Apparatus For Remote Ignition Of Explosives

Abstract

Disclosed herein is a unique non-electric ignition device for sounding rockets, explosives or the like which involves energization of a fluidic conversion device embedded in the explosive charge to be ignited. The fluidic device, consisting essentially of a convergent nozzle and resonance tube, is connected through pneumatic tubing to a remotely located pump, valve, filter network. When operated, this network develops a pre-ignition pressure level which eventually reaches a threshold level sufficient to open a relief valve. At threshold the pressurized gas is applied to the convergent nozzle, which directs the gas toward the opening of the resonance tube. A system of self-sustaining oscillations of the gas particles is created in the tube which causes the closed end of the tube to rise in temperature. The end of the tube is surrounded by a pyrotechnic-ignition interface which ignites when the tube end temperature reaches a predetermined level. This interface then ignites the main propellant resulting in the firing of the sounding rocket, detonation of the explosive, or the like.

Description

BACKGROUND OF THE INVENTION

The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85:568 (72 Stat. 435; 42 U.S.C. 2457).

This invention relates to an explosive igniting device, and more particularly to a device for igniting explosives wherein a fluidic device converts the energy stored in a remotely connected pressure system to thermal energy and to thereby ignite the explosive composition.

Sounding rockets which are low cost rockets used by various governmental agencies to determine the meteorological conditions in the upper atmosphere are typically propelled to these heights by solid propellants. These relatively small rockets (approximately 7 feet high) can be moved about and fired by a single operator. Telemetric devices located in the payload section of these rockets sense and transmit back to earth meteorological information useful for the navigation of aircraft in the area, suitable for assisting in the determination of the high altitude trajectory of land-launched missiles or manned space flights, and other obvious applications.

The ignition system which initiates the firing of these rockets ideally must be inexpensive, safe, reliable, and relatively foolproof. In the past, some form of electro-explosive technique was employed. This involved the connection of electrical wires to the propellant igniter, which in turn was connected to a source of electrical energy such as a generator or battery. Alternately, radio frequencies might be generated which would trigger an appropriate receiving device located in the propellant igniter section which, responding to the transmitted radio waves, would generate intense heat, which in turn would ignite the propellant. These prior art ignition techniques suffer from a particularly serious safety deficiency. They are susceptible to unintentional ignition through sources beyond the control of the operator. Things such as electrostatic charge buildup, lightning, radio transmitters in passing autos or aircraft, or other electromagnetic field generating devices can, pg,3 if generating sufficient energy in the area of the sounding rocket, ignite the propellant to thereby cause an unintentional firing of the rocket. This, of course, has ominous consequences.

A recent development by the assignee of this application in the area of basic fluidic to thermal conversion devices, such as that described in U.S. Pat. No. 3,630,150 and 3,360,151, has enabled the development of an ignition device which eliminates the unintentional firing of the rocket, making it strictly the operator who controls when the rocket is to be fired.

SUMMARY OF THE INVENTION

It is, therefore, the object of this invention to provide a non-electric, fluidic ignition device for a remotely located explosive charge.

It is a further object of this invention to provide an ignition device which may not be initiated unintentionally.

It is a further object of this invention to provide an ignition device which uses the surrounding air as the initiating fluidic agent.

A non-electric system for remote ignition of an explosive charge which includes in combination, an energy source for developing a supply pressure of a predetermined level, a fluidic conversion device connected to the energy source through a fluidic network which includes check valves, a line filter, pneumatic tubing and a relief valve, whereupon in response to the opening of the latter due to the supply pressure exceeding the predetermined level, the fluidic device converts the pressure waves emanating from the relief valve into thermal energy which in turn ignites a pyrotechnic-explosive charge suitable to ignite the main propellant or explosive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A simplified, pictorial drawing of the invention as used in a sounding rocket.

FIG. 2: A detailed drawing of the energy source and connecting fluidic network as used in the subject invention.

FIG. 3: A detailed sectional view of the fluidic conversion device which forms part of this invention.

FIG. 4: A schematic view of the static pressure distribution at the exit of the nozzle of the fluidic conversion device in FIG. 3.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown in combination the functional elements which comprise the substance of this invention. A source of "energy," an air pump 10, is shown connected to a length of pneumatic tubing 12, through a valve, filter arrangement 14. The pneumatic tubing 12 is connected to a relief valve 16 which is calibrated to open at the threshold operating pressure for the fluidic conversion device 18. In this illustration, the fluidic conversion device is shown set in a sounding rocket 20, but it is to be understood that this may as easily be any explosive device, even such as dynamite. The relief valve 16 is connected to the fluidic device 18 through an additional length of pneumatic tubing 22. The fluidic device 18 is embedded in the main propellant 24, and in accordance with the principals of operation for the fluidic device 18, is vented to the outside atmosphere through venting tube 26.

Referring now to FIG. 2, a more detailed description of the energy source, valve filter arrangement, and relief valve may be discussed. All of the parts depicted in FIG. 2 are essentially of a standard nature but are configured in the arrangements of FIG. 2 so as to enable a human operator to raise the level of the air pressure at the exhaust port 28, which is connected to the input of the fluidic device 18 through tubing 22, to a level of air pressure suitable for igniting the propellant 24 when the fluidic device 18 converts the air pressure to thermal energy. A suitably sized pump 10, easily operated by a human operator typically might have a bore diameter d, of 35 mm with a 51 cm stroke. The exit port 30 of pump 10 is coupled to the exhaust port 31 of check valve 32 through coupling 34. The check valve 32 has an intake port 36. Additionally, the exhaust port 31 of check valve 32, is connected via coupling 38 to the input port 40 of a similar check valve, 42.

The exhaust port 44 of check valve 42 is connected via coupling 46 to the intake port 47 of fluidic switch 48. The fluidic switch 48 is a two position switch. One position of switch 48 is as shown in FIG. 2, wherein the plunger 50 is positioned below the exhaust port 52. The second position of switch 48 is when the plunger 50 is drawn to the top of bore 54, such that plunger 50 is now above exhaust port 52.

The exhaust port 56 of switch 48 is connected via coupling 58 to the input port 60 of line filter 62. The line filter 62 has a filter element 64, which typically is suitable for filtering particles on the order of 20 .mu. meters. The exhaust port 66 of filter 62 is coupled to a variable length of pneumatic tubing 12 through coupling 68.

The pneumatic tubing 12 typically, is made from a polyethylene, of suitable strength to withstand pressures of at least 150 psig. The other end of the variable length tubing 12 is connected via coupling 70 to the input port 72 of the relief valve 16.

Input port 72 is connected to an internal port 74 within the valve 16, by a duct 76. The internal port 74 is capped by a diaphragm 78 which is spring biased against the port 74 by the action of spring 80. The spring pressure maintaining the diaphragm 78 sealed against internal port 74 is adjusted by means of screw 81. Leakage oriface 82 connects the duct 76 to the outside air. The oriface 82 might have a diameter 0.001 inch to 0.003 inch diameter. This introduces sufficient leakage to provide protection against an unintended buildup or retention of gas pressure in the valve, tubing network. In this way only the operator performing the pressurization procedure from the beginning can initiate the ignition. The exhaust port 28 of relief valve 16 is connected via coupling 84 to the pneumatic tubing 22, which in turn connects the valve, tubing network to the fluidic conversion device 18.

Referring to FIG. 3 there is shown in detail the fluidic conversion device 18 embedded in propellant 24. The fluidic conversion device 18 is the key component in the fluidic ignition system. The fluidic device 18 is similar in construction and operation to the device described in U.S. Pat. No. 3,630,150 and 3,630,151. However, the designs described in the aforementioned patents were found to be suitable for use with high pressure, other than air, gas supplies and as such are not suitable for the application envisioned here, viz, actuating a fluidic conversion device with a low pressure air supply.

For its application in an ignition system for a rocket motor, it is desirable to manufacture the fluidic conversion device 18 from a combustible plastic material such that the device 18 burns up in the heat of the ignited propellant 24. This eliminates any possibility of clogging of the rocket motor exhaust nozzle 84 (see FIG. 1). A suitable material, which also minimizes heat transfer loss to thereby contribute to a fast reaction time, was found to be a glass-filled epoxy manufactured by the Hysol Company and marketed under the trade name, MG5F. The conversion device may be molded together with the propellant 24, or it may be installed or embedded in the propellant after the propellant is packed or while the propellant is being packed.

The fluidic conversion device 18, basically consists of two essential parts, a resonance tube (hollow cavity closed at one end) 85 and an excitation nozzle 86. The nozzle of the fluidic conversion device 18 is a simple, convergent type designed to produce the proper jet cell structure, as hereinafter described, necessary to obtain resonant heating in tube 85. The nozzle diameter 88 and the pressure ratio P.sub.O /P.sub.C across the nozzle influence the length of the jet cells, so that these two parameters may be utilized to determine the proper separation distance 90, between the nozzle 86 and the resonance tube 84. It is the location of the opening 91 of the resonance tube 85 in a particular jet cell as hereinafter described which gives rise to the heating effect. The nozzle diameter 88 also determines the rate at which the supply pressure in pneumatic tubing 12 drops when the relief valve 16 is opened. If the pressure drops too quickly, i.e., too large a nozzle diameter, resonance heating will take place for a time too short to develop ignition temperatures. A suitable pressure ratio across the nozzle, P.sub.O /P.sub.C, on the order of 3 to 4, has been found to be required to give a suitable flow pattern with the appropriate jet cell structure. Experimentation resulted in the selection of a nozzle diameter 88, of 1.2 mm and a separation distance 90, of 2.0 mm.

With regard to the resonsnce tube 85, the preferable geometry where relatively low air pressure is the actuating force, was discovered to be the stepped configuration as shown in FIG. 3. This compares with the cylindrical and tapered configuration of the devices described in the aforementioned patents. A preferable tube length from the opening of the cavity 91, to the closed end 92, might be on the order of 10 mm. The internal diameter of the resonance tube 85 might preferably vary in the stepped fashion from 1.5 mm at the open end of the tube to 0.5 mm at the closed end. The length and diameter of the tube can be varied depending upon the maximum temperature sought to be achieved at the closed end and upon the time required to reach that temperature.

Since resonant heating is a flow phenomenon, no resonant heating is possible without some means of venting. The vent tube 26, allows the air being discharged from the tube 85 to leave the fluidic device 18. As hereinabove mentioned, one parameter which determines the location of the jet cell structure is the pressure ratio across the nozzle, P.sub.O /P.sub.C. With an unrestricted vent where P.sub.C = P.sub.amb the ratio is simply P.sub.O /P.sub.amb. Where the fluidic device 18 is embedded in a propellant 24, then the vent area is finite. This results in P.sub.C being greater than P.sub.amb so that the pressure ratio is reduced. Thus, the vent tube diameter and length must be considered. A suitable vent tube diameter 94 was found to be 6.4 mm. Additionally, it was found that a tube length of 1 meter could be attached to the device without reducing the pressure ratio below that needed to insure ignition.

End 92 of resonance tube 84 opens into a conical, cylindrical shaped opening 95. The cylindrical portion 96 of the opening contains a propellant igniter suitable to activate the propellant 24. In rocket motor applications BKNO.sub.3, boron potassium nitrate, would be a suitable propellant igniter. The BKNO.sub.3 is available commercially in cylindrical pellet form, where each pellet is 3 mm in diameter and 2.5 mm long.

In order that the heat generated at the end 95 of the resonance tube ignite the propellant igniter in cylindrical portion 96, it has been found that a propellant igniter interface 98, be used between the end 95 and the propellant igniter in cylindrical portion 96. This material is packed into the conical section 100 between end 95 and the BKNO.sub.3 and thereby closes off the otherwise open end 95. It has been found that in order to ignite the propellant igniter, BKNO.sub.3, the interface material must be such as to produce a hot particulate matter upon its ignition. What is needed is a pyrotechnic which will ignite at the expected resonance tube end temperature and not be dispersed by the disruptive effect of the oscillating air at the end of the tube, 95. An interface material found suitable for the described application might be nitrocellulose which has an ignition temperature of about 170.degree. C.

In operation, when the operator draws the pump handle upward, check valve 32 opens and air is drawn in through intake port 36 into the pump 10. On the downstroke the air contained in the pump 10 is under a pressure greater than the atmospheric pressure and as such closes valve 32. Instead, the air forces open check valve 42 allowing the air contained in the air pump 10 to enter the balance of the valve, tubing network. To facilitate the time for pumping this system up to the desired pressure, valves 32 and 42 have a "cracking" or opening pressure which may be on the order of 0.3 psi.

With the fluidic switch 48, in the position indicated in FIG. 2, the air channeled through the check valve 42 next proceeds through line filter 62. Depending on the size of the filter element 64, dust particles and other extraneous matter contained within the pumped air are filtered out. This prevents these particles from clogging the igniter nozzle, 86.

Next, the pumped air passes to the variable length, pneumatic tubing 12. This length of tubing forms a gas supply volume sufficient to maintain the required pressure ratio across the nozzle 86 of fluidic device 18, for a time sufficient to sustain resonant heating until the propellant igniter/propellant charge is ignited. Based on the igniter time/ignition temperature characteristics cited above, viz. a nozzle diameter of 1.2 mm, a separation distance of 2.0 mm, a propellant igniter interface, ignition temperature of 170.degree. C, and a time to ignite requirement of 2 seconds, a 100 meter line with an internal diameter of 6.4 mm was found to be adequate. By varying the conditions just mentioned the volume characteristics of the pneumatic tubing would have to be varied as well to fit the requirements of a particular configuration.

The operator continues to pump the air pump 10 until the pressure in the pneumatic tubing 12 reaches the threshold level of relief valve 16. For the combinations set out above it has been found suitable to set the opening pressure of relief valve 16 at 72 psi. Once the relief valve is opened, the supply pressure in pneumatic tubing 12 decays according to the following equation: P.sub.O = P.sub.i exp ( - .sqroot. .gamma.R To (2/.gamma. + 1 ) .gamma. + 1 /.gamma. - 1 (A*/V) t ) where:

P.sub.O -- the pressure at any time after the valve 16 opens,

P.sub.i -- the initial pressure in tubing 12,

.gamma. -- the ratio of specific heats (C.sub.p /C.sub.v) for the gas used,

R -- the gas constant

T.sub.o -- the supply flow temperature

A* -- the nozzle area of the exit of excitation nozzle 86,

V -- volume of transfer tubing 12,

t -- time

When the valve opens, P.sub.O must continue to be of sufficient level so that sufficient pressure is maintained across the excitation nozzle 86 such that resonant heating is sustained in resonant tube 85 for a period of time t, sufficient to ignite the propellant igniter interface 98. The relief valve 16 is designed such that it does not close again until the pressure drops to below 10 percent of the opening pressure. According to the above equation this insures that it will stay open long enough to insure ignition.

This ability of the relief valve 16 to maintain itself opened until the supply tube 12 pressure drops to greater than 10 percent of its opening pressure is due to a unique utilization of the standard type valve. Where normal utilization calls for exhaust port 28 to be used as the intake port and input port 72 connected to duct 76, to form the exhaust port, this invention interchanges the position of the two ports. With the air not entering port 72 and, thereafter, flowing into duct 76, an initial pressure P.sub.1 is exerted on that portion of diaphragm 78 which covers the opening of duct 76. When the relief valve opens, diaphragm 78 is pushed back from the opening of duct 76. The air in supply tubing 12, thereafter fills the volume of valve 16, exit port 28 and pneumatic tubing length 22. At this time, therefore, the total force being exerted on diaphragm 78 increases by the ratio of the total area of the diaphragm surface to the area of the open end of duct 76. This multiplication effect, therefore, keeps the cover open until the air pressure drops to a level such that the pressure times the total area of the diaphragm is equal to the force first required to uplift the diaphragm from the opening of duct 76. Through proper design, this will enable the pressure to be maintained across the excitation nozzle 86 for the period of time needed to raise the propellant igniter interface 98, to the required temperature.

The pressurized air is then carried to the excitation nozzle 86 of fluidic device 18 by the pneumatic tubing 22.

As hereinabove mentioned, the fluidic device 18 consists of two essential parts, the resonance tube 84 and the excitation nozzle 86. This device functions when the open end of the resonance tube 84 is placed in the compression region of the free jet emanating from the nozzle 86. When the flow emerges from the nozzle, it accelerates to supersonic speed and then readjusts to subsonic speed by compression through a shock wave. The process creates a series of diamond shaped cells, a b c d e f, c b' c' d' e' d, etc. of alternate supersonic and subsonic flow, see FIG. 4. The cells or conical shock waves intersect the jet axis 102 throughout the length of the jet. A plot of a typical static pressure distribution along the axis of the jet is also shown in FIG. 4. It can be seen that the pressure rises in the conical fronts of the diamonds and drops in the divergent portions to a minimum at the intersections, a f and c d. It was discovered that by placing the open end 91 of resonance tube 85, in the conical section, a b e f or c b' e' d, of the diamond shaped cells, a self-sustaining oscillation of the pressurized gas occurs within the tube.

Although there is continuous flow into and out of the resonant cavity, a portion of the gas remains trapped at the closed end 95. There it is subjected to a succession of waves producing periodic compression and rarefaction of the gas. This periodic compression and expansion of the gas produces irreversible temperature increases at the end of the cavity which raises the end wall temperature to a point sufficient to ignite pyrotechnic materials such as nitrocellulose.

Once the propellant igniter interface is ignited, a hot particulate matter is created sufficient to ignite the propellant igniter BKNO.sub.3 which in turn ignites the main propellant 24.

If it is desirous to abort a given firing, fluidic switch 48 can be actuated by depressing the plunger 50 such that it drops below opening 104. This vents any of the air built up throughout the valve, tubing network to the outside, through exhaust port 52.

Whereas the above disclosure discussed a system wherein an operator manually brought the system up to the threshold pressure, it is to be understood, of course, that where time to fire must be relatively rapid, air pump 10 may be suitably replaced by a pressurized aerosol can, or automatic pump arrangement.

Additionally, the principles of the subject invention, in addition to being used as a first stage ignition system as described above, may be utilized as part of a suitable airborne device so as to provide the mechanism for second stage firing, etc. A fluidic type computer could be programmed to divert a pressurized gas, conceivably the air outside the rocket, to a conversion device similar to device 18. This computer would be programmed to effect this diversion at an appropriate time, suitable for second staging.

Although the principles of the invention have been described in an ignition system for the firing of a sounding rocket, it should be apparent to those skilled in this art that the principles of the invention can be readily adapted to the commercial explosive market to thereby provide a safe, effective means for detonation of explosives such as dynamite or the like.

Whereas the fluidic conversion device 18 has been described as being constructed from a low heat transfer material such as Hysol Company's MG5F, it is to be understood that the device 18 can also be molded, in its entirety from nitrocellulose. This eliminates the need for the conical section 100 of opening 95, requiring only cylindrical section 96. The BKNO.sub.3 is packed in this cylindrical portion 96 as before. The wall thickness between tube 85 and section 96 might be on the order of 0.050. Now when the resonance takes place the heat transfer characteristic of the nitrocellulose is sufficiently low to allow for the end wall temperature to rise to 170.degree. C in the required time and thereby ignite the nitrocellulose. This will, in turn, ignite the BKNO.sub.3 ensuring ignition of the propellant 24.

It can also be appreciated that changes in the above embodiment can be made without departing from the scope of the present invention, and that other variations of the specific construction disclosed above can be made by those skilled in the art without departing from the invention as defined in the appended claims.

Claims

What is claimed is:

1. An apparatus for igniting a remotely located explosive charge which comprises:

A. means remotely located with respect to said explosive charge for generating a threshold gas pressure said means comprising: (a) an air pump having an exhaust port; (b) a pair of check valves with the exhaust port of one of said check valves connected to the exhaust port of said air pump; and (c) means for storing pressurized gas, the input port of said storage means being connected to the exhaust port of the second of said check valves said storage means including a relief valve having an intake port and an exhaust port, said relief valve being calibrated to open at the threshold pressure and to remain open to predetermined time thereafter with the intake port of said relief valve including a leakage orifice for venting the intake port to the outside air to provide protection against accidental build-up or retention of gas pressure in said storage means, a length of pneumatic tubing of a storage volume sufficient to maintain the pressure of the stored gas at or near the threshold level for a predetermined period of time, and a length of plastic tubing connected to the exhaust port of said relief valve;

B. a fluidic conversion device embedded in said explosive charge, said fluidic device including (a) a convergent nozzle pneumatically coupled to said plastic tubing; (b) a resonance tube, the open end of which is positioned coaxially with and at a predetermined distance from the exit port of said convergent nozzle, said fluidic device further including (c) a venting tube positioned so as to vent the space between said exit port and said opening said venting tube having a prescribed diameter and length which together with a predetermined ratio such that said fluidic device converts the energy stored in said pressurized gas to rise in temperature at the other end of said resonance tube; and (d) a propellant interface interposed between the other end of said resonance tube and said explosive charge, said propellent interface being responsive to a rise in temperature in said other end so that it ignites when the temperature reaches a prescribed ignition temperature and consequently causes the ignition explosive charge.

2. The apparatus of claim 1 wherein said threshold pressure generating means further comprises:

A. a two position fluidic switch; and

B. a line filter serially connected to said fluidic switch;

C. said serial connection interposed between the exhaust port of said second check valve and the intake port of said storage means;

D. said two position switch having a first position whereby the exhaust port of said second check valve is connected to the intake port of the storage means and a second position whereby said storage means is vented to the outside air, thereby prohibiting a pressure buildup in said storage means.

3. The apparatus of claim 2 wherein said fluidic conversion device is manufactured from a combustible plaster material.

4. The apparatus of claim 3 wherein said fluidic device further includes a cavity:

A. said other end of said resonance tube opening into said cavity, wherein said cavity contains said propellant interface;

B. said propellant interface packed into said cavity to thereby close off said other end of said resonance tube.

5. The apparatus of claim 4 wherein said propellant interface includes:

A. a pyrotechnic material positioned closest to said other end of said resonance tube; and

B. a propellant igniter material;

C. said propellant igniter material positioned between said pyrotechnic material and said explosive charge.

6. The apparatus of claim 5 wherein said pyrotechnic material is nitrocellulose.

7. The apparatus of claim 6 wherein said propellant igniter material is BKNO.sub.3.

8. The apparatus of claim 2 wherein said fluidic conversion device is manufactured from a pyrotechnic material.

9. The apparatus of claim 8 wherein said fluidic device further includes a cavity, said cavity axially aligned with the longitudinal axis of said resonance tube and positioned a predetermined distance from said other end;

A. wherein said other end is a closed end, and

B. wherein said cavity contains said propellant interface.

10. The apparatus of claim 6 wherein said propellant interface consists of a propellant igniter material.

11. The apparatus of claim 10 wherein said propellant igniter material is BKNO.sub.3.