Abstract:
A supercavitating projectile is disclosed that has deployable fins. The fins are pivotally coupled to the body of the projectile. The fins have two primary states: stowed within a recess at the surface of the projectile and deployed to a radially-extended position relative to the body of the projectile. The fins deploy as the projectile leaves its launch tube. The fins function as a control surface, interacting with the wall of the vapor cavity in which the supercavitating projectile travels.

Description:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH 
     This invention was made with Government support under Contract #N00014-07-C-1103, and the Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to underwater projectiles. 
     BACKGROUND OF THE INVENTION 
     Underwater gun systems are being developed for naval warfare. These systems often use an energetic propellant to launch a projectile from a launch tube. A challenge to the development of effective underwater guns is that a projectile traveling through water experiences a resistance or drag that is approximately one thousand times greater than the resistance experienced by the projectile traveling through air. As a consequence of this high level of drag, conventional underwater projectiles are limited to speeds of no more than about 80 kilometers/hour (km/h). 
     The high resistance presented by the water medium can be addressed via a phenomenon known as “supercavitation.” This phenomenon can occur when a projectile having a blunt nose travels at sufficiently high speeds under water. The blunt nose pushes aside water as the projectile advances. When the hydrodynamic pressure of water that is pushed aside overcomes the ambient static pressure, water vaporizes. The vaporized water forms air bubbles, which coalesce to form a “cavity” in the water. If enough bubbles are formed, the cavity will be large enough to completely engulf the projectile, with the exception of the blunt tip of the nose. This characterizes the supercavitating mode of operation, which is also referred to as “cavity-running” operation). 
     Within the vaporous cavity, the projectile is effectively traveling through air rather than water. The projectile, therefore, experiences greatly reduced drag. As a consequence, the projectile is capable of attaining a velocity far in excess of what is possible when traveling through water proper. 
     Supercavitating projectiles often collide with the walls of the enveloping cavity, which increases drag. This can be addressed by equipping the projectile with fins. When a fin contacts the cavity wall, a torque develops that steers the projectile toward the center of the cavity into a region of lower drag. 
     The fins are usually located in the aft section of the projectile body and project radially outward therefrom. The radially-extending fins prevent the projectile from being tightly packaged within a launch tube. This drawback is addressed by coupling the projectile to a sabot, which is a carrier that centers the projectile within the launch tube and falls off after launch. Use of a sabot disadvantageously increases the amount of energetic propellant required for launch and also requires an increase in launcher size. A need therefore exists for an improved supercavitating projectile that retains the in-cavity stability of known fin designs but does not require a sabot for launch. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present invention provide an improved design for an underwater projectile that is capable of operating in a supercavitating mode. 
     In accordance with the illustrative embodiment, a fin is pivotally coupled to the cylindrical body of the projectile. This pivotal coupling enables the fin to either (1) stow itself within a recess at the surface of the projectile or (2) deploy to a radially-extended position. 
     When stowed, substantially no portion of the fin protrudes beyond the circumference of the body of the projectile. In this stowed state, the projectile can be packaged inside of a launch tube or barrel without the use of a sabot. Furthermore, the fin is disposed forward of the aft end of the projectile of the booster base of the projectile. This enables multiple such projectiles to be “stacked” nose to tail within a barrel, such as in a stacked launcher configuration disclosed in U.S. Published Patent Application 2008/0022879, which is incorporated by reference herein. In this fashion, the projectile can be launched using an energetic propellant disposed within the launch tube (as per the referenced published patent application) or within the projectile. 
     Upon launch, the projectile enters the water and travels through it until a vaporous cavity is formed. In some embodiments, the water drag experienced by the projectile immediately following launch causes the fins to pivot to the deployed position. In some other embodiments, deployment via water drag is supplemented by a spring-biasing element that is used to initiate pivoting of the fins. 
     Once deployed, the fins operate in substantially the same manner as fixed-fin designs known in the prior art. In particular, the fins function as a control surface, interacting with the wall of the cavity in which the projectile travels. Contact with the cavity wall imparts sufficient torque to urge the projectile back toward the center of the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a perspective view of a projectile in accordance with the illustrative embodiment of the present invention, wherein deployable fins of the projectile are in a stowed state. 
         FIG. 1B  depicts a front end view of the projectile of  FIG. 1A . 
         FIG. 2A  depicts the projectile of  FIG. 1A  wherein the fins are in a deployed state. 
         FIG. 2B  depicts a front end view of the projectile as shown in  FIG. 1A . 
         FIG. 3A  depicts a front perspective view of one of the deployable fins. 
         FIG. 3B  depicts a back perspective view of one of the deployable fins. 
         FIG. 4  depicts an exploded view of the aft end of the projectile of  FIG. 1A . 
         FIG. 5A  depicts a side cross-sectional view of the aft end of the projectile of  FIG. 1A , wherein the fin is shown in a stowed state. 
         FIG. 5B  depicts a side cross-sectional view of the aft end of the projectile of  FIG. 2A , wherein the fin is shown in a deployed state. 
         FIG. 6  depicts a perspective view of the aft end of the projectile of  FIG. 2A , wherein the fins are shown in a deployed state. 
         FIG. 7  depicts a side view of a launcher container a plurality of the projectiles disclosed herein, wherein the projectiles are disposed in a stacked launcher configuration. 
     
    
    
     DETAILED DESCRIPTION 
     The following terms are defined for use in the description and the appended claims as follows:
         “Chord” or “Chord length” means, in the context of a fin, the distance from the front (or leading edge) of the fin to the back (or trailing edge) of the fin. The chord is parallel to the longitudinal axis of the projectile.   “Longitudinal axis” means, in the context of a projectile, an axis aligned with the length (nose to tail) of the projectile.   “Major surface” means, in the context of a fin, the (two) surfaces having an area that is a function of the span of the fin and the width of the fin, as the terms “span” and “width” are defined herein.   “Projectile” means any artificial body, either powered, such as by a motor, or un-powered, such as a bullet, etc.   “Root” means, in the context of a fin, the portion of the fin that is nearest to the body of the projectile when the fin is deployed.   “Span” means, in the context of a fin, the distance between the tip and the root of the fin.   “Substantially” means, in the context of an angular deviation, ±10 degrees.   “Supercavitating projectile” means a projectile that, when moving under water at sufficient speed, is enveloped by a gaseous cavity that the projectile itself generates.   “Tip” means, in the context of a fin, the portion of the fin that is furthest from the body of the projectile when the fin is deployed.   “Width” means, in the context of a fin, the straight-line distance between opposed edges of a fin, wherein (a) the opposed edges do not include the tip or root of the fin and (b) a line connecting the opposed edges is not parallel to longitudinal axis of the projectile. If the line connecting the opposed edges of the fin were parallel to the longitudinal axis of the projectile, the opposed edges would be the leading and trailing edge of the fin and the distance defined here as “width” would be properly characterized as the “chord” or “chord length” of the fin.
 
Definitions of other terms and phrases may appear elsewhere in this disclosure.
       

       FIG. 1A  depicts a perspective view of supercavitating projectile  100  in accordance with the illustrative embodiment. Projectile  100  comprises nose  102 , body  108 , fin recess  110 , and fin  112 , interrelated as shown. 
     Blunt forward end  104  of nose  102  is used to create the vaporous cavity that encompasses projectile  100  during supercavitating operation, in known fashion. For that reason, in the context of a supercavitating projectile, the forward end of the nose is typically referred to as a “cavitator.” In the illustrative embodiment, cavitator  104  is flat; however, in other embodiments, other structural arrangements for the cavitator may suitably be used. See, for example, U.S. patent application Ser. Nos. 12/102,784 and 12/102,781, incorporated by reference herein. 
     In the illustrative embodiment, nose  102  has “stepped” profile  106 . The stepped profile results from configuring at least the forward portion of nose  102  as a plurality of substantially right-circular cylindrical shells or segments that increase in diameter progressively moving aft. The stepped profile of the nose can provide certain advantages as a function of the projectile&#39;s yaw angle. Other structural arrangements for the nose may suitably be used. See, for example, U.S. patent application Ser. Nos. 12/102,784 and 12/102,781. 
     Body  108  is substantially cylindrical in shape. In some embodiments, body  108  houses a propellant bay (not depicted). The propellant bay contains a chemical propellant, typically (e.g., ammonium perchlorate, etc.) that is ignited to generate the energy for launch. The aft end of body  108  includes plural recesses  110  for accommodating a plurality of fins  112 . In the illustrative embodiment, projectile  100  has three fins  112 . 
     In  FIG. 1A , fins  112  lie in-plane with the exterior of body  108 , substantially parallel to longitudinal axis A-A and substantially flat against projectile  100  in recesses  110 . This orientation defines the “stowed” position or state of fins  112 .  FIG. 1B  is a front end view of projectile  100 . As seen from  FIG. 1B , when fins  112  are in their stowed position, they do not project beyond the circumference of body  108 . 
       FIGS. 2A and 2B  depict projectile  100  with fins  112  rotated out-of-plane from the surface of body  108  and away from the recess  110 . This defines the “deployed” position or state of fins  112 . As depicted in the front end view of  FIG. 2B , fins  112  project beyond the circumference of body  108  when they are in the deployed state. 
       FIGS. 3A and 3B  depict, via respective front and rear perspective views, further details of fin  112 . Referring now to both drawings, fin  112  comprises body portion  313  and shoulders  318 A and  318 B, interrelated as shown. 
     Body portion  313  of the fin  112  has two major surfaces; front surface  314  and rear surface  316 . In the embodiment depicted in  FIGS. 3A and 3B , front surface  314  has a concave shape and rear surface  316  has a convex shape. Note that in  FIGS. 2A ,  3 A, and  6 , front surface  314  is depicted as having a concave shape. In other embodiments, front surface  314  is flat, so as to mate flush with recess  110  (see, e.g.,  FIG. 5A , etc.). Regardless of the shape of front surface  314 , rear surface  316  maintains its curved shape to smoothly integrate with the exterior of body  108 . 
     Body portion  313  is characterized by tip T, root R, span S, and width W. The distance characterized as “width W” would properly be termed the “chord” of fin  112  if the fin were oriented in the manner of a typical fin, wherein the edges E 1  and E 2  were accurately characterized as the leading edge and trailing edge of the fin. But as depicted in  FIG. 2A , for example, edges E 1  and E 2  of the fin are offset by ninety degrees relative to a typical projectile fin. Normally, fin orientation is a function of aerodynamics; fins would not be oriented so as to present a major surface to “on-coming” fluid. Rather, a major surface would be oriented parallel to the direction of movement (of the fluid or projectile) to avoid what would otherwise be a large drag force. In the accordance with the illustrative embodiment, however, fin orientation is a function of the deployment mechanism; aerodynamics are not of particular concern. In fact, from the perspective of aerodynamics, the fins  112  have the worst possible orientation, in the sense that the ratio of the width of fin  112  to the chord of fin  112  is greater than 1. Note that due to the way in which fins  112  are oriented, the dimension that is referred to as the chord of fin  112  would, in a typical fin orientation, be the thickness of fin  112 . And, as previously noted, the dimension that is referred to as the width of fin  112  would, in a typical fin orientation, be the chord of fin  112 . 
     Portion  315  of front surface  314  near tip T is tapered wherein the thickness of body portion  313  decreases to a minimum at tip T. In some embodiments, a portion of rear surface  316  near tip T is tapered as well (see, e.g.,  FIGS. 5A and 5B ). Root R of body portion  313  is curved. In the illustrative embodiment, this curve precisely matches the curved shape of body  108 . As described in further detail later in this specification, the matching curved surfaces of root R and body  108  functions to support fins  112  when they are in the deployed state. 
     Shoulders  318 A and  318 B depend from root R of body portion  313 . The shoulders are separated by space  320 . Extending away from root R of body portion  313 , shoulders  318 A and  318 B enlarge to accommodate respective pivot-pin receiving holes  322 A and  322 B. 
     Edge  319  of shoulders  318 A and  318 B is a contoured surface that defines a cam, as discussed later in this specification in conjunction with  FIGS. 5A and 5B . Viewed from the front of fin  112  (i.e., as in  FIG. 3A ), edge  319  defines a smooth curve until region  324 , wherein edge  319  juts abruptly inward defining wall  326 . Edge  319  then continues at an angle (typically, but not necessarily, 90 degrees, ±about 20 degrees) relative to wall  326 , defining surface  328 . This surface is substantially parallel to the tangent of the “circle” defined by hole  322 A (or  322 B) at the point at which face  326  would intersect the hole, if face  326  were so projected. 
       FIG. 4  depicts fin-receiving region  430  disposed at the aft portion of body  108  and further depicts, via an exploded view, the fin assembly, indicated generally at  444 . The fin assembly includes fin  112 , pin  446 , and cam-follower assembly  448 . 
     Fin-receiving region  430  is physically adapted to receive fin assembly  444 . Specifically, fin-receiving region  430  includes recess  110 , channels  432 A and  432 B, and access hollow  442 . 
     Recess  110  is dimensioned and arranged to accommodate fin  112  in the stowed state. The recess is sufficiently deep so that when fin  112  is stowed, rear surface  316  of fin body  313  aligns with the surface of body  108 . In the illustrative embodiment, the curvature of rear surface  316  matches that of body  108  to provide a smooth, essentially continuous surface when fin  112  is stowed. 
     Channels  432 A and  432 B, which align directionally with longitudinal axis A-A of projectile  100  (see,  FIG. 1A ), are disposed proximal to and aft of recess  110 . The channels are spaced apart and so define tab  436 . Pivot-pin receiving hole  434 A is disposed in wall  433 A at the forward portion of channel  432 A, which is proximal to aft edge  431  of recess  110 . Similarly, pivot-pin receiving hole  434 B is disposed in wall  433 B at the forward portion of channel  432 B. Pivot-pin receiving hole  438  is disposed in tab  436  proximal to aft edge  431  of recess  110 . Holes  434 A,  434 B, and  438  are axially aligned with one another along axis B-B. 
     Fin  112  is pivotally coupled to projectile  100  as follows. Shoulders  318 A and  318 B are received by respective channels  432 A and  432 B. Fin  112  and fin-receiving region  430  are dimensioned and arranged so that when the fin&#39;s shoulders are received by channels  432 A and  432 B, pivot-pin receiving holes  422 A and  422 B in the shoulders and pivot-pin receiving holes  434 A,  434 B, and  438  in fin-receiving area  430  are axially aligned with one another along axis B-B to collectively receive pivot pin  446 . In this fashion, fin  112  is pivotally coupled to projectile  100 . Access hollow  442 , which in the illustrative embodiment is proximal to hole  434 B, provides access to the pivot-pin receiving holes to insert pivot pin  446 . 
     With continued reference to  FIG. 4 , disposed within channel  432 B is cam follower assembly  448  which, in the illustrative embodiment, comprises leaf spring  450 , locking element  454 , and fastener  458 . In the illustrative embodiment, locking element  454  is a locking wedge and fastener  458  is a set screw. Fastener  458 , which passes through hole  456  in locking element  454  and hole  452  in leaf spring  450 , is ultimately threaded into (or otherwise secured to) the base of the channel. Although cam follower assembly  448  is disposed in channel  432 B in the illustrative embodiment, it is to be understood that the cam follower assembly could alternatively be disposed in channel  432 A. Furthermore, in some embodiments, a cam follower assembly is disposed in both channels  432 A and  432 B. Cam follower assembly  448  and its operation are discussed further below with respect to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  depict a cross-sectional side view through fin-receiving region  430  along the mid-line of channel  432 B and through axis C-C of fin  112  shown in  FIG. 4 , but when the fin is actually pivotally coupled to projectile  100 . More specifically, these Figures depict recess  110  and a view into channel  432 B, as well as a cross sectional view of fin body  113 , fin shoulder  318 B, and cam follower assembly  448 . 
       FIG. 5A  depicts fin  112  in a stowed state. In this state, forward surface  314  of the fin abuts the surface of recess  110  and rear surface  316  of fin body  103  is approximately co-planar with the surface of projectile body  108 . Gap G is formed between the surface of recess  110  and tapered portion  315  of fin  112  near tip T thereof. 
       FIG. 5A  also depicts cam follower assembly  448  in channel  432 B. Leaf spring  450  of cam follower assembly  448  is spaced above bottom  560  of channel  432 B, enabling the leaf spring to deflect downward. Fastener  458  is shown in threaded engagement with base  560  of channel  432 B. 
       FIG. 5B  depicts fin  112  in the deployed state. To attain this state from the stowed state depicted in  FIG. 5A , fin  112  partially rotates about pivot pin  446 . Rotation of fin  112  from the stowed to the deployed state occurs when projectile  100  contacts water after leaving the barrel from which it is fired or launched. Rotation occurs as a consequence of the drag forces experienced at gap G. In some embodiments, a “spring-biasing” element (not depicted in  FIG. 5   b ; see  FIG. 6 ) urges fin  112  away from recess  110  to begin its rotation to the deployed state as projectile  100  leaves its launch tube. 
     In the illustrative embodiment, fin  112  rotates about 135 degrees from the stowed state to the deployed state. At some point during rotation of fin  112 , surface  319  of shoulder  318 B engages leaf spring  450 . As the fin continues to rotate, leaf spring  450  flexes downwardly (toward base  560 ), with maximum flexure occurring as region  324  of cam surface or edge  319  (see also,  FIGS. 3A and 3B ) contacts leaf spring  450 . At this point, surface  319  juts inward abruptly, releasing the flex in leaf spring  450  such that the leaf spring forcibly rebounds, engaging flat cam surface  328 . 
     Once cam surface  328  and leaf spring  450  engage, as depicted in  FIG. 5B , fin  112  is effectively prevented from rotating back toward recess  110 . This prevents fin  112  from “chattering” when the projectile is underway. 
       FIG. 6  depicts a perspective view of the aft end of projectile  100 , showing two of fins  112  in the deployed state. Drag on front surface  314  of fin  112  forces the fin back until root R engages the surface of body  108 . The body of projectile  100  itself therefore supports fins  112  once they deploy. 
       FIG. 6  depicts optional spring-biasing element  662 , which in the illustrative embodiment is a cupped spring washer or cone washer, also known as a Belleville washer. This non-flat washer has a slight conical shape that gives the washer a spring-like characteristic. As projectile  100  is loaded into its launch tube, fin  112  is forced against spring-biasing element  662  such that the spring-biasing element is compressed. When projectile  100  is fired from its launch tube, spring-biasing element  662  returns to its pre-compressed shape, releasing its stored energy. This imparts an impulse to fin  112 . Since fin  112  is pivotally attached to projectile  100 , this impulse causes the free end of fin  112  to move away from recess  110 , wherein the fin begins to rotate about pivot pin  446 . As fins  112  begin to rotate away from recess  110 , the water drag forces the fins back until root R of the fins abuts the surface of body  108 . 
     In embodiments in which spring-biasing element  662  is not used, gap G, as depicted in  FIG. 5A , permits water to contact tapered surface  315  of fin  112 , thereby causing sufficient drag to deploy the fin. 
     The following provides an example of a supercavitating projectile in accordance with the illustrative embodiment. 
     Diameter of body  108 : 40.0 mm (1.57 in) 
     Diameter of cavitator  104 : 7.62 mm (0.3 in) 
     Length of projectile  100 : 483 mm (19.0 in) 
     Center of Gravity: 279 mm (11.0 in) from cavitator 
     Fin span: 57.2 mm (2.25 in) 
     Propellant bay: 230 grams (8 ounces) 
     Mass of projectile  100  1.93 kg (4.25 lbs) 
     Material of Construction: 
     
         
         
           
             Nose: S7 Tool Steel 
             Body: S7 Tool Steel 
             Fins: Titanium
 
Leaf Spring Buckling Load: 360 Newtons (81 lbf)
 
Pivot Pin, design pressure: 620.6 MPa (90 Kilopounds/sq in.)
 
Those skilled in the art will understand that to design a supercavitating projectile, such as those described herein, requires computational fluid dynamic analysis to determine operational stability, etc. These calculations must consider nominal projectile operating depth and yaw angle. Such analysis is within the capabilities of those skilled in the art.
 
           
         
       
    
     The positioning of fins  112  forward of the aft end of projectile  100  (see, e.g.,  FIG. 1A , etc.) enables projectiles  100  to be used in conjunction with a stacked projectile launcher, a stylized representation of which appears in  FIG. 7 . Such a launcher is available from Metal Storm Ltd. Of Brisbane, Australia. 
     Launcher  770  accommodates multiple projectiles that arranged nose-to-tail within barrel  772 . In the illustrative embodiment depicted in  FIG. 7 , three projectiles  100 - 1 ,  100 - 2 , and  100 - 3  are stacked in respective positions  1 ,  2 , and  3 . 
     Within barrel  772  is a plurality of propellant bays  774 - 1 ,  774 - 2 ,  774 - 3  (collectively or generally, “propellant bay(s)  774 ”). In the illustrative embodiment, each propellant bay is configured as a ring-shaped cavity within barrel  772  that is filled with propellant. Gas ports (not depicted) lead from the propellant bay to the bore of barrel  772 . 
     Projectiles  100  are separated from one another in barrel  772  by “pusher plugs”  776 . That is, pusher plug  776 - 1  is aft of projectile  100 - 1  and forward of projectile  100 - 2 . Pusher plug  776 - 2  is aft of projectile  100 - 2  and forward of projectile  100 - 3 , etc. 
     There is one propellant bay  774  for each projectile position, such that each propellant bay contains the propellant responsible for launching an associated projectile. For example, propellant bay  774 - 1  contains the propellant that is used to launch projectile  100 - 1  in Position  1 . Launcher  770  is designed so propellant bay that is associated with a particular projectile is located just aft of the pusher plug for that projectile. 
     As previously noted, it is the positioning of fins  112  forward of the aft end of projectile  100  (see, e.g.,  FIG. 1A , etc.) that enables projectiles  100  to be used in conjunction with stacked projectile launcher  770 . In particular, if the fins folded behind the aft end of projectile  100 , there would be no way to stack the projectiles while providing sufficient structural rigidity. 
     Furthermore, in some embodiments, projectile  100  contains a booster that is ignited once the projectile leaves barrel  772 . In those embodiments, fins  112  are disposed circumferential of a propellant bay disposed proximal to the aft end of the projectile. A benefit of fins  112  disclosed herein is that when deployed, the exhaust gas from the ignited booster never impinges on fins  112  since the fins are forward of the exhaust nozzle of the projectile. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure. For example, after reading this specification, those skilled in the art will know how to design alternative embodiments of the present invention in which projectile  100  is a torpedo or a projectile that is fired in air and then penetrates the water, in which the fins are located other than at the tail of projectile  100 , etc. As a consequence, the scope of the present invention is to be determined by the following claims.