IL22959A - Igniter cord - Google Patents

Description

niiui ins Tjinie,T *n PATENTS AND DESIGNS ORDINANCE SPECIFICATION Improved igniter cord I (we) E.I. DU PONT DE NEMOURS AND COMPANY, a corporation organized and existing under the laws of the State of Delaware, U.S.A., of 10th and Market Streets, Wilmington, Delaware, U.S.A. do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the. following statement :- The present Invention concerns improvements in and relating to igniter cord.

Several means exist for igniting propellant charges, which term may include grains. When it is desired to ignite very rapidly as much as possible of a charge, e.g. in a rocket engine or artillery shell-, one means is an igniter cord fitting closely in a cavity extending along within the charge. Previous igniter cords that have been suggested and used in practice have suffered from one or more of the following defects, unreliability in performance, e.g. the igniter cord has been brisant or has not detonated with a consistent velocity so that the range and performance of the shell or rocket engine has not been consistent, lack of assurance of safety in handling and storage, expense, excessive weight of the cord itself or of the cord together with the necessary initiation means and difficulty of initiation by convenient means such as a flame or spark.

According to the present invention, there is provided an igniter cord comprising a metal sheath containing a continuous column of a mixture consisting, by weight, essentially of 50 to 95$ of lead azlde of purity substantially 98.5 > by weight, and a complementary 0 to 5# of particulate aluminium, and containing no chemical oxidant, at a loading of at least 0.25 grains of lead azlde per foot of cord.

The lead azlde must be of very high purity, i.e. of lead azlde content about 98. #, by weight,preferably such as RD-1333 or KD-15^3 lead azide, as described on pages A557 - to A565 of the Encyclopedia of Explosives and Related Items, Volume 1 , published by Picatinny Arsenal, Dover, N. J.

The particle size of the aluminium may range from finely-divided powders, e.g. about 1 micron in size, to relatively larger particles, e.g. about 100 microns in size, in such forms as ground particles, atomized pellets, band saw dust, flakes, metal shavings and rotary fillings. Rivet- grade aluminium powder, such as has a loose density of 0.76 to 0.88 gm/cc and a particle size such that 100$ passes through a 48 mesh (standard Tyler) screen, 10-20$ is held on an 80 mesh screen, 50-40$ is held on a 100 mesh screen and 2% passes a 200 mesh screen, is preferred, but other particulate, comminuted forms of aluminium, e.g. grained, atomized and/or flake-like aluminium powder, can also be used advantageously.

The proportions and loadings of the components in the mixture will depend on several factors. The ignition effect of cords containing higher percentages of particulate aluminium is generally less than that containing lower percentages, e.g. 10 to 25$. A preferred mixture comprises about 60$ lead azide and 40$ aluminium, when the cord is for use with completely-combustible artillery ammunition and smoke bombs, for example. When ignition at high velocity of an easily-ignited propellant is required, a high percentage of lead azide is used, whereas for more difficultly-ignited propellants, a lower proportion is used. At a given distribution of the mixture, the lower the lead azide content is, the lower will be the velocity of transmission of the ignition stimulus, but by increasing the distribution, i.e. -in¬ creased. For most applications, the core loading of the mixture will not exceed about 50 grains per foot (10.5 grams per meter) of sheath length, and preferably, where nonviolent, nonbrisant ignition is particularly desired, will be 1 to grains per foot (0.21 to 4.2 grams per meter) and at least 0.5 grains of lead azide per foot (0.105 grams per meter). Although larger core loadings may be used, they will not usually exceed about 200 grains per foot (42 grams per meter). Less than about 0.5 grains per foot (0.105 grams per meter) does not consistently ensure reliable ignition of the propellent charge .

The sheath is preferably of a heavy-walled ductile metal, such as lead, a lead alloy or aluminium, since, after filling such a tube with intimately mixed lead azide and particulate aluminium, it may conveniently be elongated uniformly by drawing, swaging, or rolling until substantially the desired distribution of the mixture is obtained. The metal should not react with any component of the mixture under expected conditions of storage and handling. Lead is preferred since small particles are produced upon detonation and tend to vaporize, so that only a small amount of undesirable residue generally remains. These small metal particles are believed to scour and clean an adjoining pro-pellant surface, so that the cord may be able to ignite wet, dirty, or otherwise inhibited, propellent grains.

In general, the thickness of the sheath will be about equal to the diameter of the core. At low loadings weight of lead sheath of about 3 grams per foot (9.84 grams per meter) at a core loading of 1 grain per foot (0.21 grams per meter) has given complete fragmentation of the sheath and reliable initiation of the propellant charge.

The cords may be actuated by applying to one end a conventional initiator such as an electric squib, or a beaded bridgewire arrangement, an exploding bridgewire, an arc-firing system or a conventional percussion primer. If desired, the cords may be used in conjunction with a coating or column of an igniter composition which is more easily ignited, i.e. more flame -sensitive, than is the main propellant charge. The cords have accelerated the normally slow burning rate of such igniter compositions, thus accelerating the ignition of the propellant. This is especially useful in igniting artillery cartridges having a combustible case, propellant grains, chemical grenades and clusters of chemical "sub-munitions" . Such conventional igniter compositions as black powder and compositions containing boron and potassium nitrate, or silicon, potassium nitrate, char-coal and iron oxide, have ignited at substantially the speeds of the cords, for example, velocities of the order of l800 to 3000 meters per second.

When 1he cord is initiated, the ignition stimulus is transmitted at an ultrarapid velocity, e.g. at about I8OO-5OOO meters/second, down the length of the cord, and thus of any charge adjacent to the cord, so as to initiate substantially simultaneous deflagration of the entire internal surface of the charge.

The invention is further described with reference to the accompanying drawings, in which, FIGURE 1 is a sectional view of a cord according to the invention; and FIGURES 2-6 are sectional views of typical pro-pellant charges incorporating such cords.

In FIGURE 1, 1 is a ductile metal sheath, and 2 is a mixture of lead azide and aluminium, while 5 in all the Figures is the cord itself.

In FIGURE 2, a conventional tubular rocket casing 3 has one end closed, while the other forms a nozzle 3A, and contains a conventional charge or elongated propellant grain having a central cavity throughout its entire length. The cord 5 is disposed axially within this cavity, extends substantially its entire length, and is maintained in position by washers 6 so that an air space 7 is provided between the cord 5 and the grain 4. At one end of the cord 5 is an electric initiator 8 having lead wires 9 extending to a source of electric current (not shown).

In FIGURE >, the assembly is essentially similar to that in FIGURE 2, but the cord 5 is helically coiled on a mandrel 10 spaced from the grain 4, and, in order to obtain high degree of reliability, an electric or nonelectric end initiator 8 is used to initiate each end of the cord 5. A spacing is maintained between each turn of the cord 5 to avoid cut-offs resulting from cross-propagation between the In FIGURE , direct ignition of the grain is obtained by threading the cord through longitudinal bores of several separate grains strung in equally-spaced relationship along the cord. If desired, a flexible rope or cable of suitable material, such as nylon, sisal and hemp, may extend through a bore parallel to and in close proximity to the bore for the cord, provided that the provision of this rope or cable has no detrimental effects on the burning characteristics of the grain.

Similarly, direct or semi-direct ignition of the grain can be obtained by incorporating coils or loops of the cord into the surface of the grain or merely having the cord in the immediate proximity of the coating.

In FIGURE 5, the core is introduced within an igniter coating 1 covering the surface of the grain 4 to give semi-direct ignition.

In FIGURE 6, an artillery cartridge assembly is shown, wherein the cord 5 extends through a longitudinal bore in a propellant charge 4, which may be of a composition such as smokeless powder, the cord 5 being in contact with an igniter coating 12 covering the surface of the bore.

The cord may also be used in conjunction with jelly roll or basket-type igniters. The cord may also be used to ignite gas generator .propellants for auxiliary power units, artillery gun charges, mortar charges, cartridge activated devices, oil well perforating guns, cannons and Such propellant charges, including grains, containing within suitably shaped cavities the igniter cords of the invention are also provided according to the invention.

The prior art has disclosed igniter cords comprising mixtures of 15 to 80# of crystalline high explosives, an oxidant and a fuel confined in a metal sheath at a loading of 2 to 20 grains of mixture per foot (0.42 to 4.2 grams per meter). It is very surprising that the presence of a chemical oxidant has proved unnecessary under normal atmos-pheric conditions, in view of the confined nature of the system with the igniter contained within a cavity in a pro-pellant charge, and that the oxygen from the atmosphere appears to be sufficient to allow reliable detonation to occur.

The following Examples further illustrate the invention; all parts and percentages are by weight, except where otherwise indicated.

Example 1 .2 g of a mixture, made by blending 6o# of high-purity lead azide and 40# rivet-grade aluminium in a Fisher-Kendall blender, substantially completely wetting with a 1# solution of pyronltrocellulose in butyl acetate (50 g mixture/10 cc solution), screening through 42 Tyler mesh bolting cloth, drying for 8 hours at 49° C, cooling, and passing through a 28 Tyler mesh screen, and of loading density about 1.1 g/cc, was vibrated into a lead tube 19 inches (48.26 cm) long, 0.25 inch (0.584 cm) inner diameter and 0.5 inch (1.27 cm) outer diameter. 8-inch (20.32 cm) length of the cord thus prepared was placed along the axis of a cardboard tube, 3/8 Inch (0.95 cm) inner diameter and 1 inch (2 .54 cm) long, and the annulus around the cord was filled with loose black powder. When the cord was initiated at one end by an electric squib, the cord detonated at a velocity of 2522 meters per second and ignited, substantially simultaneously, the entire charge of the powder, in spite of the absence of a chemical oxidant.

Another portion of the tube, drawn to an outer diameter of 0.05 inch (0.127 cm), and a load distribution of 1.6 grains ft (0.3 rams/meter ) , tested as described above, detonated at 2 58 meters per second and again effected substantially simultaneous ignition of the powder, in spite of the absence of a chemical oxidant.

A further portion of the tube, drawn to an outer diameter of 0.045 inch (0.114 cm), and a load distribution of 1.3 grains/ft (0.2739 grams/meter), when tested, detonated at a velocity of 2230 meters per second and ignited the powder, in spite of the absence of a chemical oxidant.

Example 2 Various cords were made from the lead azide/alumi-nium mixture of Example 1 , in a lead tube, 0.312 inch (0.792 cm) in outer diameter and 60 inches ( 1 .52 meters) long, swaged to an outer diameter of 0.073 inch (O. I85 cm), and then drawn to the load distributions shown in the Table below, and initiated by an electric initiator containing a base load of Table 1 Core Load Velocity (m/sec) grains/ft (grams/meter) 1.12 (0.235 ) 2573 1.22 (0.358) 3163 1.35 (0.279) 1 2821 1.3 (0.281 ) 2749 1.44 (0.302 ) 2848 Example 3 The procedure of Example 1 was essentially followed in preparing a cord having an outer diameter of 0.055 inch (O.139 cm) and containing an 8o/20 mixture of lead azide/aluminium at a distribution of 2.5 grains/ft (0.52 grams/meter). This cord was strung through a smoke bomb having a central core lined with Igniter mix, adjacent to the cord, of composition 26$ silicon, 35$ potassium nitrate, $ charcoal, 22% black iron oxide and 13$ aluminium, grained with a 6/4 acetone/ nitrocellulose solution. The cord detonated at 3618 meters/ second and the smoke bomb was ignited substantially simul-taneously, in spite of the absence of any chemical oxidant. i We have found that these cords have been insensitive to ordinary shock and stray electric currents, and have been difficult to initiate from the side.

Claims (24)

HAVING NOW PARTICULARLY DESCRIBED AND ASCERTAINED THE NATURE OF OUR SAID INVENTION AND IN WHAT MANNER THE SAME IS TO , BE PERFORMED? WE DECLARE THAT WHAT WE CLAIM IS

1. An igniter cord comprising a metal sheath containing a continuous column of a mixture consisting, by-weight, essentially of 50 to 9 $ of lead azide of purity substantially 9&.5%, by weight, and a,- complementary 50 to 5# of particulate aluminium, and containing no chemical oxidant, at a loading of at least 0.25 grains of lead azide per foot of cord.

2. A cord according to Claim 1, wherein the loading of lead azide is at least 0.5 grains per foot of cord.

3. A cord according to Claim 1 or 2, wherein the loading is 1 to 20 grains of mixture per foot of cord.

4. A cord according to any of Claims 1 to >, wherein the lead azide is RD-13J3 or RD-1343.

5. A cord according to any of Claims 1 to 4, wherein the aluminium is rivet -grade aluminium powder.

6. A cord according to any of Claims 1 to 5, wherein the sheath is of lead.

7. A cord according to any of the preceding claims, wherein the mixture consists, by weight, of substantially 60# of lead azide and 40# of aluminium.

8. A cord substantially as hereinbefore described.

9. A cord substantially as described in any of the foregoing Examples.

10. A cord substantially as hereinbefore described with reference to Figure 1 .

11 . A process for forming an igniter cord, comprising intimately mixing together lead azide of purity at least 98 · # by weight and particulate aluminium to form a mixture consisting, by weight, essentially of 50 to 95# of the lead azide and the complementary 50 to 5# of aluminium, packing the mixture into a lead sheath, and then uniformly elongating the lead sheath and mixture so as to form a continuous column of mixture at a loading of at least 0.25 grains of lead azide per foot of elongated sheath.

12 . A process according to Claim 11 , wherein the lead azide is RD 1333 or RD 13^3.

13. A process according to Claim 11 or 12 , wherein the aluminium is rivet-grade aluminium powder.

14. A process according to any of Claims 11 to 13 , wherein the mixture consists, by weight, of substantially 60 of lead azide and 0# of aluminium.

15. A process for preparing an igniter cord substantially as hereinbefore described.

16. A process substantially as described in any of the foregoing Examples.

17. An igniter cord prepared according to the process claimed in any of Claims 11 to 16.

18. A propellant charge containing within a suitably-shaped cavity an igniter cord according to any of

19. A charge according to Claim 18, in substantially annular form with a substantially cylindrical cavity containing the igniter cord. ■? . ; .

20. A charge accordin to Claim 18 or 19, where-in an igniter composition, such as is. more readily ignitable.;' than the propellant is disposed between the cord and the propellant.

21. A charge according to any of Claims 18 to 20, in the form of a combustible artillery shell.

22. A charge according to any of Claims 18 to 20, in the form of a rocket engine. ,

25. A charge containing an igniter cord substantially as hereinbefore described.

24. A charge substantially as hereinbefore described and illustrated in any of Figures 2 to 6. Dated this 9th day of February, 1965 3?or the Applicants DR. REIHHOLBACOEM ί