Abstract:
A casing of a solid-propellant engine comprising a core and a layer of elastomeric material, set as coating for at least part of the core to provide a thermal protection of the core itself is obtained by: inserting the core in a forming mold so as to make within the mold two annular chambers separated from one another by the core; forming a strand of elastomeric material; obtaining a defined portion of elastomeric material by cutting the strand transversely to size in an external environment; and injecting the cut portion of elastomeric material simultaneously within both of the annular chambers.

Description:
The present invention relates to a method for producing a casing for a solid-propellant engine for a rocket engine. 
     BACKGROUND OF THE INVENTION 
     As is known, a solid-propellant engine comprises a casing filled with solid propellant, a device for ignition of the propellant, and a terminal nozzle, through which the propellant, by burning, generates a desired propulsive thrust. The casing comprises a core made of steel or of a composite material, which has a cylindrical tubular portion and a closing portion shaped like a spherical cap and is insulated by being coated both internally and externally with a layer of specific thermally insulating material to define an adequate thermal protection of the core. 
     Normally, the layer of thermally resistant material is obtained by forming separately from the core two shells or caps, which, once the core has been formed in vacuum conditions, are, one, inserted within the core bringing it into contact with the internal surface of the core itself and, the other, fitted on the core causing it to adhere to its outer surface. Once the coupling is terminated, the caps are vulcanized in autoclave to complete the coating. 
     The known modality of insulation just described requires three dedicated apparatuses that are different from one another, two for making the caps and the third for assembling the caps themselves on the core. For these reasons, the mode of insulation described involves extremely long times and high costs of implementation, there being necessary three distinct working steps, rendered, on the other hand, even more complex by the particular geometry of the casing. However accurate the operations of production of the shells or caps may be, these frequently have defects and dimensional instability, randomly distributed porosities, and for this reason induce positioning errors or faults. 
     Even though the maximum care is then taken in positioning and in the subsequent operation of vulcanizing, between the caps and the core there are frequently present areas in which they are not properly stuck together, which, along with the aforesaid porosities, jeopardize sensibly the efficiency of the insulation and consequently the reliability of the casing. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide a method for the production of a casing for a solid-propellant engine, which will enable a simple and inexpensive solution to the problems set forth above. 
     Provided according to the present invention is a method for the production of a casing for a solid-propellant engine, the casing comprising a core and a layer of elastomeric material set as coating for at least part of said core to define a thermal protection of said core, the method comprising the steps of inserting said core in a forming mould so as to make, within said forming mould, two annular chambers separated from one another by said core, forming a strand of said elastomeric material, cutting transversely said strand in the presence of air to form a portion of elastomeric material, transferring said portion of elastomeric material within a transfer chamber, and injecting said portion of elastomeric material into said annular chambers. 
     Preferably, in the method defined above, said injection is performed by injecting said portion of material simultaneously in both of said annular chambers so as to fill said two annular chambers simultaneously. 
     The present invention moreover regards a plant for the production of a casing for a solid-propellant engine. 
     Provided according to the present invention is a plant for the production of a casing for a solid-propellant engine, the casing comprising a core and a layer of elastomeric material set as coating for at least part of said core to define a thermal protection of said core, the plant comprising:
         a forming mould designed to withhold said core and delimiting, in a condition of closing, an annular cavity divided by said core into two annular chambers;   forming means separate from said forming mould for causing a strand of said elastomeric material to come out;   a transfer chamber separate from said forming means and designed to receive a pre-set portion of said elastomeric material and having an inlet opening of said portion communicating with the outside; and   injection means for transferring said elastomeric material into said annular chambers.       

     Conveniently, in the plant defined above, said injection means comprise a plurality of delivery channels all communicating with said transfer chamber for filling said annular chambers simultaneously. 
     Finally, the present invention regards a casing for a solid-propellant engine. 
     Provided according to the present invention is a casing for a solid-propellant engine, the casing comprising a core and a layer of elastomeric material set as coating for at least part of said core to define a thermal protection of said core, said casing being characterized in that said layer is directly co-moulded on said core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the attached figures, which illustrate a non-limiting example of embodiment thereof and in which: 
         FIG. 1  illustrates schematically a preferred embodiment of a plant for the production of the casing according to the present invention; 
         FIG. 2  is an exploded perspective view of a detail of  FIG. 1 ; 
         FIG. 3  illustrates, in cross section and at an enlarged scale, a detail of  FIG. 1 ; 
         FIG. 4  illustrates partially in cross section a preferred embodiment of the casing made according to the dictates of the present invention; and 
         FIG. 5  is a view according to the line V-V of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , designated as a whole by  1  is a plant for the production of an insulated casing  2  ( FIGS. 4 and 5 ) of a solid-propellant engine (not illustrated) for a rocket engine. 
     With reference to  FIG. 4 , the casing  2  has an axis of symmetry  3  of its own, and comprises a cylindrical tubular intermediate portion  4  coaxial to the axis  3 , a spherical-cap portion  5  for closing a longitudinal end of the intermediate portion  4 , and an externally threaded cylindrical terminal stretch  6  opposite to the portion  5 . The casing  2  is delimited by an inner surface  8  and by an outer surface  9 , and is constituted by a core  10 , terminating with the stretch  6  and made conveniently of metal material or of other equivalent material resistant to the mechanical and thermal stresses of operation of the casing  2 , and by a layer  12  of thermally insulating elastomeric material. The layer  12  coats the core  10  completely except for the part of the terminal stretch  6  provided with the external thread, as indicated in  FIG. 4 , to define a thermal protection of insulation from the core  10  itself. 
     Once again with reference to  FIG. 1 , the plant  1  comprises a supporting structure  15 , which, in turn, comprises a base  16 , from which there rise uprights  17  for supporting an intermediate plate or platform  18 , which supports a forming mould  19  for forming the casing  2 . 
     With reference to  FIGS. 2 and 3 , the mould  19  comprises a cylindrical base portion  20 , which has an axis of symmetry  21  of its own orthogonal to the intermediate plate  18 , is stably connected to the intermediate plate  18  itself and carries, in turn, stably connected, a male body  23 , which extends upwards coaxial to the axis  21  and delimited externally by a surface  24  complementary to the inner surface  8  of the casing  2 . 
     Once again with reference to  FIG. 3 , the cylindrical base portion  20  comprises an annular resting surface  26 , which extends orthogonal to the axis  21 , surrounds a base portion of the male body  23  and defines a rest for a front surface of the threaded stretch  6  of the core  10 . The core is withheld in a position sharing the axis  21  by two semicircular inserts  27 , which surround the threaded stretch  6  and have respective bottom attachment or anchorage portions  28  inserted in a common circumferential groove  29  of the cylindrical base portion  20 . The inserts  27  are forced against one another and against the core  10  by a pair of clamping half-rings  30 , which are set on diametrally opposite sides of the cylindrical base portion  20  and are coupled to respective slides  31 , which translate on corresponding radial guides  32 . The radial guides  32  are fixedly connected to the plate  18  and carry coupled to them the slides  31 , which displace under the thrust of respective linear actuators  33  between an advanced, operative, position, in which, via the inserts  27 , they clamp the core  10  in a position sharing the axis  21 , and a resting position where they are set at a distance, in which they enable extraction of the casing  2  that has been formed. 
     When clamped in a position sharing the axis  21 , the core  10  delimits, together with the male body  23 , an annular chamber  35  having a shape and dimensions practically equal to the part of the layer  12  that extends within the core  10  itself. 
     Once again with reference to  FIGS. 2 and 3 , the mould  19  moreover comprises a top bell  37 , which extends in a position coaxial to the axis  21  above the circular portion  20  and the male body  23  and is delimited internally by a surface  38  ( FIG. 3 ) complementary to the outer surface  9  of the casing  2 . The bell  37  is mobile under the thrust of a linear actuator  39  of its own carried by a further plate  18   a  of the parallel structure  15 , raised with respect to the plate  18  between a lowered, operative, position ( FIG. 3 ) and a raised, resting, position. When the bell  37  is set in its raised, resting, position, it enables insertion of the core  10  and extraction of the casing  2 , whilst when it is set in its lowered position, it is coupled in a fluid-tight way to the half-rings  30  and delimits, together with the inserts  27  and the male body  23 , a cavity  40 , which is in turn divided by the core  10  to form the annular chamber  35 , comprised between the core  10  itself and the male body  23 , and a further annular chamber  41 , comprised between the core  10  and the bell  37 . 
     The relative positioning of the bell  37  with respect to the cylindrical base portion  20  and the stability of the bell  37  itself when it is set in its lowered position are ensured by a plurality of centring and retention pins  43 , which extend from a side wall  44  of the bell  37  in cantilever fashion towards the intermediate plate  18  in a direction parallel to the axis  21  and terminate with respective tapered end centring portions inserted in corresponding axial holes  46  of the intermediate plate  18  itself ( FIG. 2 ). 
     Once again with reference to  FIG. 3 , the bell  37  and the male body  23  are traversed by respective ducts, designated by  47  and  48 , respectively, the inlets of which give out into the annular chamber  41  and, respectively, into the annular chamber  35  and the outlets of which (not visible in the attached figures) are connected to a suction source for creating a desired negative pressure in the annular chambers  41  and  35  themselves. 
     Once again with reference to  FIG. 1  and, in particular, to  FIG. 3 , the plant  1  moreover comprises a transfer device  50  for transferring simultaneously and in a single operation a mass of elastomeric material within the annular chambers  35  and  41 . 
     The device  50  comprises a cylindrical casing  51 , which extends underneath the plate  18  sharing the axis  21  and houses, in an axially slidable way, a force plug  53  mobile in opposite directions under the thrust of a linear actuator (not visible in the attached figures). 
     The force plug  53  delimits, together with the casing  51  and a plug body  54  for closing the top end of the casing  51  itself a transfer chamber  55 . The chamber  55  communicates permanently with the chamber  35  through a plurality of calibrated passages or injection nozzles  56  made through the plug body  54  and with the chamber  41  through a plurality of passages or injection nozzles  57 , which are made through the plug body  54  and the plate  18  and form part, together with the passages  56 , of the device  50 . 
     The chamber  55  communicates directly with the outside through a side opening  57  ( FIGS. 1 and 3 ), which is closed, in use, in a fluid-tight way by a hatch and through which a pre-set amount of elastomeric material is manually inserted. 
     The elastomeric material is prepared and dispensed in a forming unit, which forms part of the plant  1  and is designated by  58  in  FIG. 1 . 
     The forming unit  58  comprises an assembly  59  for preparation of the elastomeric material and an extruder assembly  60 , which receives the elastomeric material from the assembly  59  through a duct  61  and comprises a forming head  62  or die plate designed to form a continuous strand  63  of elastomeric material outwards. At outlet from the head  62 , the strand is cut transversely to size in a known way to form a block  64  of elastomeric material, which has dimensions such as to enable insertion thereof in the transfer chamber  55  through the opening  57 . 
     Operation of the plant  1  will now be described starting from the condition in which:
         the mould  19  is set in a closed condition and houses the core  10 , the stretch  6  of which is stably clamped between the sectors  27 ;   the hatch is opened, enabling access to the transfer chamber  55 ; and   the force plug  53  of the transfer device  50  is set in a retracted position, in which it enables insertion of the body of elastomeric material  64  into the chamber  55  itself.       

     Starting from said condition, the elastomeric material used for formation of the layer  12  is prepared in the assembly  59 , and once the desired chemico-physical conditions are reached, is fed through the duct  61  to the head  62 , which progressively forms the strand  63  and causes it to advance outwards, as may be seen in  FIG. 1 . On the outside of the head  62 , the strand  63  is cut transversely to size to form the block  64 . The block  64  is transferred manually to the cylindrical casing  51  of the device  50  and inserted within the chamber  55  through the opening  57 . Once the hatch is closed, the force plug  53  is activated and displaces the block  64  into an intermediate raised position, in which it remains for a pre-set time in order to enable homogenization of the temperatures, after the force plug itself has progressively advanced towards the plug body  54 . During advance of the force plug  53 , the annular chambers  35  and  41  are set in communication with the suction source, and the elastomeric material traverses progressively the channels or nozzles  56  and  57 , coming to fill the chambers  35  and  41  progressively. The force plug  53  proceeds its travel of feed until the chambers  35  and  41  are completely filled with elastomeric material under pressure, which is, at this point, forced on the core  10  and adheres to the core  10  itself. At this point, the mould  19  is heated and the elastomeric material is vulcanized on the core  10 , where it remains stably blocked and in an unchanged position over the entire side surface of the core  10  itself so completing the casing  2 . Once vulcanizing is completed, the mould  19  is opened, and the insulated casing  2  is extracted from the mould  19  following the operation described previously in reverse. Next, holes A visible in  FIGS. 4 and 5  are made on the insulated casing  2 . 
     From the foregoing, it appears evident how the described mode of production of the casing  2  will enable, first of all, simultaneous production in a single operation of the entire thermal coating or the entire insulation of the metal core  10 . The mode of implementation described, precisely on account of the fact that it enables formation on the outside of the mould of a homogeneous mass of elastomeric material and injection of said mass directly and simultaneously into the two annular chambers  35  and  41  of the mould  19 , prevents onset of gas-holes and/or porosities in the elastomeric material, which is consequently distributed homogeneously over the core  10 , thus eliminating any possibility of localized overheating of the core  10  itself. The fact, then, of vulcanizing the elastomeric material directly in the forming mould enables a uniform adhesion of the elastomeric material to the core  10 , definitively eliminating the problem of areas that are not properly stuck together, and thus solving all the problems of reliability and functional efficiency of the casing. 
     As compared to the known solutions, the plant  1  described is extremely simple to produce at particularly contained costs. 
     Not only, but the plant  1  described, precisely because it enables separation of the step of preparation of the elastomeric material from the step of injection of the elastomeric material itself in the mould, enables injection of all those materials the chemico-physical characteristics of which are in themselves not modifiable. In other words, particularly important and advantageous is the fact of being able to extrude the elastomeric material in an external environment, as well as the possibility of it being introduced in the mould only after the exact amount thereof has been dispensed and after the thermal conditions that are optimal for the specific application have been reached. 
     From the foregoing, it appears clearly how modifications and variations can be made to the plant  1  described herein, without thereby departing from the sphere of protection defined by the annexed claims. 
     In particular, the mould could be made in a way different from the one indicated by way of example, and for instance in such a way as to enable injection of the elastomeric material at different times into the two chambers.