Patent Publication Number: US-2018030927-A1

Title: Propellant grain for a solid rocket motor

Description:
FIELD OF THE INVENTION 
     The present disclosure primarily relates to solid rocket motors, more particularly, to propellant grain configuration for use in solid rocket motors. 
     BACKGROUND 
     To meet increasingly demanding performance requirements from solid rocket motors, new propellant grain configurations are conceived. Many factors contribute towards building a high performance solid rocket motor. Two of the geometric factors are high ‘propellant volumetric loading density’ and flexibility to tailor ‘grain regression pattern’ to deliver different mission-specific thrust profiles. Conventionally grain designers have used a variety of grain configurations viz. star, finocyl and conocyl, to name a few, with varying degrees of relative merits, to meet different requirements. Among them, finocyl is a versatile grain configuration that allows for high loading density as well as thrust tailoring flexibility. A finocyl grain configuration comprises of an axial cavity surrounded by longitudinal fin cavities in cyclic symmetry. By altering the size, shape, location and number of fin, finocyl grains can be configured for a variety of thrust profiles. 
       FIGS. 1 a    &amp;  1   b  show the aft-end view and longitudinal sectional view respectively of a monolithic solid rocket motor with conventional shallow finocyl grain as in prior art. When the largest imaginary circle circumscribing the fin tips is smaller than the largest motor aperture, it is called shallow finocyl grain. The rocket motor  100  comprises an internally insulated motor casing  101  with apertures  102  and  103  at the front end (alternatively referred to as fore-end) and rear end (alternatively referred to as aft-end) respectively. Generally fore-end aperture is smaller than aft-end aperture. In a typical rocket motor, an igniter (not shown) closes the fore-end aperture  102 ; a nozzle (not shown) closes the aft-end aperture  103 . The rocket motor  100  further comprises propellant grain  104  with an axial cavity  105  and number of shallow fin  106   a  having reverse-swept leading edges  107   a . When the leading edge  107   a  makes an obtuse angle  108   a  with the forward motor axis  101   a , the fin are called reverse-swept. The propellant grain  104  further comprises a counter-bore  109  at the aft-end to accommodate a submerged nozzle (not shown) of the rocket motor  100 . 
       FIG. 2  shows the longitudinal section view of a rocket motor with conventional deep finocyl grain as in prior art. Since the largest imaginary circle circumscribing the fin tips is bigger than the largest motor aperture  102 , it is called deep finocyl grain. The deep fin  106   b  is once again reverse-swept. 
       FIGS. 3 &amp; 4  show longitudinal section view of rocket motors with yet other conventional deep finocyl grains. In either case the leading edges  107   c  &amp;  107   d  of fin  106   c  and  106   d  respectively are swept backward—characteristic of conventional finocyl grains. 
     Few of the prior inventions dealing with conventional finocyl grains follow. 
     A finocyl grain disclosed in US 00000108211 (Andrew) describes a configuration used in large monolithic boosters. The aft-end located fin is dissimilar but their leading edges are swept backwards as in any conventional finocyl grain. 
     Another finocyl grain disclosed in U.S. Pat. No. 4,936,092 (Andrew) depicts a compact, monolithic grain configuration with deep fin cavities at the aft-end of the motor. Again the leading edges of the fin are reverse-swept. 
     Yet another finocyl grain disclosed in U.S. Pat. No. 4,148,187 (Younkin) relates to a multi-piece grain configuration for modulated thrust profiles. Illustrating figures show typical finocyl grains with reverse-swept fin leading edges. 
     SUMMARY 
     The present disclosure relates to a solid rocket motor. The rocket motor comprises an internally insulated cylindrical casing with end domes having centrally located apertures at fore-end and aft-end. The rocket motor further comprises a solid propellant grain filled within the casing. The grain comprises an axial through-bore running from fore-end to aft-end along the axis of the rocket motor. The grain further comprises a plurality of longitudinal fin cavities formed in a circular array around the said axial through-bore at the aft-end of the rocket motor. Each of the plurality of longitudinal hollow fin cavities extends radially outwards with forward-swept leading edge and trailing edge. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
       The features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are therefore, not to be considered limiting of its scope. The disclosure will be described with additional specificity and detail through use of the accompanying drawings. 
         FIGS. 1 a    &amp;  1   b  illustrate aft-end view and longitudinal sectional view (along section A-A) respectively of a conventional monolithic rocket motor with conventional shallow finocyl grain in accordance with the Prior Art; 
         FIGS. 2, 3 &amp; 4  illustrate longitudinal sectional view of rocket motors with different conventional deep finocyl grain configurations in accordance with the Prior Art; 
         FIGS. 5 a    &amp;  5   b  illustrate longitudinal view and cross-sectional view (along section B-B) respectively of an exemplary solid rocket motor propellant grain configuration in accordance with an embodiment of the present disclosure; 
         FIGS. 6, 7 &amp; 8  show longitudinal sectional views of exemplary embodiments of the propellant grain with different geometry of forward-swept fin in accordance with some embodiments of the present disclosure; 
         FIGS. 9 &amp; 10  illustrate the longitudinal and cross-sectional views (along section C-C) respectively of dissimilar forward-swept fin cavities in accordance with an embodiment of the present disclosure; 
         FIGS. 11 &amp; 12  graphical representations of exemplary rocket motor chamber pressure versus time profiles achieved by the grain configuration in accordance with some embodiments of the present disclosure; 
     
    
    
     The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein. 
     DETAILED DESCRIPTION 
     While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the scope of the invention. 
     The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus. 
     In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limiting sense. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. 
     The present disclosure relates to a propellant grain configuration in rocket motors. In one embodiment, the coaxial, case-bonded, monolithic solid rocket propellant grain comprises a plurality of forward-swept, deep, longitudinal fin cavities circular-patterned about an axial conical and/or cylindrical cavity. It provides significant flexibility in thrust profile tailoring as well as high volumetric loading density. 
       FIGS. 5 a    &amp;  5   b  illustrate show the longitudinal view and cross sectional view (along section B-B) of an exemplary solid propellant rocket motor loaded with propellant grain configuration in accordance with one embodiment of the present disclosure; 
     As illustrated in  FIG. 5 a   , the solid propellant rocket motor (hereinafter referred to as rocket motor)  100  may be a monolithic rocket motor with grain configuration  104  bonded to the casing  101  of the rocket motor  100 . 
     In one embodiment, the rocket motor  100  comprises a hollow casing  101  with fore-end  102  and aft-end  103  apertures. In one example, the fore-end aperture  102  may be smaller than the aft-end aperture  103 . The casing  101  may be a cylindrical structure with convex end domes and internally insulated with a suitable insulating material. The propellant grain  104  may be cast inside the casing  101  and cured to final shape or machined to final shape. The rocket motor  100  further comprises an igniter (not shown) at the fore-end aperture  102  and a nozzle (not shown) at the aft-end aperture  103 . 
     In one embodiment, the propellant grain  104  comprises the cylindrical cavity or axial through-bore  105  along the longitudinal axis  101   a  running from the fore-end aperture  102  to the aft-end aperture  103  of the rocket motor  100 . In another embodiment, the propellant grain comprises a blind axial cavity opening towards the aft-end aperture. The shape of the central cavity or the blind axial cavity may be at least one of conical, cylindrical or combination of conical and cylindrical shapes. The propellant grain  104  further comprises the plurality of longitudinal hollow fin cavities (hereinafter referred to as “fin”)  106   e . The fin  106   e  is circular-patterned about the longitudinal axis  101   a  and located near the aft-end aperture  103 . The propellant grain  104  further comprises the counter-bore  109  near the aft-end aperture  103  to accommodate the submerged nozzle (not shown) of the rocket motor  100 . 
     As shown in  FIG. 5 a   , the fin  106   e  is formed by forward-swept leading edges  107   e , small fillet radii  110 , fin tip-chord  111 , forward-swept trailing edges  112 , large fillet radii  113  and fin root-chord  114 . 
     In one embodiment, the fin  106  is configured with their leading edges ( 107 ) making an acute angle at least one of completely  107   e ,  107   i , partly  107   f ,  107   g  and tangentially  107   h  with the forward motor axis  101   a . The fin  106   e  is configured such that the acute angular dimension  114  made by the leading edge  108   b  with forward motor axis  101   a  is greater than the angular dimension  115  made by the trailing edge  111  with the forward motor axis  101   a . The angular dimensions  108   b ,  115  made the by leading edge  107   e  and trailing edge  111  respectively with the forward motor axis  101   a  is acute (less than 90 degrees) and hence the leading and trailing edges of the fin are termed forward-swept. 
     The fin  106   e  is configured with increasing thickness from the tip to root and the maximum size of the fin  106   e  related to the diameter of the counter-bore  109  and aft-end aperture  103 . In one embodiment, the maximum size of the fin  106   e  is determined such that the diameter of the largest circle inscribed on radial side of the fin  106   e  is lesser than the diameter of the counter-bore  109  and the aft-end aperture  103  in the motor casing  101 . In other embodiment, the forward-swept leading edge  107   e  of the plurality of fin  106   e  is configured to form one or more acute angular dimensions with the forward motor axis  101   a . The tip-chord  110  is configured to run parallel to local casing insulation. 
       FIG. 5 b    illustrates distribution of the fin  106   e  in propellant grain  104  around the central cavity  105 . The extent of radial depth of fin  106   e  is illustrated in  FIG. 5 b   . The fin  106   e  is configured with circular tips. Further, the fin  106   e  is disposed about the axial through-bore  105  such that the diameter of the largest imaginary circle circumscribing the tips of the fin  106   e  is greater than the diameter of the aft-end aperture  103  in motor casing  101  and lesser than the inner diameter of the motor casing  101 . 
       FIG. 6  illustrates an exemplary embodiment of fin geometry where the forward-swept leading edge  107   f  of the fin  106   f  is at least partially perpendicular to the motor axis  101   a.    
     In another embodiment, as shown in  FIG. 7 , the inner part of the forward-swept leading edge  107   e  is reverse swept. 
     Also as illustrated in  FIG. 8 , in yet another embodiment, the leading edge  107   h  is part of an arc with the tangent at the starting point of the fin  106   h  forming acute angle  114  with the forward motor axis. 
     In one embodiment, the forward-swept leading edge  107   e  is configured to form one or more angular dimensions with the motor axis  101   a  including single, double or triple delta fin. 
       FIGS. 9 &amp; 10  illustrate longitudinal and cross sectional views of yet another embodiment where the circular patterned fin are dissimilar in dimensions. Alternate forward-swept fin are similar. Fin  106   e  and  106   i  differ in their radial depth among other dimensions. 
       FIGS. 11 &amp; 12  illustrate two of the many possible rocket motor chamber pressure profiles generated by different embodiments of the current invention.  FIG. 11  shows a ‘progressive—neutral’ type of ‘chamber pressure vs. time’ plot where the initial lower pressure ensures less loads on the thick propellant grain. An ‘M-type’ chamber pressure vs. time plot is shown in  FIG. 12 . The dip in pressure level is usually synchronized with maximum atmospheric loads in case of launch vehicles. 
     ADVANTAGES OF THE PRESENT INVENTION 
     In one embodiment, the present disclosure relates to propellant grain configuration having a plurality of deep longitudinal fin cavities with forward-swept leading edges circular patterned about an axial cavity. The grain configuration provides flexibility to tailor burn surface area regression profiles to meet different performance requirements. 
     Further, being a finocyl grain configuration it enables high propellant volumetric loading density enabling a compact rocket motor. 
     REFERENCE NUMERALS USED IN THE PRESENT INVENTION 
     
         
         
           
               100 —Rocket motor 
               101 —Motor casing 
               101   a —Motor axis 
               102 —Fore-end aperture 
               103 —Aft-end aperture 
               104 —Solid Propellant grain 
               105 —Axial through-bore 
               106   a ,  106   b ,  106   c ,  106   d ,  106   e ,  106   f ,  106   g ,  106   h ,  106   i —Fin 
               107   a ,  107   b ,  107   c ,  107   d ,  107   e ,  107   f ,  107   g ,  107   h ,  107   i —Leading edge 
               108   a —Angular dimension between the leading edge and forward motor axis 
               109 —Counter-bore in Propellant grain 
               110 —Small fillet radii of fin 
               111 —Fin tip-chord 
               112 —Trailing edge of fin 
               113 —Large fillet radii of fin 
               114 —Fin root-chord 
               108   b —Angular dimension between fin leading edge and forward motor axis 
               115 —Angular dimension between fin trailing edge and forward motor axis 
           
         
       
    
     The foregoing detailed description has described only a few of the many possible implementations of the present invention. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.