Rapid buring propellant charge for automobile air bag inflators, rocket motors, and igniters therefor

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The present invention relates to solid propellant charges. Uses of the propellant charges of the present invention include, but are not limited to, igniters and launch eject motors where it is desired that the propellant charges burn rapidly for rapid development of heat or thrust. Other uses of the propellant charges of the present invention are as gas generators for automobile air bag inflators as well as other apparatus where it is desired to produce inflating gas rapidly. Therefore, for the purposes of this specification and the claims, a "propellant charge" is meant to include gas generators for air bag inflation systems and other inflation systems. However, it should be understood that this invention is not limited to just these uses, but may find uses, for example, as main propulsion propellant charges for rocket motors.

A typical solid propellant charge includes a fuel such as aluminum particles and an oxidizer such as ammonium perchlorate which are usually bound together by a binder such as hydroxy terminated polybutadiene. The binder may also act as a fuel. Where the fuel and oxidizer are separate materials which are mixed together to form the propellant, the propellant is known as a "composite propellant."

A composite propellant is usually manufactured by blending the ingredients into a thick and viscous but still pourable mixture which is then added to the rocket motor chamber where the mixture is cast and cured into a solid mass of propellant material in position for use. For some applications such as igniters and air bag inflators, the propellant mixture may be extruded into a desired geometric shape such as, for example, pellets for an air bag inflator, as illustrated at 62 in U.S. Pat. No. 4,547,342 to Adams et al.

In a solid propellant charge, burning proceeds in a direction perpendicular to the surface at all times. Thus, in a type of rocket motor known as an end burner wherein the propellant grain is a solid mass of propellant without a perforation therein, burning is initiated at the nozzle end and proceeds in a direction toward the head end of the rocket. The burning time for an end burner type of propellant grain is relatively slow compared to those propellant grains which are perforated longitudinally usually along their longitudinal center lines. In this type of grain, burning may be initiated along the entire length of the propellant grain so that the burning proceeds from the perforation radially outwardly toward the rocket motor case. The burning time for a propellant charge is also determined by the shape of the internal perforation, the shape known as a "tube shape" or "center perforate" being relatively slower burning, for example, than the shape known as the "internal star shape" of propellant grain.

Tactical weapons such as canister fired missiles may use launch eject motors containing solid propellant charges for ejecting missiles out of their canisters before their main motors ignite. It is desirable that the propellant grain for the launch eject motor as well as the propellant charge for the igniter for the flight motor thereof be of the smokeless or minimum smoke type since large quantities of smoke or exhaust including any toxic gas therein may be injurious to the operators thereof, and the smoke or exhaust may undesirably hinder visibility of the target which visibility must be maintained after launch for control of the missile. However, smokeless or minimum smoke propellants do not usually burn as fast as is normally desired. Further, this type of propellant tends to become soft in the high temperatures typically encountered or which may be encountered in areas of the world where such tactical weapons may be used. If the propellant becomes too soft and its physical state is as a result altered such as during acceleration as the missile is ejected from the canister, the burning properties of the propellant are accordingly altered resulting possibly in an inadequate burning rate or possible explosion of the launch eject motor.

It has been suggested in U.S. Pat. No. 3,191,535 to Mulloy to prepare a solid propellant which consists essentially of a cellular fuel element having uniform interconnecting spherical voids of a metal or metal alloy, and a propellant material filling the voids.

It has also been suggested in U.S. Pat. Nos. 3,616,841 and 3,946,039 to Walz that form retaining reticulated structures of metal or the like may be used as solid propellant reinforcement and burning rate modifiers. These Walz patents, which are hereby incorporated herein by reference and made a part of this specification, describe methods for producing such a reticulated structure by using as a pattern a self-supporting reticulated polyurethane or organic foam formed of ligaments to provide a substantially homogeneous reticulated structure which may have a wide range of pore sizes, varying from 3 to 125 pores per linear inch, and the finished foam material is characterized as having ligaments which are continuous, gas-free or of low porosity, and of integral construction.

U.S. Pat. No. 4,321,220 to Camp discloses a method for strengthening a propellant charge by incorporating a support structure in the propellant charge. The method is disclosed as comprising slowly traversing a flexible perforated material through a propellant lacquer until the desired loading is obtained. Camp states that the "reinforcing" substrate is perforated or the like so that it is "permeable to propellant decomposition gases which evolve during storage" and should also be strong and have a low density. Although Camp states that the "reinforced" propellant should have a "waffle appearance" so as to have "desirably increased surface area", the disclosure in Camp addresses the problems of increasing propellant strength, and the "waffle appearance", while a step in the right direction, still does not provide a sufficiently rapid burning smokeless or minimum smoke propellant charge. The Walz patents do not disclose increasing propellant charge surface area to achieve more rapid burning thereof and do not therefore afford an adequate solution to the problem either.

It is therefore an object of the present invention to provide a smokeless or minimum smoke propellant charge which is rapid burning, i.e., has a fast burn time. "Burn time" refers to the time it takes to burn a specified volume of a propellant charge and varies depending on the physical configuration of the propellant charge as well as the type of propellant. On the other hand, "burn rate" perpendicular to the surface of a propellant is constant for a particular propellant material.

It is another object of the present invention to provide such a smokeless or minimum smoke propellant grain or charge which is also capable of withstanding high acceleration forces even when the propellant material has a tendency to become soft in high temperatures.

One suggestion for solution to the problem of slow burn times for smokeless or minimum smoke propellants has been to add burn rate catalysts such as lead and/or copper salts to the propellants to control their ballistic behavior, i.e., increase their burn rates. However, because of the toxicity of such additives, it is also desirable to eliminate them from the propellant charges. It is therefore a still further object of the present invention to provide a smokeless or minimum smoke propellant charge with decreased burn time but which does not contain such toxic substances.

Igniter propellant material which consists of pellets of boron and potassium nitrate must be placed in a housing such as a wire basket or tube which is perforated so that the propellant gases from the igniter may communicate with a propellant grain for ignition thereof. Such wire baskets or perforated tubes may get blown off during the ignition phase or get plugged up both of which conditions affect the safety of the rocket. It is a further object of the present invention to eliminate such a basket or tube for an igniter and thus also reduce the expense thereof.

It is still a further object of the present invention to provide an automobile air bag inflator which is capable of utilizing a decreased burn time gas generator propellant charge.

It is another object of the present invention to reduce the hazards to a propellant charge which may result from shock waves or static discharge.

It is still a further object of the present invention to provide decreased burn time of a propellant charge.

It is yet a further object of the present invention to provide a fast burn time propellant charge which is safe, reliable, and non-toxic.

The above and other objects, features, and advantages of this invention will be apparent in the following detailed description of the preferred embodiments thereof which is to be read in connection with the accompanying drawings.

IN THE DRAWINGS

FIG. 1 is a side view of a canister fired missile embodying the present invention with a portion of the case broken away;

FIG. 2 is a half cross-sectional view, taken in a longitudinal plane, providing a detailed view of the broken away portion of FIG. 1;

FIG. 3 is a partially cross-sectional view, taken in a longitudinal plane, of the flight motor of FIG. 1;

FIG. 4 is a perspective view of a portion of a reticulated structure for a solid propellant charge embodying the present invention;

FIG. 5 is a sectional enlarged view of a portion of a solid propellant charge embodying the present invention, including a portion of the reticulated structure of FIG. 4; and

FIG. 6 is a cross-sectional view of a gas bag inflator embodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a canister fired missile at 10 which may be shoulder launched and which includes a launch eject motor 12 and a flight motor 14. The launch motor 12 is a solid propellant motor which is used to eject the flight motor 14, which has a length typically of from about 10 inches to about 6 feet, out of a canister (not shown) after which the flight motor 14 is caused to ignite when it is a safe distance away from the operator. The size of the launch motor 12 is related to the flight motor size. For a flight motor having a length of 40 inches, the launch motor may have a length of about 4 inches.

Referring to FIG. 2, the launch motor 12 includes a generally cylindrical case 16 and nozzle 18. At 21 is shown an electric match or initiator commonly know as a squib which is fired by electric lead wires 22 which enter the nozzle opening through a conventional nozzle plug 20. The ignitor 24, fired by squib 21, expels hot gases onto the surfaces of the launch motor propellant grain illustrated at 28 to initiate burning thereof. After ejecting the flight motor 14 from the canister (not shown), the launch motor 12 is caused to separate therefrom by means of any suitable separation apparatus, generally illustrated at 30, which includes forward closure 31 and a conventional separating piston 23. The closure 31 is releasably held to the flight motor case structure 32 by circum