Abstract:
An underwater supercavitating projectile includes means to form ripples on its surrounding cavity so as to provide well-defined disturbances of the cavity boundary. As the ripples move aft of the supercavitating projectile and into the wake behind the advancing projectile, the ripples detach to form a pattern of vapor bubbles in the wake that are distinct in both size and regularity from the typical vapor bubbles formed as the cavity collapses behind the advecting projectile. Sensors record the track of the projectile along its path based on the distinct acoustic signature of the vapor bubbles. Combined with the acoustic echo from a target, the relative distance of the projectile to the target can be determined using methods known in the art. Multiple projectile trajectories are used to increase the ability to resolve the target by adjusting the aiming of the projectiles to reduce the relative distance.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to underwater targeting and tracing systems and more specifically to systems and methods for controlling and utilizing supercavitating projectile dynamics to produce a distinctive radiated noise signal. 
     (2) Description of the Prior Art 
     There exists a need for accurate localization of underwater targets for a variety of underwater systems. The basic means of identification of an underwater object or target usually relies on the transmission of an acoustic signal from a fixed location and processing of a return echo at that same location. As in the case of an in-air tracer bullet, when launching a plurality of high velocity underwater projectiles against a target, it would be desirable to trace the path of a number of such projectiles so as to localize the proximity of the projectile stream on the intended target. 
     The art for tracing underwater high speed objects previously has been limited to measuring the speed of relatively small metallic objects which travel relatively closely to a magnetic pickup. For larger, high speed, underwater projectiles, supercavitating underwater vehicles have been proposed for use. The conditions for supercavitation are known in the art. Supercavitation allows for higher speeds to be sustainable by reducing skin friction drag to a great extent at such higher speeds. 
     Proposed means for tracking larger underwater high speed objects, such as a supercavitating vehicle, rely on a number of hoops aligned on a range in the anticipated path of the high speed projectile. The hoops are sufficiently large relative to the size of the projectile and anticipated path. Each hoop contains a number of independent hydrophones. The signals from the hydrophones may be analyzed to accurately determine position and track of an underwater projectile along the plane of each hoop. The system may be used as a fixed range or as a mobile range in a remote location. However, since the hoops must be placed in the anticipated path of the projectile, such means do not aid in localizing the proximity of a projectile stream on an intended target. 
     Systems and methods are needed that can produce radiated acoustic signals from the projectile in the near vicinity of the targeted object. In order to properly distinguish the radiated acoustic signals from the projectile so as to accurately track its path, the signals can be designed to be either greater in amplitude or easier to characterize than would a transmitted signal from the receiver or target location. 
     By providing a distinguishable acoustic signal, the systems and methods can provide better resolution of underwater vehicle position and improved tracking of an underwater object. The ability to more effectively target underwater objects moving at high speed may be enhanced through better resolution of underwater objects and tracks in poor acoustic environments. 
     The distinguishable acoustic signal of the systems and methods herein can lead to decreased signal processing requirements to achieve a desired target resolution and a better ability to resolve multiple targets. Accordingly, the systems and methods may be particular effective in conjunction with projectile-based terminal defense systems, mine clearance systems, stand alone gun systems for augmenting existing targeting systems, and the like. 
     SUMMARY OF THE INVENTION 
     It is therefore a general purpose and primary object of the present invention to provide systems and methods for controlling the cavity dynamics of a supercavitating underwater projectile to produce a distinctive radiated noise signal. The distinctive noise signal may then be used in conjunction with an underwater targeting system to help identify, localize and track targets. 
     The object of the present invention is attained by modifying a supercavitating projectile to provide a well-defined, prescribed disruption of its surrounding cavity. The disruptions, in turn, can produce well characterized acoustic signals that contain unique features that interact with the acoustic environment and aid in the identification of underwater objects. 
     In operation, a supercavitating projectile can be fired toward a target from a firing platform. The projectile can include rippling means that cause one or more ripples to form on the cavity boundary so as to provide well-defined disturbances of the cavity boundary. As the projectile advects through the water, the ripples move aft of the supercavitating projectile at the speed of advance. As the ripples move into the wake, the ripples detach to form a pattern of vapor bubbles in the wake that are distinct in both size and regularity from the typical vapor bubbles formed as the cavity collapses behind the advecting projectile. This distinction in the pattern of vapor bubbles results in a distinct acoustic signature. 
     Sensors can record the track of the projectile along its path based on the distinct acoustic signature. Combining this information with the acoustic echo from the target, the relative distance of the projectile to the target can be determined using methods known in the art. The aiming of the supercavitating projectiles towards the target can be adjusted to reduce the relative distance. As in the case of using tracer bullets in air to lock onto a target, multiple projectile trajectories can be used to increase the ability to resolve the target. 
     The rippling means may be varied to produce differing patterns of ripples and hence vapor bubbles. Thus, a variety of distinct sound fields may be created. The rippling means may be in the form of a mechanical actuator that can be extended from the projectile to contact the cavity boundary and cause a ripple to be created. Depending on the extent to which the actuator contacts the cavity boundary and the shape of the actuator, various sizes of ripples and hence vapor bubbles can be formed. Additionally, the actuator can contact the cavity boundary in a specified sequence to produce a corresponding specified pattern of vapor bubbles. 
     Supercavitating projectiles with flammable cores, which are ignited upon launch, are known in the art. Combustion of the core causes a flame front to move from the aft end toward the cavitator, or forward end, of the projectile. For such projectiles, a number of small bores can extend through the outer casing of the projectile and penetrate into the core. As the flame front reaches each bore, a pressure disturbance is created, which causes a ripple to form. As in the case of a mechanical actuator, ripples from the pressure disturbance result in vapor bubbles forming and a distinct sound field can be created. The number, size and spacing of the bores are chosen to produce ripples of varying size and frequency. The rate of flame front propagation can determine the rate of ripple formation and, as such, additional distinguishing characteristics of the acoustic signal can be produced. 
     In one embodiment, an underwater targeting system comprises a supercavitating projectile launched towards a target that includes a rippling means for providing the projectile with a distinct acoustic signature. An acoustic sensor receives first acoustic signals based on the distinct acoustic signature. The acoustic sensor also receives second acoustic signals based on an echo of the distinct acoustic signature from the target when the projectile approaches the target. An acoustic processor resolves the trajectory of the projectile based on the first acoustic signals and resolves the relative distance between the projectile and the target based on the first and second acoustic signals. 
     In one variation, the acoustic processor is attuned to the distinct acoustic signature so as to preferentially resolve the trajectory and relative distance. The acoustic sensor may also or separately be attuned to the distinct acoustic signature so as to preferentially receive the first and second signals. 
     In another variation, the rippling means forms one or more disturbances at a cavity boundary of the supercavitating projectile. The disturbances separate from the cavity boundary to form vapor bubbles in the wake of the projectile. The collapse of the vapor bubbles results in the distinct acoustic signature of the projectile. 
     The rippling means can be programmed to form a predetermined series of disturbances, with the disturbances having a specified timing, shape and/or size. 
     The rippling means can include an actuator and a controller. The controller can direct the movement of the actuator so as to disturb the flow within a cavity surrounding the projectile. The disturbed flow results in the formation of the disturbances. In one variation, the controller extends the actuator into the flow. In another variation, the movement of the actuator pivots a control surface of the projectile into the flow. In yet another variation, the movement of the actuator opens a port one the projectile to the flow. 
     In a further variation, the rippling means includes a flammable core within the projectile and one or more bores in the projectile, which expose the core to the cavity surrounding the projectile. The passage of the flame front of the flammable core by a bore disturbs the flow within the cavity, which results in the formation of a disturbance at the cavity boundary. The size of the bores, the location of the bores and/or the flame rate of the flammable core can be varied to form a predetermined series of disturbances. 
     In one embodiment, a supercavitating projectile comprises an actuator and a controller, which directs the movement of the actuator to disturb a flow within a cavity surrounding the supercavitating projectile. The disturbed flow forms one or more disturbances at a cavity boundary of the projectile. These disturbances separate from the cavity boundary to form vapor bubbles in the wake of the supercavitating projectile. The collapse of the vapor bubbles results in a distinct acoustic signature. 
     In one variation, the controller extends the actuator into the flow. In another variation, the movement of the actuator pivots a control surface of the projectile into the flow. In yet another variation, the movement of the actuator opens a port on the projectile to the flow. 
     In one embodiment, a supercavitating projectile comprises a flammable core and one or more bores in said projectile. The bores expose the core to a cavity surrounding the projectile. Passage of the flame front of the flammable core by a bore disturbs a flow within the cavity and the disturbed flow forms one or more disturbances at the cavity boundary of the projectile. The disturbances separate from the cavity boundary to form vapor bubbles in the wake of the projectile. Collapse of the vapor bubbles results in a distinct acoustic signal. 
     In one variation, a size of the bores, a location of the bores and/or a flame rate of the flammable core can be varied to form a predetermined series of disturbances. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a side view of an underwater projectile advecting through a medium; 
         FIGS. 2A-2C  illustrate the formation of ripples at the cavity boundary of the supercavitating projectile and its progression along the cavity; 
         FIGS. 3A and 3B  illustrate the formation of ripples and vapor bubbles; 
         FIG. 4  illustrates a targeting, tracking system; 
         FIGS. 5A-5C  illustrate actuator rippling means; and 
         FIG. 6  illustrates a pressure disturbance rippling means. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , there is shown a side view of underwater projectile  10  advecting through a fluid medium  11  in the direction indicated by arrow A. For ease of reference, but not limitation, medium  11  may be described herein as water. As is known in the art, water  11  is accelerated over a cavitator  12  attached to a nose portion  10   a  of vehicle  10 . The downstream pressure drops below the vapor pressure of water  11  after passing cavitator  12 , resulting in the formation of cavity  13 , through which projectile  10  traverses. 
     Cavity  13  terminates in a cavity closure region  13   a . Cavity closure region  13   a  is usually well defined spatially but is not steady. Quasi-steady rupture of cavity closure region  13   a  produces a trail of vaporous bubbles  14  behind closure region  13   a . Bubbles  14  ultimately collapse and produce a large amplitude radiated acoustic signal. 
     The characteristics of radiated signals from collapsing bubbles are known in the art. However, the acoustic signature of projectile  10  does not lend itself to tracking since the formation of bubbles  14 , as illustrated in  FIG. 1 , is chaotic. To provide a more well-defined and distinguishable acoustic signature, projectile  10  includes rippling means  16 , shown schematically in  FIG. 1 , which forms disturbance  18  in boundary  13   b  of cavity  13 . The operation of rippling means  16  and its formation of a disturbance in boundary cavity  13   b  will be discussed in further detail with respect to  FIGS. 5 and 6 . For illustrative purposes and not for limitation, the formation of disturbance  18  in  FIG. 1  is taken as time t 0 . 
       FIG. 2A ,  FIG. 2B  and  FIG. 2C  illustrate projectile  10  at subsequent times t 1 , t 2  and t 3 , respectively. In  FIG. 2A , disturbance  18  is shown at a position further towards aft end  10   b  of projectile  10  as a result of projectile  10  advecting through medium  11  for time t 1 −t 0 . Additionally,  FIG. 2A  illustrates rippling means  16  forming disturbance  18   a.    
     In  FIG. 2B , disturbances  18  and  18   a  are shown at positions still further towards aft end  10   b  of projectile  10  as a result of projectile  10  advecting through medium  11  for time t 2 −t 1 . Additionally,  FIG. 2B  illustrates rippling means  16  forming disturbance  18   b.    
     Correspondingly at time t 3 ,  FIG. 2C  illustrates disturbances  18 ,  18   a  and  18   b  at positions still further towards aft end  10   b  of projectile  10  and the formation of disturbance  18   c  by rippling means  16 .  FIGS. 2A-2C  further illustrate the continuing chaotic formation of bubbles  14  as projectile  10  advects through medium  11 . 
     As can be seen in  FIGS. 2A-2C , rippling means  16  forms a series of disturbances ( 18 - 18   c ) at boundary  13   b . The operation of rippling means  16  can be such that the disturbances (generally referred to herein as  18 ) formed by rippling means  16  are regularly shaped and spaced at boundary  13   b . The advection or movement of projectile  10  through medium  11  results in such consistent disturbances  18  progressing towards cavity closure region  13   a.    
     Referring to  FIG. 3A , there is shown projectile  10  at a time t z  subsequent to time t 3  of  FIG. 2C . Rippling means  16  has formed additional disturbances  18   u - 18   y  at boundary  13   b  in the manner shown in  FIGS. 2A-2C . Disturbances  18   u - 18   y  can be formed periodically or in a time encoded manner.  FIG. 3A  illustrates disturbance  18   u  at cavity closure region  13   a , such that disturbance  18   u  is in the process of separating from cavity boundary  13   b  and forming disturbance bubble  20 . Additionally,  FIG. 3A  illustrates a trail or pattern of disturbance bubbles  22  having been formed by disturbances separated from boundary  13   b  previous to time t z . As a result of the regularity, or uniformity of disturbances  18 , the pattern of disturbance bubbles  22  is distinct from that of typical vapor bubbles  14 . This difference in size and regularity of formation result in a distinct acoustic signature for the pattern of disturbance bubbles  22 . 
     Referring also to  FIG. 3B , there is shown projectile  10  of  FIG. 3A , wherein rippling means  16  is operated to form additional disturbances  18   z . For ease of illustration, but not limitation, disturbances  18   z  are shown formed on cavity boundary  13   b , opposite from disturbances  18   u - 18   y . Disturbances  18   z  have a distinct size and shape from that of disturbances  18   u - 18   y , resulting in a pattern of disturbance bubbles  22   z  distinct from that of the pattern of disturbance bubbles  22  and further distinct from that of typical vapor bubbles  14 . For clarity, but not limitation, vapor bubbles  14  are not shown in  FIG. 3B . 
     Thus, projectile  10  can produce a variety of distinct bubble patterns, depending on the operation of rippling means  16 . Those of skill in the art can readily determine the acoustic signatures resulting from such distinct bubble patterns. Accordingly, acoustic sensors can be sensitized to the particular characteristics of the distinctive acoustic signature of the projectile. Similarly, acoustic processors can be optimized for resolving the distinctive acoustic signature amongst other acoustic input. Thus, the path of the projectile can be preferentially tracked to assist in targeting projectiles. 
     Referring to  FIG. 4 , there is shown a schematic representation of targeting system  100 . For aid in targeting, system  100  utilizes projectile  102 , which has a distinct acoustic signature produced by a rippling means, as described with relation to  FIGS. 1 ,  2 A- 2 C and  3 A- 3 B. Projectile  102  is fired from gun  104  of platform  106 . Gun  104  is aimed along a trajectory, indicated by dashed line  108 , which is estimated to intercept target  110 . Sensor  112  records the track of projectile  102  along trajectory  108 . As described previously herein, sensor  112  can be sensitized to the acoustic characteristics of projectile  102 , so as to preferentially track the distinct acoustic signal  114  of projectile  102  amidst other acoustic energy within medium  11 . 
     As illustrated in  FIG. 4 , projectile  102  does not intercept target  110 . However, projectile  102  does approach target  110  such that sensor  112  receives an additional echo (schematically illustrated as arcs  114   a ) of the distinct acoustic signal of projectile  102  along path  116 . As is known in the art, such an echo, combined with the acoustic track of projectile  102 , contains information about the relative distance of projectile  102  to target  110 . Processors  118 , optimized for the distinct acoustic signature of projectile  102 , can resolve the path of projectile  102  and hence the relative distance between projectile  102  and target  110  at closest approach. In turn, the relative distance information can be used for targeting a next set of projectiles, in a manner similar to the use of tracer bullets in resolving a target on land. 
     Referring now to  FIGS. 5A-5C  and  FIG. 6 , the operation of various rippling means is schematically represented and described.  FIG. 5A  illustrates a schematic cross-section of a nose portion  200   a  of projectile  200 . To form disturbance  202  on cavity boundary  204 , actuator  206  extends from nose portion  200   a  into cavity  208  towards cavity boundary  204 . In so doing, actuator  206  disturbs the flow within cavity  208 . In turn, the disturbed flow creates disturbance  202  on cavity boundary  204 . 
     Control mechanism  210  can be linked to actuator  206  and can control the timing and/or the extent of actuator  206  into cavity  208 . Power supply  212  provides power for the operation of control mechanism  210  and actuator  206 . The dynamics of cavity boundary  204  are well known and understood in the art. Thus, the shape and timing of disturbance  202  can be controlled. Further, control mechanism  210  can be preprogrammed to provide the well-defined series of disturbances  18 , as illustrated in  FIGS. 2A-2C  and  FIGS. 3A and 3B . 
     The shape and operation of actuator  206  can take many forms.  FIG. 5B  illustrates actuator  206   a  pivoting control surface  214  into cavity  208  to form disturbance  202   a .  FIG. 5C  illustrates actuator  206   b  opening port  216  in projectile  200 , in the direction of arrow  216   a , so as to form disturbance  202   b . Based on the particular acoustic signature required, one or more of actuators  206 ,  206   a ,  206   b , may be used. 
     The use of flammable cores in underwater supercavitating projectiles is known in the art. Such projectiles can be modified to produce distinct acoustic signatures.  FIG. 6  illustrates a schematic cross-section of a nose portion  300   a  of modified projectile  300  having flammable core  302 . Core  302  is penetrated by one or more bores  304 . Upon launch of projectile  300 , flammable core  302  is ignited. Combustion of core  302  results in flame front  306  advancing toward nose portion  300   a  of projectile  300 , as indicated by arrow  306   a . As flame front  306  reaches bore  304 , the flow within cavity  308  is disturbed. (A portion of bore  304  is shown dotted to illustrate the passage of flame front  306 .) In turn, the disturbed flow creates disturbance  310  on cavity boundary  312 . 
     The number and size of bores  304  can be chosen to produce disturbances  310  of varying size and frequency. Additionally, the rate of propagation of flame front  306  can depend on the composition of flammable core  302 . Accordingly, the rate of formation of disturbances  310  can vary depending on the composition of core  302 . Thus, a desired acoustic signature can be generated depending on the size and location of bores  304  and/or the composition of core  302 . 
     What have thus been described are systems and methods for providing well-defined, prescribed disruptions to the cavity boundary of a supercavitating projectile as it advects through a medium. The disruptions, in turn, lead to the formation of a distinct pattern of vapor bubbles, which burst as they trail behind the advecting projectile. This produces well characterized acoustic signals that contain unique features that interact with the acoustic environment and aid in the targeting of underwater objects. 
     To produce the disruptions to the cavity boundary; the projectile includes one or more rippling means, which disturb the flow within the cavity. The disturbed flow results in the disruptions of the cavity boundary. The rippling means can include mechanical actuators under preprogrammed control. The actuators can disturb the flow within the cavity by being extended into the cavity, by pivoting a control surface to interact with the flow, or by opening a port within the projectile. In a projectile having a flammable core, the core can be penetrated by one or more bores. Once the core is ignited, the passing of the flame front by such a bore results in the disturbance to the flow within the cavity. 
     In use, such a modified projectile is launched towards a target. Based on its distinct acoustic signature, acoustic sensors and processors can be attuned to better track and resolve the path of the projectile. When the projectile approaches the target, the echo of the projectile&#39;s acoustic signature from the target can also be tracked and resolved by the attuned sensors and processors. Combined with the projectile tracking information, the echo information is processed to determine a relative distance between the projectile and the target. This information can then be used for aiming additional projectiles at the target. 
     The systems and methods described herein provide for an enhanced ability to determine the near instantaneous track of an underwater object and to more accurately determine the instantaneous position of an underwater object. This is accomplished by providing means to control the spectrum and the amplitude of the radiated noise from a projectile. 
     Obviously many modifications and variations of the present invention may become apparent in light of the above teachings. As described previously, the shape of the actuator may be varied to suit the desired acoustic signature, or the extent to which the actuator interacts with the cavity flow may be varied. The location and size of the bores, as well as the composition of the flammable core can be varied to suit. Further, one or more of the actuators and the flammable core may be used in combination. 
     Additionally, it is known in the art that a tumbling projectile produces a robust, distinctive acoustic signature. The actuator or the final bore may be used to tumble the projectile at a fixed time or point in its trajectory to provide an especially strong acoustic signal for processing. 
     As another example, it is known in the art to have the projectile introduce gas into the cavity so as to maintain the closure region of the cavity further from the projectile. Such a projectile may be fitted with one, or more ports for releasing compressed gas into the flow so as to form disturbances along the cavity boundary. 
     It will be understood that many additional changes in details, materials, steps, and arrangements of parts which have been described herein and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.