Abstract:
A method and system for attenuating a shockwave propagating through a first medium. The method and system may include providing a sensor for detecting a shockwave-producing event, which may include detecting an explosive device or detecting an explosion from an explosive device, determining a direction and distance of the shockwave relative to a defended target, calculating with a computer control a firing plan, and interposing a second medium between the shockwave and a protected asset if cost effective to do so. The second medium may be different from the first medium and the shockwave may be reflected, refracted and dispersed, or absorbed as it passes through the second medium prior to reaching the protected asset, and thus may be attenuated in force as it reaches the protected asset.

Description:
FIELD 
     The disclosure relates to shockwave attenuation devices, and more particularly to a method and apparatus for interposing an intermediate medium to attenuate a shockwave. 
     BACKGROUND 
     Explosive devices are being used increasingly in asymmetric warfare to cause damage and destruction to equipment and loss of life. The majority of the damage caused by explosive devices results from shrapnel and shockwaves. Shrapnel is material, such as metal fragments, that is propelled rapidly away from the blast zone and may damage stationary structures, vehicles, or other targets. Damage from shrapnel may be prevented by, for example, physical barriers. Shockwaves are traveling discontinuities in pressure, temperature, density, and other physical qualities through a medium, such as the ambient atmosphere. Shockwave damage is more difficult to prevent because shockwaves can traverse an intermediate medium, including physical barriers. 
     Damage from shockwaves may be lessened or prevented by interposing an attenuating material between the shockwave source and the object to be protected. This attenuating material typically may be designed or selected to absorb the energy from the shockwave by utilizing a porous material that distorts as the energy of the shockwave that is absorbed. 
     U.S. Pat. No. 5,394,786 to Gettle et al. describes a shockwave attenuation device that utilizes an absorbing medium. That assembly includes porus screens that form an enclosure filled with a pressure wave attenuating medium. This attenuating medium may be an aqueous foam, gas emulsion, gel, or granular or other solid particles. However, as shown and described in the drawings of that patent, the shockwave attenuating assembly must be positioned before the explosion occurs and surround the area to be protected. For example, the assembly may be positioned on the side of a vehicle to prevent damage to the vehicle or passengers within. 
     A similar shockwave attenuation device is described in U.S. Patent Publication No. 2007-0006723 to Waddell, Jr. et al. That device includes a number of cells filled with an attenuating material, such as aqueous foams. However, like the device described in Gettle et al., the pressure-attenuating material and device must be positioned on a structure, surface, or person desired to be protected by the system before the explosion occurs. 
     One feature common among prior art shockwave attenuation systems is that they require an intermediate medium or structure that acts to attenuate the force of the shockwave by absorbing the energy of the shockwave. Although only a portion of the shockwave may pass through the medium, the energy of the shockwave is nevertheless significantly reduced by the intermediate medium. However, because these systems are structural, they must be fixed in place before a shockwave is created. Further, these shockwave attenuation systems may not protect an entire vehicle or person. For example, attenuating panels are not transparent and therefore cannot be placed over windows or used as facemasks in helmets. They also may be bulky and heavy, and therefore negatively impact the performance of a vehicle on which they are mounted. 
     Such prior art shockwave attenuation systems may not be effective to protect waterborne assets in which an incoming threat may be in the form of a torpedo, ballistic shell, bomb or a naval mine. Therefore, a need exists for a shockwave attenuation device that is capable of dynamically interposing a medium between an explosion source and a protected asset. There is also a need for an intermediate medium that effectively attenuates the energy from a shockwave and that allows for protection of a protected asset in a marine environment. 
     SUMMARY 
     According to one embodiment, a method for attenuating a shockwave propagating in a first medium may include either detecting an incoming threat or a shockwave-producing event, determining a direction of the shockwave from the threat or event relative to a protected asset, and interposing a second medium, different from the first medium, between the shockwave and the protected asset such that a shockwave produced by the event contacts the second medium and is attenuated in energy thereby prior to reaching the protected asset. 
     According to another embodiment, a system for attenuating a shockwave propagating in a first medium includes a sensor for detecting a source of the shockwave and generating a detection signal, a projectile launcher configured to launch projectiles in a pre-set quantity and direction into the first medium and thereby form the second medium, and a control configured to receive the detection signal and activate the projectile launcher to deliver at least one projectile to the first medium adjacent a protected asset. In one embodiment, the first medium may be a body of water contacting the protected asset, and the second medium may be a cavitation region formed by passage of the projectile through the first medium. In that embodiment, the protected asset may be surface ship, a barge, an offshore platform, or a submarine. 
     The control may direct the launcher to deliver the projectile at a location and in a manner that may create a transient cavitation region between the shockwave and the protected asset. The shockwave may contact the boundary between the body of water and the cavitation region and be at least partly reflected. The remaining portion of the shockwave may be partly refracted and dispersed, thereby diminishing the energy density of the shockwave. As the shockwave passes through the cavitation region, it may be at least partly absorbed by the gases in the cavitation region by viscous dissipation or by deformation of the cavitation region. 
     As the shockwave leaves the cavitation region, it may be reflected and refracted further as it passes through the curved boundary layer between the cavitation region and the denser body of water on the opposite side of the cavitation region. And finally, the shockwave may be reduced further by forward motion of the water due to momentum imparted by the projectile&#39;s recent passage. 
     According to one embodiment, the sensor may be configured to detect an incoming threat, such as a bomb, ballistic shell or torpedo. The system may include a sensor configured to detect the signature of the incoming threat, and a computer that may compare the signature to known signatures of a plurality of different threats, estimate the probability that the incoming threat is one of the known threats and then estimate the probability distribution function of explosion magnitudes and locations relative to the protected asset. Based on stored data or models about the asset&#39;s vulnerability to shockwaves of various magnitudes from various directions, together with data or models of what the launcher and projectiles can do to attenuate shockwaves in which positions and in what time interval, the computer may form a plan to counter the threat at minimum cost. The computer may then activate the control to execute a firing plan in which the projectile launcher may launch one or more projectiles to create the cavitation region. In one embodiment, the computer may determine that the potential for damage to the protected asset from the shockwave may not justify deployment of the projectile launcher at all. 
     According to yet another embodiment, the sensor may be configured to detect an explosion from a threat that has already occurred. The threat may be an incoming threat, such as a ballistic shell or torpedo, or a stationary threat such as a mine. The system may include a sensor configured to detect the signature of the explosion, and a computer that may compare the signature to known signatures of a plurality of different explosives, estimate the probability that the explosion is from one of the known explosives and then estimate the probability distribution function of explosion magnitudes and locations relative to the protected asset. 
     Based on the magnitude of the detected explosion, stored data about the yield versus signature of each threat, the measured position of the explosion, and the shape and orientation of the protected asset, the computer estimates a probability distribution function of explosion magnitudes and locations relative to the protected asset. Based on stored data or models about the asset&#39;s vulnerability to shockwaves of various magnitudes from various directions, together with data or models of what the launcher and projectiles can do to attenuate shockwaves in what positions and in what time interval, the computer may form a firing plan to counter the threat at minimum cost. 
     The computer may then activate the control to execute a firing plan in which the projectile launcher may launch one or more projectiles to create the cavitation region. In one embodiment, the computer may determine that the potential for damage to the protected asset from the shockwave may not justify deployment of the projectile launcher at all, or that the explosion is too distant from the protected asset to warrant deployment. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic elevational view of one embodiment of the disclosed system for attenuating a shockwave via cavitation; 
         FIG. 2  is a detail showing a cavitation region created by a projectile used by the system of  FIG. 1  as the projectile passes through a body of water; 
         FIG. 3  is a schematic elevational view of the system of  FIG. 1  showing attenuation of a shockwave as the shockwave contacts and passes through a cavitation region; 
         FIG. 4  is a schematic elevational view of the system of  FIG. 1  in which a shockwave from an explosion contacts a cavitation region; 
         FIG. 5  is a flowchart of a process performed by the embodiment of  FIG. 1  in which an incoming threat is detected; and 
         FIG. 6  is a flowchart of a process performed by the embodiment of  FIG. 1  in which an explosion is detected. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed shockwave attenuation method and system may utilize an intermediate medium that may be dynamically deployed between an explosion and a defended object. The intermediate medium may attenuate the energy from a shockwave through several vectors, rather than simply absorbing the energy of the shockwave. 
     As shown in  FIG. 1 , in one embodiment, the system for attenuating a shockwave via cavitation, generally designated  10 , may include a sensor  12 , a projectile launcher  14  and a computer  16 . The sensor  12  and computer  16  may be mounted on or incorporated in the projectile launcher  14 , or they may be physically separate from the projectile launcher. The projectile launcher  14  may be mounted on a protected asset  18  that may be positioned on, over (e.g., a hovercraft or air-cushion vehicle), in, under or adjacent a body of water  20 . The protected asset may be one or more of a surface ship, barge, offshore platform or submarine. In another embodiment, one or more components of the system  10  may be mounted on a beach, breakwater, pier or dock adjacent the body of water. 
     In one embodiment, the projectile launcher  14  may be mounted on a support, such as a surface ship, that is adjacent the protected asset  18 . In another embodiment, the sensor  12  and computer  16  may be mounted on a surface ship or other platform that is separate from the protected asset  18 . 
     The projectile launcher  14  may be mounted to fire a projectile  19  into the body of water  20  adjacent the protected asset  18 . As shown in  FIG. 2 , when fired into the body of water  20  by the projectile launcher  14 , the projectile  19  creates a cavitation region  21  as it travels through the body of water. The cavitation region  21  may be an area of low pressure immediately behind and trailing from the projectile  19  that may be filled with vapor and represents a second medium, different in temperature, density and composition from the body of water  20 , the first medium. This cavitation region may last for less than a second then break up into discrete cavitation bubbles. 
     In one embodiment, the sensor  12  may be configured to provide measurements that enable the computer  16  to estimate the location and time of an explosion  22 , either before or after it occurs, and direct the projectile launcher  14  to respond. In another embodiment, the sensor  12  may be configured to detect an incoming threat  24  containing an explosive device or devices, such as a ballistic shell, bomb, torpedo, depth charge, naval mine or bomb-laden surface vessel. In yet another embodiment, the sensor  12  may be configured to detect both the threat  24  and the explosion  22  from the threat. In one embodiment, two systems  10  may be deployed on a protected asset  18  in which one system is configured to detect an incoming threat  24  and the other system is configured to detect an explosion  22 . 
     In an embodiment wherein the sensor  12  is configured to detect an incoming threat  24 , the sensor may use known threat-detection technologies including radar, visible or infrared light, or passive or active acoustic sensors. The sensor  12  also may employ trajectory tracking and prediction methods. 
     In an embodiment wherein the sensor  12  is configured to detect an explosion  22  before the shockwave  26  from the explosion reaches the protected asset  18 , the sensor  12  may be configured to detect a burst or pulse of electromagnetic radiation  28  that accompanies the explosion  22 . The burst  28  travels at or near the speed of light and will reach the sensor  12  before the shockwave  26 , which travels much slower, reaches the protected asset  18 . This time lag between the pulse  28  and the shockwave  26  may be sufficient to allow the system to launch the projectile  19  into the body of water  20  to form a cavitation region  21  ( FIG. 2 ) that intercepts the advancing shockwave. 
     The sensor  12  may be configured to detect any subset of the electromagnetic radiation  28  emitted during chemical detonations, including microwave bursts, flashes of infrared, visible and ultra-violet light, and x-ray bursts. In one embodiment, the sensor  12  is configured to detect electromagnetic radiation  28  at wavelengths in which water is substantially transparent, such as visible light, near ultraviolet or near infrared. In one embodiment, the sensor  12  may be configured to detect two different bands of electromagnetic radiation in order to minimize false positives and to enhance the accuracy of identifying the signature of the explosion  22 . 
     The computer  16  is configured to receive measurements from the sensor  12 , estimate a probability distribution of where and when an incoming threat  24  will detonate to form explosion  22  (or has already detonated), and directs the launcher to fire projectiles  19  with timing and direction that provide an optimal chance to minimize damage to the protected asset  18  from the shockwave  22 . Even if the incoming threat  24  is, for example, a torpedo on a collision course with the protected asset  18 , there is a chance that the torpedo misses, explodes in the water  20  near the protected asset, and thus provides an opportunity for the system  10  to deploy to attenuate the force of the shockwave  22  reaching the asset. 
     As shown in the flow chart of  FIG. 5 , the computer  16  may be configured to develop a firing plan for the embodiment of the system  10  ( FIG. 1 ) that includes a sensor  12  configured to detect an incoming threat  24  before it explodes. As shown in box  30 , the process begins with the sensor  12  detecting the approach of an incoming threat  24 . The sensor generates data pertaining to the signature of the threat, shown by box  32 , and the motion of the threat, shown by box  34 . Data pertaining to the signature of the threat  24  may include the shape of the threat and the heat signature of a propulsion system of the threat. Data pertaining to the motion of the incoming threat  24  may include trajectory, velocity, azimuthal angle relative to the launcher  14  and altitude. 
     As shown in box  36 , the computer  16  receives the data from the sensor  12  relating to the signature of the threat  24  and accesses data on known threat object signatures, which may be stored in a database or accessible over a network as shown in box  38 . The stored threat signatures may include various known threats, such as types of torpedoes. The computer compares the observed object signature with the retrieved signatures of known threat object signatures, as indicated in box  36 . As shown in box  40 , the computer  16  then estimates, for each stored known threat, how probable it is that the incoming threat  24  is that stored threat. As shown in box  46 , the computer  16  takes or collects the stored data about the payload or warhead of each threat, shown in box  42 , the motion of the threat, shown in box  34 , and the shape, orientation and motion of the protected asset  18 , shown in box  44 . As shown in box  48 , from this collected data, the computer  16  estimates a probability distribution function (p.d.f.) of explosion magnitudes and locations relative to the protected asset  18 . 
     Next, as shown in box  50 , the computer  16  may access stored data or models about the asset&#39;s vulnerability to shockwaves of various magnitudes from various directions, including crew injuries likely to result from shockwaves, as shown in box  52 , and access stored data or models of what the launcher and projectiles can do to attenuate shockwaves in what positions and in what time intervals, as shown in box  54 . As shown in box  56 , with this information the computer  16  may formulate a firing plan to counter the explosion  22  by signaling the projectile launcher  14  to fire one or more projectiles  19  into a pre-determined intercept region between the advancing shockwave  26  and the protected asset  18 . In one embodiment, the computer  16  is configured to develop a firing plan at minimum cost. Cost may include not only the cost to operate the launcher  14 , but also the probable cost of damage to the asset  18  from the attenuated shockwave. 
     As shown in box  58 , the computer  16  then may instruct the projectile launcher  14  to execute the firing plan. In some instances, the computer  16  may determine that the explosion yield is relatively small and/or the probable distance from the asset  18  is relatively large, so that the expected cost of operating the system  10  is greater than any damage that the asset may sustain. In such cases, the optimal or lowest-cost firing plan may be not to deploy the system  10  and fire the launcher  14 . 
     As shown in  FIG. 6 , the computer  16  may be configured to develop a firing plan for the embodiment of the system  10  ( FIG. 1 ) that includes a sensor  12  configured to detect an explosion  22  from an incoming threat  24  (i.e., an explosion that has already occurred). As shown in box  60 , the process begins with the sensor  12  detecting the explosion  22  event. As shown in box  62 , the sensor  12  generates data pertaining to the signature of the explosion, and the location of the explosion, as shown by box  64 . Data pertaining to the location of the explosion  22  may include azimuthal angle relative to the launcher  14 , altitude relative to the launcher (i.e., depth in the body of water  20 ) distance from the launcher, time of the explosion, and magnitude of the explosion. 
     As shown in box  66 , the computer  16  may receive data from the sensor  12  relating to the signature of the explosion  22  and, as shown in box  68 , access signature data on known explosives, which may be stored in a database or accessible over a network from a remote database. Such explosives may include C4 and ANFO (ammonium nitrate/fuel oil). The computer  16  may then compare the observed event signature data to the accessed signature data and, for each accessed signature, assign a probability that it matches the signature of the explosion. 
     As shown in box  70 , the computer  16  then estimates, for each stored known explosion signature, how probable it is that the explosion  24  is from the explosive associated with that signature. As shown in box  72 , the computer  16  takes or collects stored data about the position of the explosion (with associated uncertainty), shown in box  64 , and the shape, orientation and motion of the protected asset  18 , shown in box  44 . As shown in box  74 , from this data, the computer  16  estimates a p.d.f. of explosion magnitudes and locations relative to the protected asset  18 . 
     Next, as shown in box  76 , the computer  16  may access stored data or models about the vulnerability of the asset  18  to shockwaves of various magnitudes from various directions, including crew injuries likely to result from shockwaves, as shown in box  52 , and access stored data or models of what the launcher and projectiles can do to attenuate shockwaves in what positions and in what time intervals, as shown in box  54 . With this information, as shown in box  78 , the computer  16  may formulate a firing plan to counter the explosion  22  by signaling the projectile launcher  14  to fire one or more projectiles  19  into a pre-determined intercept region between the advancing shockwave  26  and the protected asset  18 . In one embodiment, the computer  16  may be configured to develop a firing plan at minimum cost. Cost may include not only the cost to operate the launcher  14 , but also the probable cost of damage to the asset  18  from the attenuated shockwave. 
     As shown in box  80 , the computer  16  then may instruct the projectile launcher  14  to execute the firing plan. In some instances, the computer  16  may determine that the explosion  22  is relatively small and/or the probable distance from the asset  18  is relatively large, so that the expected cost of operating the system  10  is greater than any damage that the asset may sustain. In such cases, the optimal or lowest-cost firing plan may be not to deploy the system  10  and fire the launcher  14 . 
     As shown in  FIGS. 1 ,  3 , and  4 , when the system  10  is deployed, the launcher  14  may be actuated to propel projectiles  19  to a calculated, pre-set trajectory to an intercept region between the protected asset  18  and the oncoming shockwave  26  from an explosion  22  before the shockwave is too close to the protected asset  18  so that the cavitation region  21  created by the projectile(s)  19  may effectively attenuate the shockwave  26 . In one embodiment, the launcher  14  may be a rapid-fire repeating gun that fires in response to a signal from the computer  16 . Examples may include a machine gun, a chain gun or a rocket launcher. In such an embodiment, the launcher  14  may take the form of a single-barreled gun fixed in position to point in the general direction of the area covered by the sensor  12 . 
     In another aspect, the launcher  14  may take the form of several gun-type barrels pointed in diverse directions, each loaded with projectiles. In this aspect, the computer  16  may select one or more, but not all, of the several barrels to fire, based upon the firing plan developed by the computer, which may take into account an azimuthal estimate from the computer. This aspect may provide a wider angle of coverage than a fixed, single-barrel embodiment. 
     In another aspect, the launcher  14  may take the form of a single gun-type barrel mounted on a high-speed mechanical pointing mechanism. The pointing mechanism may aim the barrel based on an azimuth estimate from the computer  16 . This aspect may provide protection over a wider azimuth angle than the fixed single barrel embodiment, and my do so with less hardware than the multi-barrel embodiment. For applications where exceptionally fast response is needed, such as protection from close-in explosions, the launcher  14  may be gun in which a signal from the computer directly activates an igniter (or igniters) that fires the projectile (or projectiles). An example of such a gun is the FireStorm 40 mm multi-barrel grenade launcher manufactured by Metal Storm Limited of Darra, Australia. In yet another embodiment, the launcher  14  may take the form of a pre-existing projectile launcher, already mounted on a protected asset  18 , such as a surface ship, that is capable of control and firing by the computer  16 , may be employed. For example, the launcher  14  of the system  10  may take the form a Phalanx or other close-in weapons system, already present on most surface ships. 
     As shown in  FIGS. 1 ,  3  and  4 , the projectile  19  may be selected to induce a cavitation region  21  sufficiently fast, and with sufficient coverage to reduce the energy of the shockwave  26  before it reaches the protected asset  18 . In combination with the launcher  14 , the projectile  19  must be capable of being propelled through the body of water  20  at a speed and distance sufficient to create a cavitation region  21  sized and shaped to attenuate the shockwave  26  effectively. Depending upon the application, the projectile may be a regular bullet or shot, a sabot-mounted lightweight bullet to achieve high speed, a set of pellets or flechettes that spread out to give broader coverage, or a bullet that fragments upon impacting the body of water  20  (thereby creating a set of pellets that collectively may create more cavitation over a short distance than a single projectile or bullet). 
     The operation of one embodiment of the system  10  is shown in  FIGS. 3 and 4 . The sensor  12  detects an incoming threat  24  (see  FIG. 1 ), and in another embodiment the sensor  12  detects an explosion  22  in a body of water  20 . As described in detail with respect to  FIGS. 4 and 5 , the computer  16  calculates a firing plan, and if it is determined that it is cost-effective for the system  10  to be deployed, the computer  16  may send a command to the projectile launcher  14 . If the launcher  14  is positionable, the computer  16  may provide co-ordinates including azimuthal angle and elevation to the launcher  14 , or if the launcher  14  is a fixed, multi-barreled gun, the computer  16  may provide instructions on which of the barrels is to be fired. 
     Upon receipt of the signals from the computer  16 , the launcher  14  may fire one or more projectiles  19  into the water  20  between the advancing shockwave  26  and the asset  18 . As each projectile  19  passes through the water  20 , it creates a cavitation region  21 . The trajectory of each projectile  19  may be calculated to create a cavitation region  21  that is impacted by the shockwave  26 . The leading portion  82  of the boundary layer between the cavitation region  21  and the water  20  may cause at least a portion of the shockwave  26  to be reflected generally away from the protected asset  18 , as indicated by arrows A. 
     The leading portion  82  of the cavitation region  21  may be generally convex shaped, and also act as a lens, refracting and diffusing the shockwave  26  as it passes through the cavitation region, as indicated by arrows B. As the shockwave passes through the cavitation region  26 , it may deform the bubble or cavity, which may absorb energy both as potential energy associated with deforming the bubble or cavity (i.e, distorting the bubble increases its surface area, and the concomitant increase in surface tension absorbs more energy). Also, deformation of the bubble or cavity causes it to oscillate, which absorbs kinetic energy from the shockwave. 
     As the shockwave  26  exits the cavitation region  21 , the trailing interface  84  of the region may be curved and further refract the shockwave so its energy spreads over an even wider solid angle, as shown by arrows C. Another attenuation factor is the forward motion of the water  20  due to momentum imparted by the passage of the projectile  19 . Such motion may further slow the shockwave, refracting it as well as reducing its intensity. Although a single projectile  19  and its associated cavitation region may not attenuate a shockwave  26  significantly, the collective effect of multiple projectiles, creating multiple cavitation regions  21  and bubbles, may have a significant effect. 
     While the methods and systems disclosed herein constitute preferred aspects of the disclosed shockwave attenuation apparatus and method, other methods and forms of apparatus may be employed without departing from the scope of the invention. Further, the disclosed methods and systems may be used alone, or in combination with other known defensive systems.