US3086894A

US3086894A - Metallic wool ignition materials

Info

Publication number: US3086894A
Application number: US68668A
Authority: US
Inventors: Joseph M Baggett; Wallace T Mcmichael; Elmer L Pendleton
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1960-11-14
Filing date: 1960-11-14
Publication date: 1963-04-23
Anticipated expiration: 1980-04-23

Description

corporation of Delaware No Drawing. Filed Nov. 14, 1960, Ser. No. 68,668

(til. 149-5) 13 Claims.

This invention relates to a device for the ignition of solid rocket propellants.

In the field of rocket propulsion, solid rocket propellants usually have a plastic-like, caked appearance and burn on their exposed surfaces. The physical mass or body of the propellant is referred to as the grain and the ignition of the combustion surface of the propellant grain is accomplished by means of a pyrotechnic igniter, which in turn is usually started by means of electrical current or percussion action. Ideally, the ignition device should supply uniform heat at high temperature instantaneously over the entire uninhibited surface of the grain.

It is an object of the present invention to supply uniform heat at high temperature instantaneously and with negligible shock to the entire uninhibited surface of a solid rocket propellant grain.

We have found that an igniter consisting of a metallic wool, the term wool being used hereinafter to define broadly that physical state wherein a metal has been fabricated into fine metallic threads such as the well known steel wool, an adhesive, and a solid oxidizer can be prepared, which, when activated by an electric current or other suitable means, burns instantaneously with uniform heat and no shock. The term shock is used herein to describe an explosion or detonation. Further, this igniter meets the requirements of being safe handling with maximum dependability.

The process for the preparation of the igniter consists of taking a metallic wool made of a metal such as magnesium, aluminum, zirconium, iron, steel, etc., dipping it into a liquid adhesive which remains tacky after drying, and thereafter bringing the adhesive-covered metal- =lic wool into contact with a powdered oxidizer in such a manner that the oxidizer, because of the adhesive, adheres to the threads of the metallic Wool. That the igniter thus prepared is stable and safe to handle has been proven by its ability to remain inert when placed in an oven for 2 hours at 120 C.

While, in general, most metallic wools burn when in the presence of an oxidizer, some burn faster than others. Thus, though a wool made of iron will function within the scope of the present invention, magnesium has proven to be superior for igniter purposes due to its faster burning rate and is the preferred metallic Wool. The adhesive can be of a type such that a uniform layer, having the appropriate *tackiness for holding the desired amount of oxidizer, can be applied by solution dipping or other means. For obvious reasons, the adhesive must be inert, that is, unreactive at ordinary temperatures with both the oxidizer and the metallic wool. Certain low molecular weight resins and condensation products have proven satisfactory for the purpose. These adhesives we preferably hydrophobic so that atmospheric water vapor is not absorbed in the oxidizer thereby starting corrosion and thus deactivating the metal wool. Sui-table adhesives include low molecular weight polyisobutylenes, polypropylenes, polyisoprenes, and polyacrylics, and also certain polyglycols. Atactic polypropylene glycol in particular has given excellent results.

The powdered oxidizer can be selected from those well known to the rocket art such as ammonium perchlorate and lithium perchlorate. For best oxidation, the amount 3,06,804 Patented Apr. 23, 1963 of oxidizer should be stoichiometrically equivalent to the amount of metallic wool. However, it should be realized that the wool will burn in air at some rate without the oxidizer and increasing the oxidizer up to the stoichiometric amount is for the purpose of obtaining the maximum burning rate and assuring optimum combustion in a closed chamber where atmospheric oxygen is not available. Without the oxidizer, the burning is sporadic and slow, and when an excess of oxidizer is used, the excess consumes heat needed for ignition of the grain.

When igniter function includes the generation of gaseous pressure in the rocket combustion chamber, a friable organic material may be blended With the oxidizer and the blend applied to the metallic Wool. Organic materials best suited for this purpose are those containing large proportions of gas-generating agents, for example, carbon for making CO and CO nitrogen for N and nitrogen oxides, hydrogen for E 0, and especially oxygen to avoid using excessive amounts of solid oxidizer. It is useful for this organic material also to contain high energylinkages such :as acetylenic bonds and the like with the exception that linkages and groups contributing :to excessive instability or corrosive properties necessarily must be avoided. Materials suitable for the purpose include hydrophobic solid pclyalkylene oxides, for example, polypropylene oxides of molecular weight in the approximate range 10,000-25,000.

The following specific examples of our invention:

illustrate the practice Example I A pad of fine magnesium wool weighing 3 grams was lightly packed into the desired shape. The pad was lightly packed in order to leave voids within the structure. This pad was then dipped into an adhesive consisting of 2.0 weight percent atactic polypropylene glycol of approximately 20,000 molecular weight dissolved in normal hexane, shaken to remove excess adhesive, air dried until the hexane was essentially evaporated, and thereafter placed into a container containing three times the pads weight of ground ammonium perchlorate. The container was shaken at such a rate that the ammonium perchlorate penetrated throughout the pad. The pad was removed from the container and excess ammonium perchlorate not adhering to the pad was shaken off. The pad was prepared for activation by connecting two fine copper wires to the pad at separate points in such a manner as to have good electrical contact. Activation was accomplished by connecting the lead wires from v. potential to the two th-in copper wire leads and thereafter subjecting the pad to 110 v. When so activated it burned substantially instantaneously without detonation.

Example 11 A pad was prepared as in Example I using magnesium wool and lithium perchlorate. Its burning properties were similar to those of the product of Example 1.

Example III A pad was prepared as in Example I using steel wool and ammonium perchlorate. When activated it burned smoothly but not as rapidly as pads from magnesium or aluminum.

We claim:

1. A device for the ignition of solid rocket propellants which comprises a loosely packed mass of metallic wool, fabricated from a metal combustible in air in this form, and coated throughout with about the stoichiometric quantity of a powdered solid inorganic oxidizing salt, said oxidizing salt being held to the metallic wool by [an inert adhesive, and the mass of metallic wool being connected to appropriate means for activation.

2. The device of claim 1 wherein the oxidizing salt is an inorganic perchlorate.

3. The device of claim 2 wherein the metallic wool is magnesium wool.

4. The device of claim 2 wherein the metallic wool is aluminum wool.

5. The device of claim 2 iron wool.

6. The device of claim 2 wherein the metallic wool is zirconium wool.

7. A pad of metallic wool, fabricated from a metal combustible in air in this form, coated throughout with about the stoichiometric quantity of a powdered solid inorganic oxidizing salt, said oxidizing salt being held to the metallic wool by an inert adhesive.

8. A process for making a device as described in claim 1, which process 'compn'ses coating a loosely packed mass of metallic Wool fabricated from a metal combustible in :air in this form with an inert adhesive, and thereafter bringing the adhesive-covered metallic wool into contact with a powdered solid inorganic oxidizing wherein the metallic wool is salt, thereby coating said wool with about the stoichiometric quantity of oxidizing salt.

9. A process as described in claim 8 wherein the oxidizing salt is an inorganic perchlorate.

10. A process as described in claim 9 metallic wool is magnesium wool.

11. A process as described in claim 9 metallic wool is aluminum wool.

12. A process as described in claim 9 metallic wool is iron wool.

13. A process as described in claim 9 metallic Wool is zirconium wool.

wherein the wherein the wherein the wherein uhe References Cited in the file of this patent UNITED STATES PATENTS 812,195 Escales Feb. 13, 1906 2,037,101 Van Liempt Apr. 14, 1936 2,959,001 Porter Nov. 8, 1960 20 2,973,713 Burton Mar. 7, 1961

Claims

1. A DEVICE FOR THE IGNITION OF SOLID ROCKET PROPELLANTS WHICH COMPRISES A LOOSELY PACKED MASS OF METALLIC WOOL, FABRICATED FROM A METAL COMBUSTIBLE IN AIR IN THIS FORM, AND COATED THROUGHOUT WITH ABOUT THE STOICHIOMETRIC QUANTITY OF A POWDERED SOLID INORGANIC OXIDIZING SALT, SAID OXIDIZING SALT BEING HELD TO THE METALLIC WOOL BY AN INERT ADHESIVE, AND MASS OF METALLIC WOOL BEING CONNECTED TO APPROPRIATE MEANS FOR ACTIVATION.