Means for protecting electroexplosive devices which are subject to a wide variety of radio frequency

An RF attenuator for attenuating RF signals in a lead particularly for prcting against unintentional detonation of electrically initiated ammunition is presented. A firing lead is embedded in a body of ferrite material. The firing lead is formed in a planar spiral configuration with reversals of direction.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates generally to electrically fired explosive 
devices, and more particularly to control elements for protecting small 
electrically fired, fixed case ammunition from the hazards of unintended 
firing due to stray radio frequency energy. 
2. Background Art 
Electroexplosive devices such as electric blasting caps, squibs and 
detonators are used in many contexts, such as blasting operations, 
ammunition and the like. Electroexplosive devices include at least one 
electrical ignition device disposed in ignition relationship with one or 
more heat-sensitive explosive charges, such as first fire mixtures, and 
are fired by passing a D C current through a pair of leads connected to a 
filament or bridge of high electrical resistance which is in heat 
transferring contact with the first fire mixture. A sufficient flow of 
current heats the bridge wire to indescance thereby igniting the 
surrounding mixture. The energy generated from ignition of the mixture is 
then used to ignite a sequence of pyrotechnic and/or explosive charges 
which in turn can ignite or detonate other charges. 
These electroexplosive devices are subject to unintended discharge by stray 
electromagnetic or electrostatic energy. Therefore, electric firing 
techniques have included procedures intended to minimize this possibility 
and to protect individuals in the vicinity of these devices. However, the 
value of such precautionary measures is diminished because it is difficult 
to predict the extent of the electromagnetic radiation hazard from one 
moment to the next and the levels of electrical hazards are steadily 
increasing. 
Prior attempts at solving the problems associated with electroexplosive 
devices caused by stray radio frequency energy have included decreasing 
the sensitivity of the bridge by designing the bridge to require very high 
firing currents for igniting the pyrotechnic chemical disposed adjacent to 
that bridge. This approach requires the use of heavy and expensive wiring 
and requires the use of power sources providing high energy levels. In 
addition to the increased expense associated with this approach, this 
approach still fails to provide adequate safety. 
In the past, most electroexplosive devices that are suitable for use in 
radiation hazards environments have used a filter and heat sink 
combination. The filter attenuates the radiation and the heat sink 
transfers heat generated during attenuation away from the bridge wire and 
explosive components. One such filter is disclosed in U.S. Pat. No. 
4,378,738 of Proctor et al. of a common assignee. However, the '738 
devices are too large to be compatible with fixed or semi-fixed ammunition 
and therefore extensive modifications would have to be made in order to 
adapt these devices to a small size. Such modifications are unacceptable 
for many reasons, including a concomitant requirement to alter various 
procedures associated with the manufacture of such devices and perhaps a 
requirement to alter existing firing circuits. Furthermore, to be 
effective, the filter must be coupled closely to the bridge wire leads and 
shielded from electromagnetic radiation leakage paths. Furthermore, in 
fixed case or semifixed ammunition unit cost is extremely important. Under 
many conditions, the heat sink associated with the presently known devices 
may not be large enough. However, adding external heat sinks may be 
impractical due to size, cost and use considerations. Furthermore, since 
the explosive output of the device is usually buried in a booster or 
explosive, there may be no available area for an additional heat sink. 
Due to these problems, it has been proposed to use ferrite beads to 
attenuate radio frequency energy. However, such approaches require use of 
capacitors in order to obtain broadband attenuation and even then the low 
frequency attenuation may be unacceptable. In such a case it was necessary 
to use a plurality of ferrite beads in series along each of the two 
electrical leads with capacitors connected between junctions of 
corresponding beads of the two leads thus forming a low-pass attenuation 
network. Still further attenuation problems arise because the ferrite used 
in these devices had low curie temperatures so that attenuation of even 
moderate radiation caused sufficiently high temperatures to vitiate the 
attenuation properties of the device. The use of capacitors, itself 
creates problems because the combined device and capacitor is too bulky to 
fit a small primer pocket. Even then, it is questionable whether a single 
capacitor will provide the device with the capability to cover a frequency 
spectrum of from about one megahertz to about eleven gigahertz as is 
required to include the known hazards. 
Accordingly, there is a need for a device which is effective in protecting 
small devices against the hazards associated with stray electromagnetic 
energy in a cost-effective manner. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to protect electrically fired, 
fixed case ammunition from stray radio frequency energy. 
It is another object of the present invention to protect small electrically 
fired, fixed case ammunition from stray radio frequency energy without 
requiring modification of existing electrically fired device firing 
circuits, designs or procedures. In this manner, existing equipment can be 
protected without undue expense or problems. 
It is still another object of the present invention to protect small 
electrically fired, fixed case ammunition from stray radio frequency 
energy without requiring additional capacitors and/or heat sinks. 
A further object of the present invention to permit use of simple, light 
weight gun and gun pod design to be used with electrically fired primers. 
These and additional objects are accomplished when an RF attenuating 
ferrite material similar to standard ferrite formulation MN-67 or the like 
is used as an attenuating body and a firing lead made of a conductive 
material in a portion of the body forms a planar, spiral configuration in 
the ferrite body. The maximum thickness dimension of the device can be 
small as compared to its maximum planar dimension and a plurality of loops 
of the firing lead can be defined. 
In order to fully protect small, fixed case electrically fired ammunition, 
attenuation of stray radio frequencies must be accomplished, and in 
addition, an electrostatic shunt mechanism should be used in connection 
with electrostatic buildup protection. It has been found that a choke can 
be used to attenuate RF energy. However, the energy level to be attenuated 
by a choke effects the size of that choke, and in order to provide 
adequate protection, especially if the protection is to be broadband, size 
becomes a factor which is an important consideration for small, fixed case 
electroexplosive devices, such as a fixed or semi-fixed case firing 
primer. Therefore, cylindrical chokes such as disclosed in U.S. Pat. No. 
4,378,738, are too large for such applications. It has also been found 
that lossy material such as MN-67 has reasonable attenuation at broadcast 
frequencies. The MN-67 ferrite has a high curie (450.degree. F.) 
temperature, a good trade-off of low frequency attenuation and broadband 
attenuation without detected resonant frequencies and is available in many 
shapes and sizes. Since higher RF attenuation is achieved when a 
conductive path through a ferrite body is lengthened, a tradeoff between 
the length of conductor required to produce the desired attenuation band 
protection and the size requirements dictated by small ammunition size, 
cost and heat transfer characteristics is made. Therefore, merely 
combining the MN-67 with a cylindrical choke as in the '738 patent will 
produce a device which is still too large for use in small primer devices. 
It was discovered as disclosed herein that a single firing lead can be 
wound in a spiral pattern located in a single plane and still produce 
capacitance effects similar to the capacitance achieved with cylindrical 
chokes also without detected resonance frequencies. Therefore, instead of 
making a device having cylindrical form having a high length to diameter 
ratio superior attenuation can be achieved in a much smaller device by 
placing a firing lead in the body material in a planar, spiral pattern. 
The number of spiral loops is adjusted to produce the maximum length of 
lead (for maximum RF attenuation) possible for the size of the device 
which is permitted by small fixed case ammunition. Specifically, it has 
been found in an alternate embodiment that by winding the lead through the 
ferrite core a number of times with two or more reversals of direction, 
the highest attenuation is achieved for the space available. Such a 
winding pattern permits the device to have an outside diameter selected so 
that when the device can be pushed into the primer pocket during 
manufacture with a fit snug enough to produce enough heat transfer to the 
case to inhibit the problems associated with heating of the ferrite 
material during attenuation without requiring an external heat sink. 
Furthermore, the device can be manufactured to be compatible with existing 
machinery, firing circuit design and procedures thereby overcoming many 
cost related problems, and external capacitors are not required, thereby 
contributing to the cost effectiveness of the device. It has also been 
found that because the exemplary MN-67 material is electrically 
nonconductive for all practical purposes, the need for electrically 
insulating washers is eliminated thereby contributing still further 
cost-saving advantages to the present invention. For purposes of this 
disclosure the body being nonconductive means much less electrically 
conductive than the firing lead.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As a background, FIG. 1 shows the after end of a typical fixed case 
ammunition shell generally designated 10 having a primer pocket 12 housing 
a protective device or attenuation element 14 A conventional electrically 
fixed primer 16 is pressed in as usual against bottom 18 of the primer 
pocket 12. The primer pocket 12 is placed in aft end of an exemplary g 
ammunition case 22. An attenuator element 14 is pressed in place into case 
22 so that: 
a. An input button 44 is sufficiently exposed so that it can come into 
contact with the electric firing means (not shown) associated with the 
case 22. 
b. Body 28 of element 14 is pressed into contact with the metal of case 22 
to dissipate the heat generated when the ferrite material of element 14 
attenuates stray RF energy. 
c. Output from an output button 32 is in contact with electrostatic 
dissipating tape 34 so that any excessive electrostatic potential between 
primer and ammunition case is bled off before it can inadvertently set off 
the primer. 
d. The electrostatic dissipation tape 34 is pressed tightly enough so that 
D.C. current can pass freely through the element 14 through the tape and 
into the primer setting it off in a reproducible manner. 
The primer pocket 12 can be sealed with a water resistant adhesive (not 
shown) to prevent moisture intrusion and/or to help minimize blowout of 
the primer when it is fired. First fire mix element 36 boosts the output 
of the primer. Blow out disk 37 holds the first fire mix in place until 
use when it then ruptures allowing the burning particles to rapidly and 
reproducibly ignite the propellant charge 38. 
The attenuator element 14 is the subject of the present invention and is 
best shown in FIGS. 2 and 3 to which attention is now directed. The 
attenuator element 14 includes a body 40 formed of ferrite material such 
as MN-67. The body 40 is disk shaped with a thickness substantially less 
than the circular diameter. However other configurations can be used, and 
the body 40 need not be a flat circular object. Thus, the ratio of 
dimensions is much smaller than heretofore known initiators such as that 
shown in U.S. Pat. Nos. 2,821,139, and 2,991,715. 
A single firing lead 42 is embedded in the body 40. Firing lead 42 extends 
from an output button 48 which abuts the primer 16 when the attenuator 
element is in place in the cass 22. 
The firing lead extends from the input button 44 wound in a planar spiral 
form to the output button 48 whereby current passing through the input 
button from a source (not shown) via a source lead (not shown) will flow 
to the output button 48 to ignite the primer. 
The spiral winding therefore defines a path in a single plane about a 
central point coincident with the output button 48. This shape is opposed 
to a helical path which would be a three-dimensional projection of the 
spiral winding out of the plane in which it is shown and also is opposed 
to a tubular coil such as shown in U.S. Pat. No. 2,821,139. The planar 
spiral winding permits the body to have a large diameter to thickness 
ratio and achieves a distributed capacitance between adjacent parts of 
path 42 having an effect superior to the discrete capacitors, and because 
of the compactness of the device, superior to helical paths or other 
winding configurations. It is speculated that the larger the number of 
loops and the closer the spacing between adjacent loops, the higher the 
attenuation. This however can be traded-off against the temperature rise, 
unintended stray capacitance, and reliability of the device. 
An alternate design is shown in FIG. 3 wherein there is shown a reversal of 
current direction 50 such that there is a reverse current flowing in an 
adjacent wire. There can be a plurality of current reversals and data 
indicates that such a plurality of reversals gives better results. It is 
not known if there is a point of diminishing returns on the number of 
reversals. 
It is noted that while MN-67 has been disclosed, any suitable RF 
attenuating ferrite can be used so long as it has a high curie temperature 
and low frequency attenuation properties. Non-electrically conducting 
ferrites are preferred to simplify the design. The firing lead can be any 
conductive material which can be formed into a planar spiral configuration 
without breaking so that a complete electrical circuit can be maintained 
after finishing the ferrite manufacturing process. However, lead materials 
having high after-processing electrical conductivity are preferred with 
conductive ferrites being more suitable than other materials such as 
metallic wires. The input and output buttons can be sized to cover as much 
of the associated body surface as desired. The size and/or shape of the 
attenuator body can be varied whereby different devices can be identified 
without the need to color code the firing lead. 
Thus there is disclosed an RF attenuator suitable for use with 
electroexplosive devices. A spiral conductive pattern is embedded within a 
generally non-conductive disk of ferrite material. The interaction of the 
magnetic field generated by current with the body of ferrite provides a 
long distributed inductance in the lead and the distributed stray 
capacitance between adjacent closely wound leads permits superior 
attenuation of stray electromagnetic energy inputted to the lead. 
Obviously, numerous 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 herein.