Patent Publication Number: US-2004047238-A1

Title: Acoustic array deployment system and method

Description:
STATEMENT OF GOVERNMENT INTEREST  
     [0001] 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. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] (1) Field of the Invention  
       [0003] The present invention relates generally to deployment of acoustic arrays underwater, and more particularly to an acoustic array and deployment system and method for deploying an acoustic array from a moving underwater vehicle such as a supercavitating torpedo.  
       [0004] (2) Description of the Prior Art  
       [0005] Unmanned underwater vehicles typically have acoustic array sensing systems mounted in their nose structures. However, the nature of some underwater vehicles precludes the use of such sensing arrangements. For example, a supercavitating torpedo generates a cavitation bubble about the torpedo. The noise generated by the cavitation bubble severely reduces the effectiveness of any acoustic array mounted anywhere on the supercavitating torpedo. Thus, acoustic sensing for a supercavitating torpedo must be accomplished outside of the torpedo&#39;s cavitation bubble.  
       [0006] One approach for getting an acoustic array away from a vehicle is to tow it behind the vehicle. In the case of the supercavitating torpedo, this does get the acoustic array physically away from the cavitation bubble. However, since the towed array will be in line with the torpedo and its intended target (that is aligned with the torpedo&#39;s direction of travel), the cavitation bubble surrounding the torpedo will tend to mask the target in the acoustic array&#39;s endfire beam.  
       SUMMARY OF THE INVENTION  
       [0007] Accordingly, it is an object of the present invention to provide a method and system for deploying an acoustic array from an underwater vehicle.  
       [0008] Another object of the present invention is to provide a method and system for deploying an acoustic array from a supercavitating underwater vehicle to minimize the effect of the cavitation bubble surrounding the underwater vehicle on the acoustic array.  
       [0009] Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.  
       [0010] In accordance with the present invention, an acoustic array deployment system for an underwater vehicle is provided along with a method for carrying out such deployment. A weighted projectile is housed onboard the underwater vehicle. An optical fiber based acoustic array has a first end thereof coupled to the weighted projectile and has a second end thereof coupled to the underwater vehicle. Means are provided for maintaining the acoustic array in a spooled configuration while the weighted projectile is housed onboard the underwater vehicle. Means are also provided for launching the weighted projectile from onboard the underwater vehicle in a gravitationally downward direction that is not aligned with the underwater vehicle&#39;s direction of travel. As a result, the acoustic array is paid out from its spooled configuration as the weighted projectile is launched and then sinks under its own weight. By establishing the line array&#39;s broadside beam in a direction that is away from the vehicle and approximately perpendicular to the vehicle&#39;s direction of travel, the relative angular difference between the underwater vehicle and target can be used to guide the vehicle towards the target. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:  
     [0012]FIG. 1 is a schematic view of a portion of an underwater vehicle housing an acoustic array deployment system in accordance with the present invention;  
     [0013]FIG. 2 is a schematic view of a portion of the acoustic array deployment system after the acoustic array has been deployed; and  
     [0014]FIG. 3 is an isolated view of a portion of the acoustic array utilizing an amplification coating. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
     [0015] Referring now to the drawings, and more particularly to FIG. 1, a portion of an underwater vehicle  10  is illustrated. For purposes of the current description, underwater vehicle  10  is assumed to be traveling in the water along a direction of travel indicated by arrow  12 . While the type of underwater vehicle  10  is not a limitation of the present invention, the method and system for deploying an acoustic array that will be described herein is particularly useful for a supercavitating underwater vehicle (e.g., a torpedo) that generates a cavitation bubble (not shown) therearound as it moves through the water.  
     [0016] Housed within underwater vehicle  10  is a small weighted projectile  20 . More specifically, projectile  20  is housed in a launch tube  14  that is formed in a side of underwater vehicle  10 . Launch tube  14  is configured such that its launching axis  14 A will typically be approximately perpendicular to a longitudinal axis  10 A of underwater vehicle  10  (which is always parallel to direction of travel  12 ). It is to be understood that launching axis  14 A could be offset from strict perpendicularity with longitudinal axis  10 A by as much as 5-10° without departing from the scope of the present invention.  
     [0017] As used herein, the term “weighted projectile” is meant to define any body/weight that, after being launched from launch tube  14 , will be forcibly propelled through the water and then sink under its own weight. Thus, the particular size/shape/weight of projectile  20  can be adapted for a particular application.  
     [0018] An acoustic array to be deployed by the present invention is maintained in a spooled configuration prior to its deployment. In the present invention, it is preferred for the acoustic array to be constructed from an optical fiber to minimize size and weight considerations. In the illustrated example, the acoustic array is maintained in its spooled configuration using a small spool  22  mounted onboard weighted projectile  20  and a large spool  16  mounted onboard underwater vehicle  10 . More specifically, some of an acoustic array  30  is spooled on small spool  22  and some is spooled on large spool  16 . Note that either end of acoustic array  30  is coupled to a respective one of spools  16  and  22 . Further, the end of acoustic array  30  onboard underwater vehicle  10  will also be optically coupled to an acoustic array optical system  32  as would be understood in the art.  
     [0019] The use of two spools minimizes tension in acoustic array  30  when it is deployed. In addition, where deployment tension in, or deployment speed of, acoustic array  30  is a concern, motors  18  and  24  can be coupled to spools  16  and  22 , respectively, to actively pay out acoustic array  30  during deployment thereof. For example, motor  18  must pay out fiber fast enough to make up for the terminal velocity of projectile  20  and the forward velocity of underwater vehicle  10 .  
     [0020] Weighted projectile  20  is forcefully ejected from launch tube  14  by means of one or more propulsion systems. For example, a launcher  40  can be installed in launch tube  14  for purposes of supplying a launch force to weighted projectile  20 . Launcher  40  can be any type of launcher (e.g., spring-loaded ram piston, hydraulic ram piston, hydraulic pressure-based launcher, pneumatic pressure-based launcher etc.) that applies its launch force to weighted projectile  20  while remaining with underwater vehicle  10 . Additionally or alternatively, a rocket motor  42  (or other propulsion means) can be coupled to the aft end of weighted projectile  20 . In all cases, the force supplied to weighted projectile  20  should be sufficient to carry it through any cavitation bubble surrounding underwater vehicle  10 .  
     [0021] In operation, underwater vehicle  10  moves through the water along direction of travel  12 , and is oriented such that launch tube  14  faces downward, i.e., towards the sea floor. When acoustic sensing is desired, weighted projectile  10  is ejected from launch tube  14  in a direction that is substantially aligned with the earth&#39;s gravitational pull. The forceful ejection and subsequent sinking of weighted projectile  20  causes acoustic array  30  to be paid out (as shown in FIG. 2) in a direction that, in general, is not aligned with direction of travel  12 . Preferably, acoustic array  30  makes an angle with direction of travel  12  that is approximately perpendicular thereto or within approximately 5-10° of such perpendicularity. In this way, the broadside beam of acoustic array  30  is not affected by acoustic noise generated by, for example, a cavitation bubble surrounding underwater vehicle  10 .  
     [0022] As mentioned above, acoustic array  30  is preferably an optical fiber-based acoustic array. As such, optical hydrophones  34  are integrated in the array&#39;s optical fiber  36  at spaced apart locations therealong. As is known in the art, each of optical hydrophones  34  can be constructed as a Bragg grating or can be configured to produce Rayleigh scattering (at the hydrophone&#39;s location) of laser pulses passed along optical fiber  36 . The origin of such laser pulses would be acoustic array optical system  32 .  
     [0023] The advantages of the present invention are numerous. An acoustic array can now be deployed outside of a cavitation bubble in a direction that allows a target to be resolved irrespective of the noise generated by the cavitation bubble. The substantially vertical line array formed by the present invention should have sufficient vertical directivity to separately resolve both the underwater vehicle and the target in the array&#39;s broadside beam. The relative angular difference between the underwater vehicle and target can be used to guide the vehicle towards the target.  
     [0024] Although the present invention has been described relative to a specific embodiment thereof, it is not so limited. For example, as illustrated in FIG. 3, optical fiber  36  and integrated hydrophones  34  can be encapsulated or coated in a material  38  that amplifies acoustic pressure that impinges thereon. Such coating/encapsulation is disclosed in U.S. Pat. No. 4,979,798, the contents of which are hereby incorporated by reference. Thus, it will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described 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.