US 3090310 A
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M y 1963 (5. w. PEET ET A]. 3,090,310
CONDUCTIVE EXPLOSIVE PRIMER MIXTURE AND DEVICE Filed May 4, 1960 INVENTORS. GEORGE W. PEET 3,090,310 Patented May 21, 1963 Free 3,090,310 CUNDUCTEVE EXPLOSIVE PRIMER MIXTURE AND DEVICE George W. Peet, Mount Rainier, and Leo F. X. Gowen, Silver Spring, Md, assignors to the United States of America as represented by the Secretary of the Navy Filed May 4, 1960, Ser. No. 26,921 6 Claims. (Cl. 102-46) (Granted under Title 35, [1.5. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This application relates to the ordnance art and is more particularly concerned with a new and improved conductive explosive primer mixture for rapid fire guns.
Many types of modern aircraft machine guns employ a primer mixture which is electrically conductive so that when an appropriate current is passed through the mixture, the primer mixture becomes heated until its ignition temperature is attained. It then defiagrates. The deflagration of the primer mixture and the evolution of hot solids and gases ignite the propellant in the cartridge case to fire the slug.
The electrically conductive primer mixtures of the prior art were not found to be entirely satisfactory for several reasons. The primer upon ignition did not generate sufficient energy to reliably ignite the propellant powder in the cartridge. Furthermore, many of the conductive explosive primers were sensitive to the discharge of stray electrostatic energy which may come from several sources. This is undesirable since it may lead to premature firing of the cartridges.
It is an object of this invention to provide a new and improved conductive explosive primer mixture for use with rapid fire guns.
Another object is to provide a new and improved primer for machine guns which is not ignited by stray electrostatic currents.
A further object of this invention is the provision of a primer mixture for aircraft machine gun ammunition which may be fired electrically and which produces sufficient energy upon ignition to reliably ignite the propellant in the cartridge.
These and many other objects will become more readily apparent when the following specification is read and considered along with the attendant drawing which indicates a typical aircraft machine gun cartridge including the explosive primer mixture of this invention.
As seen in the figure, a cup 11 is centrally disposed in a primer pocket formed at the rearward end of the cartridge 12 which contains the propellant 13. Within the rearward portion of cartridge 12 is an electrically conducting button 14 separated from cup 11 by an insulating member 16.
Disposed forwardly of the conductive button '14 is a thin paper disc 17 which is pressed into cup 11 during manufacture to compact the electrically conducting primer mixture 18 within the electric primer assembly 10. The primer mixture is first disposed within the assembly and the disc 17, which is of slightly larger diameter than the internal diameter of cup 11, is pressed onto the primer mixture 18. It may be desirable in some circumstances to coat the disc 17 with a thin coating of varnish or shellac on the side facing the primer mixture. This serves to seal the disc and hold it firmly in place. Finally a cup support 19 is pressed Within cup 11 and protrudes slightly therebeyond in primer pocket 15. This cup support has a central aperture 21, so that upon initiation of the primer, hot gases and solids rupture disc 17 and spew through aperture 21 to initiate propellant 13 in cartridge 12. Cup support 19 serves to prevent the collapse of cup 11 and retain the primer mixture properly in place.. H
The propellant mix 13 which propels the slug of the ammunition fills the propellant cavity 23 of the cartridge 12.
The propellant cavity 23 is connected to a spit hole 24 formed in the base of cartridge case 12. The spit hole is in turn connected to the interior of cup support 19.
In operation, the device functions in the following manner: an electrical voltage power supply shown diagrammatically at 26 is applied across button 14 and case 12, both of which are electrical conductors. It should be noted that the insulator 16 serves to electrically separate the cup 11 and the button so that the only electrical contact between them is through the priming mixture 18. The resistance between cup 11 and button 14 (through mixture 18) when measured with an ohmmeter employing a 6 volt D.C. source and a short circuit current of two milliamperes, should measure at least 1000 ohms.
It should be apparent that it would be objectionable for the primer to be sensitive to random or accidental electrostatic discharge since such discharges between cup 11 and button 14 might prematurely ignite the primer.
It has been found that a propellant of the following composition is suitable for reliably igniting a typical propellant such as ball powder and isrelatively insensitive to stray electrostatic leakage while at the same time being reliably initiated by the aircraft firing system:
Zirconium, fine, 15 parts per l00+5%0% dry weight Zirconium hydride, 30 parts per :2 /2% dry weight Barium nitrate, 15 parts per 100i2 /2% dry weight Lead peroxide, 20 parts per 100i2 /2% dry weight Pentaethyeride tetranitrate (PETN), 20 parts per 100 dil /2% dry weight 7 It will be noted that this mixture differs from the conventional electroconductive explosive primer mixtures in that it contains a substantial percentage of zirconium hydride. The prior art compositions consisted of the primary electroconductive oxidizing agent, such as the lead peroxide, a finely divided metal, such as the zirconium, a secondary oxidizing salt, such as barium nitrate, and a crystalline organic high explosive, such as the PETN. It has been found that the zirconium hydride acts as an electrostatic desensitizing agent, that is, it prevents ignition by the accidental discharge of electrostatic energy through the primer. In contrast with prior art prim ers, which could be initiated with as little energy as 2000 ergs, the primer of this invention cannot be initiated by a discharge of 800,000 ergs from a 400 mmfd. condenser. Accordingly, this primer is not susceptible to premature initiation. On the other hand, thousands of rounds have been fired from aircraft machine guns without an objectionable number of misfires, therefore reliability of performance has not been sacrificed for safety. The process for manufacturing the priming mixture is shown in the following example.
The PETN was washed through an 8 mesh sieve with water and excess water squeezed from the PETN so that it contained about 30% moisture. The zirconium was washed by covering it with Water for at least 12 hours. The water was then .decanted and it was found that there was about 30% retained water in the zirconium. The zirconium hydride was sieved through a 9 xx sieve. The barium nitrate and the lead peroxide was premixed by hand rolling on a paper blanket A Hobart mixing machine was employed to blend the primer composition. Initially the wet fine zirconium was placed in the mixing bowl with the PETN and sufficient water (about 6 oz.) was added to adjust the moisture content from about 10% to about 11%. This mixture was then mixed for two minutes by remote control. The mix was tamped into the bowl and the zirconium hydride was added. Mixing then continued for two minutes. The mixture was tamped in the bowl and the blended premix of barium nitrate and lead peroxide was added and the entire mixture was mixed for three minutes. The resulting material was then cupped into the appropriate primers. It should be understood that all safety precautions Were observed during the preparation of the primer mixture and the scrap material was disposed of by burning where necessary.
Primers manufactured according to the process of Example I were tested to determine their sensitivity to electrostatic discharge. The testing apparatus consisted of charging a 400 mmfd. condenser from a DC. source and discharging the stored energy across the test primer via a small air gap. It was found that this material could not be initiated even when 800,000 ergs were in the electrostatic apparatus. In contrast, mixtures not containing the zirconium hydride were found to be initiated at low energy levels such as 2,000 ergs. In aircraft machine guns, it would not be advisable to employ these prior art mixtures which can be ignited so readily by stray electrostatic discharge.
The composition has been found to be critical. The zirconium preferably should be about 15 parts per hundred and may be increased above that figure. If the amount of fine zirconium is decreased it has been found that the functioning time, i.e. the time required for the primer to be ignited by the igniting current is too long at lower percentages of zirconium. The percentages of the other constituents may vary within acceptable industrial practice to about plus or minus 2.5% of their nominal values. The fine balance among quick functioning upon application of the igniting current, sensitivity to stray electrostatic discharge, and the ability to reliably ignite the propellant grain are disturbed if the composition is altered more than a few percent.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. An electrically conductive explosive primer mixture consisting essentially of an electrically conductive primer composition of a heavy metal peroxide as an electroconductive oxidizing agent, a finely divided metal, an oxidizing salt, and a crystalline organic high explosive and about 30 parts by weight of zirconium hydride to pre- 4 vent stray electrostatic initiation of the primer composition.
2. A composition consisting essentially of 15 parts per hundred of fine zirconium powder, about 30 parts per hundred of zirconium hydride, about 15 parts per hundred of barium nitrate, about 20 parts per hundred of lead peroxide, and about 20 parts per hundred of PETN on a dry basis.
3. The composition of claim 2 wherein the moisture content is from about 10% to 11% 4. An electrically conductive primer consisting essentially of an electrically conductive primer composition of a heavy metal peroxide as an electroconductive oxidizing agent, a finely divided metal, an oxidizing salt, and a crystalline organic high explosive and from about 27.5% to about 32.5% zirconium hydride.
5. An electrostatic discharge proof primer for use with a cartridge containing a propellant which comprises; cup means having a rearwardly disposed aperture therein, an electrically conducting button disposed within said cup and extending into the aperture thereof, insulating means electrically separating said button and said cup, an electrically conducting priming mixture having a resistance of at least 1,000 ohms disposed in electrical contact between said button and said cup, means for producing a voltage across said cup and said button through said priming mixture, said priming mixture consisting essentially of an electrically conductive primer composition of a heavy metal peroxide as an electroconductive oxidizing agent, a finely divided metal, an oxidizing salt, and a crystalline organic high explosive and about 30% of the zirconium hydride to prevent accidental initiation of the primer mixture by electrostatic discharge.
6. A discharge proof electric primer for use with a cartridge comprising a priming mixture consisting essentially of an electrically conductive primer composition of a heavy metal peroxide as an electroconductive oxidizing agent, a finely divided metal, an oxidizing salt, and a crystalline organic high explosive and about 30% of zirconium hydride to prevent accidental initiation of the primer mixture by electrostatic discharge, a primer casing for holding said mixture, an electrical conducting means attached to said casing connected to said mixture whereby an initiating electric current might be passed through said mixture.
References Cited in the file of this patent UNITED STATES PATENTS 319,628 Russell June 9, 1885 712,826 Mason Nov. 4, 1902 1,084,745 Lindsay Jan. 20, 1914 2,038,097 Burns Apr. 21, 1936 2,194,480 Pritham et a1 Mar. 26, 1940 2,293,373 Herz et a1. Aug. 18, 1942 2,380,312 Johnson July 10, 1945 2,970,047 Ciccone Jan. 31, 1961 2,988,876 Walden June 20, 1961 2,990,683 Walden July 4, 1961 OTHER REFERENCES Military Explosives, TM 9-1910, TO11A-1-34, April 1955, p. 283.