Hollow Charge Having A Directed Explosive Effect

Abstract

A hollow charge is adapted for underwater demolition operations and has a directed explosive effect, the charge being provided with a cavity at its front end relative to the direction of the explosion, the cavity enlarging towards the front end. The charge has metal walls or a metal sheathing so that, on the occurrence of the explosion, a part of the metal is formed into a thin metal stream which is directed forwards at high speed. The charge comprises an outer cone forming the shell of the charge, and a regularly shaped inner cone at the front part of the charge, enlarging towards its front end, the cones forming the cavity. A cover seals the back part of the outer cone and a base closes the front part.

Description

The invention relates to a hollow charge, adapted for underwater demolition and having a directed explosive effect, the charge being provided with a cavity at its front end relative to the direction of the explosion, the cavity enlarging towards the front end and having metal walls or a metal sheathing so that, on the occurrence of the explosion, a part of the metal is formed into a thin metal stream which is directed forwards at high speed. Specifically the charge comprises an outer cone forming the shell of the charge, and a regularly shaped inner cone at the front part of the charge, enlarging towards its front end, the cones forming the cavity while a cover seals the back part of the outer cone and a base closes the front part.

Until now it has been thought that hollow charges already known will not produce the required performance and nozzle effect unless rapid (detonation wave up to 8,000 m./sec.) explosive materials are used. However, a charge constructed in accordance with the invention will give the required performance using ammonium nitrate explosives (e.g. anite), dynamite or any of the other usual types of slow explosive, having a detonation wave speed of under 3,000 m./sec.

With the invention, safety explosives (slow civil explosives) have been used in a hollow charge for the first time, and the result achieved has been at least as effective as with rapid explosives (e.g.) nitroglycerine gelatine charge). This invention provides an effective hollow charge in which safety explosives can be used, especially appropriate for civil use, since the use of safety explosives involves no danger and the charge can, for example, be loaded on the site.

The manufacture of the charge in accordance with this invention is relatively cheap, not only because of the advantages gained in using safety explosives, but also because of the materials used in constructing the charge itself. The outer cone, cover and base are preferably made from plastic, and the inner cone can be made from steelplate.

In the types of hollow charge already known (for rapid explosives), the inner cone is usually made of copper instead of steelplate, and in fact a satisfactory performance cannot be achieved using steelplate in those known hollow charges.

The tests carried out on the hollow charge made in accordance with the invention have given surprisingly good results. It has been discovered that in a hollow charge of this type, the speed of ammonium nitrate (anite), which is a slow explosive, almost doubles on reaching the steel surface. The reason for this is still unknown. On the occurrence of the explosion approximately 60 percent of the steel melts into a steam which can penetrate, for example, through a wall of 1.5 meter thickness.

With reference to the above, it is characteristic of the hollow charge in accordance with the invention that the hollow charge contains slow civil explosives, e.g. ammonium nitrate or other similar material having a detonation speed of 5,000 m./sec.

The said hollow charge can be used in explosions both on land and underwater. On land, its general application is as a hole digging or frozen ground charge, but it can also be used for several other purposes.

Good and promising results have been obtained from the underwater tests carried out. Until now the procedure for carrying out underwater explosions has been to drill a hole in the underwater object (e.g. stone, rock) with suitable equipment, and then to place the charge in the hole and detonate it. However, the deeper the location of the object, the more difficult is the operation.

Surprisingly however, when using hollow charges in accordance with the invention, underwater explosions can be carried out simply, without danger and, above all, very effectively. The work becomes more difficult as the depth increases using the known method, but, using this method in accordance with the invention, the explosive power increases the deeper the explosion takes place. This is caused by the direct pressure wave of the water which increases the power.

Other advantages and characteristics of the invention are described in the following, with reference to the enclosed drawings, in which:

FIG. 1 shows a partial longitudinal cross-sectional view of the outer cone;

FIG. 2 is a cross section taken along line A--A in FIG. 1;

FIG. 3 shows a partial longitudinal cross-sectional view of the cover;

FIG. 4 is a partial longitudinal cross-sectional view of the base;

FIG. 5 is a partial longitudinal cross-sectional view of an assembled charge; and

FIGS. 6a-6b, 7 and 8 are diagrammatic illustrations showing the use of the charge in underwater applications.

The charge in accordance with the invention is made up of an outer cone 1, an inner cone 2, a base 3 and a cover 4. The space between the outer and the inner cone is filled with the explosive. The outer cone, the base and the cover are preferably made of plastic material such as polyethene. In the known types of charge they are manufactured from metal plate and thus the fixing and exact tight fitting of the base and cover requires accuracy and time. In accordance with the invention, the base 3, which is made of flexible polyethene, has a rim of U-shape, the upward rising edge of which has a shoulder 5, which engages a groove 6, in the front part of the outer cone, the front edges of the inner and outer cones thus becoming locked in the said U-shaped rim. The cover 4, which is also manufactured from flexible polyethene, has a central part in the form of a detonation cylinder 7 which is directed downwards, and a rim forming a vertical flange 8, the edge of which is bent outwards to form a collar 9. An inward by turned edge 10 of the upper part of the outer cone is fitted under the collar 9. The base and cover can thus be attached to the outer cone and the inner cone simply by pressing the base and cover into place, since the flexible plastic material bends. This significantly decreases the manufacturing costs.

It has previously been thought that the inner cone should be seamless. However, in the charge in accordance with the invention it can have an open seam without any effect on its operation. This significantly decreases the manufacturing costs. The seam of the inner cone need only be sealed with, for example, electrical tape or similar material. A hole can even be left in the top of the inner cone provided that it is closed, for example with a polyethene plug 11.

The thickness of the material of the inner cone 2, depends on the amount of explosive. The greater the amount of explosive the greater should be the thickness. For example, when using 1kg. of explosive the thickness is 1.5 mm. and with 1.5 kg. it is 2.0 mm.

The drawing show a charge having 1.kg. of explosive. The dimensions are as follows: Inner cone: cone angle 60.degree., height 107 mm., thickness of material 1.5 mm. Outer cone: inside diameter of the front part 127 mm., inside diameter of the back part 66 mm., height 205 mm.

The wall of the outer cone forms an angle of <30.degree. with the vertical. When the explosive between the cones is detonated, using a detonator located in the upper part, the detonation wave is directed towards the outer walls of the inner cone, causing the cone to be compressed from the back to the front, and approximately 60 percent, by weight, of the inner wall to melt to a temperature of 50,000.degree. C. The molted metal nozzle then begins to move straight along the central axis, the initial value of the speed being approximately 8,000 m./sec. when using ammonium nitrate as an explosive. The penetration ability of the nozzle, when using 1 kg. of explosive is:

frozen ground, stony approximately 0.5- 1 m. depending on stone content concrete 1- 1.5 m. armor plate 0.2 m. rock 0.3 m.

The larger the charge, the better the penetration ability.

When carrying out underwater blasting the procedure is that the diver locates the charge on the surface of the object to be blasted (on the top, at the side or below) so that the nozzle is directed in its most effective manner. The best possible result when breaking stones for example, is to place the charge under a stone, so that the nozzle penetrating the stone, together with the pressure wave transmitted by the water breaks a far larger stone than is possible in free air. To gain entrance under the stone, a charge is first exploded beside the stone with the nozzle directed beneath the stone so that all the loose material will be cleared away. The charge can then be located beneath the stone. Location of the charge under the stone by drilling is impossible. The deeper the explosion occurs the more effective it is. The size and number of the charges naturally depends upon the size and shape of the object. It can be stated, for comparison, that the breaking of a 1 m.sup.3 stone at a depth of 10 meters can cost more than 100 FMK (Finmark) using conventional methods. When using the method in accordance with the invention the cost would be only approximately 30 FMK. Furthermore, an airline diver is not required since a frogman can locate the charges in position. Neither are any machines required to break up stones or other objects.

FIGS. 6a-6b, 7 and 8 illustrate the explosion of underwater objects. In the example shown in FIG. 6a a hollow is first blasted under the stone 12, using charge 13. The actual blasting operation is then carried out with charge 15, located in the hollow 14, and charge 16 placed on the opposite side. The explosion shown in FIG. 7 is carried out using charges 17 and 18. In FIG. 8 three charges, 19, 20 and 21 are used.

Stone, rock, bedrock and other objects can be blasted using the method in accordance with the invention. It has proved to be an extremely effective and rapid method of blasting in channel-clearing operations.

Claims

We claim:

1. A hollow charge having a directed explosive effect and comprising a hollow body with a front and back, said body including an outer shell of frustoconical shape and an inner metal shell of conical shape, the shells both diverging toward the front of the body, a base of flexible plastic material including a U-shaped rim joining said shells at the front ends thereof and flexed to cover the front of said body, a cover of flexible plastic material on said outer shell at the back of the body defining a closed space between the shells, an explosive charge contained in said closed space, and detonation means carried by said cover and extending forwardly into said space between the shells for detonating the explosive charge in said space to cause the inner metal shell to be melted and a part thereof formed into a thin metal jet which is propelled through the front of the body thereof to produce vaporization of surrounding water when the charge is submerged during underwater demolition work.

2. A hollow charge as claimed in claim 1 comprising means on said cover for press fitting the same on said outer shell.

3. A hollow charge as claimed in claim 2 wherein said rim of said base includes means for locking the base on said shells when joined thereto, said U-shaped rim of the base permitting press-fit joinder of the shells and the base.

4. A hollow charge as claimed in claim 3 comprising a plastic plug in said inner conical shell at the apex thereof, said plug closing said inner shell.