US3464355A - Gas generator - Google Patents

Description

Sept. 2, 1969 AcLlN ET AL 3,464,355

GAS GENERATOR Filed June 11; 1965 2 Sheets-Sheet 1 INVENTORS' JOHN J. ACL/N JOSEPH F JUN/(ER ATTORNEY Sept. 2, 1969 AL|N ET AL 3,464,355

GAS GENERATOR Filed June 11. 1965 2 Sheets-Sheet 2 [[6 ll JOHN L ZZZTS B JOSEPH F JUN/(ER WXM United States Patent 3,464,355 GAS GENERATOR John J. Aclin, Herrin, and Joseph F. Junker, Marion, IlL, assignors to 01in Mathieson Chemical Corporation, a corporation of Virginia Filed June 11, 1965, Ser. No. 463,213 Int. Cl. F42b 5/20, 9/18, 13/46 US. Cl. 10239 9 Claims ABSTRACT OF THE DISCLOSURE A gas generator including a cartridge case and a solid propellant composition within the case and a zeolite barrier positioned between the case and the propellant composition. The zeolite barrier may be a rubber sheet having powdered zeolite embedded therein or a perforated container of granular zeolite. The rubber sheet containing the embedded zeolite may also contain embedded fibers which are capable of acting as a wick for water.

This invention relates to improved power generating units, and more particularly to means for controlling the change in dimensions of ammonium nitrate propellants employed in gas generating units.

Gas generators in which solid gas generating charges are employed have been previously used as a source of power for the operation of prime movers such as internal combustion engines, motor starters, switch closures, jet engine starters, and the like. These generators are usually formed of a cylindrical case closed at one end and open at the opposite end, the case containing an igniting means and a solid gas generating charge of suitable design enclosed in a weather seal positioned between the case and charge. Recently, ammonium nitrate granules embedded in a matrix of a resinous or plastic binder such as synthetic rubber, nitrocellulose, and the like have been employed as the solid gas generating charge in these generators. One problem that arises is that although this type of solid propellant is dependable under relatively mild temperature conditions, the dependability of firing is markedly affected when it is subjected to extreme variations in temperature conditions. Th increase in failures at extreme temperatures is generally believed to be due to marked variations in the dimensions of the solid propellant. For example, conventional gas generating charges containing ammonium nitrate have been observed to increase in dimensions when subjected to temperature cycling tests which range from between about -65 F. to about +185 F. This increase in dimensions has been thought to be associated with the changes in volume accompanying phase transistions of the ammonium nitrate.

Ammonium nitrate has been described as changing from form V (hexagonal) to for IV (orthorhombic) at about 0 F., and from form IV to form III (orthorhombic) at about 90 F. These increases in dimensions of the gas generating charges have resulted in erratic combustion and overpressurizations during operation of the units, and have also resulted in rupture of metal jet starter cartridge assemblies.

Temperature cycling of conventional gas generating assemblies is accompanied by changes in the moisture content of the atmosphere within the assembly which may result from moisture vapor transmission through the generator case surrounding the propellant from chemical reaction within the generator, or otherwise. At elevated temperatures the atmosphere inside the generator case will contain a higher moisture content than it will at reduced temperatures. Ammonium nitrate is a hydroscopic material, and the solubililing of this material in condensed moisture followed by recrystallization when the moisture evaporates may also constitute part of the 3,464,355 Patented Sept. 2, 1969 mechanism of propellant growth or increase in dimenslons. The recrystallization of ammonium nitrate may be a function of the breathing characteristic of ammonium nitrate. The nitrate is solubilized while the relative humidity within the case atmosphere is relatively high. When the relative humidity drops, the adsorbed moisture is squeezed out or released as vapor within the case. The water may literally be squeezed from the propellant at lower temperatures. The end result is a leaching of ammonium nitrate from the propellant surface. Small crystals and larger salt deposits are found on the grain surface and the bottom of the case.

A number of materials have been tested with temperature cycled ammonium nitrate propellants in an attempt to reduce or eliminate the change in dimensions during cycling.

Various known desiccants such as magnesium oxide, calcium oxide, calcium carbonate and the like have been included in the gas generating container, but these have not been effective over the entire range of extreme cycling temperatures (between 65 F. and +185 F.) to which the gas generators are subjected, particularly under test conditions. The main disadvantage inherent in these desiccants is that although they absorb moisture from the atmosphere when the relative humidity is high, they release moisture into the atmosphere when the relative humidity is low.

There is a need in the industry at the present time for an effective means for controlling the dimensions of solid propellants in gas generators subjected to extreme variations in temperature conditions, for example, through the range between 65 F. and +l F.

It is a primary object of this invention to overcome the disadvantages inherent in conventional gas generators, particularly those in which a water soluble or hygroscopic material such as ammonium nitrate is employed as a component of the solid propellant.

Another object of the invention is to provide an improved solid propellant type gas generator.

It is another object of this invention to provide an improved means for controlling the dimensions of solid propellants containing water soluble or hygroscopic materials such as ammonium nitrate.

It is another object of the invention to provide a solid propellant gas generator in which the penetration of the propellant grain by water is reduced.

Still another object of the invention is to provide a thermal barrier in solid propellant gas generators which assists in shielding the generator case from exposure to high temperatures during firing by providing a coolant gas during combustion.

These and other objects of the invention will be apparent to one skilled in the art from the following detailed description.

FIGURE I is a plan view of a typical gas generator assembly of this invention.

FIGURE II is a longitudinal, cross-sectional view through lines IIII of FIGURE I of a preferred embodiment of the gas generator of this invention.

It has now been discovered that the foregoing objects of the invention are accomplished when a barrier of zeolite is positioned between the weather seal and the solid propellant charge of a gas generator. Improved gas generators of this invention not only resist penetration by water from the atmosphere outside of the generator, but also are provided with means to absorb moisture from the atmosphere within the generator case and retain this absorbed water even when subjected to extreme temperature variations. As a result, there is a marked inhibition of dimension changes in the solid propellant grain.

The barrier containing zeolite may be positioned within the gas generator by a variety of techniques. In one em- 3 bodiment, a sheet of rubber having powdered zeolite embedded therein is wrapped around all of the outside surface, except for each end, of the solid propellant grain and the covered grain is inserted into the case of the gas generator. The discharge end of the generator is sealed with a suitable moisture barrier.

In another embodiment, when propellants are required which generate extremely clean combustion gases, a barrier is placed between the flame surface of the propellant and the rubber sheet to prevent combustion of the rubber sheet and to prevent the zeolite from being carried into the gas discharge.

In another embodiment of the invention, irregular fibers capable of acting as a wick for water are incorporated into the rubber sheet with the zeolite powder, thereby enhancing the contact of moisture with the zeolite particles. In still another embodiment of the invention a suitable cooling salt, such as ammonium oxalate, melamine, oxamide, ammonium formate, mixture thereof, and the like may also be dispersed in the rubber sheet with zeolite powder in order to reduce the adverse temperature effects upon the generator case when the generator is ignited.

In still another embodiment of the invention, the solid propellant grain wrapped with rubber sheet containing zeolite, with or without fibers and/or cooling salt, as the case may be, is inserted into an envelope of a laminate of aluminum and polyethylene terephthalate. In still another embodiment of the invention, the barrier of zeolite is a perforated container of granular zeolite particles positioned within the generator case adjacent to the solid propellant grain. Such a container may be utilized as a supplement or replacement for the aforesaid rubber sheet containing powdered zeolite.

More in detail, any natural or synthetic zeolite having desiccating properties, and being capable of adsorbing and retaining water from the atmosphere in the range between about 65 F. and +18-5 F. may be employed. The synthetic zeolites disclosed in United States Patent Nos. 2,882,243 and 2,882,244, both of which are entitled Molecular Sieve Adsorbents and which issued to Robert M. Milton on Apr. 14, 1959, illustrate the types of zeolite that are particularly suitable for use in the instant novel invention. The synthetic zeolites in United States Patent No. 2,882,243 are crystalline synthetic materials having a composition expressed in terms of oxides as follows:

wherein M represents at least one of the materials in the group consisting of hydrogen, ammonium, metals in Groups I and II of the Periodic Table, and the transition metals of the periodic table, n represents the valence of M, and Y may be any value up to about 6, the atoms of said material being arranged in a unit cell in such a manner that the X-ray powder diffraction pattern of the material is essentially the same as that shown in Table A of the patent.

The synthetic zeolites of United States Patent No. 2,882,244 are crystalline synthetic materials having a composition expressed in terms of oxides as follows:

wherein M represents at least one cation having a valence of not more than three, n represents the valence of M, and Y may be any value up to about 8, the atoms of said material being arranged in a unit cell in such a manner that the X-ray powder diffraction pattern of the material is essentially the same as that shown in Table A of the patent.

The particle size of the zeolite compositions employed in the preparation of the novel rubber sheet is generally in the range between about 0.1 and about 10 microns, and preferably in the range between about 0.5 and about 5 microns but slightly smaller or slightly larger particles .4 may be employed if desired. When granular particles of zeolite are employed for use in the perforated containers or retaining devices, the particles may be irregular in size, for example, rods of irregular length, up to about Mr inch, or more, having an outside diameter between about inch and about inch and preferably between about inch and about /s inch diameter. However, granules having a diameter less than & inch may also be employed, providing the openings in the perforations of the retaining device are of a smaller diameter.

In the preparation of rubber sheet containing zeolite, it is preferred to employ powdered zeolite having pores with an average diameter in the range between about 3 A. and about 6 A. units, but powdered materials with larger or smaller pore openings can be used if the material to be removed from the atmosphere has a molecular size that is small enough to pass into the pores of the zeolite.

The rubber component of the rubber sheet is any anhydrous rubber capable of maintaining strength and resilience over wide tempertaure ranges. Typical examples of suitable rubbers which may be employed for this purpose include anhydrous coagulated or precipitated latex, (natural or synthetic solid rubber latex) such as C-rubber (acrylic acid-polybutadiene copolymer), openchain conjugated diolefins having from 4 to 8 carbon atoms exemplified by butadiene-l,3; 1,4-dimethyl butadiene-l,3; 2,3-dimethyl butadiene-l,3, and the like, or of rubbery copolymers of such diolefins and similar conjugated diolefins with each other or with copolymerizable monomeric materials containing a single ethylenic linkage exemplified by styrene, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, acrylonitrile, or similar materials, or of rubbery polymers of chloroprene and the like, or a blend of a natural latex with a synthetic latex or a blend of synthetic lattices. In general, any elastomeric system capable of being formed into a desired shape, which is not affected by water or zeolite and which does not form products on ignition that would be deleterious to the system may be employed.

The novel rubber sheeting of this invention is prepared, for example, by adding the rubber mixture to steam heated rolls, heated to a temperature sufficient to soften the rubber, and the rubber is then worked between the rolls until a uniform tacky sheet coats the roll and a good soft bead is formed between the nip. The dry particles of zeolite are then slowly added to the rubber on the rolls and gradually worked into the sheet. In a preferred embodiment the zeolite and rubber are preblended in a suitable mixer prior to forming the sheets. The proportion of zeolite added to the rubber is preferably equivalent to between about 40 and about percent by weight of the rubber, but may range between about 1 and about percent by weight of the rubber. Working of the mixture of soft rubber and zeolite is continued until a substantially uniform dispersion of the zeolite in the soft rubber is obtained. The sheet is rolled until it has a substantially uniform thickness of between about 30 and about mils, but may be thinner or thicker if desired.

In a preferred embodiment of the invention, fibers capable of acting as a wick for water are admixed with the rubber and zeolite prior to or during the formation of the rubber sheet. Suitable fibers include vegetable fibers such as cotton fibers as well as synthetic fibers such as nylon fibers, acrylic fibers, polyester fibers, and the like. Asbestos is undesirable for this purpose since it acts as a curing agent for the rubber in certain instances and imparts undesirable rigidity to the resulting rubber sheet. The proportion of fibrous material added to the rubber is equivalent to between about 0.5 and about 40, and preferably between about 1 and about 20 percent by weight of the rubber composition.

In another embodiment, particles of a cooling salt such as ammonium oxalate are added to the mixture of rubber and zeolite, with or without the fibrous materials,

as the case may be, prior to or during the formation of the rubber sheet. The proportion of cooling salt added to the rubber is equivalent to between about 0.2 and about 60, and preferably between about 5 and about 40 percent by weight of the rubber composition.

After the desired dispersion of solids in the rubber and the desired thickness of rubber sheet are obtained, the sheet is removed from the rolls, with or without cooling, and cut into the desired shapes. The resulting rubber sheet containing the zeolite composition is capable of absorbing water in a proportion approaching that which can be absorbed by the original powdered zeolite alone. This novel rubber sheeting is particularly useful as a component of solid propellant gas generators in order to render the propellant grain impervious to penetration by water, and also to control the size of the solid propellant containing water soluble or hygroscopic material such as ammonium nitrate.

FIGURE I is a plan view of a typical gas generator 100 of this invention having a case 1 formed of a suitable material such as rubber, reinforced plastic, wood, steel and the like. Secured to the top of case 1 is an ignition assembly contact 2 having an ignition safety 3 which must be removed prior to use.

FIGURE II is a longitudinal cross-sectional view through lines II of FIGURE I of a novel gas generator of this invention. Case 1, having ignition assembly contact 2 and ignition safety 3 positioned at one end, is se-' cured at the opposite end by crimping or otherwise to case cover 4. An igniter assembly 5 is secured to ignition assembly contact 2 within case 1. Any conventional igniter assembly may be employed as igniter assembly 5, such as the type shown in United States Patent No. 2,979,896 issued to Harold E. Perkins, Jr. et al. on April 18, 1961, which contains a conventional ignition composition and an electric squib having suitable lead wires and electrical connections for receiving an electrical charge when gas generator 100 is inserted into the breach mechanism (all of which are not shown except for gas generator 100).

Igniter assembly 5 is positioned adjacent to an uninhibited portion of propellant grain 6.

When desired, an electrical charge is imparted to the igniter assembly 5 of a gas generator 100 positioned in a suitable breach (not shown). The resistance generated within the electrical squib (not shown) of the igniter assembly 5 ignites the complex ignition train which in turn ignites the exposed uninhibited portion of propellant grain 6, such as the perforation along the central longitudinal axis thereof. Secured to the opposite end of the propellant grain 6 within the perforation along the central longitudinal axis is a sustainer *9 which is a very fast burning propellant system, which is more easily ignited by igniter assembly 5 than the propellant grain 6 and provides a flame for permitting continuous combustion of the propellant grain 6 along the perforation at its central longitudinal axis. It also assists in directing the flame and gas to the outer periphery of the propellant grain 6 by acting as a momentary barrier to gas and flame flow through the perforation to the discharge end of gas generator 100. Propellant grain 6 is secured to the interior of case 1 by means of suitable flange 7. Attached to flange 7 is flame shield 8 which surrounds the exterior cylindrical surface of propellant grain 6 and acts as a flame barrier to decrease exposure of case 1 to high temperatures.

In contact with flame shield 8 is a rubber sheet 10 having powdered zeolite dispersed therein. As indicated previously, this rubber sheet may also have fibrous materials dispersed therein to act as a wick for water, and particles of a cooling salt such as ammonium oxalate. A flexible plastic envelope 11 surrounds the igniter assembly 5 and the propellant grain 6 having flame shield 8 and rubber sheet 10 adjacent thereto. The flexible plastic envelope is preferably a laminate of aluminum and polyethylene terephthalate, but any other substantially water impervious material may be employed.

Propellant grain 6 has secured at each end suitable flame restrictors 12 and 13 to prevent combustion of these surfaces. Secured between propellant grain 6 and sustainer 9, as well as adjacent to flame restrictor 13, is spacer assembly 14. Positioned between case cover 4 and spacer assembly 14 is zeolite packet 15 containing zeolite granules 16. Secured to the top of zeolite packet 15 is zeolite packet cover 17 having openings 18.

It is particularly advantageous to employ as a zeolite barrier both a rubber sheet containing powdered zeolite positioned as described above, as Well as a packet of zeolite. However, if desired, either rubber sheet containing zeolite or the packet of zeolite may be omitted. Employing a barrier of zeolite in this manner not only inhibits the penetration of water into the solid propellant grain but also absorbs water that may be volatilized or generated by chemical reaction of the propellant grain when it is exposed to extreme temperature conditions. Once the water is absorbed by the zeolite, extremely high temperature and/or vacuum are necessary to release it from the zeolite. As a result, no equilibrium is set up in the systems in which moisture is transferred between the zeolite and solid propellant grain as the temperature varies and thus, leaching of ammonium nitrate from the grain is inhibited. A gas generator is obtained having a solid propellant grain that is relatively stable under a wide variety of temperature conditions and substantially no misfires occur due to modifications of the propellant grain configuration.

Gas generators in which the novel desiccating system of this invention may be employed are also disclosed in United States Patent No. 2,942,547, issued June 28, 1960 to Joseph William Rabern et al. and in United States Patent No. 2,979,896 issued April 18, 1961 to Harold E. Perkins, J r. et al.

The gas generating cartridge of United States Patent No. 2,942,547 referred to above is a cylindrical resilient casing closed at one end and opened at the opposite end, the opened end being covered with a perforated plate. Contained within the gas generator is a solid propellant grain comprised of granular ammonium nitrate embedded in a matrix of synthetic rubber. The grain is perforated in the center and inhibited at each end, permitting combustion of the grain at the internal perforation as well as at the outer periphery. Combustion is effected by energizing an electric squib which ignites a plurality of ignition charges positioned within the gas generator. A thin layer of polyethylene terephthalate is positioned on the outside of the perforated plane at the opened end of the gas generator and adjacent to the inhibited end of the solid propellant grain to provide a Weather seal for the cartridge. The novel rubber sheeting of this invention is wrapped around the outer periphery of the propellant grain prior to inserting it into the casing.

The gas generator described in US. Patent No. 2,979,896 employs a propellant grain which is inhibited on the outer periphery and on the inner perforation, but each end is uninhibited. Grains of this type may also be wrapped with the novel rubber sheeting of this invention. In addition, gas generators such as those described in either of the aforesaid patents may be provided with a container of granular zeolite positioned in the perforation of the grain to supplement or replace the rubber sheeting containing zeolite. When the zeolite barrier of this invention is employed within gas generators, not only is water from the outside atmosphere prevented from penetrating the interior of the gas generator but also water either initially present or subsequently present in the atmosphere within the gas generator is absorbed by the rubber sheet, thereby inhibiting dimension changes in the solid propellant grain. As a result greater reliability is imparted to the gas generator when subjected to extreme variations in temperature conditions.

The following examples are presented to define the invention more fully without being limited thereby. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 Two hundred parts of C rubber (acrylic acid-polybutadiene copolymer) were soaked in hexane to soften the rubber and then added to a Sigma blade mixer containing 400 parts of powdered zeolite and 400 parts of ammonium oxalate. These ingredients were admixed for about one hour at a temperature of about 155 F. A vacuum was then applied to the mixer to effect removal of hexane from the resulting mixture. Mixing was continued under vacuum for about 45 minutes, after which mixing was stopped and the resulting dry granular solvent free particles were removed from the mixer.

The dry granular particles were added slowly to a cold two roll mill. The nip in the rolls was kept at a minimum until a continuous sheet was formed around one roll and a head was present. The rolls were then heated and the nip gradually adjusted to form a thin sheet of uniform thickness of about 0.070 inch. This sheet was cut into pieces having a length of about 28 inches and a width of about inches.

EXAMPLES 2 AND 3 A solid propellant containing ammonium nitrate was prepared by admixing the following ingredients in the following proportions:

Ingredient: Proportion, parts Ammonium nitrate 74 Acrylic acid modified polybutadiene rubber 14 Guanidine nitrate 3 Sodium barbiturate 2 Carbon black 2 Ammonium oxalate 5 When these materials were thoroughly mixed, the resultant mixture was transferred to molds where it was formed into grains having a diameter of one inch and a length of about two inches. The resulting molded grains were cured to solidify them.

One of these grains was placed in a container with two grams of the sheet containing zeolite prepared in Example 1. The results obtained with this grain are set forth below as Example 2.

A second grain was placed in another container with 1.6 grams of granular zeolite. The results obtained with this grain are set forth below in Example 3.

A third grain was placed in another container. The results obtained with this grain are set forth below as Comparative Test.

The three containers were each hermetically sealed and subjected to temperature cycling conditions in which each container was maintained at 65 F. for two hours and then maintained at a temperature of 160 F. for two hours. The containers were then subjected to a total of 30 temperature cycles under these conditions. At the end of this period the respective grains were examined and the dimensions were measured. The results were as follows:

These tests show that the increase in dimensions of the propellant grains were markedly reduced when subjected to severe temperature conditions in the presence of zeolite and surface salting was eliminated.

8 EXAMPLES 4 AND 5 Percent increase in Sixty cycles Ninety cycles Diameter Length Diameter Length 1. 0 1. 7 0. 8 l. 6 0. 9 0. 9 1. l l. 7 Comparative test 5. 4 5. 7 10. 4 9. 9

These tests also show the marked effect of the zeolite upon inhibiting the change of grain dimensions when subjected to extreme cycling temperature conditions.

EXAMPLE 6 A gas generator of the design shown in FIGURES I and II was prepared having an over-all length of about 8.5 inches and an outside diameter of about 6.46 inches. The initial outside diameter of the propellant grain was 5.620 inches. The grain was then subjected to 30 temperature cycles, in which each temperature cycle comprised storing at percent relative humidity and sea level pressure under the following conditions:

(A) Stored for 6 hours at 50 F.

(B) Stored for 4 hours at F. (C) Stored for 2 hours at F. (D) Stored for 4 hours at 140 F.

At the end of the 30 cycles the gas generator was disassembled and the outside diameter of the propellant grain was found to be 5.62 inches which was an increase of only about 0.4 percent in the diameter of the propellant grain.

Various modifications of the invention, some of which have been referred to above, will be apparent to those skilled in the art. Therefore, we do not wish the invention to be limited except as defined in the appended claims. What is desired to be secured by Letters Patent 1s:

1. In a gas generating cartridge having a case with a gas discharge and solid gas generating propellant composition within the case, an ignition means adapted to ignite said solid propellant and a weather seal positioned between said solid propellant and said case, the improvement which comprises a zeolite barrier positioned between said weather seal and said propellant charge, said zeolite barrier being a rubber sheet having powdered zeolite and fibers capable of acting as a wick for water embedded therein.

2. The gas generator of claim 1 wherein the proportion of powdered zeolite is between about 1 and about 90 percent by weight of the rubber and the proportion of the fibers are between about 0.5 and about 40 percent by weight.

3. The gas generator of claim 1 wherein the proportion of fibers is between about 1 and about 20 percent by weight of the rubber.

4. In a gas generating cartridge having a case with a gas discharge and solid gas generating propellant composition within the case, an ignition means adapted to ignite said solid propellant and a weather seal positioned between said solid propellant and said case, the improvement which comprises a zeolite barrier positioned between said weather seal and said propellant charge, said zeolite barrier being a rubber sheet having powdered zeolite, fibers capable of acting as a wick for water and particles of a cooling salt embedded therein.

5. The gas generator of claim 4 wherein said cooling salt is ammonium oxalate.

6. The gas generator of claim 4 wherein the proportion of cooling salt is between about 0.2 and about 60 percent by weight of the rubber.

7. In a gas generating cartridge having a case with a gas discharge, a solid propellant composition within said case, ignition means adapted to ignite said solid propellant, the improvement which comprises a zeolite barrier positioned between said case and said propellant composition, said zeolite barrier being a rubber sheet having powdered zeolite and fibers capable of acting as a wick for water embedded therein.

8. In a gas generating cartridge having a case with a gas discharge, a solid propellant composition within said case, ignition means adapted to ignite said solid propellant, the improvement which comprises a zeolite barrier positioned between said case and said propellant composition, said zeolite barrier being a rubber sheet having powdered zeolite, fibers capable of acting as a wick for Water and particles of a cooling salt embedded therein.

9. The gas generator of claim 8 wherein said cooling salt is ammonium oxalate.

References Cited UNITED STATES PATENTS 2,469,350 5/1949 Lauritsen 102-49 X 2,793,492 5/ 1957 Sage et al. 102-49 X 3,044,254 7/1962 Adelman 252-194 X 3,204,558 9/1965 Jacobson et al. 102-38 3,245,946 4/1966 OConnor 252-194 X ROBERT F. STAHL, Primary Examiner US. Cl. X.R. 60-253; 260-41

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