Explosives booster

An explosives booster (10) intended for igniting a main explosive charge is provided which includes a first explosive charge (15) which is sensitive enough to be ignited by the explosion of a charge from a detonator (13); and a second explosive (16) charge which is less sensitive to ignition that the first charge, (15) but more sensitive to ignition than the main charge. The first charge may comprise a 60/40, and the second charge a 40/60, PETN/TNT pentolite mixture.

BACKGROUND OF THE INVENTION
 This invention relates to an explosives booster.
 Explosives boosters are used for amplifying the explosion from a detonator
 in the direction of the main charge.
 PRIOR ART TO THE INVENTION
 The known boosters usually comprise an elongated hollow body, usually of
 cardboard or plastics, which is charged with an explosive material which
 is more sensitive to detonation than the main charge.
 A common material for such booster charge is pentolite, which is a mixture
 of pentaerythritol tetranitrate ("PETN") and trinitrotolune ("TNT").
 It is also known to employ a booster body which is of cone shape, with the
 arrangement such that the detonator is located towards the base of the
 cone, and the wider end of the cone faces the main charge.
 The known detonator boosters suffer from various disadvantages.
 Thus, for example, because of the relatively high concentration of the PETN
 component of the pentolite mixture, the known boosters are relatively
 expensive.
 Other disadvantages relate to the configuration of the booster body and the
 dimensional interrelationship of its components, both which may adversely
 influence the effectiveness of the explosion and the resultant shock wave.
 OBJECT OF THE INVENTION
 It is accordingly an object of this invention to provide an explosives
 booster with which the aforesaid disadvantages may be overcome or at least
 minimised.
 SUMMARY OF THE INVENTION
 According to the invention an explosives booster intended for igniting a
 main explosive charge is provided which includes a first explosive charge
 which is sensitive enough to be ignited by the explosion of a charge of a
 detonator charge; and a second explosive charge which is less sensitive to
 ignition than the first charge, but more sensitive to ignition then the
 main charge.
 With such an arrangement the detonator in use will ignite the said first
 charge, which in turn will ignite the said second charge, which in turn
 will ignite the said main charge.
 It will be appreciated that with such an arrangement the said first charge
 may comprise a conventional 60/40 PETN/TNT pentolite mixture, while the
 said second charge may comprise a pentolite mixture with a lower
 concentration of PETN, such as, for example, a 40/60 mixture, which
 accordingly makes it less expensive.
 Furthermore, because the only purpose of the said first charge is to ignite
 the said second charge, a relatively small first charge can be employed
 which further serves to reduce the costs of the arrangement.
 Further according to the invention the booster includes a hollow body which
 is of substantially ice-cream cone configuration, in longitudinal cross
 section and which comprises a hollow cup of substantially semi-dome shape
 and a hollow elongated stem which projects radially outwardly from the
 apex of the dome; the stem being adapted to carry the detonator and said
 first charge in its bore, and the cup being adapted to carry said second
 charge.
 Further according to the invention the said bore of the stem of the booster
 is of circular configuration in cross section, and its diameter where it
 is ignited by the detonator is at least twice that of the detonator.
 Thus, for example, if a conventional type of detonator with a diameter of 7
 mm is employed, the effective diameter of the bore of the stem at the
 ignition point is in the order of at least 14 mm.
 Still further according to the invention the said cup is also of circular
 configuration in cross section, and its diameter at its widest part is at
 least four times that of the detonator.
 Still further according to the invention the diameter of the cup at its
 widest part is not greater than the total height of the booster measured
 from the free end of the said stem to said widest part of the cup.
 Applicant has found a booster of the aforesaid configuration to have the
 following advantages:
 1. because the detonator is located in a position spaced from, but
 centrally relative to, the said second charge, optimum use is made of the
 directional detonation wave caused by the detonator explosion, while an
 advantageous pressure duration is also obtained;
 2. the shape of the booster ensures that a planar shock wave is obtained;
 3. the presence of dead zones in the charge behind the point of innitiation
 of the explosion is avoided, or at least minimised;
 4. because of the aforesaid interrelated dimensions of the booster
 components, a full detonation wave, as well as proper reversed detonation
 (retonation), is obtained which, because of the gradual build up until the
 shock wave has fully formed and flattened out, ensures better and
 prolonged results.
 Still further according to the invention the stem of the booster is
 releasably securable to the cup of the booster, so that differently sized
 cups may be employed with the same stem in order to provide boosters of,
 say, 200 g, 400 g and 800 g.
 The cup and the stem may each be provided with said second and first
 charges respectively in any suitable manner such as, for example, by means
 of a casting operation.
 Preferably, also, an inwardly extending detonator well may be provided
 towards the one side of the said second charge through which the detonator
 may be passed in order to allow it to be loaded from below into the bore
 of the stem of the booster while the latter is in position in the blasting
 hole.
 Because of its particular location, such a well has no influence on the
 shockwave being formed by the explosion. Preferably, the hollow body of
 the booster is made of a suitable material such as cardboard or plastic.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
 In this embodiment of the invention an explosives booster 10, which may be
 of any suitable material such as card board or plastics, has an elongated
 body comprising a semi-dome shaped hollow cup 11, which is of circular
 configuration in cross section, and an elongated hollow stem 12, which is
 also of circular configuration in cross section, and of which the one end
 can releasably be clipped onto the apexed end of cup 11. The assembled
 booster 10 is accordingly of substantially ice-cream cone configuration in
 longitudinal cross section.
 Stem 12, which is open at its other or bottom end, i.e. the end opposite
 the one being clipped on to cup 11, is adapted to hold a conventional type
 of detonator 13, which may be inserted from below through said open end
 into the bore of stem 12.
 The upper end of the bore of stem 12 flares outwardly in the upward
 direction to define a compartment 14 into which a first explosive charge
 15 comprising a 60/40 PETN/TNT pentolite mixture may be received.
 Cup 11 is filled with a second explosive charge 16 comprising a 40/60
 PETN/TNT pentolite mixture.
 Charge 16 is provided towards its one side with a detonator well 17 through
 which detonator 13 may be passed in order for it to be located from below
 into the bore of stem 12 through its said bottom end.
 The diameter of compartment 14 at point 18, ("the ignition point") which is
 in line with the upper end of detonator 13, is approximately twice the
 size of the diameter of detonator 13, while the diameter of cup 11 at its
 widest part 19 is more than four times the diameter of detonator 13.
 The diameter of cup 11 at its widest part 19 is less than the overall
 height of booster 10, measured from point 19 to said bottom open end of
 stem 12.
 In use, a cup 11 loaded by means of a casting operation with an explosive
 charge 16, comprising a 40/60 PETN/TNT pentolite mixture, is clipped onto
 stem 12, which is loaded by means of a casting operation with an explosive
 charge 15 comprising a 60/40 PETN/TNT pentolite mixture, and the assembled
 booster 10 lowered into the blasting hole (not shown).
 When booster 10 is in position in said blasting hole (not shown), detonator
 13 is passed from above through well 17 in charge 16 and then through said
 open bottom end of stem 13 until its upper end is imbedded in charge 15,
 as shown in the drawing.
 The said blasting hole (not shown) is then in conventional manner loaded
 with the main explosive charge (not shown).
 When detonator 13 is ignited, it ignites explosive charge 15, which in turn
 ignites explosive charge 16, which in turn ignites the said main charge
 (not shown).
 Because of the lower concentration of PETN in charge 16, and the relatively
 small mass of charge 15 compared to that of charge 16, booster 10 is less
 expensive than the conventional arrangements of comparable size.
 Furthermore, because of said dimensional interrelationship between its
 various components, the use of booster 10 yields all the various
 advantages referred to above compared to the conventional arrangements.
 As will be appreciated, there are no doubt many variations in detail
 possible with an explosives booster according to the invention without
 departing from the spirit and/or scope of the claims.