Method of Case Bonding Propellant

A solid propellant grain must be securely fastened in a rocket motor case if problems are to be avoided when the rocket motor is fired. Even when the solid propellant grain appears to be securely bonded to the motor case, problems relating to separation and uncontrolled burning sometimes develop after ignition, and particularly in flight, where the forces caused by acceleration forces and other forces surpass the holding strength of the bonding material between the motor case and the propellant or the holding strength of bonding material between the propellant and liner material.

There is a need for a mechanical method to hold a solid propellant grain in place in addition to a chemical and/or physical bond, i.e., surface adhesion, chemical bonding, surface tension etc., which is generally employed in prior processes.

Therefore, an object of this invention is to provide a mechanical method of fastening a solid propellant grain in a rocket motor case.

A further object of this invention is to provide a mechanical method of fastening a solid propellant grain in a rocket motor case which additionally employs a chemical and/or physical method of fastening a solid propellant grain in a rocket motor case.

A mechanical method of bonding a solid propellant grain to a rocket motor case comprises the steps of applying an adhesive or liner composition to the internal surface of a rocket motor case or the internal surface of the rocket motor case liner or insulation material, embedding in the liner composition, while the liner is uncured, a mechanical structural material in the form of a wire grid, wire hooks, rubber spikes, steel wool, expanded metal grid, polypropylene fabric, or the like. The embedded mechanical structural material is allowed to protrude above the uncured liner composition. The liner is allowed to gel and cure, and the propellant is cast against the liner and flows around the protruding mechanical structural material. On curing, the propellant attaches itself to both the liner and the mechanical structural material thereby employing the mechanism of mechanical fastening along with chemical and/or physical bonding by surface tension forces and adhesive forces. The cured propellant formed in this manner requires a higher tensile adhesion load to cause failure of the bond securing the propellant grain in the rocket motor case.

The FIGURES of the drawing are sectional views of a portion of a rocket motor which illustrate typical approaches, in accordance with this invention, for mechanical bonding of a solid propellant in a rocket motor case.

An uncured liner material (e.g., comprised substantially of hydroxyl terminated polybutadiene with a inert filler material of carbon black and containing a curative and crosslinking agent of a diisocyanate) is applied to the internal surface of a rocket motor case. Prior to curing the liner material, a mechanical structural material such as a wire grid, wire hook, rubber spikes, steel wool, metal grid or polypropylene fabric is embedded in the uncured liner material, but a portion of the mechanical structural material is allowed to protrude above the uncured liner material. The liner material is allowed to gel or cure, and then a solid propellant composition (e.g., a hydroxyl terminated polybutadiene propellant employing a diisocyanate curing and crosslinking agent) is cast against the cured liner material and allowed to flow around the protruding mechanical structural material. The propellant when cured yields a propellant grain having an improved propellant to case bond. The described method is particularly attractive for use for obtaining improved propellant to case bond for propellants that are difficult to bond to conventional liners. A propellant grain formed as described provides improved bond strength between the propellant grain and the rocket motor case for high acceleration rocket motors.

In further reference to the Figures of the drawing, FIG. 1 shows a sectional view 10 of a portion of a rocket motor having a case 12 with insulation material 14 bonded thereto. A mechanical structural material 18 in the form of wire hooks is shown bonded in a cured barrier or liner material 16. A propellant grain 19 is shown which has been cast around the wire hooks, bonded to the wire hooks and to the liner material 16 to form a propellant grain having an improved propellant to case liner bond.

FIG. 2 shows a sectional view 20 of a portion of a rocket motor case with associated parts as numbered in FIG. 1 except FIG. 2 shows another mechanical structural material 22 in the form of rubber spikes. All other like numerals are as described in FIG. 1.

It is understood that if no requirement exists for insulation material, then the liner material is applied to the rocket motor case. The internal surface can mean the surface of the insulation or the surface of the rocket motor case as applicable.

In addition to the mechanical structural materials illustrated in the drawing, additional materials have proven to be useful. Some of the materials have proven to be superior in performance as indicated in Table I which summarizes a mechanical bond study-propellant to case bond.

In reviewing the mechanical structural materials of Table I, a wide range of materials are shown to be useful for improving the bond strength and raising the psi load level for tensile adhesion failures to take place. These materials which showed significant contribution include rubber spikes, metallic and nonmetallic materials in the form of fine or coarse screen mesh, and grating, phenolic honeycomb, metal springs and hooks, polyamide loops and hooks, and polypropylene fibers. The preferred nonmetallic materials of construction for screen, mesh, and grating material are polypropylene and polyamide.

The propellant formulation which was used in the evaluation of the mechanical structural materials was a hydroxyl terminated polybutadiene formulation which is conventionally employed as a casting formulation. The main criterion of the propellant is that it be capable of being cast and that it is compatible with the liner formulation and the mechanical structural materials.

The control liner material as shown in Table I serves as the base of comparison at which tensile adhesion fails. For example, at a psi load of 44.4 for the control two failures of the bond plus propellant occurred while one failure at the bond interface occurred. The highest psi load for failure (132.4) occurred when employing polypropylene fibers as the mechanical structural material. Three failures were observed on three samples evaluated which included fibers plus propellant. In other words, the strength of the bond between the propellant, the mechanical structural material, and the motor case was so secure that to cause failure, a load sufficiently high enough to tear the polypropylene fibers and the propellant apart was required. In an actual flight environment, this value would indicate that a rocket motor having this strength of propellant to case bond would meet the requirements for high acceleration use where the rocket motor would be subjected to high G values. Although the control propellant for tensile adhesion failure test where failure occurred at 44.4 is considered an acceptable bond for many rocket motors, the use of mechanical structural material provides a wider margin of reliability for the high performance rocket motors. The level to which the rocket motor could perform is considered directly related to the level of tensile adhesion load required to cause a propellant bond to case failure or combination of bond and/or propellant failures.

A mechanical structural material employed in accordance with this invention increases the shear strength; therefore, the mode of failure due to low shear strength is lessened. Also, the modes of other failures, such as by peeling or other separations, are lessened by the use of the disclosed mechanical structural materials.