segment. 
         [0018]    Finally, inserting pins in the radial orifices, each pin presenting a first axisymmetric surface that is to be housed in a radial orifice of the first nozzle segment and a second axisymmetric surface of the same connection part, that is eccentric relative to the first axisymmetric surface and complementary to the inside axisymmetric surface of one of said eccentric bushings. The first axisymmetric surface of the pin is aligned with the radial orifice of the first nozzle segment by turning the pin and the eccentric bushing in the corresponding radial orifice of the second segment. 
         [0019]    Thus, thanks to the eccentricity of the pin and of the bushing, it is possible to adapt the geometry of the connection device formed by each bushing-and-pin pair to different relative positions in the axial direction of the nozzle of the radial orifices of the first segment relative to the radial orifices of the second segment, thus at least maintaining prestress between the two segments in that direction. 
         [0020]    In order to obtain accurate prestress between the two elements, the prestress may be applied by external tooling while bringing the radial shoulder of the first segment to bear against the radial shoulder of the second segment. By way of example, the external tooling may comprise traction fingers or clamps. Nevertheless, as an alternative, it is also possible to envisage applying the prestress by turning the pin and the eccentric bushing in the corresponding radial orifice of the second segment. 
         [0021]    The method may also include an additional step of putting into place at least one axial retention member for axially retaining said pins, in order to retain them in the radial orifices of the nozzle segments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention may be well understood and its advantages appear better on reading the following detailed description of an embodiment given by way of nonlimiting example. The description refers to the accompanying drawings, in which: 
           [0023]      FIG. 1  is a partial schematic view in longitudinal section of a rocket engine comprising a nozzle made of two segments of thermally dissimilar materials; 
           [0024]      FIG. 2A  is a cutaway perspective view of the junction between the two nozzle segments connected together by a connection device according to a first embodiment; 
           [0025]      FIG. 2B  is an exploded perspective view of the  FIG. 2A  junction; 
           [0026]      FIG. 3  is a perspective view of the bushing and of the pin of the connection device of  FIG. 2 ; 
           [0027]      FIG. 4  shows the two nozzle segments of  FIG. 2  being brought to bear one against the other; 
           [0028]      FIGS. 5A to 5C  show the  FIG. 2  connection device being adjusted by turning the bushing and the pin; 
           [0029]      FIG. 6  is a cutaway perspective view of the junction between the two nozzle segments connected together by a connection device according to a second embodiment; and 
           [0030]      FIG. 7  is a cutaway perspective view of the junction between the two nozzle segments connected together by a connection device according to a third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]      FIG. 1  shows a rocket engine  1  in part, and more specifically an assembly comprising a propulsion chamber formed by a combustion chamber  2  extended by a convergent-divergent nozzle  3 . In order to lighten this assembly, the convergent-divergent nozzle  3  is made up of two segments  103   a,    103   b:  a throat  103   a  and a divergent portion  103   b.  The throat  103   a  is formed integrally with the combustion chamber  2  that is made of high-temperature resistant metal material, and in the example shown, it presents regenerative cooling ducts  104  for exchanging heat with a propellant of the rocket engine  1 . In contrast, the divergent portion  103   b  is made of composite material, e.g. a carbon/carbon (C/C) ceramic matrix composite, of the carbon silicon carbide (C-SiC), or of the silicon carbide silicon carbide (SiC-SiC) type, using fibers of carbon or of silicon carbide. 
         [0032]    Because of the greater thermal conductivity of the metal material of the throat  103   a,  and because it is subjected to regenerative cooling by the propellant flowing through the ducts  104 , the temperature of the throat  103   a  in the proximity of its junction with the divergent portion  103   b  may be substantially lower than the temperature of the divergent portion  103   b  in the same zone. Furthermore, the metal of the throat  103   a  normally presents a coefficient of thermal expansion that is substantially different from that of the composite material of the divergent portion  103   b.  This gives rise to particular stresses for the mechanical connection between these two segments  103   a  and  103   b.    
         [0033]    Thus, in a conventional connection using radial flanges together with bolts, during operation of the rocket engine, the bolts suffer firstly from high levels of shear stress because of the difference of thermal expansion between the two adjacent segments of the nozzle, and secondly from nonuniform heating that tends to expand the bolts and thus to loosen the connection. Such a connection is thus normally unsuitable for this application. 
         [0034]      FIGS. 2A and 2B  show a connection according to a first embodiment that seeks to solve those drawbacks. This connection between the throat  103   a  and the divergent portion  103   b  is provided by a series of connection devices  105 , each comprising a pin  106  and a bushing  107 , the devices being arranged all around the nozzle. These connection devices  105  maintain prestress F between a radial shoulder  113  of the divergent portion  103   b  pressing against a corresponding radial shoulder  114  of the throat  103   a.  A sealing ring  115  between these shoulders  113  and  114  provides sealing for the connection between the throat  103   a  and the divergent portion  103   b . Each pin  106  is housed at one end in a radial orifice  108  in a ring  109  of the throat  103   a,  and at the other end inside the bushing  107 , which is itself housed in a corresponding radial orifice  110  of a ring  111  of the divergent portion  103   b.  This radial orifice  110  presents a shoulder  112  against which the bushing  107  comes into abutment. 
         [0035]    The pin  106  and the bushing  107  may be seen more clearly in  FIGS. 2B and 3 . Thus, the pin  106  presents two surfaces  106   a,    106   b  that are axisymmetric, and more specifically cylindrical, and that are eccentric relative to each other. Its first surface  106   a,  which presents a diameter d 1 , is to be housed in the radial orifice of a first nozzle segment, specifically in the radial orifice  108  of the ring  109  of the throat  103   a,  while its second surface  106   b,  which presents a diameter d 2  that is greater than the diameter d 1  of the first surface  106   a,  is to be housed inside the bushing  107 . The offset s 1  between the axes of the first surface  106   a  and the second surface  106   b  is equal to or smaller than the difference between these two diameters d 1  and d 2 . The bushing  107  is likewise eccentric, with an internal axisymmetric surface  107   a  and an external axisymmetric surface  107   b,  which have axes of symmetry that are substantially parallel and that are offset by a distance s 2 . In the embodiment shown, the offset s 1  between the axes of the pin  106  is substantially equal to the offset s 2  between the axes of the bushing  107 . Alternatively, they could nevertheless be different. 
         [0036]    The second axisymmetric surface  106   b  of the pin  106  is housed with a small radial clearance inside the inside axisymmetric surface  107   a  of the bushing  107 , in such a manner as to allow relative rotation between these two parts, but without allowing significant relative movement in a direction perpendicular to the axis of rotation. In analogous manner, the first axisymmetric surface  106   a  of the pin  106  and the outside axisymmetric surface  107   b  of the bushing  107  are also housed with a small radial clearance respectively inside of the orifice  108  of the ring  109  and inside the corresponding radial orifice  110  of the ring  111 . 
         [0037]    In order to avoid of the pins  106  being able to escape from the radial orifices  108 , the assembly also includes an axial retention member in the form of an annulus  117  fastened by screws  119  to the ring  109  of the throat  103   a.  Axial projections  117   a  from the annulus  117  engage in an annular groove  118  around an inside end  106   c  of each pin  106  projecting from the orifice  108 , in order to retain each pin  106 . 
         [0038]    The connection shown in  FIG. 2  may be put into place using the following method: 
         [0039]    In a first step, the bushings  107  are received inside the radial orifices  110  of the ring  111  of the divergent portion  103   b,  each coming into abutment against the shoulder  112  of the corresponding orifice  110 . Thereafter, the divergent portion  103   b  is caused to press against the throat  103   a  as shown in  FIG. 4 . To do this, three fingers  116  are inserted from the outside in three of the orifices  110  in the ring  111  of the divergent portion  103   b.  The three fingers  116  may be situated at intervals of 120° in a transverse plane so as to ensure they are mutually balanced, and they exert a prestress force F on the divergent portion  103   b . Alternatively, or in addition to these fingers  116 , other means may be envisaged for introducing and initially applying this prestress F, such as for example, conventional clamps. The selection of the means for applying prestress depends in particular on the geometry of the two parts caused to press against each other. 
         [0040]    Thereafter, while this prestress F is being maintained between the opposite radial shoulders  113  and  114  of the throat  103   a  and of the divergent portion  103   b , the pins  106  are inserted through the remaining orifices  110 . For the purpose of bringing each pin  106  into exact alignment with the corresponding orifice  108  in the ring  109 , the pin  106  and the bushing  107  may be turned in the orifice  110  in the manner shown in  FIGS. 5A to 5C . As can be seen in the figures, the eccentricity of the pin  106  in the bushing  107 , and the eccentricity of the first axisymmetric surface  106   a  of the pin  106  relative to its second axisymmetric surface  106   b  make it possible specifically to adjust the position of the first axisymmetric surface  106   a  vertically by an amount h in the direction of the prestress F using the following formula: 
         [0000]        h=s   1  sin α+ s   2  sin β
 
         [0000]    in which the angles α and β are the angles of rotation respectively of the pin  106  and of the bushing  107  starting from the position shown in  FIG. 5A . The concept “vertical” is used herein to designate a direction parallel to the central axis of the nozzle  3 . 
         [0041]    If the offsets s 1  and s 2  are substantially equal, and if the adjustment is purely vertical, as in the example shown, then the angles of rotation α and β will be substantially identical, and the value of the adjustment distance h will comply with the following formula: 
         [0000]      h=2s 1  sin α
 
         [0042]    After insertion of the pins  106  through the orifices  110  that are not occupied by the fingers  116  and into the corresponding orifices  108 , the fingers  116  are removed and the pins  106  that have been installed take up the prestress F. The three orifices  110  now released of the fingers  116  may still receive respective pins  106 , with their inside ends  106   c  being put into in alignment with the corresponding orifices  108  in the same manner. Each connection device  105  is self-locking, in the sense that the dimensions of the pin  106  and of the bushing  107 , and the coefficients of friction between the various contacting surfaces, are such that neither the prestress F nor the additional stresses during operation of the rocket engine  1  may cause them to turn any more in order to relax the prestress. 
         [0043]    Finally, the annulus  117  is put into place, engaging the annular grooves  118  of the pins  106  in order to retain them, and it is fastened to the ring  109  by means of screws  119 . 
         [0044]    The member for axially retaining the pins may be of a form other than the annulus  117  in this first embodiment. Thus, according to a second embodiment as shown in  FIG. 6 , each pin  206  is held individually by a bracket  217  bearing against the inside edge of the ring  211  and connected to an outside end  206   d  of the pin  206  by a screw  220 . In the connection method of this second embodiment, each bracket  217  is put into place individually on the ring  211  and thereafter it is connected to the corresponding pin  206 . This serves not only to retain the pin  206  axially, but also, given the friction between the head of the screw  220  and the surface of the bracket  217 , this serves simultaneously to create additional resistance to rotation of the various elements of the connection device  205  in the orifices  208  and  210  of the rings  209  and  211  after the device has been put into place, thereby maintaining the prestress between the shoulders  213  and  214  of the nozzle segments  203   a  and  203   b.  Apart from that, the other elements of this nozzle are equivalent to those of the nozzle of the first embodiment, and they are installed in analogous manner. 
         [0045]    Although in the first and second embodiments the ring of the downstream nozzle segment, i.e. of the divergent portion, surrounds the ring of the upstream nozzle segment, this arrangement may also be inverted. In a third embodiment, as shown in  FIG. 6 , the connection between the throat  303   a  and the divergent portion  303   b  of a nozzle also is provided by a series of connection devices  305 , each comprising a pin  306  and a bushing  307 , the devices being arranged all around the nozzle. As in the first two embodiments, these connection devices  305  maintain prestress F of a radial shoulder  313  of the divergent portion  303   b  pressing against a corresponding radial shoulder  314  of the throat  303   a.  A sealing ring  315  between these shoulders  313  and  314  also provides sealing for the connection between the throat  303   a  and the divergent portion  303   b.  Nevertheless, in this third embodiment, each pin  306  is housed at one end in a blind radial orifice  308  in the divergent portion  303   b,  and at the other end inside the bushing  307 , which is itself housed in a corresponding radial orifice  310  of a ring  311  of the throat  303   a  placed around the divergent portion  303   b.  This radial orifice  310  presents a shoulder  312  against which the bushing  307  comes into abutment. Both the pin  306  and the bushing  307  are eccentric in analogous manner to the pins and the bushings in the two above-described embodiments. Thus, the eccentricity of the pin  306  in the bushing  307 , and the eccentricity of the first axisymmetric surface  306   a  of the pin  306  relative to its second axisymmetric surface  306   b  make it possible specifically to adjust the position of the first axisymmetric surface  306   a  vertically by an amount h in the direction of the prestress F in a manner analogous to the first and second embodiments. In this third embodiment, the connection devices  305  do not include axial retention members for retaining the pins  306 . Nevertheless, in order to increase the resistance to rotation of the pin  306  and of the bushing  307  after each pin  306  has been adjusted vertically, each pin  306  houses a nut  321  on an outside thread of the outside end  306   d  of the pin  306 . This nut  321  bears against an outside surface  322  of the ring  311  via a washer  323  so as to increase the friction resistance against movement of these various elements of each connection device  305 . 
         [0046]    Although the present invention is described above with reference to a specific embodiment, it is clear that various modifications and changes may be applied to those embodiments without going beyond the general ambit of the invention as defined by the claims. Furthermore, individual characteristics of the various embodiments mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.