Abstract:
A system for determining the velocity of detonation of a mild detonation fuse mounted on the surface of a device includes placing the device in a predetermined position with respect to an apparatus that carries a couple of sensors that sense the passage of a detonation wave at first and second spaced locations along the fuse. The sensors operate a timer and the time and distance between the locations is used to determine the velocity of detonation. The sensors are preferably electrical contacts that are held spaced from but close to the fuse such that expansion of the fuse caused by detonation causes the fuse to touch the contact, causing an electrical signal to actuate the timer.

Description:
The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 

   CROSS REFERENCE TO RELATED APPLICATIONS 
   (Not Applicable) 
   BACKGROUND OF THE INVENTION 
   Mild detonating fuze (MDF) is a metal clad linear explosive usually drawn or extruded in a round configuration. Grain sizes can vary from 2.5 gr/ft to 100 gr/ft, and the metal sheath can be aluminum, tin, lead or silver. The diameter of MDF ranges from 40 mils or smaller to 250 mils or larger. 
   D. Smith, U.S. Pat. No. 6,561,101, shows a typical application where a length of MDF is used to delay a detonation after an input. The MDF is placed in a groove spiraling around the exterior surface of a metal case of a device. An electrically controlled igniter ignites one end of the MDF and the other end ignites an actuator. The length of MDF is chosen to provide a predetermined delay between ignition and actuation, the delay being a function of the length of MDF and the velocity at which the MDF detonates. 
   One problem that arises is that the velocity of detonation (VOD) for MDF may change over time. Such a change would change the actuation delay for devices such as ordinance that typically have a long shelf life between date of manufacture and date of use. For a quality check, VOD can be measured in a laboratory after the MDF is removed from a few stored devices. The average VOD for these devices is imputed to the remaining stored devices. However, handling the MDF introduces another variable that is not present in the stored devices, so the accuracy of this test is not certain. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to provide a system for measuring VOD directly from the stored devices without handling the MDF in those devices. 
   To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, is a method of determining velocity of detonation of a metal-clad detonation fuze that expands in diameter as it detonates along its length, comprising the steps of placing a sensor spaced from and adjacent a first location on the fuze and placing a second sensor spaced from and adjacent a second location on the fuze. The fuze is ignited at a third location and it detonates along its length past the first and second locations. The expanded fuze is detected by each sensor as the fuze detonates at that sensor&#39;s location; and the velocity of detonation is determined from the time difference between the actuation of each sensor. 
   Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. 
       FIGS. 1 and 2  show top and cutaway side views, respectively, of the test fixture holding a device prior to measurement of VOD. 
       FIG. 3  is an enlargement of a portion of  FIG. 2  during a measurement of VOD. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIGS. 1 and 2 , a test apparatus  10  and a device  50  to be tested are held together as discussed hereinafter. The test apparatus  10  includes at least two sensors  21   a ,  21   b  that are held at different positions slightly above an MDF  40  on device  50 . MDF  40  expands when ignited and sequentially actuates each sensor. The time between actuation of the first and second sensors, coupled with the distance between the sensors, provides a measurement of VOD in MDF  40 . 
   Apparatus  10  includes a body  20  that is preferably constructed from a rigid electrically insulating material. Body  20  is illustrated as being sized somewhat larger than the exterior dimensions of device  50  and has an interior surface  22  facing and interacting with device  50  and an opposed exterior surface  24 . Apparatus  10  may also include a clamp  30  for holding device  50  tightly in a predetermined position against interior surface  22 . 
   The particular device  50  illustrated includes a body formed as a metal block  52  having a generally rectangular shape rounded at one end  51  and squared at opposite end  53 . Block  52  has a uniform thickness around its perimeter  54  and a thinner inner portion  56 . This construction creates an open volume  55  between block portion  56  and body surface  22  when device  50  is clamped in position to body  20 . 
   As further illustrated, a length of MDF  40  extends from an input igniter  42  at one end to an output igniter  44  at the opposite end within a groove  58  (shown in  FIG. 3 ) formed in the surface of inner portion  56  that faces open volume  55 . These igniters are conventional in the art (typical igniters are discussed in the Smith &#39;101 patent) and may be embedded in holes inside device  50 , on the surface  56  of device  50 , or in any other location. The input igniter may be electrically actuated by a squib or similar device, while the output igniter typically is operationally connected to another explosive device. The output igniter and subsequent explosive device are not part of this invention. 
     FIGS. 2 and 3  illustrate the relationship among MDF  40 , groove  58 , and conductors  21   a, b , and  g . In order to provide for in situ measurement of the VOD, apparatus  10  further comprises at least two spaced conductors  21   a, b , for making contact with MDF  40  only after that MDF is ignited at end  42 . MDF  40  is seen in cross-section in the referenced Smith &#39;101 patent and  FIG. 3  to have a thin aluminum or other metal cladding  41  surrounding an explosive  43 . As illustrated under conductor  21   b  in  FIG. 3 , MDF  40  may be embedded in a room temperature vulcanizing adhesive  59  to hold it in place in groove  58 . As illustrated in  FIG. 2 , a first conductor  21   a  is spaced above MDF  40  at a location near ignition end  42  and a second conductor  21   b  is similarly spaced above MDF  40  at a second location that is further along MDF  40  from ignition end  42  than the location of first conductor  21 . The spacing is such that each conductor  21   a, b  does not make contact with cladding  41 . 
   As illustrated in  FIG. 3  with reference to conductor  21   a , MDF  40  expands as the MDF explosive burns. Since the spacing between each conductor  21   a ,  21   b  and MDF  40  is less than the distance that an ignited MDF expands, the expanded MDF makes electrical contact with conductor  21   a . A third conductor  21   g  can be placed in contact with ground, preferably by touching MDF  40  at a third location further along MDF  40  than the location of second conductor  22 . 
   In the disclosed embodiment, each conductor  21  is a conventional  4 - 40  metal screw extending through a threaded hole in body  20  and having a tapered end  22 . Each conductor further includes a tension nut  25  tightened against surface  24  through a conductive loop terminal lug  27  to put the screw under tension, so that it&#39;s depth may be accurately controlled. A wire  61  extends from each lug  27  to control circuitry  60 . This structure permits screws  21   a, b  to be tightened against MDF  40  to ensure that a circuit is closed, and then backed off ¼ turn to provide for a 0.006 inch gap between MDF  40  and screw ends  22   a,b . This gap is shown with respect to screw  21   b.    
   Alternatively, as is apparent to those skilled in the art, the conductors could be pins or any other elongated structure that tightly and adjustably fit through an insulating sleeve in a metal block. In addition, the pin or screw could be an electrically insulating material with only an electrically conductive portion at the end for contacting MDF  40 . 
   Device  50  must be precisely placed with respect to apparatus  10  in order for conductors  21  to contact MDF  40  at the desired locations. This precision can be attained by providing registering projections from interior surface  22  that interact with the edge of device  50  at different locations that limit the position of device  50  relative to interior surface  22  to the desired position. In the disclosed embodiment, projection  25  is an L-shaped member that positions one corner of device  50 , while projections  26  and  27  are pins that hold opposing sides while still permitting device  50  to be slid into the proper position. These projections are disclosed as metal projections from a plastic block  20 ; but they also could be plastic. One of ordinary skill will recognize that many other registering techniques also can be used, such as pins on either block  20  or device  50  that fit into holes on the opposing structure, or indentations formed in the interior surface  22  that conform to the outline of block  50 . 
   As disclosed, once device  50  is properly positioned against interior surface  22 , it is held in place by a clamp  30  that is formed by a rigid bar  32  that spans the back side  58  of block  50  and which is held by screws  34 ,  36  that are tightened into interior surface  22 . One of ordinary skill will recognize that many other holding techniques could be used, ranging from screws that extend through holes in device  50  into the interior surface  22  to other mechanical clamps that hold device  50  tightly against block  20 . 
   In the preferred embodiment of the invention, circuitry  60  is a Digital Igniter Tester from Valhalla Scientific Co., San Diego, Calif. This device accurately measures the resistance as pins  21   a  and  21   b  contact MDF  40 ). However, for purposes of understanding the invention, the disclosed circuitry provides the desired function. 
   The basic idea is that an electrical circuit is sequentially completed at each conductor  21   a ,  21   b  when the detonation wave moves along and expands MDF  40  to contact the conductor, as shown at  21   a  in  FIG. 3 . Conductor  21   g  is grounded against MDF  40  and its wire  61   g  provides the ground for circuitry  60 . A series battery Va and resistor Ra are connected to wire  61   a  and the ‘start’ input of digital timer  66 . A similar battery Vb and resistor Rb are connected to wire  61   b  and the ‘stop’ input of timer  66 . When either conductor is spaced from the MDF, as illustrated by end  22   b  in  FIG. 3 , no current flows through R and the voltage at timer  66  is V volts. However, when a conductor touches the MDF, as illustrated by end  22   a  in  FIG. 3 , the voltage at timer  66  is 0 volts. 
   To make a measurement in accordance with the invention, device  50  is clamped into apparatus  10  and screw end  21   g  is tightened against MDF  40 . Each one of screw ends  21   a  or  21   b  is similarly tightened against MDF  40  and, after a closed circuit is observed using circuitry  60 , the screw is backed off about ¼ turn to provide the desired gap. MDF  40  is then ignited and the time for the detonation wave to pass from screw  21   a  to screw  21   b  is determined by timer  66 , which timer is turned on when the wave causes MDF  40  to contact screw end  22   a  and turned off when the wave causes MDF  40  to contact screw end  22   b . Since the distance between these screws may be accurately measured, the VOD of detonation is easily calculated. 
   In one measurement made with the disclosed embodiment of the invention, the time for a detonation wave to move from conductor  21   a  to conductor  21   b  along 40 mil diameter MDF was 7.18 μs. Since the distance along MDF  40  was accurately determined to be 53.54 mm, the VOD is calculated to be 7.46 mm/μs. 
   The accurate measurement of the distance can be determined by replacing the MDF  40  with a drawn-hardened, identical diameter copper wire in device  50 . This copper wire segment is marked by lowering pins  21   a  and  21   b  to slightly indent the copper wire column. The copper wire column is removed from the device  50  and straightened. The distance between the indent marks on the copper wire is measured using a calibrated optical comparator. 
   It should be apparent that there are many modifications possible with this invention, as long as the concept of using the expansion of MDF to actuate a sensor at multiple places along the MDF is followed. 
   Other sensors are also contemplated for use with this invention, such as self-contained switches being closed or capacitive sensors being actuated by the expanding MDF. In addition, different versions of the apparatus may be configured to work with any device shape. 
   If the device has a round configuration similar to that of the Smith &#39;101 patent with MDF on the outside of a cylinder, the test apparatus can also have a similar configuration with spaced conductors extending from a concave body that fits over a portion of the cylindrical device. As before, the body holds spaced conductors over spaced locations along the MDF. 
   Furthermore, while it simplifies timing calculations for the sensors to be actuated sequentially as disclosed above, as long as the distance between the ignition point and each sensor is known, the ignition point could also be between the sensors along the MDF. 
   It is intended that the scope of the invention be defined by the appended claims.