Abstract:
A multi-mode fuze for a munition has at least one sensor that generates an electrical output dependent on a rate of deceleration when the munition impacts a target, a logic circuit electrically coupled to the at least one sensor effective to discriminate between a soft target and a hard target dependent on the electrical output and a fuze that transmits a detonation signal to an initiating explosive to thereby detonate the munition. The detonation signal is transmitted at a time dependent on target discrimination. The multi-mode fuze of the invention may be incorporated into an explosive projectile that includes an aerodynamically shaped metallic casing, an explosive contained within the metallic casing and an initiating explosive contacting the explosive. The multi-mode fuze communicates with the initiating explosive to trigger detonation of the explosive either on impact with a hard target or following a delay on impact with a soft target.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION  
       [0001]    This patent application relates to and claims priority to U.S. Provisional Patent Application Serial No. 60/341,157 entitled “Medium Caliber High Explosive Dual-Purpose Projectile With Dual Function Fuze” by F. M. Bone that was filed on Dec. 14, 2001. The subject matter of that provisional patent application is incorporated by reference herein in its entirety. 
     
    
     
       STATEMENT OF GOVERNMENT RIGHTS  
         [0002]    This invention was conceived and developed under internal research and development finding and was not government funded.  
         BACKGROUND  
         [0003]    1. Field of Invention  
           [0004]    This invention relates to a fuze for initiating detonation of an explosive projectile. More particularly, the fuze includes a target-sensing element that is capable of varying detonation delay time in response to the hardness of the target.  
           [0005]    2. Description of Related Art  
           [0006]    The current maximum effective range angle for ground attack aircraft using medium caliber ammunition is 4000 to 5000 feet at a slant angle of between 5° and 15°. Typically, the munition employed by the ground attack aircraft is an armor piercing incendiary (API) projectile, the effectiveness of which depends on kinetic energy. API projectiles are effective against hard (armor plated) targets but that effectiveness is reduced against softer, unarmored, threats because the energy is not distributed within the target. The current maximum effective range places the attacking aircraft in, danger by exposing the pilot and airplane to small arms fire and man-portable anti-aircraft missiles.  
           [0007]    Chemical energy projectiles produce effective terminal shaped charge or explosively formed fragments. Though not used for fixed wing ground attack aircraft, the projectiles can be effective at extended ranges, 9000 to 12,000 feet, for armored target defeat. The chemical energy projectiles are less effective against soft targets due to the near instantaneous reaction of the fuze that distributes energy on the surface of the threat.  
           [0008]    One factor that influences the effectiveness of an explosive munition against a target is the detonation delay following impact. When a target is relatively soft, for example, if detonation occurs after the projectile has entered the target, damage is enhanced by placing the blast, fragments and incendiary effect inside the target. In contrast, hard targets are more effectively defeated by detonating a munition on impact with the target surface to produce a chemical defeat by plasma jet or explosively formed fragment. Accordingly, explosive projectiles frequently include a fuze that is capable of delaying detonation dependent on the most effective impact of the projectile.  
           [0009]    One such fuze is disclosed in U.S. Pat. No. 5,872,324 by Watson et al. A tri-mode fuze assembly includes a casing, that serves as a structural containment for a booster pellet, which, in turn, provides the initiation of a main warhead. A hard target impact detonator is located on a forward end of the fuze assembly providing a mechanical, instantaneous detonation of the warhead in the event that the target impact results in a physical crushing of the warhead. A second detonator located within the casing provides a pyrotechnic timer-operated delay for penetration of hardened targets. This timer is initiated by initial impact and continuous deceleration of the warhead when the integrity of the warhead is maintained. A third detonator provides an instantaneous, void-sensing detonation capability for the fuze. Operation of the third detonator is initiated by initial impact and the interruption of the continuous deceleration required by the delay detonator. Once target penetration begins, any variation in the rate of deceleration results in an immediate initiation of the main warhead by the void sensor.  
           [0010]    U.S. Pat. No. 5,872,324 utilizes a firing pin to detonate the explosive when an impenetrable target is encountered and a sensitive pyrotechnic time delay for hard but penetrable targets. In the case of soft targets, a mechanical apparatus requiring very strict adherence to small manufacturing tolerances is used to sense deceleration. Successful actuation of a mechanical firing pin is dependent on axial loading to drive the pin into a stab detonator. If the pin enclosure receives a side load due to high oblique impact, the energy can crush the pin or produce a load that is not along the centerline to cause sufficient pin motion to perforate the stab detonator. In addition to impact reliability, the difficulty in manufacturing these components, temperature and launch loads can cause unwanted variations in performance.  
           [0011]    U.S. Pat. No. 5,255,608 by Min et al. discloses an intelligent hard-target weapon providing a real-time determination of target consistency during weapon penetration. Input signals are provided by an accelerometer used as a primary sensor. On-line concurrent processing of the data of a specific length facilitates a few different modes of feature extraction. The processor provides a robust, real-time decision making for the fuze utilizing sensor signals (accelerometer data). The feature sets utilized include (1)amplitude profiles of the signals, (2) their derivative profiles, and (3), the measure of their abrupt changes. The purpose is to provide for detonation at the proper point as the high-speed penetrator passes through various layers such as concrete, steel, dirt, sand, etc. on its way to a valuable buried target. Real-time decision making is provided for the fuze utilizing accelerometer data.  
           [0012]    U.S. Pat. No. 4,799,427 discloses an ignition device for a projectile, in particular a guided missile, where the ignition moment is controllable as a function of the impingement delay and of the flight time of the projectile. This allows compensation for the type of material comprising the target, e.g., hard or soft, and for the amount of time the projectile has been airborne, thus compensating for reduced projectile velocity at the time of striking the target.  
           [0013]    U.S. Pat. Nos. 5,872,324, 5,255,608, and 4,799,427 are incorporated by reference in their entirety herein.  
           [0014]    U.S. Pat. No. 4,375,192 by Yates et al., that is incorporated by reference in its entirety herein, discloses a fuze which is designed to initiate detonation upon penetration of a target a preselected distance or after a preselected number of cavities have been perforated by the warhead. Other, default or salvage, modes are based upon breakup of the warhead, or ricochet.  
           [0015]    In the abovementioned patents, unreliable or expensive mechanical or pyrotechnic means are used to trigger detonation, or complex algorithms are employed to determine the status of the projectile&#39;s penetration. There remains, therefore, a need for an easily manufactured and reliable fuze that is capable of initiating detonation on impact with a hard target or after a time delay on impact with a soft target. This fuze is incorporated into a projectile that combines the terminal effects of a chemical energy projectile for hard target defeat with a delayed reaction to effectively defeat soft targets.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, it is an object of the invention to provide a reliable fuze capable of detonating a projectile instantaneously or after a time delay based on the magnitude of deceleration of the projectile.  
           [0017]    It is a feature of the invention that the fuze is useful for projectiles impacting both hard and soft targets. It is another feature that accelerometers are used to detect projectile deceleration and communicate the magnitude to fuze logic that determines whether to detonate the projectile instantaneously or after a time delay.  
           [0018]    It is an advantage of the invention that the use of accelerometers and solid state logic to determine detonation makes the fuze more reliable and less expensive than other fuzes. It is a further advantage that terminal effects of the projectile are maximized.  
           [0019]    In accordance with the invention, there is provided a multi-mode faze for a munition having at least one sensor that generates an electrical output dependent on a rate of deceleration when the munition impacts a target, a logic circuit electrically coupled to the at least one sensor effective to discriminate between a soft target and a hard target dependent on the electrical output and a fuze that transmits a detonation signal to an initiating explosive to thereby detonate the munition. The detonation signal is transmitted at a time dependent on target discrimination.  
           [0020]    The multi-mode fuze of the invention may be incorporated into an explosive projectile that includes an aerodynamically shaped metallic casing, an explosive contained within the metallic casing and an initiating explosive contacting the explosive. The multi-mode fuze communicates with the initiating explosive to trigger detonation of the explosive either on impact with a hard target or following a delay on impact with a soft target.  
           [0021]    The above stated objects, features and advantages will become more apparent from the specifications and drawings that follow. 
       
    
    
     IN THE DRAWINGS  
       [0022]    [0022]FIG. 1 illustrates in partial cross-section representation a projectile of the invention having a nose located dual function fuze.  
         [0023]    [0023]FIG. 2 illustrates in cross-sectional representation a projectile of the invention having a base located dual function fuze.  
         [0024]    [0024]FIG. 3 graphically illustrates the rate of deceleration following impact with either a soft or a hard target.  
         [0025]    [0025]FIG. 4 is a block diagram Illustrating the application of a fuze logic sequence.  
         [0026]    [0026]FIG. 5 graphically illustrates the relationship between impact acceleration and voltage output for a piezoelectric crystal. 
     
    
     DETAILED DESCRIPTION  
       [0027]    25-mm through 76-mm medium caliber ammunition is used on a number of existing and future gun systems including assault vehicles, amphibious assault vehicles, fixed wing aircraft, ships and tanks. The targets may be soft targets such as lightly armored vehicles, including personnel carriers, trucks and airplanes, ground support communication stations and radar installations. These targets are generally supported by 0.039 inch (1 mm) aluminum in the form of aircraft or 0.039 to 0.250 inch (1.0 to 6.4 mm) steel for vehicles and ground support equipment. Other targets are hard targets such as heavily armored vehicles, tanks and bunkers. These targets are generally supported by 0.5 to 1.5 inch (12.7 to 38.0 mm) rolled homogeneous armor (RHA) plate with a hardness in the range of 300 to 360 BHN (Brinell Hardness Number).  
         [0028]    BHN is a number related to the applied load and to the surface area of the permanent impression made by a ball indenter computed from the equation: 
           BHN =2 P/πD (( D −( D   2   −d   2 ) 1/2 )) 
         [0029]    Where P is the applied load in kgf, D is the diameter of ball in mm, and d is mean diameter of the impression in mm.  
         [0030]    [0030]FIG. 1 illustrates in partial cross-section representation a projectile  10  of the invention having a nose-located dual function fuze. The projectile  10  has a metallic casing  12 , typically formed from steel, that forms fragments when an explosive  14  within the casing  12  is detonated. The fragments enhance terminal effects within the threat along with a shaped charge jet or explosively formed fragment as described below. One suitable explosive  14  is a plastic bonded explosive (PBX).  
         [0031]    Housed within a nose portion of the projectile  10  is a soft target sensing element  16  that may be a mechanical switch or a piezoelectric crystal. When the projectile  10  impacts a relatively soft, unarmored, target such as an aircraft or ground support equipment, deformation of the nose  18  is small as compared to hard target engagement. In one embodiment, the soft target nose deformation closes a mechanical switch in the soft target sensing element  16 . This switch starts a timer to delay projectile  10  reaction until the projectile is well inside the target. A suitable delay is from 150 to 300 microseconds.  
         [0032]    Alternatively, the soft target sensing element  16  includes a piezoelectric crystal having an output proportional to the impact shock wave carried into the projectile nose  18 . The signal wave form from the piezoelectric crystal is analyzed by a logic circuit contained within fuze  20  to start the time delay for reaction inside the target.  
         [0033]    Further, the piezoelectric crystal in soft target sensing element  16  can be used to detect harder targets as described below. This approach simplifies projectile design and enhances reliability by using one sensing piezoelectric crystal located in the projectile nose  18 .  
         [0034]    Located rearward of the of the soft target sensing element  16  is a hard target sensing element  22 . Typical armored or hardened threats that can be effectively engaged with medium caliber ammunition are protected with between 0.5 inch and 1.5 inch of Rolled Homogeneous Armor Plate with a hardness ranging between 300 and 360 BHN. The harder target resistance increased projectile nose  18  deformation actuating the hard target sensing element. Hard target sensing element  22  may be a mechanical switch or second piezoelectric crystal that sends a signal to the logic circuit of the fuze  20  when nose  18  deformation reaches the hard target sensing element. Alternatively, as described above, a single piezoelectric crystal may be utilized that generates a different waveform from that generated on soft target impact and the fuze logic discriminates between the two.  
         [0035]    One suitable piezoelectric device for determining acceleration is a piezoelectric element such as those made by Kinetic Ceramics, Inc. which has an output proportional to the impact shockwave carried into the projectile nose  18 . The signal waveform from the piezoelectric element is analyzed by the fuze logic to instantaneously detonate the projectile or start the time delay.  
         [0036]    An effective way to defeat a hard target is with a penetrating jet that can be either a shaped charge plasma jet or an explosively formed fragment. Shaped charge liner  24  is formed from a suitable liner material such as copper, tantalum or tungsten. Disposed rearward of the convex surface of the shaped charge liner  24  is the explosive  14 . When detonated, the explosive generates a shock wave that collapses the liner expelling a plasma jet formed from liner material forwardly from the projectile  10 . There is a set-off distance between the shaped charge liner and the target at which the plasma jet has maximum momentum (a combination of jet length and jet speed). A distance “d” between the hard target sensing element  22  and shaped charge liner  24  is set such that the liner is collapsed as close to the set-off distance from the target as possible. A more detailed explanation of shaped charge liners is found in U.S. Pat. No. 6,393,991 to Funston et al., that is incorporated by reference in its entirety herein.  
         [0037]    If the hard target sensing element  22  generates a signal, the fuze logic overrides any delay remaining from the soft target sensing element  18  signal to insure the shaped charge liner is collapsed at approximately the set-off distance.  
         [0038]    Additional elements of the projectile  10  include a safe and arm device  26  to prevent premature detonation and detonation of projectiles that miss the target. The projectile  10  must be safely armed before the fuze is activated. The arming of the projectile a safe distance after expulsion from a launch muzzle may achieved by a combination of a mechanical action out-of-line rotor supplemented by an electrical timer. Predetermined levels of linear acceleration, commonly referred to as setback, and radial forces, commonly referred to as spin load, must be met to satisfy the dual environment safe and arm functions for aligning a primary fuze rotor enclosed detonator with a secondary fuze energetic element or booster. After mechanical safe and arm conditions are satisfied, the arming distance is further extended by an electronic time delay started at launch. After approximately 0.5 second time of flight from the muzzle, the electrical circuit for fuze function is closed to await a detonation signal. If no signal is received within the time required for the projectile to reach the target plus some margin of error, the projectile is disarmed.  
         [0039]    An initiating explosive  28 , such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) is detonated by an electric signal received through leads  30  transmitted from fuze  20 . The shock wave from detonation of the initiating explosive  28  detonates explosive  14 .  
         [0040]    [0040]FIG. 2 illustrates in cross-sectional representation a projectile  40  in accordance with a second embodiment of the invention having a base-located dual function fuze. A number of the elements of this projectile are similar to the elements of preceding projectile  10  and those similar elements are identified by like reference numerals. Communicating with, and preferably contained within, fuze  20  is an accelerometer  42 . The accelerometer is capable of detecting the rate of projectile deceleration and generating an electrical signal proportional to the rate of deceleration. Accelerometer  42  may be a mechanical or piezoelectric device, but micromechanical systems (MEMS) are preferred.  
         [0041]    MEMS is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate using microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. MEMS accelerometers are typically much smaller, more functional, lighter, more reliable, and are sold for a fraction of the cost of the conventional macroscale accelerometer elements.  
         [0042]    While the MEMS accelerometer has been disclosed in combination with a base-loaded fuze, the MEMS accelerometer may also be utilized with a nose-load fuze system as well.  
         [0043]    [0043]FIG. 3 graphically illustrates the rate of deceleration following impact with either a soft target (reference line  52 ), a hard target (reference line  54 ) or a miss (reference line  50 ). The velocity of a projectile in normal flight experiences relatively smooth acceleration due to variables such as gravity or drag force (a function of the velocity squared). Once the flight has exceed a first threshold time (reference point  56 ) the projectile is armed. If the flight exceeds a second threshold time (reference point  58 ) without a sudden decrease in velocity, a miss is determined and the projectile is disarmed.  
         [0044]    When the projectile impacts a soft target, a calculation of the rate of change of acceleration, Δa, over a time interval, Δt, yields a first value for Δa. When the projectile impacts a hard target, the value of Δa is considerably larger. A simple embodiment of the fuze logic algorithm can be described as follows.  
         [0045]    If |Δa|≧× then y=0;  
         [0046]    If |Δ|&lt;× then y=150 microseconds.  
         [0047]    Where a equals acceleration of projectile;  
         [0048]    x equals the predetermined threshold magnitude for discriminating between a hard target and a soft target. X must exceed some minimal value to indicate that a target has been impacted.; and  
         [0049]    y equals the time delay.  
         [0050]    Considering a maximum impact velocity at 3000 feet per second (914.4 meters per second), a sampling rate of 1 microsecond is adequate to assure correct logic function prior to destruction of the sensing elements of the projectile.  
         [0051]    The fuze logic is best understood with reference to FIG. 4. The fuze logic may utilizes a pre-programmed microprocessor such as those made by KDI Precision Products, Inc. of Cincinnati, Ohio. The microprocessor is a solid state device powered by electrical energy provided by a set back generator such as those made by Miltec SA. The electrical energy stored in a capacitor sets the threshold levels in the fuze for delay or instantaneous reaction decision. The microprocessor with storage capacitor is encapsulated in a molded polymer to resist the affects of acceleration and spin loads.  
         [0052]    An accelerometer  42  is electronically connected to fuze logic  60 . The accelerometer  42  is capable of transmitting a proportional signal  62  to fuze logic  60 . Fuze logic  60  receives proportional signal  62 , compares it to magnitude  64 , and transmits either: (1) signal  66  instantaneously to detonate the initiating explosive  28  if signal  62  meets or exceeds magnitude  64 , or (2) signal  68  after a time delay  70  if signal  4  falls below magnitude  64 .  
         [0053]    A medium caliber projectile will defeat a soft target with impact obliquities up to 75 degrees NATO. At these obliquities, the axial component of the acceleration should be adequate to trigger detonation. For a hard target or a greater obliquity however, a triaxial sensing element would be useful to assure function if the axial component is very small.  
         [0054]    The use of a single piezoelectric crystal to determine target type and to provide that information to the fuze logic is illustrated by the Example that follows.  
       EXAMPLE  
       [0055]    A piezoelectric crystal from Kinetic Ceramics, Inc. of Hayward, Calif. having a rated sensitivity of 0.37 mV/g was incorporated into a simulated projectile. Weights were dropped on the nose of the projectile from varying heights to simulation impact accelerations of varying g-forces. An impact acceleration force of from 1,000 g to 10,000 g was deemed to simulate impact with a soft target and an impact acceleration in excess of 20,000 g was deemed to simulate impact with a hard target. The voltage generated by the piezoelectric crystal following these simulated impact was recorded. As shown from FIG. 5, the target type was readily determined from the voltage output. An output of about 3 volts or less corresponded to a soft target and an output of about 4.4 volts or more corresponded to a hard target. There was a standard deviation in the measured voltages of about ±17%.  
         [0056]    It is apparent that there has been provided in accordance with the invention a fuze that fully satisfies the objects, features and advantages disclosed hereinabove. While disclosed in accordance with specific embodiments of the invention, it is apparent that many alternatives, modifications and variations are equally applicable to the invention and these alternatives, modifications and variations are equally encompassed within the scope of the claims that follow.