US2574479A - Propellant having an opacifier for preventing self-ignition by radiant energy radiations - Google Patents

Description

Nov. 13, 1951 c. N. HICKMAN 2,574,479

PROPELLANT HAVING AN OPACIFIER FOR PREVENTING SELF-IGNITION BY RADIANT ENERGY RADIATIONS Filed Jan. 30, 1945 lmunmb PAY LOAD I! COMBUSTION CHAMBER a FIGJ w IO i Z v 3 H62 (TRANSLUOENT) INVENTOR CLARENCE NJHIGKMAN ATTORNEYS time curve.

Patented Nov. 13, 1951 PROPELLANT HAVING AN OPACIFIER FOR PREVEN Tm G SELF-IGNITION BY RADI- ANT ENERGY .RADIATIONS Clarence N. Hickman, Jackson Heights, N. Y., assignor to the United States of America as represented by the Secretary of War Application January 30, 1943, Serial No. 474,213

(Granted under the act of March 3, 1883, as

1 Claim.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon. I

This invention relates to jet propulsion devices and to propellants therefor, and has for its principal object the provision of a propellant which, when burned underthe conditions existing in a rocket combustion chamber, will produce a pressure-time curve which can be controlled and predicted by the initial form and dimensions of the powder.

The principal object of this invention can be achieved only when the propellant which is used possesses, during the major portion of the burning, a closely defined burning surface which varies in accordance with the law of burning by uniform layers. This requires that the burning take place only on the surfaces exposed by the burning away of the outer layers, and that there be no additions to the burning surfaces such as those produced by internal burning or the breaking away of the outside surfaces. I Since the propellant burns'only on its exposed surfaces, the rate of gas evolution, and the pressure developed, will depend at any instant upon the area of the burning surface. The initial burning surface will depend upon the form and dimensions of the grain. If as the burning continues, there is an increase in the effective burning area for any reason, this increased area upon burning will evolve gas at a higher rate thus producing a higher pressure. I have found that a pressure-time curve of the desired shape can be obtained by the use of a grain form such that the curve of the burning area against time is substantially similar in shape, during equilibrium burning, to the shape desired in the pressure- It, therefore, becomes another object of this invention to provide a rocket propellant in the form of a powder grain having a substantially constant burning surface throughout the burning time.

' In order to accomplish the above object- I prefer to use a cylindrical stick of powder having a single concentric cylindrical perforation formed therein.

One of the most suitable propellants from the point of view of energy, content and availability is a so-called "double-base powder. These double-base powders are formed into grains" by any one of several well known methods, whic need not be describedherein. The motor weight restrictions ina iet propelled amended April 30, 1928; 370 0. G. 757) device, such as a rocket, place rather severe limitations on the burning characteristics of propellants. They must, after ignition, burn evenly and uniformly with time, under the pressure conditions existing in the chamber, without developing pressures exceeding the allowable limit for the chamber in which the propellant is disposed. The pressure developed in the motor chamber should remain fairly constant or increase slightly so that the propellant will burn completely, but, as stated, the pressure must not increase to high values. If the latter should happen, the motor chamber walls may burst. If

such failure occurs in the projector tube during the firing of the rocket, the tube itself may burst. Increases in pressure during burning after the rocket has left its projector tube may burst the motor chamber. Either type of bursts results in hazards to the firing personnel and equipment.

To provide a propellant in which the burning area may be regarded as constant, I utilize a cylindrical stick of powder having a single cylindrical axial perforation and space the powder grain from the inner Walls of the combustion chamber so that the inner and outer cylindrical surfaces may be exposed for burning. The burning surface area of a cylindrical stick of powder having a concentric cylindrical perforation may be regarded as constant only if the burning surface area of the ends is neglected. The ends can be neglected when the length of the stick is large compared to the diameter. If, however, the length of the stick is short the degressive effect due to burning at the ends may be great. In order to overcome this disadvantage the ends may be treated with a material which will inhibit combustion. A material which I propose is tar. If the ends are painted with tar the burning area will be constant, and substantially constant pressures will be maintained. Other inhibitors such as cellulose acetate or rubber compounds could be used.

In the course of my researches on the use of large grains of double-base, translucent powder for rocket motors and other like jet-propelled devices, I discovered that such grains almost always developed abnormally high pressures during burning in the rocket motor. I used translucent grains because any cracks or flaws therein which produce a non-uniform burning surface may thus be determined by visual inspection prior to loading in the combustion chamber. Translucent grains consisting of cylindrical tubes of about V inch outside diameter and inch inside diameter appeared to be uniform throughout, no holes,

and that double-base propellants for use in rockets and like jet propelled devices must be opaque to the light rays which appear to cause flssuring. H I

Itshould be understood; however,-

there I may be applications in rocket propulsion wherein.

break the stick into pieces andto expel partially burned pieces of powder from the rocket motor;

I was finally able to examine, partially burned powder, and to subject such powder grains to partial burning tests, using apparatus as described in my copending application SL'NI50'L002',

filed Sept 2, 1943, now matured to U; S. PatentNo. I

2,464,179. I discovered that partially burned,

light translucent powder grains had surfacesmarked by fissures, worm holes and evencracks- These fissures were frequently of substantial depth and area. Thus it was apparentvwhy' the pressure increased, frequently by several thousand pounds, during burning. For some unknown reasonthe translucent powder fissured during burning, thefissuresformed resultedin increased burning area and,, as; a consequence, V excessive increases in pressure.

The increase in pressure, sometimes exceeding the bursting strength ofv the motor.chamber, is a kindof progressive process. Thatis tosay, when ansmall -fissure or worm hole forms, the burning surface of the powder'grain is slightly increased. This leads to. anincreased volume -of gas, which in turn increasesthe pressure inthe motor chamber. The increase in pressure increases the rate of burning, more fissures form, again increasing the pressure with further increase in burning speed so,,in a fraction of a second, the propellant, instead of burning evenly,,burnswith explosive force-and the-development of very high pressures in the motor chamber. x

Gin the. other hand whenfissuring is prevented, in accordance with my invention, the. powder grains of my invention burn smoothly from the surfaces inwardly, and theburning rates,,.;pr,essures developed, etc., can be correlated with the structural design of therocket motor chamber,

nozzle area, etc. 7 1

Many explanations of the cause of fissuring were advanced. A shock wave theory wasproposed, and ultimately'd-iscarded. Others attr-ihuted it to some peculiarity in the physical structure of the powder grain, i e., its brittleness.

I finally hit upon an explanation based on radiation phenomena. I felt that as the, surfaces of the powder burned, thelight energy developed was radiated below the burning surface into the interior of the grain where, selectiveignition oi nitroglycerin molecules could possibly-occur; If that happened it would bereasonable to-believe that the, combustion products wormed through to'the' surface of the grain, thus creating chansnels or fissures, and exposing more surface area I figuration producing a constant area of burning,

configurations other than that disclosed may be desired to. produce; for example a slightly progressive powder to reduce burning time. How-- ever, in; these specific instances it is still important, that an opaque powder grain be used in therocket chamb'erto prevent fissuring which if occurring. would disrupt the calculated burning. we time and produce pressures higher than those which: therrocket combustion chamber was designed to withstand.

In order that my invention may be more clearly V understood, I have-,,on the appended single sheet of drawings, illustrateda powder grain of cylindrical configuration, haying a single axial cylindrical perforation: formed therein, so thati when burnedonboth: surfacesit will-offer a, su,-b st=,r.n1 V

tially constant; bur-nine area; The surface 7 or, the propellant shown in one view has-Jdeveloped fis: sures and, wormholes. after partial burning. -It has also been noted that the: worm-holes =;whigl1 depot-extend: tof-the surfacexare pbservable with; in th ihl3fif1ifl1i of. the grain. wilrmore' clearly illustrate'what issmeantbyterms worm holes andrffissures. I. have, also shown a schematic;;representation of. a rocket-device con: taining the propellants of the present invention.-

,In-the-drawingss- Fig. L is: a poWderg-rain of the, presentlinven tion,'. L A 2;- shows-a powder grairrwhich.has fissured, and- 1 Y ,lFfig. Bis azrepresentation ofa rocket containing the opaque.priopellantsv of; my; invention.

In- Figs 2; the g na-in Lhasrits surface marked andqscored-by fissuresslfl and worm-holes2. This grain-is-translucent tolig-ht. In Fig. 1 the grain 3 shows no fissuring after partial burning.v This grainis opaque to: the light. rays; v t In Rig. '3 the rocket motor; chamber 445' pro,

videdwith plurality of cylindrical sticks f 1 opaque powder ,3 each of. which has a'single axial cylindrical perforation 6,. and which, if of small: diameter compared. to their length, have their end: surfaces paintedwithtan. The; pro-j pellant sticks, are: heltlinv place by the trap wires 1 threaded through theperforations 6. For purpose 0t clarity of illustration, these. grains 1 are I shown-spacedfromeach other, although this is notlgenerallycarried, out in. practice; ,Thenozzle 1 or. venturisof. the-motor is. shown at and. -,the payloadflor highexplcsiveheadat 9.-

The present invention, therefore, comprisesthe use: ofa-propellan-t. in. a. rocket chamber, said I propellant :being characterized-by, the fact, that it-;is opaque, and also by-the fact-.thatit differs fromqthe propellants originally tried but which were found to be unsatisfactory since. they prok duced" fissures and {excessive pressures, only: in

thatuit' isiopaque whereas:- the: otherpropellants reiierredt-towere: translucent. -I prefer. to rnakej the propellant opaque by the addition ,of a suit-s' V 1 ableropa ifv naasent; t inyention-alsoincludes .the combination of alrocket mutorpombustion chamber-and: propel V constant;- surfaceeanem? 1 ithe -ca'se fifi short I grains or in other cases where a more exactly constant burning area is desired, the invention may include the treating of the end surfaces with an inhibitor such as tar.

This invention also includes the combination of a rocket motor combustion chamber and propellant having the opaque character stated, but not limited to any specific geometrical configuration. For the propellants involved in this invention I prefer to use the double-base powder.

By double-base powder or propellant I mean any mixture of nitrocellulose and nitroglycerin in those proportions which will give propellant characteristics. Such proportions are well known in the art and I need not elaborate on them in this specification. As a rule the nitrocellulose exceeds the amount of nitroglycerin, but propellants containing equal quantities of such constituents have been made and are useful. I do not wish to be limited to any particular proportion of nitrocellulose and nitroglycerin, since my invention does not depend upon such proportions. Likewise, the amount of opacifier to be incorporated in the mixture for the purpose stated can vary over wide limits. The final result is a powder which is opaque to light energy which appears to cause fissuring in the absence of the opacifier. The opacifier is quite uniformly distributed throughout the powder grain. In the case of soluble coloring agents, such as nigrosine, the entire grain is colored. In the case of solid opacifiers such as lampblack or carbon black the finely'divided particles thereof are quite uniformly distributed throughout the powder grain, although there may be zones having a higher concentration of the solid; but visual inspection of any surface reveals that the solids are well distributed. Photo micrographs of surfaces produced by cutting the grain at various points, at fairly high magnifications, show the finely divided solid particles scattered more or less uniformly through the body of the powder. Obviously, the use of lampblack, graphite and the like in my invention bears no similarity to the coating or surface treatment of powder grains after they have been formed for the purpose of modifying the burning characteristics of the grain surface.

Those skilled in the art will appreciate that many different opacifiers can be used in the practice of my invention. For example, lampblack is an excellent material to incorporate in the propellant along with the other major constituents. Nigrosine, a, black aniline dye, is useful. Any colored, or coloring material, capable of screening out those light rays which appear to cause fissuring, can be used. Thus the desired end may be achieved through the use of dyes, stains or pigments.

Thus a double-base powder containing about 40 percent nitroglycerin, 58 percent nitrocellulose, and 2 percent ethyl centralite is prepared in sheet form for extrusion. This powder, if extruded to grains in the form of cylinders inch outside and 4 inside diameters will fissure when burned. In the practice of my invention I incorporate about .15 to 0.5 weight percent of lampblack in the powder. The lampblack can simply be rubbed on the sheeted powder before extrusion thereof. When the lampblack coated sheet is extruded the coloring agent is sufiiciently disseminated through the grain to render it opaque to light. Alternatively the lampblack can be added to the nitroglycerin prior to colloiding nitrocellulose.

When using dyes such as nigrosine, not so much opacifier is required. I need only add 0.1

weight percent of nigrosine to the powder constituents to render the final grain opaque. Black p-aminophenol is a suitable opacifier, and there are many others. The object, in each instance, is so to color the final grain that light energy developed on the surface during burning is not permitted to penetrate below the burning surface. This is most easily accomplished by simply coloring the grain throughout with any suitable agent which will' inhibit the penetration of the light energy which appears to cause the fissuring. I believe that the red end of the light spectrum is the particular cause of fissuring and therefore coloring agents or opacifiers which shield only against the penetration of red rays will be operative. But coloring agents such as carbon black, lampblack, aniline dyes, etc., are so readily available and useful that I find them quite advantageous.

Having thus described my invention, what I claim is:

A propellant grain for jet-propelled devices, comprising a double base powder containing 58% nitrocellulose, 40% nitroglycerin and 2% ethyl centralite, an opacifier consisting of .15 to 0.5 weight percent of lampblack incorporated in said powder, said opacifler being dispersed uniformly throughout the powder to form a composite opaque mixture which will prevent internal selfignition of the powder by radiant energy radiations produced by surface burning of the powder.

CLARENCE N. HICKMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 175,735 Nobel Apr. 4, 1876 328,919 Chambers Oct. 27, 1885 437,499 Mindeleff Sept. 30, 1890 440,921 Mindelefi Sept. 30, 1890 1,003,411 Bales Sept. 19, 1911 1,308,343 Du Pont July 1, 1919 1,376,029 Olsen Apr. 26, 1921 1,611,353 Lepinte Dec. 21, 1926 1,709,868 ONeil Apr. 23, 1929 1,788,438 Norton Jan. 13, 1931 1,838,984 Berkowitz Dec. 29, 1931 2,008,889 Wagner July 23, 1935 2,038,700 Woodbridge Apr. 28, 1936 2,062,495 Brayton Dec. 1, 1936 2,131,354 Marsh Sept. 27, 1938 2,131,383 Marsh Sept. 27, 1938 2,235,298 Olsen Mar. 18, 1941 2,408,252 De Ganahl Sept. 24, 1946 2,440,327 Crawford Apr. 2'7, 1948 FOREIGN PATENTS Number Country Date 12,325 Great Britain of 1895 15,553 Great Britain of 1898 21,482 Great Britain of 1903 516,865 Great Britain Jan. 12, 1940

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