US2865726A - Explosive charge for delay fuze - Google Patents

Description

Dec. 23, 1958 TIME (MICROSECONDS) H. P JENKINS ET AL 2,865,726 EXPLOSIVE CHARGE FOR DELAY FUZE Filed June 3, 1952 A 0 2o 40 so so 95 I00 CONCENTRATION (WEIGHT LEAD Azms) INVENTORS CHARLE S H. SHONATE HUGH P. JENKINS, JR

ATTO NEYS tat 2,865,725 Patented Dec. 23, 1953 EXPLOSIVE CHARGE F OR DELAY FUZE Hugh P. Jenkins, Jr., and Charles H. Shomate, China Lake, Calif.

Application June 3, 1952, Serial N 0. 291,586 3 Claims. (Cl. 52-2) (Granted under Title 35, U. S. Code (1952), see. 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 invention relates to a method for controlling the rate of detonation of explosives; to new explosive compositions, and articles made therefrom.

There are many applications of explosives wherein control of their detonation rate is highly advantageous. This is particularly true in the use of military explosive missiles where penetration of the target before explosion is often necessary in order to realize the maximum effect. The ordinary primary explosives such as lead azide when used alone have too short a delay for use in fuzes to give maximum etfect against certain targets which are frangible, such as airplane wings.

In the past, delay in fuzes has usually been obtained by such devices as pyrotechnic trains, bafiie elements, me chanical devices and others. These prior devices have many disadvantages for this use, including the fact that they do not give reproducible delays and, generally, short delays cannot be obtained with them. In addition, the mechanical devices have restricted application because of space requirements. In general, delays have not been obtained in fuzes for military explosive missiles by modification of the composition of the initiating explosive.

it is an object of this invention to provide a method for controlling the rate of detonation of initiating explosives.

The above and other objects are accomplished by adding to the primary explosive a waxy material in amounts determined by the rate of detonation required.

The waxy materials include fatty acids such as stearic acid, metallic salts of fatty acids such as aluminum stearate and many others, the essential requirement being that the material have waxy properties so that the particles of explosives are coated and the interstices between the particles are filled up. The invention has direct applicability to primary explosives such as lead azide and mercury fulminatc, but could conceivably be used to modify the detonation rate of other types of explosives.

The invention is illustrated by reference to the tabulated results set forth in Tables I and IT below and the accompanying drawings hereby made a part of this specification in which,

Fig. 1 is a vertical section of the apparatus used to test the explosive mixtures, and

Fig. 2 is a graph of the results given in Table I in which delay time is plotted against percent lead azide in the mixtures.

Various mixtures of lead azide and aluminum stearate and lead azide and stearic acid were tested by measuring the time elapsing between initiation and detonation of a small column of each mixture.

Referring to Fig. 1, the numeral designates an outer wall of strong material such as brass or stainless steel having a cylindrical chamber 11 therein. The exit of the chamber is partially closed with a metal plug 12 2 which is crimped in place as shown. The initiating device 13 such as a graphite igniter bridge is placed at the opposite end of the chamber, and is provided with an ignition device 14 which may be of the electrical or other type. The lead azide mixture 15 was placed inside the chamber 11 in contact with the graphite igniter bridge. The length of the column, as shown in the tables, was about .9" long. The described test device could be used as a delay element for a fuze. The column of explosive was initiated by a condenser discharge which also started two timing devices, each adapted to record the time interval between initiation of the explosive, and the arrival of a detonation wave at the exit of the column. Two RCA type WF99B counters, connected in parallel, were set to start on one electrical pulse and to stop on the second. A make-type switch was used at the exit of the column. It consisted of two leaves of 0.004-inch copper foil, separated by a 0.004-inch-thick paper washer fastened together with a single Wrapping of scotch tape. This switch was then clamped down on top of the bomb with the vent hole in line with the center of the paper washer. The blast from the vent effected a contact between the two copper strips, completing the stop-circuit and sending a pulse to stop the counters. This arrangement, when tested, always gave a positive, reproducible operation. The timing circuit is not shown as it is considered standard apparatus. The interval between two pulses was read directly from the indicating decades of the counters. The accuracy claimed for the counters is one millionth of a second. The two counters were used as checks against each other and when one of the counters failed to start or stop, or the readings disagreed by more than 5 microseconds, the readings were discarded.

It should be noted that the instruments measure the lapse of time between a sending of a pulse to fire the explosive and a receiving of a return signal. In addition to the time required to detonate the column of lead azide after initiation, this time interval includes, of course, the times required to send a pulse to the igniter bridge, to explode the graphite film, to initiate the lead azide, to send the shock wave through the brass plug, to close the make-switch, and to send a stop pulse back to the counters. In order to ascertain the magnitude of these additional time intervals, at special bomb with an explosive column only inch long was fired. The time for the detonation to travel through the inch column was considered negligible. The total time lapse, as read on the counters, was only 16 microseconds. This indicated that the timing system was satisfactory in that this time interval was small compared with the recorded times of several hundred microseconds usually obtained.

Standard military lead azide was used in all the compositions. Usually one gram of azide was weighed accurately and mixed thoroughly with a calculated amount of the diluent. Mixing was accomplished by a small tumble mill designed for the purpose. The azide mixtures were loaded into the bombs by increments, the increment being compressed on top of the igniter bridge by a polished As-inch steel plunger, the second increment on top of the first, and so on, until the column was filled under 12 tons per square inch pressure. A greater degree of homogeneity in the column was obtained by this method of loading. In general, the compressed material was a hard mass, which did not lose'its form when the pressure was removed. The aluminum stearate powder used was a Braun Corporation product. The stearic acid was a reagent-grade material purified by distillation.

The results obtained on a number of compositions are given in the following tables.

TABLE I Lead azide-aluminum stearate mixtures Mixture" I ength Time 7 Average Stand ard Composition of Chargetmicrosec.) (microseel) Deviation (wt. percent azide) (int) (microsee;)

Y 0 923 173 0.891 461 (i8 4 14s 57.14, 0. 910 83 108 35 TABLE II Lead azzde-stearzc aczd mixture Mixture i Weight Length Time Average Composition of Charge of Charge (microsec.) (mici-osec.) (wt. percent azide) (g.) (in.)

0. 292 0. 890 no fire 0.324 0015- i 652 i 591 0.317 0.808 530 It will beInoted from the tables that'the optimum compositionof lead'azidc and aluminum stearate is one containing about144.44 percent lead azide wherein an average-delay of 816 micro-seconds is obtained with a standard deviation of 108 microseconds-between the tests. With a 50; percent. mixture of lead azide, an average delay 06354., microseconds is obtainedWith a standard deviation of l6l'microseconds. With a composition containing 57.14 percent azide, an average delay of 108 microsecondsis obtained with a standard deviation of 35 microseconds. The standard deviation is lower than any heretofore obtained for this short delay and is quite small for this type of statistical phenomenon.

Referringlto Fig. 2, it will be noted from the graph that, as would be expected with nearly pure lead'azide, extremely short delaysare obtained This delay does not increase appreciably until'the percentage of lead azide decrease to-about60. At somewhere between 60' and 40 percent-lead azide, there is a'sharpdeviation inthe delay time and delays of 800 seconds and above'are reached-within this percentage range. It will be noted that ifthe results for stearic acid were-also plotted on the 'graph, the critical range wouldalso fall within the 40' and 60 percent lead azide range, as;th emixture did notfire'at 44e44 percent lead azide "and two firings of a 50 percent-lead azide mixture gave an average delay of-59l seconds.

Although the cause of this unexpected phenomenon is not definitely known, it is believed to be due to a aseeyrae 1,488,787 Harle Apr. 1, 1924 7 2,146,033 Seavey et al. Feb. 7, 1939 2,402,235 Burrows et a1. June 18, 1946 d changeover from detonation to defiagration at somewhere above 60 percent of lead azide; This would seem to be substantiated by the fact that, as is well known, lead azide can be dead pressed to the point where it cannot be detonated but can be deflagrated. This result has been explained by the fact that loose explosives are most readily detonated because of the possibility of formation of pressure gradients in the interior ofthe mass. Applying this theory to the instant invention, it is believed that the use of the waxy materials fills the interstices between the particles, thus preventing the formation of pressure gradients so-that deflagration and not detonation occurs with the result that delays are obtained.

As is illustrated by the above results, it is an advantage of the invention that the rate of detonation of a primary explosive such as lead azide can be reproducibly con trolled, depending upon the desired delay, by the addition of the type diluents disclosed herein. This is a far more reliable method ofobtaining reproducibility than that of compressing the explosive and the use of the other prior methods mentioned above with their attendant disadvantages. Another advantage is the fact that extremely short controlled delays can be obtained with this invention; A further advantage of the invention is that its use reduces space requirements for fuzes, as the space for the, delay-train is dictated onlyby the amount of explosive mixture used.

Obviously, many modifications and variations of the 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 explosive charge for use as a short time. delay fuze consisting essentially of a compressed mass of homogeneously mixed particles of lead azideand a material chosen from the class consisting of aluminum stearate and stearic acid, the particlesof'said material being present inan amount from about40% to about 60% by weight. v

2. An explosive charge for use as a short time delay fuze consisting essentially of a compressed mass of homogeneously mixed particles of lead azide and aluminum stearate, the aluminum-stearate particles being present in an amount from about,40% to about 60% by weight.

3. An explosive charge for use as aishort timedelay fuze consisting essentially, of, ancompressed. mass of homogeneously mixed particles of lead azide and .stearic acid, the stearic acid particles being; present in an amount from about.40% to about 60% byweight.

ReterencesCited in thefile of this .patent UNITED'STATES PATENTS

Claims (1)

1. AN EXPLOSIVE CHARGE FOR USE AS A SHORT TIME DELY FUZE CONSISTING ESSENTIALLY OF A COMPRESSED MASS OF HOMOGENEOUSLY MIXED PARTICLES OF LEAD AZIDE AND A MATERIAL CHOSEN FROM THE CLASS CONSISTING OF ALUMINUM STEARATE AND STEARIC ACID, THE PARTICLES OF SAID MATERIAL BEING PRESENT IN AN AMOUNT FROM ABOUT 40* TO ABOUT 60% BY WEIGHT.

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