Patent Publication Number: US-7712419-B1

Title: Hand grenade fuze

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 USC 119(e) of U.S. provisional patent application No. 60/747,448 filed on May 17, 2006, which application is hereby incorporated by reference. 

   STATEMENT OF GOVERNMENT INTEREST 
   The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
   BACKGROUND OF THE INVENTION 
   The invention relates in general to hand grenades and in particular to fuzes for hand grenades. 
   Grenades, such as the M67 fragmentation hand grenade, are widely used in the field by the US Army and US Marine Corps. The present fuze for the M67 hand grenade does not comply with the Insensitive Munitions (IM) requirements. This problem relates to the C70 Detonator used in the fuze. The detonator contains large quantities of lead-based primary explosives (lead azide, lead styphnate) as well as RDX. These explosives initiate the grenade under many of the IM test conditions. Safety issues, in combination with the environmental compliance requirements for use of lead compounds, make the manufacture of these detonators an unattractive investment for US-based manufacturers. 
   A need exists for a new hand grenade fuze. The new fuze must not impact the lethality of the present design (M67), while meeting the IM safety requirements. It also should reduce the item&#39;s total life-cycle cost, and the soldiers&#39; and environment&#39;s exposure to lead. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a hand grenade fuze with improved IM performance. 
   It is another object of the invention to provide a hand grenade fuze that contains much less lead than existing fuzes. 
   One aspect of the invention is a detonator for a hand grenade fuze comprising a detonator case; a slider that is transversely reciprocable in the detonator case from an unarmed position to an armed position, the slider including a longitudinal through-hole filled with a primary explosive; a spring that biases the slider to the unarmed position; an arming mechanism attached to the slider for moving the slider to an armed position, the arming mechanism comprising a shape memory alloy; an explosive lead disposed below the slider; and a booster charge disposed below the explosive lead. The arming mechanism may comprise a pull strip or rod that shrinks when heated. 
   Another aspect of the invention is a grenade fuze comprising the inventive detonator and a delay mix disposed above the slider. 
   A further aspect of the invention is a grenade comprising the inventive grenade fuze. 
   Yet another aspect of the invention is a method of arming the inventive grenade comprising igniting the delay mix; heating the arming mechanism with heat from the delay mix; and using the arming mechanism to move the slider in-line with the explosive lead. 
   Still another aspect of the invention is a method comprising externally heating the grenade until the arming mechanism moves the slider in-line with the explosive lead; and further externally heating the grenade until the arming mechanism melts and the spring forces the slider out-of-line with the explosive lead. 
   The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
       FIG. 1  is a side view of a known hand grenade. 
       FIG. 2  is a sectional view of a known hand grenade fuze. 
       FIG. 3  is a sectional view of one embodiment of the inventive hand grenade fuze. 
       FIGS. 4A and 4B  are enlarged views of a portion of  FIG. 3  showing the fuze in an unarmed and armed state, respectively. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To produce an IM compliant detonator, the invention uses an out-of-line slider with a memory metal mechanism that acts as the arming mechanism for the fuze train. When the safety pin is pulled, the delay column is set off. Heat produced by the burning of the delay column activates the memory metal mechanism, which moves the out-of-line slider to an in-line position. The memory metal mechanism may take a variety of forms, such as strips, rods, etc. 
   With memory metal strips, heat produced by the delay column causes the memory metal strips to push or pull the slider to the in-line position, thereby initiating the primary explosive. The primary explosive initiates the lead explosive, which sets off the booster. In one embodiment, a memory metal restraining rod shrinks due to the heat produced from the delay column, thereby releasing the slider. The primary explosive is a greatly reduced quantity (compared to known detonators) of lead azide or a heavy-metal free compound. The invention may use the existing C70 detonator case. 
     FIG. 1  is a side view of a known hand grenade  10 , specifically the M67 hand grenade. The M67 hand grenade is a traditional pull-pin grenade.  FIG. 2  is a sectional view of the known M213 fuze  14  for the M67. Pulling the pin  12  ( FIG. 1 ) in the fuze  14  releases the spoon and the hammer (not shown), which hits the primer  16  at the top of the fuze body  18 . The primer  16  then ignites the delay mix  20 . The delay mix  20  burns several seconds before initiating the attached C70 detonator  22 . 
   The C70 detonator  22  includes a column of lead styphnate  24 , lead azide  26  and RDX  28  in a detonator case  30 . The detonator  22  is massive, containing approximately 10 times more lead styphnate  24 , lead azide  26 , and RDX  28  than other detonators. The massive size of the detonator  22  is not simply a case of over engineering. The length of the detonator  22  is required to properly initiate the grenade&#39;s explosive fill for proper fragmentation. The diameter of the detonator  22  is dictated by the dimensions of the fuze body  18 . 
   The known fuze train is simple and has functioned well and reliably in grenades for decades. Unfortunately, the known fuze train has major safety issues. Any unwanted stimulus that causes the primer  16  to function, like fire, initiates the entire fuze train. The large quantities of primary explosive in the detonator  22  can also be detonated by external stimuli with enough energy to function the entire grenade  10 . These problems are exacerbated through sympathetic detonation when many grenades are in the same location. 
     FIG. 3  is a sectional view of one embodiment of an inventive hand grenade fuze  40 .  FIGS. 4A and 4B  are enlarged views of a portion of  FIG. 3  showing the fuze  40  in an unarmed and armed state, respectively. Fuze  40  replaces the contents of the C70 detonator  22  with an out-of-line detonator  38  including a booster  46 . The fuze  40  uses the detonator case  30  to preserve the fragmentation pattern of the grenade. While the size of the detonator case  30  is large compared to other detonators, it only has an approximate inner diameter of 0.3 inches, so extremely small interior parts are required. Detonator  38  slides the explosive  52  in line only when the delay mix  20  functions under normal conditions. Detonator  38  has drastically smaller quantities of primary explosives. Fuze  40  is expected to have improved IM performance and can use heavy metal-free primary explosives, thereby eliminating lead from the grenade. 
   In the embodiment of  FIG. 3 , detonator  38  includes a detonator case  30 , a slider assembly  42 , an explosive lead  44  and a booster  46 . The slider assembly  42  comprises a slider  48  that is generally transversely reciprocable in detonator case  30 , a spring  54  that biases the slider  48  out-of-line with the delay mix  20  and explosive lead  44 , and an arming mechanism  56  made of shape memory alloy (SMA). Slider  48  has a vertical through-hole  50  filled with primary explosive  52 . An explosive lead  44  and booster charge  46  may both be PBXN-5, for example. 
   Arming mechanism  56  may comprise one or more of pull strips, rods, or other suitable shapes. Any shape memory arming mechanism may be used as long as it responds to the heat generated by the delay mix  20  by moving the slider  48  to the in-line position. For simplicity, it is assumed that the arming mechanism  56  comprises pull strips. 
   In the unarmed state,  FIG. 4A , the spring  54  biases the slider  48  to one side such that the primary explosive  52  is not aligned with either the delay mix  20  or the explosive lead  44 . In the armed state,  FIG. 4B , the pin  12  has been pulled and the primer  16  has ignited the delay mix  20 . Heat from the ignition of the delay mix  20  causes the SMA pull strips  56  to shrink. The shrinkage of the SMA pull strips  56  overcomes the force of spring  54  and pulls the slider  48  to a position where the primary explosive  52  is aligned with the delay mix  20  and the explosive lead  44 . The delay mix  20  sets off the primary explosive  52 , which sets off the explosive lead  44 , which ignites the booster  46 . The booster  46  initiates the explosive fill in the grenade. 
   To eliminate lead from the fuze  40 , a lead-free primary explosive  52  must be used in place of the lead styphnate  24  and lead azide  26 . An exemplary lead-free primary explosive  52  is cyanuric triazide (referred to as triazide). Triazide is a heavy metal-free material containing three azide groups bonded to a ring of carbon and nitrogen. Tests have shown triazide to behave like a primary explosive and have demonstrated its ability to function as part of a fuze train. 
   One IM test involves shooting a bullet into the most sensitive portion of a munition. The most sensitive portion of the inventive fuze  40  is the primary explosive  52 . If a bullet is shot into the triazide with the slider  48  in the unarmed position,  FIG. 4A , the triazide is expected to ignite but neither the delay mix  20  or the explosive lead  44  should be initiated because the triazide is not in the armed position. 
   Another IM test is cook-off. Cook-off of a grenade with fuze  40  initially results in the pull strips  56  pulling the slider  48  into the armed position,  FIG. 4B . The material comprising the pull strips  56  is selected so that the melting temperature of the strips  56  is significantly less than the initiation temperature of any of the explosives in the grenade. Therefore, as the temperature increases, the pull strips  56  will melt and fail and the spring  54  will force the slider  48  back to the unarmed position,  FIG. 4A . Then, when the primary explosive  52  finally ignites, it is out of line with both the delay mix  20  and the explosive lead  44 . 
   While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.