US2942964A

US2942964A - Stable gas-generating composition

Info

Publication number: US2942964A
Application number: US504048A
Authority: US
Inventors: Theodore A Burgwald; Linsk Jack; Edwin F Morello
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1955-04-26
Filing date: 1955-04-26
Publication date: 1960-06-28
Anticipated expiration: 1977-06-28

Description

June 1960 T. A. BURGWALD ET Al. 2,942,964

STABLE GAS-GENERATING COMPOSITION Filed April 26, 1955 \A A. l/ r F lg. 2

Fig. 1

12L Y Theodgggfifiggg wald Edwin E. More/l0 INVENTORS:

2,942,964 STABLE GAS-GENERATING COMPOSITION Theodore A. Burgwald, Whiting, and Jack Linsk, Highland, Ind., and Edwin F. Morello, Juliet, 111., assignors to Standard Oil Company, Chicago,glll., a corporation of Indiana l i 7 Filed Apr. 26, 1955, Ser. No. 504,048

8-Claims. c1.s2-.s

This invention relates to a composition for the generation of a gas. The invention pertains more particularly to an improved gas-generating composition consisting essentially of ammonium nitrate as the primary gas-producing component of the composition, intimately mixed with a combustible bindermaterial. The composition also contains an inorganic combustion catalyst, a synergist to accelerate the burning rate of the composition and other components to stabilize thermally the grain composition; Still more particularly, the invention relates to a composition for thefgeneration of gasby'the combus- :tion of a shaped grain consisting essentially of the ammonium nitrate, a combustion catalyst, a combustible bindery-a carbon synergist to accelerate burning rate and additives to stabilize the grain ballistically when stored under: high temperature conditions. Such composition is useful for the propulsion of rockets for ground-to-ground missiles, ship-to-shore'missiles, air-to-air missiles and airto-ground missiles and the composition may also be used as'propulsionmeans in assist take-off in military and commercial aircraft. Grains made from the composition can be stored for long periods of time at relatively high atmospheric temperatures without damage to the ballistic properties of the grains.

Ammonium nitrate iswidely used as a component of high explosives, particularly the so-called safe explosives. Even though ammonium nitrate is classified as a high explosive, it is extremely insensitive to ordinary heating and toshock and cannot be readily detonated by the local application of heat or by a blasting cap. Further, when ignited, ammonium nitrate alone does not burn uniformly and has a tendency to go out. prove the burning quality, to increase the sensitivity, and to utilize the excess free oxygen available from the decomposition of the ammonium nitrate and to provide shaped grains suitable for use in rocket motors and assist take-off motors, combustible binder material is used in the ammonium nitrate composition. a

The use of ammonium nitrate-base compositions as solid propellants for rockets and assist take-01f units is attractive because of the 10W cost and-availability of the ammonium nitrate, because of the relatively low flame temperature of decomposition, of the nitrate, that is, be-- tween 3150" and 2900 F. and because the excess free oxygen available from the decomposition permits the use of oxidizable material to increase the energy available from the decomposition. However, the physical characteristics of ammonium nitrate and grain material produced therefrom introduces problems with respect to shaped grains. Thus ammonium nitrate exists in diiferent crystalline forms, the transition from one form to a different form involving a volume change of the ammonium nitrate. Volume changes which occur at about 90 F. and also at about F. involve 3.5% and about 3% increase respectively. 'It is, therefore, obvious that an ammonium nitrate base composition could be seriously affected by storage where large volume changes occur at temperatures common to the storage conditions. One

the principal gas-generating material.

In order to im- 2,942,964 Patented June 28, 1950 requirement for solid propellant suitable for military use is that it'be ballistically stable after prolonged storage at temperatures as high as 170 F. or following storage at temperatures as low as F.

Another requirement is that the grain fabricated from the composition will not shatter as a result of being subjected to alternate high and low temperatures, i.e., 170 F. followed immediately by a temperature of -75 F. in a series of at least two repeated cycles and also that the gram must burn uniformly following such alternate high and low temperature treatment. Still another requirement of the grain material is that it may be ignited at extremely low temperatures, i.e., -75 F. and temperatures as high as to F. as Well as at intermediate ambient temperatures following hot or cold storage.

That is, a grain composition must be reliable with respect to firing after being subjected to variable temperature environment. Another requirement is that the grain material does not gas appreciably when subjected to a temperature encountered in service. tive of chemical change in the composition and it is usually necessary to use a gassing inhibitor to meet the requirement of low gassing properties. i

An object of this invention is the preparation of a gas-generating composition using ammonium nitrate as Another object of the invention is to produce a gasgenerating composition suitablefor use in rockets and assist take-off units, which composition is ballistically stable after being subjected to high and low temperature environment.

' Still another object of the invention is to produce a gas-generating composition which is relatively stable chemically at temperatures encountered in service.

Yet another object of the invention is to produce a gasgenerating propellant composition which exhibits good ballistic performance following storag at high temperatures. Other objects will be apparent as the detailed description of the invention proceeds.

The composition of this invention consists essentially (1) At least 70% by weight of ammonium nitrate, usually from about 72% to, 76%. g

(2) From about 1% to about 6% by weight of an inorganic combustion catalyst consisting essentially of a mixture of ammonium dichromate and insoluble Prussian blue;

(3) Between about l8%-and 27% by weightof a combustiblebinder, which binder consists essentially of:

(a) From about 18% to' about 35% by weight of cellulose acetate which analyzes between about 51% and 57% by Weight of acetic acid; a

(b)' Between about 20% and 40% by weight of the liquid polyester condensation product of ethylene glycol and diglycol-ic acid, said condensation prodnot having a molecular weight within therange of from about 250 to about 1000; and a a a (0) Between about 20% and 40% by weight of a nitrodiphenyl ether; I

(4) Between about 0.3% and 3% by weight of an amine gassing inhibitor, based on the total weight of the gram;

(5) From about 0.3% to about 3% by weight of finely divided'carbon i.e., carbon black or finely divided activated carbon; and

(6) From about 0.05% to about 0.5% by weight based on the total weight of the grain of a non-ionic surfactant material selected from the class consisting of polyoxyalkylene glycol-fatty acid esters having molecular weights of about 1000 to about 2000, polyoxypropylenepolyoxyethylene glycol having molecular weights of 2000 Gassing rate is indicato about 3300 and a sorbitan oleate containing from 1 to 3 oleate radicals.

The term ammonium nitrate as used in the specification and claims is intended to mean either ordinary commercial grade ammonium nitrate which may or may not contain small amounts of impurities. The nitrate may be coated with a small amount of anti-caking agent such as petrolatum or paraffin wax. It also includes military grade ammonium nitrate as well as mixtures of minor amounts (usually less than of other inorganic nitrates such as sodium nitrate or potassium nitrate with the ammonium nitrate. .A mixture of finely ground and unground or coarsely ground ammonium nitrate is beneficial to the ability of solid propellant grains to pass cycling tests described hereinbelow. Usually the major proportion of the ammonium nitrate is ground, a minor proportion i.e., up to about 25% or 30% being unground. Finely ground nitrate is desired in order to 'attain maximum grain density. The ammonium nitrate is of such particle size that at least about 50% will pass through a #100 U.S. Standard sieve and at least 90% will pass through a #20 US. Standard sieve.

The inorganic combustion catalyst of this invention consists of a mixture of ammonium dichromate and insoluble Prussian blue catalyst. The insoluble Prussian blue is a more effective catalyst at high pressure than soluble Prussian blue. Thus when operating the combustion chamber containing the solid propellant at pressures between about 500 and 2000 p.s.i., a higher burning rate is obtainable when using the insoluble Prussian blue catalyst than is obtainable when using soluble Prussian blue as catalyst for an otherwise similar composition. In gen.- eral, the ratio of ammonium dichromate to Prussian blue of the composition should vary Within the range of from about 1 to 3 and 3 to 1 and preferably about two parts by weight of ammonium dichromate catalyst per part by Weight of insoluble Prussian blue catalyst in the composition. The total amount of inorganic combustion catalyst will usually be within the range of from about 1% to 6% by weight of the grain composition, and prefer-ably about 3% by weight.

The binder in the composition of this invention consists essentially of cellulose acetate plasticized with a mixture of a nitrodiphenyl ether and the condensation product of a dihydric alcohol with a dicarboxylic acid such as ethylene glycol with diglycolic acid. An amine inhibitor is added to the binder material.

Cellulose acetate is used as the polymeric material to be plasticized to form binder material. It is known as a partially esterified material and is described as having an acetic acid content between about 51% and 57% by weight. The term percent by weight acetic acid denotes the amount of acetic acid obtained upon saponification of the cellulose acetate and is expressed as percent of the initial material. A particularly suitable cellulose acetate is one which analyzes between 54% and 56% by Weight of acetic acid. Lacquer grade cellulose acetate is a particularly good polymeric material for incorporation in the grain composition. Lacquer grade cellulose acetate is described in addition to its acetic acid content by its viscosity when dissolved in acetone, the viscosity ranging from 2 to 80 centipoises at 25 C. for a 20% solution in acetone.

The preferred cellulose acetate of this invention analyzes between about 54 and 56% by weight of acetic acid and has a viscosity between about 2 and 10 centipoises. A binder having the proper characteristics to provide a shaped explosive with the ammonium nitrate contains between about 18% and by weight of the defined cellulose acetate. Preferably the binder contains about 33% cellulose acetate. The complete grain composition contains from about 6% to about 9% by weight of the cellulose acetate.

The preferred plasticizerutilized in the binder consists essentially of 2 components, that is, V

(a) The product of the polyesterification of a dihydric alcohol such as ethylene glycol with a dicarboxylic acid such as diglycolic acid, a molar excess of the alcohol being used in producing the polyesterification product; and

(b) A dinitrodiphenyl ether such as 2,4-dinitrodiphenyl ether.

In order to obtain a polyesterification reaction product satisfactory as a plasticizer component, a polyester having substantially no cross-linkages is desired. It has been discovered that the dihydric alcohol used in the preparation of the plasticizer must be selected from at least one of the dihydric alcohols in the class consisting of ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol and in general, the polyglycols must have a molecular weight less than about 400 in order to produce a polyester of the desired properties.

The dicarboxylic acids used in the preparation of a plasticizer must be selected from at least one of the class consisting of aliphatic dicarboxylic acids and aliphatic oxydicarboxylic acids, which acids have between 2 and 6 carbon atoms in the molecule. Examples of dicarboxylic acids are malonic acid, succinic acid, glutaric acid and adipic acid. Examples of the oxydicarboxylic acids are diglycolic acid (a,ot' oxydiacetic acid) and oxydipropanoic acid.

The condensation product of ethylene glycol with diglycolic acid described in the copending application of Norman J. Bowman and Wayne A. Proell, entitled Polyester Plasticizer, filed November 30, 1954, Serial No. 471,992, now abandoned, is preferred. It has been discovered that the molecular weight of the productof the polyest'erification reaction has a considerable effect on the plasticizing properties of the product. A relatively low molecular weight, that is, Within the range of about 250 to about 600, is desirable and is obtained by using a molar excess of alcohol over acid. The molar ratio of alcohol to acid should be between about 1.02 and 1.3, preferably not less than about 1.15 to obtain the most satisfactory condensation product, with respect to plasticizer properties. The binder of this invention usually contains from 20% to 40% by weight of the condensation product based on the total weight of the cellulose acetate, dinitrodiphenyl ether and the condensation product i.e., exclusive of the amine inhibitor in the binder. The com plete grain composition contains from about 5% to about 9% by weight of the condensation product.

The second plasticizer component utilized in the preparation of the binder of this invention is a nitrodiphenyl ether, preferably 2,4-dinitrodiphenyl ether which may be prepared in high purity and high yield by reacting 2,4- dinitrochlorbenzene with phenol in aqueous caustic medium, as taught in copending application of Wayne A. Proell and Norman J. Bowman, entitled Thermoplastic Compositions, filed October 27, 1954, Serial No. 465,132. Other nitrodiphenyl others may be used but in general the average nitro content is not greater than about 2.5 nitro groups per molecule and not more than about 2 nitro groups substituted on a given phenyl group of the molecule. Dowtherrn A, a commercial product containing about 73% by weight of diphenyl ether, the remaining 27% being all diphenyl may be nitrated to contain such distribution of nitro groups and the product may be used as the nitrodiphenyl ether plasticizer component. However, 2,4-dinitrodiphenyl ether prepared from 2,4-dinitrochlorbenzene is preferred. The complete grain formulation contains from about 6% to about 9% by weight of the nitrodiphenyl ether plasticizer component.

Certain aromatic amines when introduced into the ammonium nitrate-based grain catalyzed by Prussian blue and ammonium dic'nromate catalyst, have the very desirable effect of decreasing the gassing of the grain at relatively high temperature. Different classes of amines may be used as gassing inhibitors. However, diphenylamine is preferred. A marked improvement in the gassing en istees g tendency offthe composition isobtained by the use of relatively small amounts of the amine, i.e., from about 0.5% to 3% by weight based on the total weight of. the

grain and preferably from about 1% to about 3 of the a'mine'base'd on the total weight-of the grain. f

'iThe carbon component, which is added as a synergist to increase the burningrate of the composition, may be finely divided highly adsorptive' carbons" such as Nor'iti and Nuchar. However, it is preferred to use carbon naces.

The carbon blacksare characterized by low ash. content and by having extremely small particle size, that is, 50 to 5000 A., and they contain adsorbed hydrogen and oxygen. The Bead type carbon blacks may be used.

Examples of bead type carbon blacks suitable as catalyst synergists are Micronex Beads (channel blacks) and StatexBeads (furnace blacks). The carbon component of the propellant composition of this invention contains not more than 5% ash and preferably less than 0.5% ash. From about 0.3% to 3% .byweight of carbon black may be used in the grain composition of this in-' vention. f

It has been discovered that certain non-ionic surfactants (sorbitans) derived from sorbitol, the sorbitan oleates being mixtures containing an average of from 1 to 3 oleyl radicals foreach molecule of anhydrized sorbitol. Sorbitol is dehydrated to give condensed link structures of 'sorbitans which, when esterified with oleic acid, produce the preferred Spans or Arlacels. Arlacel C, which issorbitan sesquioleate, has been found to be particularly' effective as an additive to promote ballistic dependability of the'grain compositions. This material is an oily liquid at C. having a specific gravity of 0.95-1.00, a flash point of about 450 F. and a fire point of about 530 F.

Arlacel C has a viscosity at 25 C. of 900-1100 centipoises. Another very eifective non-ionic surfactant isSpan 8S, which is a sorbitan trioleate having a viscosity at 25 C. of 100250 centipoises. It has a specific gravity of 0.92-0.98, a flash point of 500 and a fire point of 570. F.

Another type of non-ionic surfactants which we have found to be eifective in stabilizing the compositions are polyalkylene oxide condensation products and the ester derivatives thereof. These are known as non-ionic block polymers and are prepared by condensing propylene oxide in the presence ofmoisture or in the presence of a catalytic amountof sodium hydroxide. The polyoxypropylene glycol product is then reacted with ethylene .oxide to produce the ethylene oxide-propylene oxide block copolymenl The polyoxyalkylene glycols and the ester den'vatives thereof contain from 2 to 3 carbon atoms per .alkylene oxide unit, An example of polyalkylene oxide 7 Percent Ammonium nitrate, at least 70 Combustion catalyst consisting of a mixture of ammonium dichromate and. insoluble Prussian blue 1.0 to 6.0 Cellulose acetate 6.0 to 910 Ethylene glycol diglycolate 5.0 to 9.0 2,4-dinitrodipheny1 ether 7.0 to 9.0 Diphenylamine gassing inhibitor 0.3 to 3.0 Finely divided carbon 0.3 to 3.0

Non-ionic surfactant material'selected from the a class consisting of 'polyoxypropylene-polyoxy-" ethylene glycol having molecular weights within'the range of 2000 to about 3300, monoester derivatives. of polyoxyalkylene glycols having molecular weights within the range of about 1000 to about 2000 and at least one sorbitan oleate containing an average of from 1 to 3 oleyl radicals per mole of sorbitol intermediate used to obtain sorbitan 0.05 to 0.5

a position consists essentially, on a weight basis,'of: v2 V condensationproduct is the polyo xypropylenepolyoxy ethylene glycol, Pluronic L- 62. It corresponds to the general formula HO(C H O) ,(C H O) ,(C H O),,H and has a molecular weight of approximately 2000 of which the base unit, polypropylene oxide, provides from about 1500 to 1800 of the molecular weight. Pluronic L-62 is a liquid having a viscosity at 25 'C. of 300 to 500 centipoises.

' -An example of an ester derivative of a polyoxyalkylene glycol productv is Nonisol-250, the oleyl monoester of polyethylene oxide. The fatty acid radicals of the monoester contain 12 to 20 carbon atoms. It is a liquid having a molecular weight of about 1000.

The amount of surfactant material added to the composition is relatively small, i.e., usually within the range of 0.05 to 0.5% by weight of the composition, preferably 0.1% to 0.2%. 1 e I Thus to recapitulate, the preferred gas-forming com- The composition of this invention can be prepared by several methods. The preferred procedure is as follows:

The binder is prepared by heating the mixture of plasticizer components, that is, ethylene glycol-diglycolic acid condensation product and dinitrodiphenyl ether to a temperature between about 120 C. and 150 C. The mixture is agitated while being maintained at an elevated temperature above the melting point of the mixture, agitation being continued until a substantially homogeneous mixture has been obtained. To the homogeneous mixture is then added the cellulose acetate synthetic polymer and the mixture is stirred to homogeneity to complete the fabrication of the binder. The desired amount of ammonium nitrate in finely powdered state is admixed with the desired amount of catalyst, also in finely powdered condition.

is continued until a homogeneous solid phase has been obtained and the'mass is formed into shapes and configurations suitable for solid propellant uses. The configurations are commonly called grains. These configurations or shapes may be formed by introducing the pasty mass into suitable molds and molding the material under pressure or shapes may be made by extrusion. The compositions flow under 7 pressure at temperatures above C. and become dimensionally stable at temperatures below about 90 C.

Burningrate test strips of the gas-forming compositions are prepared by extruding or molding the homogeneous composition at a temperature below about C. under a pressure of about 2.000 p.s.i., the test strips being. about '7 A" to about /3 in diameter and about 5" long. Ihe burning rate of these test strips is determined at series of pressures, i.e., 600, 800, 1000, 1200, 1400, 1600 and 1800 pounds per square inch nitrogen'pressure in a Crawford bomb.

The burning rates in inches per second at the different pressures are plotted on log-log paper and this plot gives a straight-line relationship, the burning rates being plotted as ordinates and the pressures being plotted as abscissa. The slope of this straight line is defined as the pressure exponent or as the exponent of the burning rate asrelated to pressure in the formula I hite) wherein B is the linear burning rate at pressure p, [3 is the linear burning rate for the composition at 1000 p.s.i., p is pressure in p.s.i. in the burning chamber and n is the pressure exponent showing the dependence of the burning rate on pressure. Thus the exponent is the numerical value equal to the slope of the curve of burning rate in inches per second vs. pressure obtained by plotting 'the burning rate at various pressures on log-log paper. Ammonium nitrate compositions usually have a pressure exponent of about 0.7 or higher. The lower the value of n, the less is the detonating character of the decomposition of the gas-producing composition and the more even and smooth is the burning of the propellant grain. Thus a sustained thrust rather than a detonation is produced in the burning of the satisfactory propellant grain and sustained flow of gas from thegas generator is obtained if the pressure exponent of the composition is low. In general, pressure exponents lower than about 0.7 are desired.

One of the specifications for an acceptable solid propellant grain is satisfactory ballistic performance following prolonged storage at 77 C. The chief object of this invention is to overcomethe effects of prolonged high temperature storage relative to detonation. This is accomplished by incorporation in the grain of a small amount of the surfactant in the grain composition, which also contains the diphenylamine.

Gas-producing grains are prepared from the pasty compositions containing these ammonium nitrate combustion catalysts and binder by molding the composition into cylindrical grains under pressure of about 2000 to 4000 psi. The size and shape of the grains are dependent upon their intended use. In one application grains for airplane assist take-E service are usually about 30" in length by 3" to 6 in diameter. These are provided with centrally located holes of different shapes, that is, star form, cruciform, circular, etc. Test grains which were motor were 2.75" in diameter and 4 /2" to about in length and were provided with a starform centrally located hole. These test data obtained by burning such grains in the test motor indicate the thrust or impulse, uniformity of the burning rate and over-all performance of the compositions when used in assist take-off operations. For these operations, the grains are mounted in a conventional case and are ignited or fired by electrical or other known means. The temperature of the gases produced by firing of the grain may be of the order of 1500 F. to 3000 F. and the pressure or impulse thrust by the hot gases will be dependent upon the grain size, diameter of the nozzle and other factors. The gas-producing grain material may be molded into disc form, stacks of discs being used as gas-forming propellant material for missile rockets.

Test grains of 2.75" in diameter by 5" in length are given a thermal shock test, which is referred to herein as thermal cycling test. In this test, several of the grains of the given composition are subjected, in an oven, to a temperature of 175 F. for a period of two hours, following which the grains are immediately subjected for a period of two hours to a temperature of F. to complete one cycle. Immediately following this period the grains are again subjected to the 175 F. temperature .for another period of two hours and then to a second cold temperature treatment at 75 F. for two hours, after which at least one of the grains is allowed to come to room temperature, i.e., about 75 F. to F. This or these grains are examined for resistance to deformation and shattering. Part of the grains are tested for firing qualities, the temperature of the grain being -75 F., 175 F. and normal ambient temperatures following the cycle treatment. The grains of this invention pass these cycle tests, ignite at low, high and normal ambient temperatures following the cycle test and do not shatter. They burn uniformly following the thermal cycle treatment.

Another test to which the finished grains are subjected is that of hot aging. In this test, a number of grains are maintained at a temperature of 170 F. in an oven in the presence of circulating air. Grains are withdrawn periodically which are tested with respect to firing and burning qualities. In order to pass this test, the grains must be unaffected with respect to firing qualities. It must burn uniformly and not detonate after being subjected to the high temperatures for a period of 30 days. The addition of the sorbitan oleate, particularly the sorbitan sesquioleate, known commercially as Arlacel C, to the grain composition causes a grain'to pass this 30-day hot aging test, as indicated hereinbelow. Like results are obtained in this test by the incorporation of Span 85, Pluronic L62 and Nonisol-25O.

Another requirement of the gas-forming compositions of this invention is that of chemical stability at relatively high temperatures as indicated by the substantial absence of gassing of the composition. The gassing tendency is measured by an arbitrary test which has been made more severe with respect to temperature than would be imposed on the composition at any atmospheric temperatures.

This testis commonly designated as the C. Gas Stability Test and is carried out as follows:

Three grams of a finely divided ammonium nitrate base composition is placed in a vessel. The vessel is connected by tubing to a mercury manometer system which is calibrated so that readings are in units of volume. The vessel is inserted to an opening in a metal block; this metal block is provided with electrical heating elements and controls which permit the block to be maintained at a temperature of 135 C. A period of 15 minutes is allowed for thesample to come to 135 C. At this time, the manometer is set to zero. The gas rate in cc./g./hr. after each 15-minute interval is then plotted against the time of heating of the sample.

In general, it is considered that a composition which has an essentially zero gassing rate during the first hour of heating will be substantially free of gassing tendency in storage at atmospheric temperatures. It is considered that a composition which has a gassing tendency not in excess of about 2 cc./g./hr. after the first hour of heating at a temperature of 135 C. is satisfactory with respect to hot storage i.e., temperatures as high as 77 C.

The effect on hot storage stability of including in the gas-forming composition a small amount of sorbitan sesquioleate is illustrated by the comparison of the burning properties of grains prepared in Examples 1 and 2 below.

Example 1 diglycolate plasticizer component was prepared by congem-pet tiensing ethylene glycol with diglycolic acid in a mol ratio of glycol to acid of 1.2. The polyesterification reaction product had a specific gravity of 1.35, a'refraction index of 1.475, and had a molecular weight within the range of from about 300 to 400. The mixture of cellulose acetate and two component plasticizer was heated and stirred to obtain a homogeneous mixture and to this mixture was then added 2.02 parts by Weight of diphenylamine and 0.10 part by weight of Arlacel C (sorbitan sesquioleate). To this mixture was then added 73.9 parts by weight of finely ground ammonium nitrate with which was thoroughly mixed 0.5 part by weight of carbon black (ash-free), 1 part by weight of insoluble Prussian blue and 2 parts by weight of commercial grade ammonium dichromate. The ammonium nitrate was Special nitrate, 70% of which had been ground at 14,000 r.p.m., thus giving an ammonium nitrate product, atleast 50% of which pass through a #100 US. Standard sieve. This mixture was stirred and milled at temperatures with' in the range of 110 to 120 C. for'a,fperiod of several hours until a homogeneous product was obtained.

A part of the grain composition prepared in Experiment 1 was molded at 2000 pounds pressureinto burning rate test strips, A" x A by 5" in length. The burning rate of this-composition at 1000 p.s.i. was 0.115 in./sec and the pressure component was 0.54. A small amount of-the gas-producing composition was tested in the 135 C. Gas Stability Test. No gassing was observed for two hours. The remainder of thegas-producing composition produced in Experiment 1 was molded at4000= pounds pressure to form cylindrical grains, each grain being provided with a starform hole throughout the length of the grains. These grains were about 2.75" in diameter and about 5". in length and hada density of about 1.5 8 grams per cubic centimeter. The grain material molded very well. Several of the grains were cycled through high 175 F.) and low (-75". F.) thermalshock treatment. Followingsuch treatment, the grains ignited and burned satisfactorily in the rocket motor at grain temperatures of 175 F., -75 F. andat 70 F.

A number of the grains produced from, material of Example 1 were subjected to the hot aging test, i.e., storage at 77C. (170 F.) overan extended period of time. It was found that these grains could be stored at 170 F. for more than 80 days without afiecting the burning properties of the grains, that is, grains stored for one day up. to more than 85 days burned uniformly upon ignition in a test motor. The weight loss upon storage of'these grains, which weighed about 600 grams each, was less than about 0.5% even after storage for more than 80 days at 170 F.

Example 2 In the second example gas-forming grain material was compounded according to the same procedure as that. outlined in Example 1', except that the composition contained no surfactant as defined hereinabove. The composition of this grain material compounded in Example 2'was as follows:

t Percent Cellulose acetate LL-l grade 1 6.90 2,4-dinitrodiphenyl ether 8.05 Glycol diglycolate 5.98 'Diphenylamine 1 2.07 Insoluble Prussian blue 1.00 Ammonium dichromate 200 Carbon black 0.50

Ammonium nitrate 73.50

, duced in Example 2 above was the sameas the composition';ofthe gas-forming material produced in Example 1 except that no sorbitan sesquioleate surfactant was present in the composition. Although the material of Example 2 which contained diphenylamine showed no gassing during a 1% hour heating of a 3 gram sample at C., grains of this material which were stored'at F. malperformed after 7 days of the hot storage. The grain wormholed and the resulting high pressure developed on 'firing ruptured a blowout disc. The burning rate of a strand of this grain propellant composition, at 1000 pounds pressure was 0.14 inch per second with a pressure exponent of 0.74. Although the above composition hasbeen designated as Example 2, it is to be understood that the composition is a control composition containing no surfactant and the hot storage test shows the characteristics of the propellant grain when the surfactant is not present in the composition.

' Example :3

p In .a third example, a gas-forming grain was compounded according to the procedure outlined in Example 1 except that a' larger proportion of carbon was used in the grain'and Span 85, sorbitan trioleate, was included in the grain composition in an amount of 0.1% by weight Two additional grain formulations were compounded according to the procedure described in Example 1 to correspond to the composition of the base control grain Example 2 except that in one formulation 0.1% of Nonisol-250 was included as a non-ionic surfactant and in the other formulation 0.1% of Pluronic' L-62 was used as the non-ionic surfactant. Gas-forming propellant grains fabricated from these formulations passed the 30-day hot aging test, that is, they were fired successfully after being subjected to a temperature of 77 C. i.e., 170 F., for a period of 30 days.

The drawings show a particular application of this invention to an assisted take-ofi unit. The ATO unit illustrated is designed to be hung-under the wing of an aircraft; normally at least two units, i.e., one under each wing, will be used. In Fig. l, the body of the unit is made up of a tubular member 11 which is closed at one end and Whichis provided with threads at the open end. Member 11 is provided with two loops, 12 and 13. These loops are used to hang the unit from a carrier, not shown, which is attached to the wing of the aircraft. This carrier makes it possible to jettison the unit after take-off. A somewhat funnel shaped member 14 is attached to member 11 by engagement of the threads at the large open end of member 14 with the threads on member 11. Member 14 is provided with a. nozzle 16 through which the decomposition'products pass. The size of nozzle '16 determines in part the pressure maintained inside of the chamber formed by members 11 and 14.

The solid propellant =fills the cylindrical portion of member 11. The solid propellant in this illustration consists of seven tubular grains, 17, 18, .19, 20, 21, 22 and 23; each having an OD. of about 3" and having a centrally located cylindrical opening 1" in diameter, the full length of the grain; the grains are approximately 30" long. The grains used herein contain about 2% of ammonium dichromate catalyst, 1% of insoluble Prussian blue catalyst, about 2% diphenylamine, 20% of binder and 73.5% of ammonium nitrate. Each grain has the annular area at each end inhibited against burning by a coating consisting of a mixture of vistanex resin and carbon black in order that the burning may take place on the cylindrical surfaces only. For some uses it is desirable to have a grain which burns cigarette fashion in which case the outer surface and one end of the grain will be inhibited to prevent combustion.

Although a tubular grain is illustrated herein, the invention is not limited to such a grain. Any particular shape may be utilized. Examples of other shapes are cylinder, cruciform, triform, hexaform, octaform and slab. In the case of perforated grains, the perforation may be circular or star-shaped with various numbers of points in the star. Furthermore, a single cylindrical grain having one or more longitudinal perforations may be utilized in some cases, instead of the multigrain unit shown. a

The grains are held in longitudinal position and prevented by sliding back and forth in the combustion chamber by means of a wire grid 26. Wire grid 26 consists of a ring cut to fit the threads of member 14 and provided with a grid work of metal wires that will resist the high temperature existing in the combustion chamber.

On one side of the conical portion of member 14 there isprovided for the combustion chamber a safety venting means 23. Venting means 28 comprises a tubular member fastened to member 14, which tubular member has full access to the combustion chamber and is provided with a rupture disc, not shown. The rupture disc is of such construction that excess pressure in the combustion chamber will blow out the disc, whereby the pressure in the combustionchamber will be held below the point of serious damage to the unit.

An igiiter means is positioned with-in member 14 so as to close off the nozzle 16. The igniter means consists of a container 31 filled with black powder, or some other easily ignited material, which can produce a large volume of gases at elevated pressure. A squib 32 'for igniting the powder is attached to the container 31 in communication with the powder contained therein. Electrical wires 33 connect a wire in the squib to the electrical system of the aircraft and a switch therein (the connections to the aircraft are not shown).

The ATO unit, is assembled as follows: the grains are inserted into member 11. Venting means 28 are fastened to member 14. Igniter 31 is inserted through the large open end and fitted so as to close the nozzle, the wires 33 having first been passed through the nozzle 16. The wire grid 26 is screwed into the large open end of member 14. The assembled nozzle portion is then securely screwed onto member 11.

The assembled unit is then attached to the wing of the aircraft by loops 12 and 13; wires 33 are connected to the electrical operating assembly in the aircraft. When the pilot desires to obtain the assisted take-off, he throws the switch which causes the current to pass through wire 33 and to heat up the firing wire in squib 32, which in turn ignites the powder in the container 31.

The container is of sufficient strength to withstand the initial pressure generated by the gases from the powder. The hot gases raise the pressure in the combustion chamber high enough to permit the grain to ignite. The combustion of the grain causes the pressure in the chamber to rise to a point which cannot be resisted by container 31. The container disintegrates and the pieces are discharged through nozzle 16. The total time from throwing the switch to full operation of the unit is on the order of less than 0.5 second.

As the gases pass out of the nozzle the reaction acts on the aircraft and adds its thrust to assist the aircrafts propeller; a marked increase in forward speed results and permits the aircraft to take off in a shorter space of time or it permits lifting a load heavier than could be airborne by the use of the propellers alone.

The conventional placement of the igniter may be used with the lower catalyst content grains. However, it is necessary to use a much heavier powder charge in the igniter or, preferably, the nozzle is provided with a rupture disc, which is set to blow out at about 500 psi. Other methods of igniting the grain can be readily devised.

While the utility of the shaped grain composition has been illustrated by means of an assisted take-off operation, it must be understood that the solid propellant of this invention can also be used for other purposes. Some of these are air-to-air missiles, air-to-ground missiles, ground-to-ground missiles, etc. An important use of the invention lies in the production of gases at elevated pressures in a stationary or a portable system; discontinuous operation is readily obtained when using about 2% of catalyst as the composition can be extinguished readily by merely depressuring the combustion chamber.

Having thus described this invention, what is claimed 1. A gas-forming composition consisting essentially of at least about 70% by weight of ammonium nitrate; from about 1% to about 6% by weight of an inorganic combustion catalyst consisting essentially of a mixture of ammonium dichromate and insoluble Prussian blue; from about 18% to about 27% by weight of a binder material, which bindermaterialconsists essentially of from about 18 to about 35% by weight of cellulose acetate, which analyzes between about 51 and 57% by weight of acetic acid, from about 20 to about 40% by weight of a liquid polyester condensation product of ethylene glycol with diglycolic acid, said polyester condensation product having a molecular weight within the range of from about 250 to about 600, said binder material also containing from about 20% to about 40% of a nitrodiphenyl ether based on the total weight of the binder; from about .3 to about 3% of diphenylamine as a gassing inhibitor, based on the total weight of a gas-forming composition; from about .3 to about 3% byweight of finely divided carbon and from about 0.05 to about 0.5% by weight of a nonionic surfactant material selected from the class consisting of polyoxypropylene-polyoxyethylene glycols having molecular weights within the range of 2000 to about 3300, poiyoxyalkylene glycol fatty acid monesters having molecular weights of about 1000 to about 2000, the fatty acid radicals of which contain 12 to 20 carbon atoms and at least one oleyl ester of mono'anhydro sorbitol.

2. The gas-forming composition described in claim 1 wherein the nitrodiphenyl ether is 2,4-dinitrodiphenyl ether.

3. The composition of claim 1 wherein the finely divided carbon is carbon black.

4. The gas-forming composition as described in claim 1 wherein the oleyl ester of anhydro sorbitol is sorbitan sesquioleate.

5. The gas-forming composition as described in claim 1 wherein the oleyl ester of the anhydro sorbitol is a sorbitan trioleate.

6. The gas-forming composition as described in claim 1 wherein the non-ionic surfactant material is a polyoxypropylene-polyoxyethylene glycol having a molecular weight of about 2000.

7. The gas-forming composition as described in claim 1 wherein the non-ionic surfactant material is a polyoxyalkylene glycol-fatty acid ester having a molecular Weight of about 1000.

8. A shaped grain propellant composition consisting essentially on a weight basis of about 73.9% ammonium nitrate; about 6.8% of cellulose acetate, which analyzes between about 54 and 56% by weight of acetic acid, about 7.9% of 2,4-dinitrodiphenyl ether; about 5.9% of the condensation product of ethylene glycol with digly- $942,964 l3 7 l4 colic acid; about 2% of diphenylamina; about 1% of References Cited in the file of this patent l I msolub c Prusslan blue and about 2% of ammomum d1 UNITED STATES PATENTS chromate catalyst; about 0.1% of sorbitan sesquioleate having a specific gravity at 25 C. of 0.95-1.00 and about 2,159,234 Taylor May 23, 1939 0.5% of a carbon black. 5 2,628,561 Sage et a1. Feb. 17, 1953

Claims

1. A GAS-FORMING COMPOSITION CONSISTING ESSENTIALLY OF AT LEAST ABOUT 70% BY WEIGHT OF AMMONIUM NITRATE, FROM ABOUT 1% TO ABOUT 6% BY WEIGHT OF AN INORGANIC CONBUSTION CATALYST CONSISTING ESSENTIALLY OF A MIXTURE OF AMMONIUM DICHROMATE AND INSOLUBLE PRUSSIAN BLUE, FROM ABOUT 18% TO ABOUT 27% BY WEIGHT OF A BINDER MATERIAL, WHICH BINDER MATERIAL CONSISTS ESSENTIALLY OF FROM ABOUT 18 TO ABOUT 35% BY WEIGHT OF CELLULOSE ACETATE, WHICH ANALYZES BETWEEN ABOUT 51 TO 57% BY WEIGHT OF ACETIC ACID, FROM ABOUT 20 TO ABOUT 40% BY WEIGHT OF A LIQUID POLYESTER CONDENSATION PRODUCT OF ETHYLENE GLYCOL WITH DIGLYCOLIC ACID, SAID POLYESTER CONDENSATION PRODUCT HAVING A MOLECULAR WEIGHT WITHIN THE RANGE OF FROM ABOUT 250 TO ABOUT 600, SAID BINDER MATERIAL ALSO CONTAINING FROM ABOUT 20% TO ABOUT 40% OF A NITRODIPHENYL ETHER BASED ON THE TOTAL WEIGHT OF THE BINDER, FROM ABOUT .3 TO ABOUT 3% OF DIPHENYLAMINE AS A GASSING INHIBITOR, BASED ON THE TOTAL WEIGHT OF A GAS-FORMING COMPOSITION, FROM ABOUT .3 TO ABOUT 3% BY WEIGHT OF FINELY DIVIDED CARBON AND FROM ABOUT 0.05 TO ABOUT 0.05% BY WEIGHT OF A NONIONIC SURFACTANT MATERIAL SELECTED FROM THE CLASS CONSISTING OF POLYOXYPROPYLENE-POLYOXYETHYLENE GLYCOLS HAVING MOLECULAR WEIGHTS WITHIN THE RANGE OF 2000 TO ABOUT 3300, POLYOXYALKYLENE GLYCOL-FATTY ACID MONESTERS HAVING MOLECULAR WEIGHTS WITHIN THE RANGE OF 2000 TO ABOUT ACID RADICALS OF WHICH CONTAIN 12 TO 20 CARBON ATOMS AND AT LEAST ONE OLEYL ESTER OF MONOANHYDRO SORBITOL.