Patent Publication Number: US-8530809-B2

Title: Ring gear control actuation system for air-breathing rocket motors

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to guided missiles with air-breathing rocket motors, and more particularly to a control actuation system (CAS) for manipulating tail fins to maneuver the guided missile. 
     2. Description of the Related Art 
     Flight vehicles such as self-propelled missiles, gun or tube launched guided projectiles, kinetic interceptors and unmanned aerial vehicles require command authority to maneuver the vehicle to perform guidance and attitude control. Each of these vehicles may operate over a speed range encompassing both subsonic and supersonic Mach numbers and within the atmosphere and exo-atmosphere during a single mission. The differing speed and atmospheric conditions present different problems for effectively maneuvering the vehicle under volume, weight and cost constraints imposed by the vehicle and mission. 
     One approach used in a majority of if not all missile products employs a Control Actuation System (CAS) for guidance to the target. Typically the CAS employs a set of four fin control surfaces actuated by individual motors. The motor drives an input gear train that actuates an output gear that is coupled to a radially oriented fin control surface. Actuation of the fin control surfaces into the onrushing free stream produces drag and directional forces to maneuver the vehicle. Control surfaces are effective at supersonic speeds above Mach 1 in an atmosphere where sufficient drag and force is produced to quickly maneuver the vehicle. 
     An air-to-air missile (AAM) is a guided missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fueled but sometimes liquid fueled. Air-breathing rocket motors such as a ramjet are emerging as propulsion systems that will enable medium-range missiles to maintain higher average speeds across their engagement envelopments and achieve higher ranges. 
     In a typical rocket motor, there exists sufficient volume in the annular region between the exhaust tube and the missile airframe to locate the CAS. Typically, four tail fins are oriented radially about the circumference of the missile and spaced 90 degrees apart. The CAS includes four direct actuation assemblies positioned in the annular region within the missile airframe under the fin. A rotary drive motor drives an input gear train that rotates the output gear on which the tail fin is mounted to pivot the fin about its axis. 
     In guided missiles with air-breathing rocket motors, and in particular ramjet engines, the exhaust tube is contracted only slightly from the diameter of the rocket motor and substantially fills the cross section of the missile airframe. Air-breathing rocket motors operate with relatively low operating pressures and thus require relatively large flow cross sections through the exhaust tube. Consequently, a conventional CAS cannot be located within the missile airframe. 
     U.S. Pat. No. 5,904,319 entitled “Guided Missile with Ram Jet Drive” describes a CAS for use with a guided missile with ram jet drive. The missile has two outer air intakes in the lower area of the airframe, which lead to the tail with wake shafts, with a tail plane including four separately pivotable vanes in the form of a diagonal cross. A rigid wing arrangement is provided in or in front of the center of the missile. One drive unit with linear movement is provided for each vane. Two of the drive units are arranged longitudinally offset in the longitudinal and circumferential directions of the guided missile in each wake shaft. A kinematic connection from the drive unit to the lower vane is formed by a coupling rod each with joints at both ends. A kinematic connection from the drive unit to the upper vane is formed by a pivotable double lever each and a coupling rod with ball joints at both ends. This configuration produces non-linear (non-constant) gear ratio to pivot the vanes (see col. 4, line 43). 
     SUMMARY OF THE INVENTION 
     The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later. 
     The present invention provides a CAS for an air-breathing rocket motor propelled guided missile. 
     This is accomplished by positioning the drive motors and input gears in an inlet fairing extending aft of the air inlet towards the tail of the missile. The output gears are positioned coincident with and mechanically coupled to their respective tail fins spaced around the circumference of the missile. At least one of the tail fins is offset in a circumferential direction of the missile from its corresponding input gear and the inlet fairing. At least one ring gear is positioned around the exhaust tube to rotate in the circumferential direction of the missile. The ring gear comprises input and output teeth that engage the input and output gears, respectively, to actuate the tail fin. 
     In an embodiment, the air-breathing rocket motor comprises a ramjet. 
     In an embodiment, the ring gear is part of a ring gear subassembly that also comprises bearings and a bearing housing that allow the ring gear to rotate in the circumferential direction of the missile around the exhaust tube. The ring gear is subjected to both axial and lateral forces that are not evenly distributed due to the locations of the input and output gears thereby producing a net moment on the ring gear. The subassembly may comprise a thrust bearing such as a needle bearing for axial forces and a pair of radial bearings for the lateral forces and moment. 
     In an embodiment, the guided missile comprises four tail fins and a single air inlet and inlet fairing. Two remote tail fins are offset in the circumferential direction of the missile from their corresponding input gears and the inlet fairing. The other two direct tail fins are offset in the circumferential direction from each other but coincident with the inlet fairing. The two remote tail fins are independently actuated by a pair of ring gears that mechanically couple the fins&#39; respective input and output gears. The two direct tail fins are independently actuated by a direct coupling of their input and output gears. In an embodiment, the pair of ring gears face each other with all four output gears there between. 
     In an embodiment, the guided missile comprises four tail fins (two remote and two direct) and a pair of air inlets and inlet fairings. The drive motors and input gears from one remote and one direct tail fin are positioned within each of the inlet fairings. The two remote tail fins are independently actuated by a pair of ring gears that mechanically couple the fins&#39; respective input and output gears. The two direct tail fins are independently actuated by a direct coupling of their input and output gears. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  are a perspective view of an air-breathing rocket motor propelled guided missile provided with an embodiment of a ring gear CAS to manipulate the tail fins and a partial sectional view of the air inlet and its fairing and air-breathing rocket motor in accordance with the present invention; 
         FIGS. 2   a  and  2   b  are diagrams of an embodiment of the drive motor and gear mechanisms of the ring gear CAS; 
         FIG. 3  is an exploded view of an embodiment of a ring gear assembly; 
         FIGS. 4   a  and  4   b  are a transparent and exploded view, respectively, of the ring gear CAS; and 
         FIGS. 5   a  and  5   b  are end and section views of the tail section of the air-breathing missile. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention describes a control actuation system (CAS) for an air-breathing rocket motor propelled guided missile. The CAS positions the drive motors and input gears in an inlet fairing extending aft of the air inlet towards the tail of the missile. The output gears are positioned coincident with and mechanically coupled to their respective tail fins spaced around the circumference of the missile. At least one of the tail fins is offset in a circumferential direction of the missile from its corresponding input gear and the inlet fairing. At least one ring gear is positioned around the exhaust tube to rotate in the circumferential direction of the missile. The ring gear comprises input and output teeth that engage the input and output gears, respectively, to actuate the tail fin. 
     The ring gear CAS may be configured for use with any air-breathing rocket motor propelled guided missile. A “missile” may be considered to encompass all varieties of air, ground or sea-launched missiles and short, medium or long-range missiles. The “missile” may be a self-propelled missile, gun or tube launched guided projectile, kinetic interceptors or unmanned aerial vehicles that require command authority to maneuver the airframe. The missile may have three or more, typically four tail fins, that pivot about a radial axis to maneuver the airframe at sub or supersonic speeds. The fins may be fixed in their deployed position to pivot or may deploy upon or shortly after launch. 
     The missile must use an air-breathing rocket motor for propulsion. An air-breathing rocket motor requires the intake of air for combustion of its fuel to produce hot exhaust gases that propel the missile. The air-breathing rocket motor may, for example, comprise a ramjet, scramjet, or turbojet. The rocket motor&#39;s exhaust tube will typically substantially occupy the cross section of the missile airframe. The missile comprises at least one air inlet with a streamlined inlet fairing extending aft towards the tail of the airframe. There are many different air inlet configurations that satisfy this criterion. The underslung two-dimensional and underslung axisymmetric are examples of single air inlet configurations. The twin two-dimensional and under wing axisymmetric are examples of dual air inlet configurations. The streamlined inlet fairing serves to provide a smooth transition from the trailing edge of the air inlet to the tail of the missile to reduce aerodynamic drag. 
     Without loss of generality, an embodiment of a ring gear CAS will be described for use with an air-breathing ramjet rocket motor propelled medium-range air-to-air missile with an underslung two-dimensional air inlet. A ramjet uses the forward motion of the missile to compress incoming air without the necessity of a separate compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move the missile from a standstill. A booster motor is suitably used to launch the missile and bring the missile up to speed. Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3 and can operate to speeds of Mach 6. Ramjets are particularly useful in applications requiring a small and simple engine for high speed such as air-to-air missiles. There are many different types of ramjets including solid fuel ramjets, liquid fuel ramjets and ducted rockets. 
     An air-breathing guided missile  10  with a ramjet engine is shown in  FIGS. 1   a  and  1   b . The missile airframe  12  has a largely cylindrical shape. A nose cone  14  forward of airframe  12  has a generally ogive shape for aerodynamics. A seeker and a guidance system are located in the nose cone and forward section of airframe  12 . A warhead (e.g. explosive or kinetic rod) is typically located in airframe  12  aft of the guidance system. 
     A ramjet  16  is positioned aft of the warhead in airframe  12 . In this embodiment, ramjet  16  is a variable-flow ducted rocket that comprises solid fuel  18  containing both a fuel and an oxidizer. An igniter  20  starts the solid fuel burning to produce a fuel-rich mixture that is injected into a combustion chamber  22  through a throttle valve  24  that controls the air/fuel mixture. An underslung two-dimensional air inlet  23  draws air into the combustion chamber and, together with the incoming fuel, combusts to produce a stream of hot gases that is expelled through an exhaust tube  25  and a ramjet nozzle  26  to expand to atmospheric pressure to produce a high velocity jet to propel the missile. A streamlined inlet fairing  28  extends aft from the trailing edge of air inlet  23  to the tail of the missile to improve aerodynamic performance. 
     For the ramjet, the exhaust tube  25  is contracted only slightly from the diameter of the rocket motor and substantially fills the cross section of the missile airframe  12 . Ramjets operate with relatively low operating pressures and thus require relatively large flow cross sections through the exhaust tube. For example, a missile airframe with a 7″ diameter may be fitted with an exhaust tube having a 6″ diameter leaving only ½″ of spacing between the exhaust tube and the airframe defining an annular volume. 
     Because the ramjet cannot operate at zero airspeed, the missile is provided with a booster rocket. In this missile configuration, the combustion chamber  22  is filled with a solid booster propellant and an ejectable booster nozzle is mounted inside the ramjet nozzle  26  as best shown in  FIG. 5   b . At launch, an igniter ignites the solid booster propellant to produce thrust that propels the missile up to speed. At the end of the boost phase, the combustion chamber  22  is empty and the booster nozzle is jettisoned. 
     Four tail fins  32 ,  34 ,  36  and  38  are spaced circumferentially about the missile and extend radially therefrom. Each tail fin can pivot independently about a radial axis. Tails fins  32  and  34  are offset in a circumferential direction of the missile from inlet fairing  28  (e.g. inlet fairing  28  occupies a lower section of the missile airframe and fins  32  and  34  occupy an upper section of the airframe). Tail fins  36  and  38  are coincident with inlet fairing  28  (e.g. both the inlet fairing  28  and tail fins  36  and  38  occupy the lower section of the airframe). In this embodiment, for storage in a launch bay fins  36  and  38  are shortened so that their total length from the airframe matches fins  32  and  34 . 
     A ring gear CAS  40  is positioned around exhaust tube  25  in the small annular volume and within the volume of inlet fairing  28 . CAS  40  includes four actuation assemblies to independently actuate the four tail fins  32 ,  34 ,  36  and  38 . Two assemblies are “remote” actuation assemblies that remotely actuate tail fins  32  and  34  on the other side of the missile from the inlet fairing. The other two assemblies are “direct” actuation assemblies that directly actuate tail fins  36  and  38  located with the inlet fairing. CAS  40  also comprises electronics  214 , batteries  216  and wiring harnesses (for connection to the missile guidance system). 
     CAS  40  is suitably a standalone unit that slides over exhaust tube  25  and is secured (e.g. bolted) to the rocket motor. Ramjet nozzle  26  is threaded onto exhaust tube  25 . The CAS&#39; wiring harnesses are connected to like missile wiring harnesses to establish communication between the missile and CAS. Inlet fairing  28  is secured in place over the CAS components and fins  32 ,  34 ,  36  and  38  are mounted onto the CAS. 
     As depicted in  FIGS. 2   a  and  2   b , in an embodiment for the underslung two-dimensional inlet fairing CAS  40  includes four actuation assemblies  42 ,  44 ,  46  and  48  to independently actuate the four tail fins  32 ,  34 ,  36  and  38  (not shown), respectively. Assemblies  42  and  44  are “remote” actuation assemblies that remotely actuate tail fins  32  and  34  on the other side of the missile. Assemblies  46  and  48  are “direct” actuation assemblies that directly actuate tail fins  36  and  38  located with the inlet fairing. 
     Direct actuation assemblies  46  and  48  each comprise a drive motor ( 50  and  52 ) that rotates a drive shaft ( 54  and  56 ) to turn an input gear train ( 58  and  60 ). As shown, the drive motors are positioned axially. The drive motors may be positioned radially although this may require smaller diameter, less efficient drive motors. The input gear train comprises one or more compound gears (i.e. a large diameter gear attached to a small diameter gear) to increase the gear ratio. The input gear train mechanically couples the drive shaft to an input gear ( 62  and  64 ) to rotate the input gear about an input axis. As shown, the input axis is oriented radially. The input gear may or may not be part of one of the aforementioned compound gears. An output gear ( 66  and  68 ) engages the input gear ( 62  and  64 ) to rotate about a radial axis and actuate the fins ( 36  and  38  not shown). When the CAS is secured to the missile, all of the direct actuation assembly components are external to the exhaust tube and lie within the interior volume of the inlet fairing with the exception of the output gear that extends through the fairing along a radial axis to the missile to actuate the fin. 
     Remote actuation assemblies  42  and  44  each comprise a drive motor ( 70  and  72 ) that rotates a drive shaft ( 74  and  76 ) to turn an input gear train ( 78  and  80 ). As shown, the drive motors are positioned axially. The drive motors may be positioned radially although this may require smaller diameter, less efficient drive motors. The input gear train comprises one or more compound gears (i.e. a large diameter gear attached to a small diameter gear) to increase the gear ratio. The input gear train mechanically couples the drive shaft to an input gear ( 82  and  84 ) to rotate the input gear about an input axis. As shown, the input axis is oriented radially. The input gear may or may not be part of one of the aforementioned compound gears. The input gear could be rotated 90° to orient its input axis axially. An output gear ( 86  and  88 ) is positioned coincident with and mechanically coupled to rotate the tail fin ( 32  and  34 ) about a radial axis. The output gear is offset in a circumferential direction from the input gear ( 82  and  84 ). A ring gear ( 90  and  92 ) is positioned axially to rotate in the circumferential direction. The gear comprises input teeth ( 94  and  96 ) that engage the input gear ( 82  and  84 ) and output teeth ( 98  and  100 ) that engage the output gear ( 86  and  88 ) to rotate the output gear and remotely actuate the tail fin ( 32  and  34  not shown). In general, the input teeth and output teeth have a different geometry to engage the different sized input and output gears. The teeth may, for example, comprise face teeth, spur teeth, or bevel teeth (which are mirrored by the input and output gears). In this embodiment, ring gears  90  and  92  each have two sectors of face teeth and are positioned with their teeth facing each other so that the four output gears and their fins can lie in a plane orthogonal to the missile between the two ring gears. 
     When the CAS is secured to the missile, all of the remote actuation assembly components are external to the exhaust tube. The drive motor, input gear train and input gear lie within the interior volume of the inlet fairing. The output gear and its fin are offset in the circumferential direction of the missile from the inlet fairing. The ring gear lies both within and external to the inlet fairing, which allows the ring gear to affect remote actuation of the fin. 
     In an alternate embodiment, minor modifications can be made to the ring gear CAS to accommodate an air-breathing rocket motor that has a pair of air inlets and inlet fairings such as a twin two-dimensional intake. For example, a direct and a remote actuation assembly can be paired and positioned within one of inlet fairings. The direct actuation assembly would actuate a fin coincident with (and extending from) that inlet fairing. The remote actuation assembly would actuate a fin offset in a circumferential direction of the missile from that inlet fairing. Furthermore, the ring gear CAS may be reconfigured for use with various combinations of one or more air inlets and inlet fairings and three or more fins positioned circumferentially about the missile as long as one of the fins is offset in the circumferential direction of the missile from an inlet fairing. 
     As shown in  FIG. 2   b , in remote actuation assembly  42  ring gear  90  rotates about a ring axis  110 . Output gear  86  and fin  32  are positioned to rotate about an output axis  112  that is radial with respect to ring axis  110 . Drive motor  70  is positioned axially with respect to a ring axis  110 , offset by in the axial direction. Drive motor  70  rotates drive shaft  74  to turn the compound gears of input gear train  78 , which in turn rotate input gear  82  about an input axis  114  that is radial with respect to ring axis  110 . Rotation of input gear  82  engages input teeth  94  on ring gear  90  causing the ring gear to rotate in a circumferential direction about ring axis  110 . Rotation of the ring gear  90  displaces output teeth  98  in a circumferential direction engaging output gear  86  and causing the output gear and fin  32  to rotate about output axis  112 . 
     The ring gear CAS and particularly the remote actuation assembly will both fit into the limited annular space available around the exhaust tube and the volume provided by the inlet fairing and provides the performance required by a supersonic missile. The ring gear is an idler gear and as such its geometry does not affect the overall gear ratio. However, the gear ratio at the ring gear is higher than the overall gear ratio (over twice as high in this embodiment). Therefore, the ring gear is characterized by low equivalent friction and low equivalent inertia when measured at the drive shaft, due to the relatively high gear ratio at the ring gear. The gear design of the CAS also provides a high stiffness throughout the gear train to actuate the fins. The gear design provides a constant gear ratio over a range of motion of the fin. 
     An embodiment of a ring gear assembly  120  of which ring gear  90  is a part is illustrated in  FIG. 3 . Rolling elements such as needle, ball or roller bearings support loads from both the input and output gear teeth allowing the ring gear to rotate with low friction. Forces on the ring gear are in both the axial and the lateral directions. Due to the locations of the two mating gears the forces are not evenly distributed which produces a net moment on the ring gear. In this embodiment, the assembly uses a thrust bearing  122  to address the axial forces and a pair of radial bearings  124 ,  126  to address the lateral forces and moment. Radial bearings  124  and  126  separated by a bearing spacer  128  are placed into an annular ring on the backside of ring gear  90 . This subassembly is inserted into a bearing housing (not shown). Thrust bearing  122  is placed between a pair of races  130 ,  132  and held in place by the bearing housing. Thrust bearing  122  may comprise needle or roller bearings. A race is a hardened steel ring designed to support the loads of rolling elements. It will be apparent to one of ordinary skill in the art that there are many different configurations for assembling rolling element bearings to support low friction rotation of ring gear  90 . Some bearing types will support axial, lateral and moment loads. 
       FIGS. 4   a  and  4   b  illustrate assembled and exploded views of an embodiment of ring gear CAS  40  sans the electronics, batteries and wiring harnesses. As mentioned previously, the CAS is a separate unit the slides over the tail of the missile, is bolted to the rocket motor and connected to the missile electronics through its wiring harness. 
     To assemble CAS  40  from its parts, ring gear assemblies  120  and  138  are slid over a main housing  140  from opposing ends. Aft housing  142  is attached to main housing  140  to secure ring gear assembly  120 . Forward housing  144  is attached to the other side of main housing to secure ring gear assembly  138 . Remote output gear  88  and its bearings  146  are mounted to main housing  140 . A remote thrust plate  148  is placed over output gear  88  and bolted to the main housing, forward housing and aft housing. The process is repeated for remote output gear  86 , bearings  150 , and remote thrust plate  152 . 
     A gear housing  154  is attached to the main housing, forward housing, and aft housing. For the two remote assemblies, first and second stage compound gears and their bearings that make up the input gear trains ( 78  or  80 ) and input gears ( 82  and  84 ) are installed into the gear housing. Intermediate thrust plates  156  are placed over the compound gear stages and bolted to the gear housing. For the two direct assemblies, the first and second stage compound gears that make up the input gear trains ( 60 ), input gears ( 64 ) and output gears ( 68 ) and their associated bearings  157  are installed into the gear housing. Direct thrust plates  158  are placed over the direct gear trains and bolted to the gear housing. 
     The drive motors  50 ,  70 ,  72 , and  52  are first separated into motor rotor and motor stator. The drive shaft bearings and motor rotor are assembled onto their respective drive shafts. The four motor drive shaft assemblies are installed into gear housing  154 . Motor stators are installed over the drive shaft assemblies until end of motor stators contact gear housing. One or more motor housings  160  are placed over the drive motor/shaft assemblies. A forward plate  162  is placed over motor housings  160  and attached with bolts  164  through motor housings to gear housing  154 . A metal skin  166  is placed over the entire assembly and bolted to main housing  140 , forward housing  144 , aft housing  142 , and forward plate  162 . 
     As shown in  FIGS. 5   a  and  5   b  of the tail section of guided missile  10 , ring gear CAS  40  is positioned around exhaust tube  25  and bolted to the rocket motor  16 . Ramjet nozzle  26  is threaded on to the aft section of exhaust tube  25 . Inlet fairing  28  is bolted to the missile airframe and extends aft from the air inlet to the tail of the missile. Tail fin  36  extends from inlet fairing  28  (fin  36  is suitably shortened to accommodate the inlet fairing). Tail fin  32  extends from the tail of the missile opposite the inlet fairing. As previously mentioned, ramjet combustion chamber  22  is initially provided with solid booster propellant  200  and ejectable booster nozzle  202  inside ramjet nozzle  26 . At launch booster propellant  200  is ignited and burns until gone to produce thrust through booster nozzle  202  to bring the missile to a sufficient speed for the ramjet to takeover at which point booster nozzle  202  is jettisoned. 
     Ring gear CAS  40  fits in the small annular region around exhaust tube  25  and within the volume defined by inlet fairing  28  so that the CAS&#39; skin  166  has approximately the same diameter (cross section) as the missile airframe. The drive motors  50 ,  70 ,  72  and  52  and their gear assemblies reside towards the tail of inlet fairing  28 . As shown drive motor  70  rotates drive shaft  74 , which in turn rotates input gear train  78  (e.g. compound gears  210 ,  212 ) to rotate input gear  82 . Input gear  82  engages ring gear  90  causing the ring gear to rotate around exhaust tube  25  in a circumferential direction of the missile to rotate the output gear (not shown) to pivot tail fin  32 . Ring gear CAS  40  also includes electronics  214  and batteries (not shown) positioned forward in inlet fairing  28  and a wiring harness  218  to connect to a missile wiring harness. 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.