US3435764A - Dormant explosive device - Google Patents

Description

April 1, 1969 S. R. KELLY T AL DORMANT EXPLOS IVE DEVI CE Filed Nov. 15

SUPPLY OF EXPLOS IVE OPTIONAI:"*

coNmgpNINc EXPLOSIVE DRY FORMATION OF DORMANT EXPLOSIVE COLUMN COVERING OF DRY COLUMN TO FORM SEMI-FUSE EXAMINATION OF DORMANT SEMI-FUSE APPLICATION OF MOISTURE BARRIER ACTIVATION OF F'USE OPTIONAL-7 FINISFIINO OPERATIONS I N V ENTORS STANLEY R. KELLY JOHN IVI. SMITH ATTORNEYS United States Patent Int. Cl. F42b 3/10 U.S. Cl. 10227 6 Claims ABSTRACT OF THE DISCLOSURE A dormant explosive fuse is comprised of a covered column of coarse dry high explosive material having a particle size sufficiently large to render the explosive material incapable of initiation and propagation at the core load utilized,

This is a continuation-in-part of our copending patent application Ser. No. 511,554, filed Dec. 3, 1965, now U.S. Patent No. 3,381,568.

The present invention relates to high explosives and is principally concerned with dormant explosive devices and a new and improved process for their fabrication and activation. More particularly, the invention is directed to a new method for producing detonating fuses, cords and the like by which the fuse is fabricated while the explosive material is in the dry state.

As is well known, detonating fuses, cords and the like are generally produced in continuous lengths as flexible explosive columns, consisting essentially of a central core of high explosive material encased in a protective sheath. The amount of explosive within the core and the makeup of the sheath may vary somewhat; however, detonating fuses generally contain 40 to 60 grains of explosive per linear foot and sufficient coverings to minimize whatever damage might be encountered in use, such as abrasion, water penetration and the like Heretofore, it has generally been the practice in the explosives industry to manufacture detonating fuses by a dry or wet process. In the dry procedure, the raw core of the fuse is usually provided by forming a column of dry high explosive and spinning about the column a plurality of textile yarns or the like. These yarns circumscribe and confine the explosive column in a longitudinally oblique or spiraling manner but do not usually provide a complete barrier or sheath for the column of dry explosive. At the moment the column is formed and throughout all of the subsequent fabricating operations the explosive is present in an active state, that is, it is at all times fully responsive to a detonating stimulus and fully capable of initiation and explosive propagation. Thus, the dry spinning operation is unavoidably associated with substantial safety hazards requiring isolation and barricading of the entire manufacturing facilities.

The wet method, on the other hand, has the definite advantage of introducing into the fabricating operations a large number of safety factors. In accordance with this process, the core is formed from a wet slurry of explosive material and is wrapped in a textile covering prior to subsequent operations such as ins ection and final wrapping. As can be readily appreciated, the handling of wet slurries of explosive rather than the dry materials substantially reduced the hazards associated with the manufacturing operation. Unfortunately, the more complex machinery necessitated by the wet operation together with its greater maintenance and slower production rate constitute a substantial disadvantage from a commercial point of view.

3,435,764 Patented Apr. 1, 1969 Accordingly, it is a principal object of the present invention to provide a manufacturing technique for the production of detonating devices which is capable of utilizing the advantages of both the wet and dry methods while eliminating many of the disadvantages thereof.

Another object of the present invention is to provide a new and improved manufacturing process for detonating fuses, cords and the like which obviates many of the safety hazards associated with the conventional dry manufacture of detonating fuse, while retaining the substantial commercial advantages thereof.

Still another object of the present invention is to provide a new and improved method for safely manufacturing detonating fuse and the like which method facilitates greater control over the core load of the fuse, provides increased production rates and other substantial economic and manufacturing advantages while requiring a reduced number of process cuts in the fuse as well as reduced labor, machinery and maintenance costs.

A further object of the present invention is to provide a new and improved process for the dry manufacture of detonating fuses and the like which permits the safe assemblage, examination and storage of a detonating fuse and eliminates the necessity for isolating and barricading the operation yet is capable of providing in a facile and economical manner a detonating fuse or the like having excellent reliability and performance characteristics,

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

In the drawing:

FIG. 1 is a flow diagram indicating the typical process steps in the method of the present invention; and

FIG. 2 is an elevational view, partly broken away and partly in section, of a portion of a dry spinning machine suitable for use in the process of the present invention.

In accordance with the present invention, the above and related objects are, in general, effectuated by first forming an inactive or dormant core for an explosive device and subsequently activating the explosive within the core. The dormant or potentially active explosive core may be safely handled during fabrication operations and advantageously stored in its inactive or passive state until such time as it is necessary to activate or energize the explosive within its core to provide the desired initiation and propagation characteristics. It is a further advantage of the present invention that activation can take place after fabrication of the device and without removing the explosive core therefrom, the dormant core being incapable of initiation or explosive propagation yet being characterized by an ability to be activated to a high velocity detonating state in a facile and effective manner.

Referring now to the drawing in greater detail and particularly to the flow diagram of FIG. 1, there is illustrated in a general manner the various procedural steps in-the process of the present invention. In accordance with that process there is initially provided a suitable supply of explosive material, as indicated by the numeral 10. This material might be any of the conventional solid high explosives and should be in a form suitable for the preparation of the desired explosive core or column.

In the preferred embodiment of the invention the explosive column is formed by gravity feed of the dry explosive, followed by confinement in a textile wrapping.

In such an operation it is essential that the explosive material exhibit proper flow characteristics. This is particularly important in fuse manufacturing operations where it is necessary to maintain close control over the core load to the fuse. In general, better flow characteristics are obtained from the coarsest grade of each explosive employed. In fact, in many instances explosives of the finer grade will not produce the desired effects and should not be employed. For example, the necessary flow properties for a center thread gravity feed operation are obtained from the explosive pentaerythritol tetranitrate (PETN) only at coarse grain sizes. More particularly, grain sizes which are coarser than those normally used in conventional PETN detonating fuses of core loads from 40 to 60 grains per foot are preferred. Accordingly, cap grade PETN has been used with repeated success. Cap grade PETN generally conforms to a screen analysis (Tyler Standard Sieves) of:

Percent Maximum retention on a mesh screen 1 Minimum retention on a 48 mesh screen 35 Maximum passed through a 100 mesh screen 20 The coarse high explosives may in certain instances be used as received from the supplier; however, as indicated generally at 12, best results are obatined when the explosive is suitably conditioned in order to enhance its characteristics, particularly its free flow properties. In the case of PETN, the How properties of the explosive may be beneficially affected by suitable chemical treatment. This treatment may take the form of a rinse with an aqueous solution of an anti-static or wetting agent, such as the saturated, long chain or fatty alcohol sulfates sold by E. I. du Pont de Nemours & Company (Inc.) under the tradename Duponol G. Such anti-static or wetting agents tend to enhance and promote the fiow characteristics of the dry explosives treated therewith. The washed explosive is, of course, filtered to remove the excess treating liquid and then dried prior to its formation into the core of a fuse.

Exemplary of the high explosives which may be efficaciously employed in the present invention are: pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene trinitramine (HMX), trinitrotoluene (TNT), tetryl and the like or suitable mixtures thereof.

As indicated by the numeral 14, the dry explosive material possessing the requisite size and flow properties is formed into a dormant column, that is an explosive column which is incapable of initiation or propagation. This may be accomplished by forming the dry explosive material into a column having a core load below the critical detonating core load for the particular explosive and particle size utilized. It will of course be appreciated that the column may contain a suitable mixture of explosives or a mixture of high explosive material plus adulterants, the latter being of either a combustible or noncombustible nature. Accordingly, the critical core load will vary depending on the composition of the explosive mixture employed. The critical detonating core load for the particular core composition employed is the weight of explosive or mixture per unit of length below which the explosive will not initiate or propagate when primed with conventional detonators or boosters.

The critical load will vary directly with the grain size or screen analysis of the explosive and is different for each explosive material or mixture. For example, the critical core load for cap grade PETN is between about 35 and 42 grains per foot when the core is made-up entirely of PETN. At the higher core load cap grade PETN will initiate and propagate while at the lower core load it is incapable of iniation and propagation and would result in a dormant fuse. If a mixture of 72 percent cap grade PETN and 28 percent sodium carbonate is used, the critical core load is increased to above 75 grains per foot (54 4 grains per foot of PETN). However, in both instances, as the grain size is reduced, the critical core load is also lowered. Further variation in critical core load can be effected by using other explosives, such as RDX which is less sensitive than PETN and at the same grain size has a higher critical core load.

As indicated by numeral 16, the dormant explosive column is formed into a semi-fuse or raw core by providing the column with a textile covering, such as by the conventional dry spinning operation. This may be accomplished by utilizing a center thread, gravity feed, fuse making apparatus, such as that illustratively depicted in FIG. 2 and generally designated 30. In operation, the coarse, granular explosive 32 is fed into the tapered funnellike hopper 34 together with the single center thread 36 and, via gravity, flows into and through the neck 38 of the hopper. As the explosive flows through the neck 38 it is formed into column 40 and is circumscribed by the paper tape 42 which has been suitably curved along its longitudinal axis under the influence of the tubular mold 44. The tape 42 is overlapped, as shown, to form a cylindrical sleeve 46 enclosing a portion of neck 38, the column of explosive 40 and the center thread 36. As illustrated, the textile spinning machine 30 is further provided with a pair of flat, vertically spaced, counter-rotating platforms 48, 50 provided with central apertures 52, 54, respectively, in which is positioned the tubular mold 44. Each of the platforms are adapted to support a plurality of freely rotating textile bobbins 56 adjacent the periphery thereof for feeding strands of yarn 58 through the slots 60, 62. to the interior of the mold. As the paper-enclosed explosive column moves downwardly through the tubular mold 44, the platforms 48, 50 revolve in opposite directions causing the yarns 58 to wrap the explosive column in a continuous spiraling manner thereby forming the semifuse 64. As mentioned hereinbefore, the textile covered semi-fuse, also referred to as the raw core, is at this stage of the manufacturing operation, substantially passive, dormant or inactive, is safe for handling and is incapable of undergoing initiation or propagating of an explosive signal.

The textile covered semi-fuse 64 while in its dormant state is fed to an examination station, as indicated by the numeral 18. There the entire length of the fuse is physically examined to ensure that the core weight of all portions of the fuse falls within the desired range and below the critical load for the explosive material utilized. This examining process may be eifectuated in accordance with standard fuse making techniques. In accordance with the preferred method, a fuse size or diameter inspection is made by passing the raw core between a pair of sensing rollers which measure the dimensional characteristics of the raw core. Since the examination is by mechanical means, there is necesarily some reduction in the particle size of the explosive column. However, care is generally taken so that such size reduction is maintained at a minimum. Additionally, the size of the explosive material initially employed should take into consideration such reduction in order to prevent premature activation of the fuse.

Following examination, the raw core is provided with a suitable moisture barrier, as indicated by the numeral 20. The barrier may take a number of forms but a covering of plastic or the like is generally preferred. Such coverings or layers of plastic can be easily applied by extrusion techniques. Since the application of hot plastic over the raw core tends to increase the ambient temperature of the explosive material in the fuse, it is particularly advantageous to apply the plastic prior to activation of the explosive within the semi-fuse. Accordingly, the extrusion process takes place while the core is incapable of initiation or explosive propagation thereby obviating the safety hazards associated with the heating of an active explosive column.

As indicated by the numeral 22 of FIG. 1, the continuous fuse may at this stage of the process be wound on a take-up spool for safe storage. Alternatively, the fuse may be fed directly to the activation station. In any event, it will be appreciated that the entire fuse making operation from the column formation to the application of a moisture barrier has involved the handling of an inactive or only potentially active explosive column. In fact, numerous tests have been conducted on the plastic jacketed inactive fuse and these indicate that the fuse is incapable of initiation and propagation by end priming with detonating cord, by side priming with dynamite or by side priming with heavy-duty detonating cord. Thus, it has been clearly established that the dormant fuse of the present invention is free from the safety hazards previously associated with the dry spin formation of detonating fuse and the like.

The prefabricated dormant fuse may be activated, as indicated by the numeral 24 of the flow diagram of FIG. 1, in a relatively facile and economical manner. Although various types of activation might be employed it is preferred in accordance with the present invention to activate the explosive column by causing a change in the size distribution of the explosive material and thereby effectively lower or decrease the critical core load of a prefabricated fuse. The revised size distribution is elfectuated without changing the core load or dimensional configuration of the fuse itself. The method of producing the desired activation which has been most successfully employed is to pass the fuse through a series of crushing rollers. These have the efiect of reducing the particle size of the explosive in the raw core, resulting in a substantial shift in the size distribution of the explosive. The result is a grain size which is capable of initiation and propagation at the core load employed by the prefabricated fuse. It will be appreciated that the core load or other dimensional characteristics of the fuse need not be altered during the activation operation and in accordance with the preferred method of the present invention, should not be so altered. It is, of course, absolutely necessary that all portions of the explosive fuse be subjected to the activating operation in order to ensure the proper operation of the resultant product along its entire length.

The vitalized or activated fuse may finally be encased, if desired, in further textile coverings or sheaths depending upon the environmental conditions under which the detonating fuse will be used. Such coverings provide not only a protective function but also act to reinforce the fuse and protect it from abrasion during usage. Additionally, such added wrapping tends to round-out the fuse after the crushing operation and beneficially affects the velocity of detonation of the explosive column. The wrapping may comprise countering layers of textile wrapping covered by wax or by other suitable protective materials. It will, of course, be appreciated that the final reinforcing and protecting coverings may be applied to the fuse prior to the activation operation. Generally, however, it is prepared to activate the fuse prior to finishing, since the final countering and waxing operations have the beneficial effects mentioned heretofore.

In order to provide a fuller understanding of the present invention, the following specific examples are given by way of illustration and are not intended to constitute any limitation thereon.

Example I A supply of pentaerythritol tetranitrate (PETN) conforming to E. I. du Pont de Nemours specification for cap grade PET-N and having the screen analysis set forth in column A of Table I was washed with Duponol G, filtered and dried. The dry explosive was placed in the hopper of a gravity feed, center thread, dry spinning apparatus and formed into a column having a core load of approximately grains per foot. The semi-fuse was then examined and covered with an extruded plastic jacket. A screen analysis of the explosive core of the jacketed fuse is set forth at column B of Table I.

Initiation of the plastic jacketed fuse thus formed was attempted by end priming with 50 grain reinforced Primacord detonating fuse and by side priming with Gelex No. 2 dynamite initiated by reinforced Primacord. In each instance, the jacketed fuse failed to initiate, indicating its dormant or inactive character.

A portion of the dormant fuse was subjected to the activation step of passing the cord through a series of barrel rollers. A screen analysis of the explosive within the fuse following the activation step is set forth in column C of Table I. The resultant fuse exhibited the ability to be initiated by blasting caps and other detonating devices as well as the ability to initiate itself by a lap connection. It further evidenced the ability to initiate all cap sensitive explosives and to initiate and propagate when wet or dry.

TABLE I.SCREEN ANALYSIS OF PE'IN Percent retained Screen size (mesh) A B C Propagation of the explosive signal was tested by activating in the same manner as above portions of two dormant fuses as made hereinbefore. First, one end of a 10 foot piece of dormant fuse was activated along a length of 5 feet. Second, a 4 foot length in the middle of a 10 foot piece of dormant fuse was activated. The fuses were marked at the division points between the activated and unactivated portions. The activated portions of the fuses were then initiated. In each instance the fuses failed to propagate beyond the marked borders.

Example 11 A fuse was prepared in accordance with the procedure of Example I except that a core load of 17.5 grains per foot was used. The fuse was incapable of initiation and propagation prior to the activation step. After activation by crushing, the fuse successfully initiated and propagated.

Example III A fuse was prepared in accordance with the procedure of Example I except that the explosive core was formed from a mixture of 72 percent by weight cap grade P=ETN and 28 percent by 'weight sodium carbonate at a core load of 75 grains per foot (54 grains per foot of PETN). Prior to activation by crushing, the fuse would not initiate or propagate. After crushing, the explosive column initiated and propagated at a detonation velocity of 5300 meters per second.

Example IV Example I was repeated using RDX as the explosive material. The starting material had the screen analysis given in column A of Table II. After being spun into a fuse having a core load of 51.6 grains per foot, it was covered with a plastic jacket. At this stage the fuse was tested and found to be incapable of initiation and propagation. It exhibited the screen analysis set forth in column B of Table II. The f-use was then activated by crushing and resulted in a cord capable of initiation and propagation. The screen analysis after activation is shown at column C of Table III.

TABLE II.SCREEN ANALYSIS OF RDX A B C Example V A fuse was prepared in accordance with the procedure of Example I except that a core load of 6.5 grains per foot was used. The fuse was incapable of initiation and propagation prior to the activation step. After activation by crushing, the fuse successfully initiated and propagated at velocities of detonation ranging from 6500-6700 meters per second.

The cord propagated through square knots when several lengths were square-knotted together and fired with a blasting cap.

Particle size distribution of the PETN in the activated cord as measured by the micromerograph was:

Particles, percent: Microns 44 94 20 63 10 16 5 4 3 Average particle size of the PETN in the activated cord by the Fischer sub-sieve sizer was 8.0 microns.

Example VI A fuse was prepared in accordance with the procedure of Example 1 except that a core load of 4 grains per foot was used. The fuse was incapable of initiation and propagation prior to the activation step. After activation by crushing, the fuse successfully initiated and propagated at velocities of detonation ranging from 480 04000 meters per second.

As can be seen from the foregoing detailed description, the present invention provides a novel and improved method for safely manufacturing detonating fuses, cords and the like. The method eliminates the costly isolation of manufacturing facilities as heretofore required and facilitates the safe fabrication and examination of high explosive detonating devices in a facile and economical manner. Additionally, it provides a dormant detonating fuse or the like which can be safely held in storage until such time as its use is required. From a commercial standpoint the new and improved method faeilitiates greater control over the core load than was previously possible in accordance with the wet braiding method,

while requiring a reduced number of processed cuts and lower labor, machinery and maintenance costs.

We claim:

1. A dormant explosive device capable of being activated to a condition of ready initiation and propagation independently of its confining environment comprising a core of dry high explosive material and a covering confining the core, said explosive material being present in a quantity and condition rendering it incapable of initiation and propagation and being characterized by the ability of being readily activated Without removal from said covering and independently of its confining environment.

2. The dormant explosive device of claim 1 wherein the explosive material is granular and is present at a core load less than that necessary for propagation of an explosive signal in the granular size employed, said material being characterized by the ability of being readily activated by altering the granular size without removal from said covering.

3. The device of claim 1 in the form of an elongated fuse, cord or the like wherein the explosive material is coarse and granular and is present at a core load below the critical core load necessary for initiation and propagation of an explosive signal at the coarse grain size employed, said material being readily activated by causing an effective reduction in the critical core load without removal of the explosive from said covering.

4. The device of claim 3 wherein the high explosive material is PETN.

5. The device of claim 3 wherein the high explosive material is RDX.

6. The device of claim 3 wherein the explosive material is cap grade PETN having a core load of about 35 grains per foot and less.

References Cited UNITED STATES PATENTS VERLIN R. PENDF-GRASS, Primary Examiner.

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