IL26660A - Processing method and apparatus for gas-generating propellant compositions - Google Patents

Description

ΜΕΠΙΠ malum *na*srt> jpnm π«·» PROCESSING METHOD AND APPARATUS FOR OAS-GENERATINO PROPELLANT COMPOSITIONS This invention relates to methods and apparatus for aligning elongated metallic heat conductors within a viscous propellant matrix and for making solid propellant grains containing discontinuous, elongated metallic heat conductors longitudinally aligned in the direction of flame propagation of the grain.

The incorporation of elongated metallic heat conductors in solid propellant grains is known to effect substantial increases in the mass-burning rate of the grains. A detailed discussion of the use of elongated metallic conductors is given in United States Patents 3., 109, 374 , issued to Rumbel et al, dated 5 November 1 63 3 , 109, 375 , issued to Rumbel et al, dated 5 November 1963 ; 3 , 116, 692 , issued to Rumbel et al, dated 7 January 19644 h^-ow-.Hafe* As disclosed in the aforementioned patents, the maximum increase in propellant burning rate is obtained when the elongated metallic heat conductors are longitudinally aligned in the direction of flame propagation of the grain.

Discontinuous, short, elongated metallic heat conductors can be economically incorporated into propellant compositions by conventional mixing techniques. Discontinuous heat conductors thus incorporated are, however, disposed in irregular orientation within the propellant grain and maximum burning rates are not obtained. Furthermore, variations in orientation of the discontinuous conductors throughout the length of the grain often result in non-uniform linear burning rates.

Until now, satisfactory techniques have not been available for longitudinally aligning discontinuous heat conductors within viscous propellant matrices to produce solid propellant grains having maximum burning rates.

Processes known to the plastics industry for aligning particles within a plastic mass to provide reinforcement or decorative effects have not proven satisfactory for propellant processing. For example, such methods as illustrated by United States Patents Numbers 1 , 700, 208 ; 2 , 682 , 081 ; 1 , 918,848; 2,332, 829; and 2 , 149 , 066, produce orientation of particles in a plastic mass by passing the plastic mass through reduced Q-pertlngsJ diameter die openings or through f%epn-fcnge in plates positioned in the flow path of the mass. The particles which contact the walls defining such openings are aligned longitudinally in the direction of flow by a "stroking" effect. Longitudinal alignment of particles not contacting the walls is accomplished only if the mass is of such low viscosity that substantial laminar flow, that is, an increasing gradient in flow velocity from the walls to the center of the mass, is induced by passage through the opening. Since many propellant matrices are highly viscous and move through such openings in substantially plug flow, e.g. uniform cross sectional flow rates, only limited orientation of particles within the matrices is obtainable by such methods.

Furthermore, in such methods the flow path is significantly obstructed by the die or perforated plate and areas of stagnation are induced in the flowing mass. Prolonged exposure s.tagnatedjj of fafeang¾-»tte<¾- areas of many propellant compositions to the temperature and pressure conditions of extrusion and casting operations creates hazards of explosion, decomposition and ' premature hardening.

The need for a satisfactory method of aligning discontinuous elongated metallic heat conductors in a viscous propellant matrix to produce propellant grains having maximum burning rates is, therefore, readily apparent.

Accordingly, it is an object of this invention to provide methods and apparatus for longitudinally aligning discontinuous matrix. Another object of this invention is to provide methods for making inhibited solid propellant grains containing such heat conductors longitudinally aligned in the direction of flame propagation of the grain. Other objects and advantages of this invention will be apparent from the drawings and the following detailed description.

Referring to the drawings: FIGURE 1 is an illustration, partly broken away, of an apparatus for aligning elongated, discontinuous, metallic heat conductors.

FIGURE 2 is a longitudinal sectional view of the apparatus shown in FIGURE 1 wherein the orientation of particles according to this invention is schematically illustrated.

FIGURE 3 is an illustration, partly broken away, of an alignment apparatus employing alternate embodiments of alignmen means.

FIGURES 4 , 5 / 6 , and 7 are longitudinal views, partly in section illustrating the production of inhibited propellant grains according to this invention.

It has been discovered that discontinuous, elongated, metallic heat conductors can be longitudinally aligned in a propellant matrix by effecting relative motion between a matrix and a plurality of alignment means disposed in a containment means. FIGURE 1 illustrates an apparatus utilized in the practice/ f^ie-of the invention. The use of this apparatus is illustrated in FIGURE 2 wherein a viscous propellant matrix 5 containing short wire heat conductors 6 is forced through containment means comprising a passageway 1 which is serially transversed along its length by alignment means 2 , 3 / and 4 each comprising o parallel elongated members 2a , 3a , and 4a respectively. It is longitudinally aligned in a direction more nearly parallel to the longitudinal axis as they pass successive alignment means.

Alignment of the wires is accomplished by contact with the elongated members of the alignment means and/or by laminar flow patterns induced in the matrix b the elongated members. When a wire contacts an elongated member, the force of the moving matrix turns the wire about the point of contact into a direction more nearly parallel to the longitudinal axis of the passageway. Additionally, the alignment means induces regions of laminar flow even in highly viscous compositions that would normally approach plug flow patterns.

It is seen that each alignment means defines channels in the passageway and that the boundaries of the channels defined by different alignment means are not longitudinally colinear. That is, longitudinal planes subtending the different alignment means are laterally and/or angularly displaced from each other. Thus, the elongated members 3a are positioned to be contacted by wires which did not contact elongated members 2a. Also, the elongated members 3a induce additional regions of laminar flow to effect alignment of wires which do not actually contact the elongated members. In addition, wires already partially aligned by preceding alignraent means are further aligned into a direction more nearly parallel to the longitudinal axis of the passageway by the successive alignment means.

The containment means can be, for example, a passageway as described above or a stationary container such as a propellant grain mold or inhibitor beaker in which the alignment means is movable relative to the walls of the mold or beaker and any matrix disposed therein. The containment means can be of any desired cross-sectional design. The cross-sectional dimensions preferably are substantially uniform along the length of the propellant matrix.

The alignment means of this invention are transverse to the longitudinal axis of the containment means and define channels through which the matrix passes or which are passed through the matrix. It is necessary that the channels have a transverse dimension at least equal to the longest distance subtending any heat conductor adjacent to the alignment means in order to prevent entrapment of the heat conductors and channel obstruction or blocking. To effect additional alignment by additional alignment means it is necessary that the boundaries of channels defined by the additional alignment means not be longitudinally colinear with the boundaries of channels defined by previous alignment means through which the matrix is moved.

The alignment means of this invention preferably comprise at least one elongated member transversing the longitudinal axis of the containment means. Elongated members of alignment means may be arranged in a variety of patterns. For example, the elongated members in each alignment means may be arranged in a substantially parallel non-intersecting relationship as illustrated in FIGURE 1. Alternatively as shown in FIGURE 3 alignment means 7 consists of an elongated member 7 in the form of a spiral and alignment means 8 is formed by elongated members 8a arranged in the form of a screen. Other arrangements of elongated members to form alignment means within the spirit of this invention will be readily apparent.

Preferably the elongated members will be relatively narrow in order to prevent stagnation of propellant flow. Generally, narrow, elongated wires or strips are execellently suited for use as components of alignment means. If the matrix is very viscous and additional strength is required of the elongated members of alignment means/ thin elongated bands having their widths the containment means may be advantageously utilized. The elongated members of different alignment means are caused to define different channels in the containment means by varying the spacing or arrangement of elongated members of different alignment means or by directing the members of different means across the containment means at varying radial angles.

The use of alignment means comprising elongated members arranged in non-intersecting relationship as shown in FIGURE 1 minimizes '^blocking" problems since there are no corners to trap the heat conductors. Such an arrangement is, therefore, particularly preferred.

If only one alignment means, defining channels having transverse dimensions sufficiently large to prevent blocking, is utilized, uniform alignment of heat conductors across the cross-section of the grain is not obtained. Instead, the heat conductors are preferentially aligned only in zones corresponding to longitudinal planes defined by the boundaries of the channels and along the perimeter of the grain due to contact with and/or laminar flow patterns created by the walls of the containment means. Conductors in other zones in the cross-sectional area of the grain retain substantially random orientation. The burning of such grains proceeds more rapidly in the zones of conductor alignment and the burning surface recesses along such zones until an equilibrium point is reached. The time required to achieve such equilibrium is undesirably long when only the few zones of alignment produced by a single alignment means are present in the grain. Also, variations in alignment of conductors in zones of random orientation hinder the production of grains having reproducible burning charac-teristics. Therefore, to obtain grains having maximum burning rates and reproducible burning characteristics, the use of a It is often desirable that propellant grains be provided with inhibitor casings in order to restrict the burning of the propellant grain to desired surfaces.

To produce heat-conductor containing inhibited grains conveniently and economically, it is necessary to load the propellant matrix containing aligned heat conductors into the inhibitor casing without disturbing the orientation of the heat conductors. This may be accomplished as shown in FIGURE 4. An inhibitor casing 9, attached to a heat plate 10, is positioned around a longitudinal passageway 1 containing alignment means. As the propellant matrix is forced through the passageway and alignment means, the inhibitor casing 9 and head plate 10 are simultaneously displaced with respect to the alignment means and passageway to the position indicated by dotted line 11. The displacement is effected at about the same rate at which the propellant matrix enters the inhibitor casing. Thus, there is substantially no relative motion between the inhibitor casing and the propellant matrix which would serve to disorient the aligned staples.

The matrix is cured or hardened within the encasement in such a manner as to effect an intimate bond between the hardened propellant grain and the inhibitor casing. This generally can be accomplished by curing the matrix within the insulating encasement under pressure.

In order to provide structural support and/or dimensional stability to the inhibitor casing during the loading or hardening step of the process, it may be desirable to position a mold 12 around the inhibitor casing 9a as shown in FIGURE 5, in which case the mold and inhibitor casing are simultaneously displaced to the position indicated by dotted line 13· Alternatively, v./ . formed of flexible material, the inhibitor casing 9c may be inverted over a mold 12b as shown in FIGURE 7· When the propellant matrix is forced into the inhibitor casing 9c the casing will be reinverted and forced into the position shown by dotted line 15· The inhibitor casing may be formed of natural or synthetic polymers which may, if desired, be reinforced with fibers, wires, fabrics or the like. A wide variety of inhibitor casings and materials and methods suitable for making the same are well-known in the propellant art, and accordingly, no attempt will be made to give a detailed discussion of inhibitor casings in this application. However, a particularly preferred propellant grain made according to this invention utilizes a combustion-restrict¬ ■^t^-Jtoi^d--^-- ofo r Unite States Patent Appjrfeafcion Oeriat- The inhibitor casing described therein comprises an inhibitor sleeve formed of elastic material. The sleeve has restraining means embedded therein to provide a preferential direction of elasticity of the sleeve. Preferably the sleeve is restrained so as to inhibit stretch in the longitudinal direction while permitting relatively free stretch in the radial direction of the sleeve. Such an inhibitor casing having an internal dimension equal to that of the internal dimension of the passageway can be stretched over a passageway as in FIGURES and 6 , or a mold as in FIGURE 7; and its elastic properties will return it to an internal dimension substantially the same as the internal dimension of the passageway when the loading process forces the encasement from the passageway or mold.

Thus, relative motion between the propellant matrix and the inhibitor casing is further minimized. Furthermore, hardening the mold thereby providing a propellant grain whose dimensions are as accurate as those of the mold.

The elongated heat conductors can be made of any heat conducting material suitable for effecting improved performance of gas-generating compositions. For example, staples, e.g. thin flat metal strips, or short wires of aluminum, magnesium, beryllium, zirconium, titanium, silver, copper or alloys thereof can be effectively employed. If desired, such conductors can be coated with a self-oxidant composition having a higher burning rate than the gas-generating matrix to provide gas-generating compositions having even higher burning rates than are obtainable with uncoated conductors. Alternatively, the conductors can be coated with non-self-oxidant compositions having substantially lower heat conductivity than the elongated conductors to provide gas-generating compositions having burning rates intermediate the rate of a non-conductor containing composition and a composition containing uncoated conductors. A more detailed discussion of coated and uncoated heat conductors is found in aforementioned United States Patent Numbers 3 , 116 , 692 ; 3 , 109 , 374 ; The size of the heat conductors is determined by consideration of desired performance characteristics of the propellant. The maximum length of the heat conductors is limited only by the size of the passageway and by techniques of mixing conductors into the matrix. Generally, the conductor length should be no greater than one-half the internal diameter of the passageway. Since breakage of longer conductors may occur during mixing, a conductor length of less than 2 inches is preferred and a length of 1/2 inch or less is particularly preferred.

The following examples are presented to further illustrate the invention.

Example 1 A viscous propellant matrix having the following composition was prepared: Polybutadiene binder 12% burning rate catalyst 2% ammonium perchlorate 70 aluminum powder l % elongated aluminum heat conductors (average size .05in x .002 in.

The mix was forced through a passageway intersected by a series of screens of gradually decreasing mesh size into a mold cavity and cured at elevated temperature. Sample strands of the cured material were cut in the direction of flow through the passageway or orientation direction,, and in the transverse^, or anti-orientation direction.

Burning rates for the strands at various pressures were as follows : Burning rate of strand Burning rate of strand cut in anti-orientation Pressure cut in orientation direction direction 1000 psia I.65 in/sec 1-35 in/sec 2000 psia 2.30 in/sec 1-70 in/sec 3000 psia 2.8Ο in/sec 1-95 in/sec Thus it is seen that the process is effective to orient the heat conductors sufficiently to provide significant increases in burning rate.

Example 2 A propellant mix having approximately the following J Carboxy terminated polybutadiene binder 10% by weight Ammonium perchlorate 72% by weight Burning rate catalyst 4 by weight Aluminum powder 11.5 by weight Elongated aluminum heat conductors as described in Example 1 2.5$ by weight The mix was forced into an inhibitor casing through a tubular passageway serially transversed by twelve alignment means. The first eight alignment means were longitudinally spaced 1/4 inch apart and the last four were spaced 1/8 inch apart. Each alignment means comprised a plurality of parallel wires. The wires were spaced 1/4 inch apart in the first eight alignment means and 1/8 inch apart in the last four alignment means. Wires of successive alignment means were radially rotated and right-biased with respect to wires of preceding alignment means as follows: Radial angular rotation Right bias with with respect to preceding respect to preced- Aliqnment means aliqnment means inq aliqnment mea; second 90° 0 third 22.5° l/lb inch fourth 900 0 fifth 22.5° l/lb inch sixth 90° 0 seventh 22.5° l/lb inch eighth 90° 0 ninth 22.5° I/16 inch tenth 90° 0 eleventh 22.5° 1/16 inch twelfth 90° 0 This arrangement was chosen to provide a large number The mix was cured in the encasement to form a solid grain.

Several grains were prepared in this manner and showed essentially no variation in burning rate when tested. Thus, it is seen that the invention permits production of grains having reproducible burning rate characteristics.

Strands were cut from one of the above-described grains in longitudinal and transverse directions. At about 2000 psig, the "longitudinal" strands burned at an average rate of about 4.2 inches per second as compared to an average burning rate of about 2.7 inches per second for strands cut in the transverse direction. Thus, it is seen that heat conductor alignment in the longitudinal direction and improved burning rates were effected by the procedure.

Claims (12)

HAVING HOW particularly described and ascertained the nature! of our said invention and in what manner the earn© is to he informed, we declare that what we claim io t

1. A method of aligning elongated metallic heat conductors within a viscous, combustible, gas-generating matrix, said method being characterized by the sequential steps of: a. introducing a viscous, combustible, gas-generating matrix containing said heat conductors randomly dispersed therein into an elongated containment means, b. effecting relative motion between said matrix and a first alignment means disposed in said containment means transversely to the longitudinal axis thereof and defining a plurality of first channels therein, each of said first channels having a transverse dimension at least equal to the longest distance subtending any of said heat conductors adjacent to : said first alignment means, thereby aligning at least some of said heat conductors into a direction more nearly parallel to the longitudinal axis of said containment means, and ✓ c. effecting relative motion between said matrix and a second alignment means disposed in said containment means transversely to the longitudinal axis thereof, said second alignment means bein spaced from said first alignment means along the longitudinal axis of said containment means and defining a plurality of second channels in said containment means, each of said second channels having a transverse dimension at least equal to the longest distance subtending any of said heat conductors adjacent to said second alignment means, each of said second channels having a boundary not longitudinally colinear with the boundaries of said first channels, thereby effecting alignment of said heat conductors into a direction more

2. The method of claim 1 further relative motion between said matrix and a plurality of additional alignment means each disposed in said containment means transversely to the longitudinal axis thereof, each of said additional alignment means being spaced from other alignment means along the longitudinal axis of said containment means and defining a plurality of additional channels therein, each of said additional channels having a transverse dimension at least equal to the longest distance subtending any of said heat conductors adjacent to the additional alignment means defining said additional channels, said additional channels having boundaries not longitudinally colinear with the boundaries of channels defined by other alignment means, until a major portion of said heat conductors are aligned substantially parallel to the longitudinal axis of said containment means.

3. The method of claim 1 or 2 further characterized by a step of hardening said viscous combustible gas-generating matrix, thereby preserving the alignment of said elongated metallic heat conductors .

4. The method of any of claims 1, 2, or 3 further characterized in that each of said alignment means comprises a plurality of substantially parallel, elongated narrow members separated by a distance at least as great as the longest distance subtending any of said heat conductors adjacent to each of said alignment means, the elongated members of each alignment means transversing said containment means at a radial angular displacement from the elongated members of other alignment means.

5. A method for making an inhibitor encased solid pro-pellant grain containing elongated metallic heat conductors aligned in the direction of flame propagation of said grain, said method being characterized by the steps of: a. forcing a viscous propellant matrix containing elonated metallic heat conductors randomly dis ersed f b. simultaneously effecting displacement between said means for aligning said heat conductors and said inhibitor casing at a rate substantially equal to the rate at which said matrix is passed into said inhibitor casing and c. hardening said matrix and effecting a bond between said matrix and said inhibitor casing.

6. A method for making an inhibitor encased solid pro-pellant grain containing elongated metallic heat conductors aligned in the direction of flame propagation of said grain, said method being characterized by the sequential steps of: a. introducing a viscous propellant matrix containing said heat conductors randomly dispersed therein into a passageway, b. forcing said matrix through a first section of said passageway containing a first alignment means disposed therein transversely to the longitudinal axis thereof and defining a plurality of first channels in said passageway, each of said first channels having a transverse dimension at least equal to the longest distance subtending any of said heat conductors adjacent to said first alignment means, thereby aligning at least some of said heat conductors into a direction more nearly parallel to the longitudinal axis of said passageway, c. forcing said matrix through a second section of said passageway containing a second alignment means disposed therein transversely to the longitudinal axis thereof, said second alignment means being spaced from said first alignment means along the longitudinal axis, of said passageway and defining a plurality of second channels in said passageway, each of said second channels havin a transverse dimension at least e ual to said second channels having a boundary not longitudinally colinear with the boundaries of said first channels, thereby effecting alignment of said heat conductors into a direction more nearly parallel to the longitudinal axis of said passageway, d. forcing said matrix into an inhibitor casing, e;. simultaneously effecting displacement between said alignment means and said casing at a rate substantially equal to the rate at which said matrix is passed into said casing and f. hardening said matrix and effecting a bond between said matrix and said inhibitor casing.

7. The method of claim 6 further characterized by forcing said matrix through a plurality of additional sections of said passageway containing a plurality of additional alignment means disposed therein transversely to the longitudinal axis thereof, each of said additional alignment means defining a plurality of additional channels in said passageway, each of said channels having a transverse dimension at least equal to the longest distance subtending any of said heat conductors adjacent to the additional alignment means defining said additional channels, said additional channels having boundaries not longitudinally colinear with the boundaries of channels defined by other alignment means, until a major portion of said heat conductors are aligned substantially parallel to the longitudinal axis of said passageway, prior to the step of forcing said matrix into said inhibitor casing.

8. The method of claim 6 or 7 wherein each of said alignment means comprises a plurality of substantially parallel elongated narrow members separated by a distance at least as great as the longest distance subtending any of said heat conductors adjacent to each of said alignment means, the elongated members of each alignment means transversing said passageway at a radial 26660/2 '

9. The method of any of the claims 6, 7, or 8 further characterized by the step of positioning the inhibitor casing around the exterior of the passageway in stretched relationship thereto prior to introducing the matrix into the passageway and still further characterized in that said inhibitor casing comprises a substantially cylindrical sleeve, said sleeve being preferentially elastic in its radial dimension and having its longitudinal elasticity restrained,

10. The of any of claims 5, 6, 7, 8, or 9 further characterized by the step of enclosing the inhibitor casing in a mold prior to hardening the propellant matrix. method^

11. The pyerecoo of any of claims 5, 6, 7, 8, 9, or 10 further characterized in that the hardening of the propellant matrix is effected at superatmospheric pressure.

12. An apparatus for aligning elongated metallic heat conductors within a viscous, combustible, gas-generating matrix, said apparatus being characterized by a passageway having a plurality of alignment means serially disposed therein transversely to the longitudinal axis thereof, each of said alignment means being spaced from other alignment means along the longitudinal axis of said passageway, each of said alignment means comprising a plurality of substantially parallel elongated members, said members being separated by a distance sufficient to permit passage of said heat conductors, and the elongated members of each of said alignment means being radially angularly displaced from the elongated members of other of said alignment means thereby defining channels in said passageway having boundaries not longitudinally colinear with the boundaries of channels defined by other alignment means. %ea tfci© Hftnh day of Q<$%