United States Patent 3731584

Apparatus and process for making filament reinforced solid propellant grains by automatically extruding uncured propellant and winding filaments onto a rotating grain-forming mandrel. Heatable compactor blades, underlying the mandrel, spread the propellant evenly and lower its viscosity if necessary as the propellant is applied from a plurality of extruders. The compactor blades and extruders are interconnected to simultaneously move toward and away from the mandrel.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
156/429, 264/3.3, 264/211.12
International Classes:
F02K9/24; F42B5/188; (IPC1-7): B31C1/00; F42B33/10
Field of Search:
86/1,20 264
View Patent Images:
US Patent References:
3278960Adhesive processes1966-10-18Nardone
3224317Method of constructing a miniature solid propellant rocket1965-12-21Gould
3067686Type of propellant grain1962-12-11Coover et al.
3043738Applying thermosetting resin to porous support by using vacuum1962-07-10Demeter et al.
3022735Solid rocket propellants1962-02-27Eberle
1771749Brake lining1930-07-29Eisenhardt

Primary Examiner:
Borchelt, Benjamin A.
Assistant Examiner:
Tudor H. J.
I claim

1. A device for making a reinforced structure comprising;

2. The device according to claim 1 wherein the feeding means comprises an assembly of plural structural material extruders having extrusion nozzles whose combined lengths constitute the end to end length of the structure being formed, said assembly being spaced from said core so that extrusions from said nozzles merge on said core in a single ribbon.

3. The device according to claim 1 wherein the compacting means comprises at least one compacting blade whose length is at least as long as the longitudinal length of the grain to be formed.

4. A device for making a reinforced propellant grain comprising;

5. The device according to claim 4 further comprising pressure detection means connected to each individual extruder for detecting pressure overload conditions and discontinuing extruding when such condition exists.

6. The device according to claim 5 wherein the pressure indicating means comprises a compression spring disposed between the lower end of each ram rod and the actuator, and an electrical switch positioned adjacent the rod that is activated between the rod when the spring attains a predetermined compression.

7. A device for making a reinforced propellant grain comprising;

8. The device according to claim 7 wherein the longitudinal axes of the individual extruders are inclined in the order of from 0° to 15° relative to a radial line of the core passing through the area of the core where the uncured grain material is being deposited, the inclination being sufficient to achieve optimum adherence.

9. A device for making a reinforced propellant grain comprising;

10. The device according to claim 15 wherein the linking means is a pulley system having a cord fixed at its opposite ends and looped at intermediate portions over idler pulleys fixed to the compacting means and to the extruder assembly.

11. A device for making a reinforced grain comprising;

12. The device according to claim 11 further comprising linking means interconnecting the compactor blade and the extruders for simultaneously adjusting them between respective retracting positions and operative positions adjacent the core.


This invention relates to a procedure and apparatus for fabricating reinforced solid propellant grains of the type that may be used in constructing rocket motors. The concept of reinforcing solid propellant grains with filament windings to improve the structural integrity and ballistic characteristics of the grain is well established. Casting techniques attempted have generally proven inadequate for producing high performance reinforced grains. From a practical standpoint, the matrix propellant cannot be uniformly spread into a pre-arranged mesh of reinforcing filaments. Forcing the propellant into the mesh results in greatly distorting the mesh pattern and the development of voids due to poor adherence between the filaments and propellant. Resulting high porosity may cause catastrophic results by producing an unpredictably broad burning area. Such a burning area will produce intense combustion product pressures that may burst the rocket motor casing.

Another approach to fabricating reinforced grains, with which the instant invention is most closely related, is a process involving numerous hand manipulations and great time consumption while exposing the workmen to severe risks due to the volatile nature of the process. A customary technique involves the use of a caulking gun containing a charge of pressurized propellant which is manually deposited upon a grain-forming mandrel rotatable about its longitudinal axis. Unsymmetrical grains often result and therefore must be rejected due to the difficulty in synchronizing the rate of extrusion and the rotation rate of the mandrel. Attempts to smooth and evenly spread the propellant soon after it is deposited on the mandrel with hand spatulas or the like is difficult, especially in cases where the propellant is highly viscous. During the periods of extrusion and non-extrusion, reinforcing filaments are spirally wound over the mandrel or onto the preceding covering of propellant. When propellant coverings are of non-uniform thickness, subsequently wound filament layers will also be irregular, thus producing a defective grain.


Briefly described, the instant invention comprehends a procedure and apparatus for fabricating tubular shaped reinforced propellant grains of a type that may be incorporated in a rocket motor, for example. A continuously operating filament winding device travels back and forth relative to the axis of a rotatable grain-forming core winding filaments onto the core. Disposed on one side of the core are a plurality of upwardly inclined grain extruders having edge abutting extruder nozzles whose combined overall lengths define a discontinuous narrow slot constituting the length of the grain to be formed. The slot is substantially coextensive with the end to end length of the core. In its operating position the slot is spaced a slight distance from the core sufficient to permit the grain material being extruded from the individual nozzles to merge and become a continuous ribbon that is deposited onto the core, covering its entire periphery through one complete revolution of the core. Beneath the core is positioned a pair of spaced compacting blades that serve the two-fold purpose of compacting and evenly distributing the uncured propellant and also smoothing out furrows and irregularities in the grain caused by the penetration of the filaments being wound onto the core. Optionally the compactor blades are heatable so that the viscosity of fast curing materials and the like may be sufficiently lowered to make the material more amenable to shaping. To avoid the tendency of the grain material to pull loose, fall away and become wasted immediately after it is deposited onto the core, the axes of the extruders are inclined within a range from 0 degrees to 15 degrees relative to the radial line of the core passing through the zone on the core periphery where the propellant is being deposited.

The filament winding is continuous while the feeding is intermittent. The feeding is commenced after a predetermined thickness of filament build-up has been accomplished and the feeding cycle is discontinued after a full revolution of the core at which time a complete cover of uncured grain will have been deposited onto the core. The compactor blades and grain extruders are interconnected so that they may be simultaneously moved to or retracted from their operating positions. Inasmuch as the apparatus and procedure in accordance with this invention can be practiced from a remote location, the peril to the operators who heretofore had to be in the immediate proximity of the explosive material is eliminated. A desired symmetrical grain of high integrity and reliability is fabricated at a substantial diminution of costs in terms of funds and man hours.


FIG. 1 is a perspective view of apparatus for fabricating reinforced grains with a section of the plural extruder assembly removed to show the details of a single extruder;

FIG. 2 is a schematic representation of a filament winding device shown winding reinforcing filaments onto a grain-forming core;

FIG. 3 is a schematic end view of the apparatus showing the compactor blade assembly and more details of the extrusion assembly;

FIG. 4 is a detailed view, partially in section, showing a safety electrical switch capable of discontinuing the extruding operation;

FIG. 5 is a sectional view taken along a plane that passes laterally through an individual extruder and the grain and core assembly showing the extruder orientation relative to the surface portion of the grain onto which new grain is being deposited; and

FIG. 6 is a perspective fragmentary view showing a plurality of extruders whose individual nozzles define a discontinuous slit that is slightly spaced from the core.


Referring now to the drawings and especially to FIG. 1 there is shown a solid propellant grain making apparatus 10 having a rotatable mandrel assembly 12 and a propellant feed assembly 30. Extending through the longitudinal axis of mandrel 12 is a rotatable spindle 14. As shown schematically in FIG. 2, spindle 14 has one end 16 connected to a motor drive shaft 21 connected to a drive assembly 23. Its other end 18 is suitably journalled for rotation in a stationary bearing 24 formed in a rigid upright plate 25. Rigidly mounted on the intermediate region of spindle 14, as shown in FIG. 1, is a grain-forming core 20 on which a tubular shaped propellant grain 28 may be fabricated. When grain 28 is fully built, its interior periphery will be flush with the exterior periphery of core 20.

Propellant feed mechanism 30 is provided for extruding a matrix propellant onto core 20 and gradually building up grain 28 to the desired dimensions. Feed mechanism 30 has a base plate 32 which may be mounted on wheels (not shown) and an upright frame 34. Anchored to base plate 32 are two fluid actuated cylinders 40 (only one being shown) each having a piston 42. As schematically depicted in FIG. 3, piston 42 is connected to a ram head 44 which in turn is fixed to a yoke 50. The arms of yoke 50 are erect and secured to a pair of upwardly diverging blades 52 and 54. Blades 52 and 54, as will be more fully explained below, are heatable and operate to compact and evenly spread extruded propellant as the propellant grain is being constructed. The line of intersection of the planes in which blades 52 and 54 lie is parallel to and lies in a vertical plane with the longitudinal axis of spindle 14. The longitudinal lengths of blades 52 and 54 are substantially coextensive with the end to end length of core 20.

Connected to the top of frame 34 is a bracket 35 that serves as a guide-way through which propellant feed assembly 30 may slide. A pulley assembly 60 includes a tension cord 62 whose opposite ends are rigidly secured to an eyebolt 63 on base plate 32 and an eyebolt 64 on frame 34. Intermediate portions of cord 62 are looped over an idler pulley 66 attached to yoke 50 and an idler pulley 67 secured to extrusion assembly 30. To promote superior extrusion, assembly 30 may be inclined to a vertical plane through core at between 25 and 65 degrees. When yoke 50 that supports compactor blades 52 and 54 is thrust upwardly by cylinder 40, slack in chord 62 will be taken up causing propellant extrusion assembly 30 to slide upwardly toward core 20. Conversely, retraction of yoke 50 automatically causes propellant feed assembly 30 to be withdrawn from core 20 under the force of gravity and a spring loaded mechanism (not shown).

On the opposite sides of propellant feed assembly 30 is a pair of pneumatic cylinders 72 and 74. As shown in FIG. 1, the lower distal ends of piston rods 73 and 75, associated with cylinders 72 and 74 respectively, are connected to a horizontally extending base bar 78. Parallel to base bar 78 is a transverse bar 80, most clearly shown in FIG. 3, whose opposite ends are threadably connected to the inner walls of cylinders 72 and 74. Disposed between cylinders 72 and 74 is a bank of individual propellant extruders 84. Each extruder 84 has a cartridge 86 for containing a charge of matrix propellant material to be extruded and a piston head 88 connected to a ram rod 92. As most clearly shown in FIG. 4, the lower end 93 of each rod 92 is connected to base bar 78 by way of a coiled compression spring 95 and lock 97. If a pressure overload condition arises in the extrusion assembly during extrusion, the grain being fabricated may be defective. From the standpoint of quality control and safety it is desirable therefore to temporarily terminate the extrusion operation until the trouble source is remedied. This type of condition to be avoided may, for example, result from excessive force being applied to ram rods 92 and bar 78 or when propellant flow is clogged or irregular, e.g., due to high viscosity propellant causing a pressure backup. To detect and correct these conditions, a safety switch 97 is positioned slightly below the tip of rod end 93. Excessive compression stress exerted on spring 95 forces end 93 to slide through a bearing 99 until it contacts safety switch 97. Then through a suitable electrical circuit (not shown) the extruding operation is terminated so that the trouble source can be investigated.

The upper end of each individual extruder 84 as shown in FIGS. 3 and 5 terminates in an extrusion nozzle 102. As best shown in FIG. 6, the nozzles 102 of extruders 84 are aligned in edge abutting relationship constituting discontinuous or interrupted slot 120. Slot 120 is separated by a distance ΔX, preferably 1/16 inch, from core 20. As the grain fabrication continues, the distance ΔX is maintained relative to the built-up portion of the grain. It should be noted that for the sake of clarity and to avoid duplicated descriptions, the term core is often intended to comprehend both core 20 and the built up portion of uncompleted grain 28. Referring again to FIG. 5, extruder axis 108 of extruder 84 is shown slightly inclined by an angle θ relative to radius line 109. Radius line 109 passes through the zone of grain 28 onto which propellant is being deposited. Differently stated, angle θ is defined by the tangent plane passing through the zone of grain 28 onto which propellant is being fed and a plane 112 that is oriented perpendicularly to extruder axis 108. Maintaining the inclination of angle θ during extrusion tends to prevent the spilling or falling away of propellant from the outer surface of grain 28. Angle θ is adjustable and is dictated by such parameters as the surface velocity of grain 28, feed rate and viscosity of the matrix propellant and type of propellant being employed. Thus the function of maintaining the angle θ is to assure an optimum mechanical interlock, i.e., adherence between the matrix propellant material being extruded with the grain and reinforcing filaments. It has been found that if angle θ is considerably less than optimum then the matrix propellant may not become sufficiently pressed onto the moving surface of grain 28, the resulting poor adherence causing propellant to fall away and become wasted. To the other extreme, if angle θ is opened too wide, then the pressurized propellant will tend to flow backwardly over the top of the extrusion nozzles and away from the grain producing considerable waste. Depending upon the above mentioned parameters, angle θ may be optimum within a range of from 0° to 15°.

As a newly extruded covering of propellant encounters first compacting blade 54 and then compacting blade 52, as shown in FIG. 3, uneven or irregular areas of propellant are spread out so that a covering of uniform thickness is achieved. The upward pressure from fluid cylinder 40, exerted on blades 52 and 54, causes them to lightly engage the adjacent grain surface being formed so that the newly applied propellant covering is intimately commingled with the reinforcing filaments and grain surface. It is contemplated that while the winding operation may be continuous, the propellant feed operation will be intermittent. During periods when the propellant feed operation is suspended the reinforcing filaments being applied sink into the grain surface tending to develop furrows and rippling. If these resulting surface irregularities and voids were not eliminated, they would drastically impair the overall grain performance and integrity. Therefore, it is the function of blades 52 and 54 to smooth over the grain surface so that the desired intimate bonding and grain geometry is attained. For some types of propellant it may be desirable to heat the compacting blades 52 and 54, for example, by way of a hot fluid circulating system or electrical heating system (not shown) so as to lower the viscosity of the propellant to make it more amenable to shaping.

Before the first covering of matrix propellant is fed onto core 20, a suitable criss-cross pattern of load bearing reinforcing filaments is spirally wound on core 20. While the particular technique or apparatus for winding the filaments onto core 20 is not an important part of this invention, an adequate technique and apparatus is disclosed in a U. S. Patent application, Ser. No. 179,466, filed Mar. 19, 1962, which application has been assigned to the assignee of this invention. In part, the invention of the application concerns a device and method for winding reinforcing filament into a propellant grain which may revolve about its longitudinal axis as the device travels longitudinally relative to its axis or vice versa. Core 20 may be constructed of detachable Teflon coated steel components or in the alternative may be constructed of water soluble composition such as sand with polyvinyl alcohol. In the latter case, core 20 would be dissolved in accordance with the well known lost-wax techniques after propellant cure. As schematically depicted in FIG. 2, reinforcing filament wires 131 are wound over grain 28 by a winding device 133. Filaments 131 issue under pretension from a multi-wire delivery head 135 that is supplied from a supply spool (not shown). During the winding operation a carriage 137 that mounts delivery head 135 travels back and forth upon a rotatable track 139. As mentioned in the U. S. Application, the winding device may be stationary as the solid grain being fabricated simultaneously rotates and travels back nd forth relative to its longitudinal axis.

In constructing a reinforced propellant grain in accordance with this invention, first winding device 133 is operated until a pattern of filament wires of a predetermined thickness is applied to core 20. Then, referring to FIG. 3, cylinder 40 operates to elevate compacting blades 52 and 54 into engagement with the bottom side of core 20. At this point with the winding operation either continuing or suspended and with propellant extruders 84 advanced to their feeding positions, base bar 78 is actuated to cause piston heads 88 to penetrate deeper into their respective cartridges 86. The uncured matrix propellant charges now under compression are expressed from their respective noozles 102. Referring to FIG. 6, the propellant material passes through discontinuous slit 120 and across the slight gap ΔX as a continuous ribbon of uniform thickness. The ribbon is driven into the pores and voids defined by the filament sections of the winding pattern. Propellant extrusion continues through one complete revolution of core 20 at which time extruding is temporarily terminated while the winding operation continues through another cycle. The feed rate at which the propellant is discharged initially onto core 20 and subsequently onto grain 28 is predetermined so that proper adherence is attained. After a second filament pattern of proper thickness is wound onto grain 28 the extrusion operation is commenced once again and continues throughout another revolution of core 20. When filament wire of 0.0075 inch diameter is used, the winding operation will proceed through fifty or more revolutions before a cover of propellant is fed onto the grain.

As grain 28 is continuously built-up, its outer diameter becomes enlarged forcing blades 52 and 54 to gradually descend so that their pressure against the grain remains constant. Since yoke 50 is interlinked by pulley assembly 60 with feed assembly 30, the individual extruders 84 are simultaneously withdrawn from grain 28 so as to maintain a constant gap ΔX. By maintaining a constant compacting pressure against grain 28 and a constant feed condition of propellant onto grain 28, a grain of high integrity and the desired contouring is achieved.

Since the grain fabrication technique is essentially fully automatic it can be operated from a remote location thereby eliminating the exposure of personnel to potential hazards. The simultaneous operations of the individual extruders 84 to produce continuous ribbon of propellant that extends over the entire length of the grain serves to greatly improve built-up uniformity while reducing the hazard of air inclusions. As mentioned in the case of propellant compound of relatively high viscosity, the heat supplied from compactor blades 52 and 54 lowers the propellant viscosity so that the propellant can be more easily spread and worked. Due to the fact that in the individual extruders 84 are inclined at a predetermined optimum angle θ the potential waste of propellant is substantially eliminated. Once the propellant is initially adhered to the surface of grain 28, its tendency to roll and pull loose from the grain is eliminated because of the spreading and compacting force exerted by blades 52 and 54.

While the particular composition of the reinforced solid grain does not form an important part of this invention, for purposes of illustration, the reinforcing filaments may be metallic wires used as fuel such as aluminum or magnesium constituting approximately 5 to 16 percent of the overall grain weight. The grain may also include an ammonium perchlorate oxidizer constituting 60 to 80 percent of the grain weight and a suitable binder and other additives such as curing agents in the order of 1 to 30 percent of the overall grain weight. The use of any other suitable fuels, oxidizers, binders and curing agents such as those mentioned in the co-pending application is also within the scope of this invention.

While a particular embodiment has been chosen for best illustrating the particular uniqueness and advantages of the instant invention, it should be made clear that the scope and spirit of the invention is to be limited only by the following claims.