Abstract:
A projectile having a cavity-running mode is provided with a means for changing the diameter of its nose. Based on changed conditions, the diameter of the nose can be actively reduced or increased, as required, to maintain a desired value for the nose-to-body ratio of the projectile.

Description:
STATEMENT OF RELATED CASES 
       [0001]    This case claims priority of the following U.S. Provisional Patent Applications: Ser. No. 60/911,419 filed on Apr. 12, 2007, Ser. No. 60/992,025 filed on Dec. 3, 2007, and Ser. No. 61/033,418 filed on Mar. 3, 2008. All of these Provisional Patent Applications are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to underwater projectiles having a supercavitating running mode. 
       BACKGROUND OF THE INVENTION 
       [0003]    Research and development is currently underway to produce underwater projectiles that travel at very high speeds using the phenomenon of “supercavitation.” A progenitor of such projectiles is the “Shkval,” which is a rocket-propelled torpedo that was developed by Russia and achieves a velocity of 250 knots (288 mph). 
         [0004]    A supercavitating projectile&#39;s main features are a specially shaped nose and a streamlined, hydrodynamic, and aerodynamic body. The nose has a blunt leading surface that is referred to as a “cavitator.” When the projectile travels through water at speeds in excess of about one hundred miles per hour, the cavitator deflects water outward so fast that the water flow separates and detaches from the surface of the projectile. Since water pressure takes time to collapse the wall of the resulting cavity, the nose opens an extended bubble of water vapor. 
         [0005]    Given sufficient speed, the cavity can extend to envelop the entire projectile except the nose. One engulfed by the bubble, the drag experienced by the projectile is significantly reduced. As a consequence, a projectile moving in the cavity (“cavity-running”) can travel at far greater speeds for a given amount of thrust than a projectile that is moving in a conventional manner through water. A cavity-running projectile quite literally “flies” through the surrounding gas. In the absence of sustaining propulsion, the projectile loses supercavitation and eventually stalls due to drag. A secondary benefit of cavity running is that the motion stability of the projectile is enhanced. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a way to increase the distance over which a projectile is cable of sustaining a cavity-running mode. 
         [0007]    There is an “optimum” value for the nose-to-body ratio of a projectile when operating underwater in a cavity-running mode. This optimum value is a function, primarily, of the prevailing pressure (i.e., depth and water density) and velocity. Therefore, as long as the projectile maintains a horizontal trajectory and constant speed, the optimum value for this ratio does not change. Once the projectile deviates from a horizontal path, the optimal value will, of course, change. 
         [0008]    It has been recognized by the inventor that it would be advantageous to be able to actively and dynamically adjust the nose-to-body ratio to maintain an optimum value. Doing so maximizes the distance for which the projectile remains in the cavity-running mode. And that maximizes the speed of the projectile and distance that it can ultimately travel. 
         [0009]    In accordance with the illustrative embodiment, a projectile having a cavity-running mode is provided with velocity and/or depth/pressure sensors and a means for changing the diameter of its nose. Based on changes in conditions, as measured by the sensors, the diameter of the nose can be actively reduced or increased, as required, to maintain a diameter that best satisfies the optimum value for the nose-to-body diameter ratio. 
         [0010]    In some embodiments, the forward-most portion of the nose is configured as a plurality of nested, right-circular cylindrical shells or segments. By withdrawing or adding segments in the manner of the movement of a “spy-glass” or telescoping antennae, the diameter of nose (i.e., cavitator) is changed. 
         [0011]    The means for changing nose diameter can be suitably implemented as a MEMS device configured as an actuator and coupled to a drive train, wherein the drive train couples to the various segments of the forward portion of the nose. In conjunction with the present disclosure, those skilled in the art will be able to design and build such a mechanism. 
         [0012]    In some other embodiments, a simplified (i.e., less “intelligent”) version of an actuator, such as a series of spring latches, can be used to adjust the diameter of the nose by sequentially releasing segments of the nose, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  depicts a schematic of a projectile in accordance with the illustrative embodiment, wherein the cavitator portion of the nose comprises a plurality of nested cylindrical segments, and wherein the cavitator section is shown at maximum diameter. 
           [0014]      FIG. 2  depicts the projectile of  FIG. 1  wherein the nested segments of the cavitator are partially retracted, and wherein the cavitator section is shown at a minimum diameter. 
           [0015]      FIG. 3  depicts the projectile of  FIG. 1  wherein some of the segments of the cavitator are retracted such that the cavitator section has an intermediate diameter. 
           [0016]      FIG. 4  depicts an end-on view of several of the segments of the cavitator, wherein the segments are separated from one another for pedagogical purposes. 
           [0017]      FIG. 5  depicts an end-on view of the segments of  FIG. 4  when the segments are in their nominal nested arrangement. This Figure illustrates that the forward surface of the cavitator is a continuous surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  depicts projectile  100  in accordance with the illustrative embodiment. The projectile includes nose  102  and body  112  (both shown in partial section). Nose  102  comprises forward-most section or cavitator  104 . 
         [0019]    Forward face  108  of nose  102  is blunt so that when projectile  100  achieves suitable velocity, a cavity-running mode of operation is created. In particular, the blunt face  108  pushes aside water as it advances. When the hydrodynamic pressure of water that is pushed aside overcomes the ambient static pressure, the 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). Since the projectile is then surrounded by air, rather than water, hydrodynamic drag is substantially reduced. 
         [0020]    For the foregoing reason, forward face  108  and/or segment  104  is therefore referred to as a “cavitator.” For the purposes of this specification, including the appended claims, reference to “the diameter of the nose” means the diameter of cavitator  104 . 
         [0021]    As depicted in  FIG. 1 , cavitator  104  comprises one or more nesting segments  106 - i , i=1, n (collectively referenced as “segments  106 ”). In the illustrative embodiment, i=9; that is, there are nine segments  106 . 
         [0022]    To permit nesting, each segment  106 - i  has a diameter that is different than every other segment  106 - i . In the illustrative embodiment, the nesting segments are cylindrical; however, in other embodiments, they are right-circular conical segments. Note that in the illustrative embodiment, forward face  108  is a substantially flat, substantially continuous surface. As such, and as will become clearer in conjunction with  FIGS. 4 and 5 , the forward face of all segments  106 - i  in  FIG. 1  are co-planar or flush with one another (although this is not depicted in  FIG. 1  for pedagogical purposes). 
         [0023]    As depicted in  FIG. 2 , each segment  106 - i  is independently movable via the operation of actuator  216  and drive line  214 , to which the segments are operatively coupled.  FIG. 2  depicts the segments  106 - 2  through  106 - 9  retracted to a successively greater extent, such that segment  106 - 1  becomes the forward most segment and, in fact, defines cavitator  104 . This has the effect of reducing the diameter of nose  102 . 
         [0024]    Thus, in the state that is depicted in  FIG. 1 , projectile  100  has its maximum nose diameter wherein all segments  106  are fully extended. In such a state, cavitator  104  comprises all segments  106 . By contrast, in the state depicted in  FIG. 2 , projectile  100  has its minimum nose diameter, and cavitator  104  comprises only segment  106 - 1 . 
         [0025]      FIG. 3  depicts and embodiment in which segments  106 - 4  through  106 - 9  are retracted so that cavitator  104  is defined by segments  106 - 1 ,  106 - 2 , and  106 - 3 . In this state, projectile  100  has a nose diameter that is intermediate between that shown in  FIGS. 1 and 2 . 
         [0026]    As previously noted, forward surface  108  of cavitator  104  is substantially continuous. One way to accomplish this is depicted in  FIGS. 4 and 5 .  FIG. 4  depicts front views of segments  106 - 1 ,  106 - 2 , and  106 - 3 . The segments are separated from one another in the  FIG. 4  for explanatory purposes. Segment  106 - 1 , which has the smallest diameter, is “solid,” or otherwise has a continuous forward face  107 . Segment  106 - 2 , which is the next largest segment after  106 - 1 , has an open region  430 - 2  that receives face  107  of segment  106 - 1 . Segment  106 - 2  also includes a solid-surface marginal regional  432 - 2  that represents the increment in diameter beyond segment  106 - 1 . Likewise, next-largest segment  106 - 3  has an open region  430 - 3  for receiving the surface that is collectively defined by marginal region  430 - 2  and face  107 . Segment  106 - 3  also includes marginal region  432 - 3  that represents the increase in diameter over segment  106 - 2 . 
         [0027]      FIG. 5  depicts these three segments end on in the normal nested arrangement. In the state depicted in  FIG. 5 , face  107  (of segment  106 - 1 ), marginal region  432 - 2  (of segment  106 - 2 ), and marginal region  432 - 3  (of segment  106 - 3 ) are flush, defining face  108 . This is representative of a “front view’ of  FIG. 3 . It is face  108  that “sees” water and creates the supercavitation phenomena, as previously described. 
         [0028]    All other segments are configured in the manner of segments  106 - 2  and  106 - 3  to enable nesting and to provide, a “solid” or continuous forward face  108  to cavitator  104 , regardless of how many segments  106 - i  define the cavitator. 
         [0029]    Returning to  FIG. 2 , the telescoping operation of nose  102  is controlled via processor  218 , based on input from one or more sensors. In some embodiments, projectile  100  includes velocity sensor  220 . In some other embodiments, projectile  100  includes depth (or pressure) sensor  222 . In yet some further embodiments, projectile  100  includes both velocity sensor  220  and depth (pressure) sensor  222 . As described further below, processor  218  uses velocity and/or depth measurements obtained by the sensors to calculate an “optimal” nose-to-body diameter ratio for those conditions. 
         [0030]    The nose-to-body diameter ratio can be “optimized” on a variety of different bases. For example, it can be optimized based on providing the maximum range. Or it can be optimized based on minimizing the amount of thrust required to maintain supercavitation operation (i.e., the cavity-running mode). Those skilled in the art will recognize that additional bases for optimization exist. 
         [0031]    For example, in some embodiments, processor  112  calculates an optimal nose diameter (i.e., cavitator diameter) using the expression: 
         [0000]        D   n   *=D   b /([0.550783× V   0 ]+0.157122)  [1]
       Wherein:
           D n *=optimal nose diameter;   D b =body diameter; and   V o =velocity of the projectile.
 
The foregoing equation was obtained by curve fitting solutions of D n * against different values of V o , wherein V o  is the initial velocity of a projectile normalized to the characteristic velocity V c  at the depth of the (horizontally moving) projectile. V c =(2P/ρ water ), where P is the static drag and ρ water  is the density of water at the relevant temperature. Interpreting V o  as the current velocity of projectile (as if the projectile was just launched at the current depth), the instantaneous optimal nose diameter D n * is given by expression [1].
   
               
 
         [0036]    Since body diameter D b  is fixed and known, and the velocity is obtained from sensor  114 , a new nose diameter is readily calculated. 
         [0037]    Processor  218  determines which one or more segments  106 - i  should be moved to achieve the new “optimal” nose-to-body diameter ratio. Signals indicative of which segment(s) to move and by how much are generated by processor  218  and transmitted to a driver (not depicted). The driver generates signals appropriate for controlling actuator  216  to extend or withdraw drive line  214 , as appropriate. 
         [0038]    It will be appreciated that a variety of “nose” configurations can be adopted for any specified nose diameter. In particular, the nose diameter is determined by the diameter of cavitator  104 . But that does not dictate the extent to segments  106 - i  that are not part of the cavitator  104  must be retracted. For example,  FIG. 2  depicts a minimum nose diameter wherein only segment  106 - 1  is fully extended to serve as cavitator  104 , while the remaining segments are incrementally retracted. In other states, the remaining segments can be retracted to a different extent. 
         [0039]    The extent to which the remaining segments are retracted will be dependent upon aerodynamic considerations, among other factors, and is within the competency of those skilled in the art. 
         [0040]    Alternate expressions are available for determining optimal nose diameter. For example, for a cavity-running projectile that is under thrust, F, the optimal nose diameter (the diameter that sustains a cavity-running mode for the available thrust and the operating depth) is given by: 
         [0000]        D   N *=0.29×( F /(ρ water   gH+ATM ) 0.5 ,  [2]
       wherein:
           F is the applied thrust;   H is the depth of the projectile under water;   ATM is the water pressure bearing on the projectile;   ρ water  is the density of the water at the relevant temperature; and   g is the acceleration due to gravity.   
               
 
         [0047]    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 and that the scope of the present invention is to be determined by the following claims.