Pulse forming apparatus

J. V. MCNULTY ETAL

PULSE FORMING APPARATUS

3,255,366

Sheets-Sheet 1

June 7, 1966

Filed Nov. 25, 1960

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INVENTORS JOHN V. McNULTY LOUIS J. KNUDSON DAJ/D J. WRJGHT

ATTORNEY

June 7, 1966 ^33,000

J. V. MCNULTY ETAL

PULSE FORMING APPARATUS

Sheets-Sheet 2

Filed Nov. 25, 1960

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INVENTORS / McNulty J. KNUDSON

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JOHN LOUIS ^m rx DAV/D J.WR/GHT

ATTORNK'Y

3,255,366

Patented June 7, 1966

United States Patent Office

3,255,366 PULSE FORMING APPARATUS

John V. McNulty, Louis I. Knudson, and David J. Wright, Norwich, N.Y., assignors to General Laboratory Associates, Inc., Norwich, N.Y., a corporation of New York Filed Nov. 25,1960, Ser. No. 71,807

Claims. (Cl. 307—106)

This invention relates to apparatus for initiating an explosion or igniting combustible material, by the use of heat and/or shock waves, and particularly to electrical apparatus for producing heat and/or shock waves at a location remote from the source of electrical energy.

Electrically-operated devices for producing heat and/ or shock waves are generically termed initiators and have been in use for many years for initiating combustion or for detonation of explosives. One common form of initiator is termed an exploding bridge wire, and consists of an unsupported section of wire bridging a gap. The bridge wire has physical characteristics such that when it is energized by an impulse of given intensity (i.e., relatively steep wave front, relatively high current), it is immediately vaporized, thereby releasing heat and causing a shock wave in its immediate vicinity. Such bridge wires, when so energized, are used to initiate combustion or detonation of explosives which are relatively insensitive to shock and heat. Similar bridge wires are sometimes energized with impulses of lower intensity (i.e., smaller currents of longer duration) to ignite combustible materials and to ignite relatively sensitive explosives.

A transmission line between a source of electrical energy and a bridge wire tends to distort and attenuate a steep wave front impulse produced at the source. As the distance between the source and the bridge wire increases, the distortion and attenuation in the transmission line increases. This distortion and attenuation is of no importance in connection with bridge wires which are energized by low intensity impulses. On the other hand, such distortion and attenuation has in the past limited the practical distance between the source and the bridge wire, where high intensity energization is used. One particular factor in the distortion which is troublesome is the decrease in the steepness of the wave front. Another troublesome factor is the decrease in amplitude which is characteristic of attenuation.

One of the many uses for initiators of the type described is for starting rocket engines. As rocket engines increase in size, the point of initiation is moved farther and farther from the source of electrical energy. In many current rockets, the distance in question is of the order of 50 feet. In other types of installations, the distance may be even greater. A transmission line of that length may. severely· distort and attenuate a steep front energy pulse so that when the pulse reaches the initiator, its wave front may be reduced in slope. Consequently, the shock wave produced by the initiator may be reduced in intensity so as to result in a failure to fire.

The electrical transmission system between the source of electrical energy and the initiator is subject to accidental pickup of electrical energy from external sources. Such external sources may be either static electricity, or electromagnetic fields, particularly from communication and radar systems, or power circuits, which may be either alternating or direct current. When electrical energy is accidentally picked up by the transmission system, there are two possible types of failure at the initiator.

The most serious type of failure which may occur at the initiator is an accidental explosion or initiation of combustion at a time when it is not desired. The other type of failure is serious although not usually dangerous. It is commonly termed “dud-out” and consists of a destruction of the initiator structure, for example, the exploding bridge wire. Such destruction may take place by electrical energization with a current insufficient in magnitude to produce the heat and/or shock waves necessary to start an explosion. If an exploding bridge wire is destroyed in this manner by stray energization, then when it is desired thereafter to produce an explosion, the explosion will not be initiated because of the absence of the bridge wire.

An object of this invention is to provide an improved system for transmitting a steep front electrical pulse, with minimum distortion and attenuation, to an electrical device responsive to such .a steep front pulse and located remotely from the source of electrical energy.

Another object is to provide an improved transmission system of the type described including means to inhibit the delivery to the load device of accidentally picked up electrical energy.

Another object is to provide an improved transmission system of the type described, in which the load device is an initiator.

The foregoing and other objects of the invention are attained in the apparatus described herein. This apparatus includes a transmission system having at the receiving end of the transmission line a receiving unit including a capacitor connected between the conductors of the transmission line, and, in parallel with the capacitor, an enclosed spark gap connected in series with a steep front pulse responsive initiator. When an electrical pulse is sent over the transmission line, it must first charge the capacitor, before any energy can be delivered to the initiator. When the capacitor charges to a potential equal to the breakdown potential of the enclosed gap, the energy stored on the capacitor is discharged through the initiator in the form of a pulse having a steep front. Thus even though the original pulse may be attenuated in the transmission line, the pulse which is actually delivered to the initiator does have a steep wave front.

In one modification of the transmission system, after taking into account the distributed inductance and capacitance of the transmission line, an inductance coil is coordinated with the line resistance and the capacitance of the capacitor at the receiving end of the line in order to provide a potential iricreasing effect.

In another modification of the transmission system, the source of energy feeding the transmission line is controlled by a potential which is fed back from the capacitor in order to maintain the charge of the capacitor at a predetermined potential approaching the breakdown potential of the enclosed gap.

Other modificaitons of the transmission system are shown, including arrangements for supplying a plurality of initiators over one transmission line.

Several of the transmission systems disclosed include further improvements with respect to prevention of accidental energization of the initiator by stray fields, etc.

Other objects and advantages of the invention will become apparent from a consideration of the following specification and claims, taken together with the accompanying drawing.

In the drawing:

FIG. 1 is an electrical wiring diagram of a transmission system which is a feature of the present invention;

FIG. 2 is an electrical wiring diagram of a modification of the circuit of FIG. 1;

FIGS. 3, 3A and 4, 5, 5A, 6 and 7 are electrical wiring diagrams of further modifications of the circuit of FIG. 1;

FIG. 8 is a somewhat diagrammatic illustration of a prior art type of initiator; and

FIG. 9 is a cross-sectional view of another form of initiator of the semiconductor gap type, which may be used in the systems of the present invention.

3,255,366

FIG. 1

This figure illustrates an electrical circuit including a charge generator shown diagrammatically at 1, a storage capacitor 2, a switch 3, a shielded transmission line 4 including a central conductor 5 and a shield 6 connected to ground at 7, and a receiving unit including a capacitor 8 connected between the wire 5 and ground and an enclosed spark gap 9 connected in series with an initiator 10. The series group including the gap 9 and the initiator 10 is connected in parallel with capacitor 8 between the receiving end of the conductor 5 and ground.

The charge generator 1 may be of any suitable type. For a simple illustration, it may include a battery and a conversion device for changing the energy available at the battery to more appropriate potential levels, and charging the capacitor 2 with the converted energy. A suitable charge generator is illustrated in FIG. 7 and described in detail below.

The switch 3 may be any device suitable for opening and closing the circuit through the transmission line 5. For instance, switch 3 may be a mechanical switch or any other device capable of shifting between a low impedance condition (e.g., closed circuit) in which the capacitor 2 sends a relatively large current through the wire 5 and a high impedance condition (e.g., open circuit) in which substantially no current is sent through the wire 5. For example, switch 3 might be an electronic switch or a trigger and gap arrangement as shown in FIG. 3 and described below.

The initiator 10 may be one of the devices illustrated in FIGS. 8 and 9.

OPERATION (FIG. 1)

When the switch 3 is closed, a current impulse starts flowing through the conductor 5 to the capacitor 8. This impulse may appear as having a steep wave front at the switch 3, but it is attenuated by the transmission line 4, so that at the capacitor 8 it appears with a wave front having a more gradual slope. As the capacitor 8 charges, the potential across it eventually reaches the breakdown potential of the gap 9, at which time the capacitor 8 discharges and transmits an impulse with a steep wave front to the initiator 10. Capacitor 8, gap 9, and initiator 10 should be in as close proximity to each other as is conveniently possible, to ensure maximum steepness of the wave front at the initiator 10.

It may be seen that the transmission system of FIG. 1 delivers a steep front electrical impulse to the initiator 10 regardless of the fact that the steep front impulse which appeared at the output side of the switch 3 may have had its wave front substantially distorted before it reached the receiving end of the line 3. The system may be regarded as a pulse forming system for delivering a pulse of desired shape to the initiator 10.

FIG. 2

This figure illustrates a modification of the circuit of FIG. 1 in which an inductor 11 is inserted in series with the transmission line conductor 5. The inductor 11 is coordinated with the line resistance and the capacitance of capacitor 8 so as to produce a potential increasing or “voltage doubling” effect, thereby enabling the use of a charge generator 1 having an output potential lower than the potential of the enclosed gap 9.

The inductor 11 reduces the amplitude of the current flowing through the transmission line. Since the losses in the transmission line vary as the square of that current, this indicative loading is effective to reduce the power losses in the line and thereby to insure the delivery of a greater potential to the receiving units.

FIG. 3

The circuit of FIG. 3 differs from the circuit of FIG. 1 by the substitution of a different transmission line generally indicated at 12 for the transmission line 4 of FIG.

and by the replacement of the generalized switch 3 of FIG. 1 with a trigger input transformer 13 and an enclosed gap 14.

The transmission line 12 comprises two conductors 16 and 17 enclosed within a shield 18, which is grounded at 12α. The shielded transmission line 12 provides a balanced line with both conductors 16 and 17 at the same potential with respect to ground. By virtue of the shield 18, stray ground currents do not pass through the conductors 16 and 17 and hence do not supply any energy to the initiator 10. The shielding is also effective to keep out stray electromagnetic fields and to prevent accidental contact between conductors 16 and 17 and power circuits.

By using the gap 14 and the trigger transformer 13, the load of the transmission line is kept off the storage capacitor 2 until the breakdown potential of gap 14 is reached. That breakdown potential is selected so that it is not exceeded by the charge generator 1, but is only exceeded by a breakdown signal supplied through the trigger transformer 13. Accidental supply of energy from the charge generator 1 to the initiator 10 is thereby prevented.

The transformer 13 is effective to load the line 12 inductively in the same manner that inductor 11 of FIG. 2 loads the line 4.

FIG. 3A

This figure illustrates a modification of the circuit of FIG. 3, which utilizes a three-point gap 14α in place of the two-point gap 14 of FIG. 3. A three-point gap may be desirable in some installations because it can be fired with less energy than in the case of the two-point gap. However, the two-point gap has a higher breakdown potential and hence an increased safety factor. For that reason, the two-point gap is preferred for most installations.

FIG. 4

This circuit illustrates an arrangement for energizing two initiators 10α and 10&amp; from a single source through a single transmission line. The transmission line is indicated diagrammatically at 4 and may be the same as the transmission line 4 of FIG. 1. Alternatively, a balanced and shielded transmission line such as that shown in FIG. 3 may be used. The receiving end of the transmission line is connected to the mid-point of a balancing transformer 19. The opposite end terminals of the transformer 19 are connected respectively to receiving units 20 and 21. The receiving unit 20 comprises a capacitor 8α, a gap 9α and an initiator 18α, while the receiving unit 21 comprises a capacitor 8Z&gt;, a gap 9b, and an initiator 10Z&gt;. The capacitors, gaps and initiators of the respective receiving units correspond to the capacitor 8, gap 9 and initiator 10 of FIG. 1.

The transformer 19 of FIG. 4 acts as a splitting transformer and maintains equal currents to the load capacitors 8α and Sb. This insures that the two capacitors charge at nearly the same rate, so that the time differential between the breakdown of the initiator spark gaps will be very small, i.e., of the order of a few microseconds. Furthermore, if one gap fires before the other, the current in that one half of the balancing transformer is effective to induce additional potential in the other half of the transformer winding so as to increase the current into the receiving unit which has not yet fired.

FIG. 5

In this figure, there is shown an arrangement for supplying four receiving units numbered respectively 22, 23, 24 and 25 over a single transmission line 4. The transmission line 4 is connected to the mid-point of the primary winding 26 of a transformer 27 having secondary windings 28 and 29 mounted on separate legs of the transformer core. The respective end terminals of the primary winding 26 are connected to the mid-points of the secondS,255,366

S ary windings 28 and 29. The receiving units 22 and 23 are connected to the end terminals of winding 28, and the receiving units 24 and 25 are connected to the respective end terminals of the winding 29.

The transformer 27 acts in the same manner as the balancing transformer 19 to balance the load currents between the two groups of receiving units 22 and 23 on the one hand and 24 and 25 on the other hand. Furthermore, it acts to balance the load currents between receiving unit 22 and receiving unit 23 and also between receiving unit 24 and receiving unit 25.

Flux induced in the transformer core by primary winding 26 has two parallel paths, one through winding 28 and one through winding 29. If either secondary winding 28 or secondary winding 29 is heavily loaded, as when the initiators fire, the other parallel path remains effective to energize the unloaded secondary winding.

The transformers 19 and 27 of FIGS. 4 and 5 act as inductive loads on the transmission line in the same manner as the winding 11 of FIG. 2.

There is shown in FIG. 5A, a transformer 27α corresponding to the transformer 27 but having a somewhat different core structure. The center leg of the core on which the winding 26 is wound actually consists of two separate cores spaced apart by an air gap. The winding 28 is wound on one of the two cores and the winding 29 on the other.

A plurality of loads may be connected to a single source in multiples of two, as shown in FIGS. 4 and 5. In the case of an odd number of loads, one half of one of the transformer windings such as 28 or 29, may be left open without affecting the operation of the system.

Although the circuits of FIGS. 4 and 5 are shown with the grounded transmission line of FIG. 1, they may be used with equal facility with the balanced and shielded transmission line of FIG. 3.

FIG. 6

Another possible method of connecting two receiving units to the same transmission line is illustrated in this figure. One of the receiving units is connected to the transmission line 4 through an inductor 30 and the other receiving unit is connected to the transmission line 4 through an inductor 31.

While this arrangement will effectively divide the load between the two receiving units, it does not have the effect of providing additional triggering voltage to a receiving unit which may be lagging.

FIG. 7

This figure shows a circuit similar to that of FIG. 3. This circuit differs from FIG. 3 in that the details of the circuits in the charge generator 1 are shown. The charge generator .1 includes an oscillator generally indicated at 32 supplied from battery la through a voltage regulator 33. The voltage regulator is controlled by a feed-back from the potential across the capacitor 8. In this modification, capacitor 8, instead of being normally discharged, is maintained nearly at the breakdown potential of the gap 9. Thus the trigger pulse from transformer 13 required to break down the gap and initiate the explosion is reduced in magnitude as compared to the trigger pulse required in FIG. 3 and the other modifications. Consequently, the time required between the application of the trigger pulse to the transformer 13 and the initiation of the explosion may be substantially reduced in this figure below the time required in the other figures.

The voltage regulator 33 comprises a transistor 34 having a collector 34c, an emitter 34c and a base 34b. Collector 34c is connected to the positive terminal of battery la. Base 34Z&gt; is connected through a resistor 35 to the grounded negative terminal of battery la. Emitter 34e is connected to oscillator 32 at the center tap of a winding 36 on a feed-back transformer 37.

The oscillator 32 includes two transistors 38 and 39 having emitters 38e, 39c, bases 386, 39b and collectors 38c, 39c, respectively. The collectors 38c and 39c are connected to the opposite end terminals of primary winding 36 of the feed-back transformer 37. Feed-back transformer 37 has a secondary winding 49 whose opposite end terminals are connected to the bases 386, 39b of transistors 38 and 39. The emitters 38e and 39e are connected to the opposite end terminals of a primary winding 41 of an output transformer 42. Primary winding 41 has a midpoint connected to the midpoint of the secondary winding 40 of the feed-back transformer and through a wire 43 to ground.

The output transformer 42 has a secondary winding 44. A capacitor 45 and a diode 46 are connected in series across the secondary winding 44. A diode 47 has its anode connected to the junction between the capacitor 45 and the cathode of diode 46. The anode of diode 46 is connected to conductor 17. The cathode of diode 47 is connected to the other conductor 16 of the transmission line 12.

A bleeder resistor 48, which may be several megohms, is connected across the capacitor 8. A movable tap 48a on the resistor 48 is connected to an amplifier 49. The output of the amplifier 49 is impressed across the resistor 35 in series with the base 34b of transistor 33.

OPERATION OF FIG. 7

The oscillator 32 produces a square wave output which is impressed on the terminals of the primary winding 41. The oscillator 32 is essentially a relaxation oscillator. The core of the feed-back transformer 37 is of material having a substantially rectangular hysteresis loop. Consider that the current through the transistor 38 is zero and increasing in the positive sense. As the current increases no change in flux in the core of transformer 37 takes place until the current reaches a corner of the rectangular loop. Beginning at that time, the flux reverses rapidly and at a rate of change which is substantially constant. Because of this constant rate of change the potential induced in the secondary winding 40 is substantially flat-topped, i.e., it is a square wave. When the flux in the core of transformer 37 reaches a point where the core is saturated, no potential is induced in the secondary winding 40. Consequently, the input current to the transistors is cut off and the output current falls off, rapidly reversing due to the collapse of the magnetic field in the transformer 37 and inducing a potential in the secondary winding 40 in the opposite sense. The square wave in the windings of transformer 37 controls the output wave form. The transformer 42 has closely coupled windings and does not saturate, so that it has little effect on the wave form. The current in the windings of transformer 37 is determined chiefly by the load on the secondary winding 44 and is substantially constant during each half cycle.

During the half wave when the upper terminal of secondary winding 44 is negative, current flows from that winding through the diode 46 and charges the capacitor 45 with its right hand terminal positive. The winding 44 and capacitor 45 together act as a voltage doubler, to develop across that capacitor a transient inverse potential equal to twice the applied potential. During the opposite half waves, current flow through the diode 46 is blocked but the current can flow from winding 44 and capacitor 45, which now act as potential sources in series aiding, through diode 47 and the transmission line 12, thereby charging capacitor 8.

A proportion of the potential on capacitor 8 is fed back at a low potential and substantially zero current to the amplifier 49 whose output is impressed across the resistor 35. The potential across resistor 35 controls the current flow from, battery la through transistor 34 and thereby determines the amplitude of the oscillations of the oscillator 32. The circuit is effective to maintain

3.255.366 /

the charge on capacitor 8 at a potential determined by the setting of tap 48a on resistor 48.

The discharge of capacitor 8 through gap 9 and initiator 10 is controlled by a trigger signal through transformer 13, as described above in connection with FIGS. 1 and 3.

FIG. 8

This figure illustrates an exploding bridge wire initiator typical of the prior art, and such a device may be used as the initiator 15 in any of the circuits of FIGS. 1 to 7. It consists of a wire 5G of material having a high temperature coefficient of resistance, bridging a gap between two support wires 51, 52. The wire 50 may be welded or soldered to the support wires. The wires 51, 52 pass through and are supported by an insulating block 53. Upon subjection to an electrical impulse of predetermined energy, the wire 50 vaporizes, producing heat and a shock wave in its immediate vicinity.

FIG. 9

This figure illustrates a modified form of initiator device, with which the initiator systems described above may be used. This initiator is described in greater detail and claimed in a copending application for United States Letters Patent of John V. McNulty, Louis I. Knudson and Edward L. Mooney, Serial No. 71,650, filed November 25, 1960, entitled “Initiator Devices,” now Patent No. 3,196,042.

The initiator of FIG. 9 includes a hollow cylindrical metal shell 60, open at its left-hand end and having an inwardly projecting flange opening 60α at its right-hand end defining a central opening 606. An annular member 61 of semiconductive material, for example, ferrite, is inserted through the open end of the shell 60 and abuts the flange 60α, with the central opening of the annular member 61 aligned with the opening 606. An electrode 62 having an enlarged head 62α extends axially of the shell •60, with the head 62α inside the central opening in the annular semiconductive member 61, but with its end spaced from the flange 60α. An insulating member 63, of ceramic material, is inserted through the open end of the shell 60 and abuts against the annular member 61 and head 62α. The several parts may be held firmly together by appropriate bonding material.

It may be seen that the initiator of FIG. 9 provides a gap between the head 62α of the electrode 62 and the flange 60α on the metal shell 60, which serves as the other electrode of the gap. The gap is defined on one side by the inner surface of the annular semiconductive member 61.

The initiator of FIG. 9 may have its gap end immersed in a solid, explosive material which it is desired to ignite or detonate. When an appropriate potential is applied across the electrodes, the gap first breaks down along the surface of the semiconductive member 61. After that initial breakdown,, a typical spark discharge occurs between the two electrodes. The spark discharge is accompanied by production of substantial heat and by a substantial shock wave in the explosive material, resulting in the initiation of combustion or explosion, depending upon the nature of the material in which it is immersed.

The semiconductor initiator of FIG. 9 has a substantial advantage over the bridge wire initiator of FIG. 8, in that the semiconductor initiator can be tested with a full scale spark discharge without destroying its effectiveness. When the bridge wire type of initiator is tested with full scale energization, the bridge wire is destroyed, and it thereafter cannot initiate an explosion. The semiconductor initiator of FIG. 9 is consequently much more reliable and dependable than the bridge wire initiator shown in FIG. 8.

The pulse forming systems described above for energizing initiators may use either the bridge wire type of initiator shown in FIG. 8 or the semiconductor initiator of FIG. 9. Other suitable types of semiconductor initiators are disclosed and claimed in the copending application of McNulty, Knudson and Mooney, Serial No. 71,650, referred to above.