Extinguishable Propellant Composition

This invention relates to solid rocket propellants, and more particularly, to a solid rocket propellant capable of being readily extinguished and subsequently re-ignited.

While conventional solid rocket propellants provide important advantages over liquid propellants for certain applications, they are in general subject to the disadvantage that once they are ignited, they cannot readily be extinguished and re-ignited. Since there are numerous rocket applications wherein it is desirable or necessary to start and stop the rocket motor one or more times, the difficulties encountered in extinguishing and re-igniting solid propellant rockets constitute a serious limitation on their utility.

It is known that solid propellants burning in a rocket casing at an elevated pressure can be extinguished by an abrupt reduction in the pressure within the casing. The ability of a solid propellant to be extinguished in this manner can be conveniently evaluated in terms of its low pressure deflagration limit (Pdl). This limit is defined as the pressure at which burning of the propellant ceases when it is subjected to a relatively low specified pressure decay rate. In order to achieve rapid extinction the Pdl value of a propellant should desirably be of the order of 30 to 200 psia.

It is an object of the present invention to provide a solid propellant composition that can be readily extinguished and re-ignited. It is another object of the invention to provide a solid propellant having a relatively low burning rate and a relatively high low pressure deflagration limit (Pdl), say 50 psia or greater. It is still another object of the invention to provide an extinguishable solid propellant composition having processing properties which permit it to be conveniently mixed and cast in fluid form and subsequently cured at an elevated temperature. It is a still further object of the invention to provide an extinguishable solid propellant having good mechanical properties. Other objects of the invention will be in part obvious and in part pointed out hereafter.

The objects of the present invention are achieved in general by preparing an intimate mixture of finely divided oxidizer, preferably ammonium perchlorate, and liquid curable fluorocarbon binder, casting the mixture in a desired configuration and curing it at an elevated temperature. The preferred liquid fluorocarbons used as binders in the present compositions are fluoroalkyl acrylates and methacrylates of the general formula ##STR1## wherein R is hydrogen or methyl and n is 1 to 4.

As conducive to a clearer understanding of the present invention, it may be pointed out that solid propellants comprising ammonium perchlorate as an oxidizer and a fluorocarbon binder to provide a high density propellant are known in the art and are disclosed, for example, in U.S. Pat. Nos. 3,235,422, 3,255,059 and 3,565,706. However, the disclosures of these prior patents do not deal with the problems of providing an extinguishable propellant.

It has been found that in order to achieve the combination of acceptable processing and mechanical properties, as well as a low burning rate and a relatively high extinction pressure, the relative proportions of oxidizer and liquid fluorocarbon binder must be kept within rather narrow limits. More particularly, the weight ratios of ammonium perchlorate to binder should be in the range of about 4.5:1 to about 6:1, i.e., about 14 to 18% by weight of binder and 82 to 86% of ammonium perchlorate. Also as indicated in the specific Examples given below, the ammonium perchlorate should have a particle size distribution that reduces the void space between the particles to a relatively low value. If the ammonium perchlorate/binder ratio is increased above 6:1 with such a particle size distribution, the consistency of the mixture is not sufficiently fluid to permit rapid and thorough mixing and acceptable casting. On the other hand, if the ratio is below about 4.5, the propellant has inferior flame extinction properties.

It has been further found that the processing properties of the compositions, in particular the fluidity of the uncured compositions for a given ratio of ammonium perchlorate to fluorocarbon binder, can be improved by incorporating in the binder a small amount of an acrylate or methacrylate having an oxygen-containing functional group. Suitable acrylates for this purpose are hydroxyalkyl, epoxyalky, and glycerol acrylates and methacrylates, and mixtures thereof. It appears that the acrylates having oxygen-containing functional groups, when incorporated in the binder as described below, may be preferentially adsorbed on the surfaces of the ammonium perchlorate particles. In any event, the presence of a small amount of such acrylates in the mixture reduces its viscosity significantly, improves its mixing properties and castability, and also improves the mechanical properties of the cured composition.

It has been still further found that the rheological properties of the uncured castable composition and its uniformity can be improved by incorporating therein a minor amount of a high molecular weight fluorocarbon polymer. A copolymer of hexafluoropropylene and vinylidene fluoride sold under the trade designation "Viton A" is suitable for this purpose. Such a polymer promotes stable dispersion of the particulate ammonium perchlorate in the liquid binder during preparation, casting and curing of the propellant composition.

Curing of the composition is effected by polymerizing the acrylate binder components through the ethylenically unsaturated groups thereof, using conventional free radical catalysts such as, for example, benzoyl peroxide, dicumyl peroxide, tertiary butyl perbenzoate or cumene hydroperoxide. Curing can be effected at temperatures of say 120.degree. to 135.degree. F. for periods of several hours to several days.

The preferred ranges of proportions of the binder components on a weight basis are as follows:

The mixing, de-aerating, casting and curing of the present compositions may be effected by the conventional procedures known in the art.

In order to point out more fully the nature of the present invention, the following specific Examples are given of formulations embodying the invention.

A propellant formulation was made by mixing the following ingredients in the indicated proportions, the percentages being given by weight.

The high molecular weight fluorocarbon polymer was dissolved in the 1,1,7-trihydrododecafluoroheptyl acrylate and the resulting solution was mixed with the benzoyl peroxide and the other liquids, namely, the hydroxypropyl acrylate and the glycidyl methacrylate. Thereafter the ammonium hexafluorophosphate and the ammonium perchlorate were added to and mixed with the composition.

To facilitate admixture of the ammonium perchlorate with the pre-mixed liquids, the temperature of the formulation was increased to 110.degree. to 115.degree. F. and near the end of the mixing period, to 125.degree. F. Mixing was carried out over a period of about an hour. At the end of this time the viscosity of the mix was about 7 Kps.

Upon completion of the mixing the composition was cast and cured. Curing was effected at about 135.degree. F. over a period of 24 hours. After curing the following properties were measured:

A propellant formulation was made by mixing the following ingredients in the indicated percentages by weight:

The mixing, casting and curing steps were carried out as in Example 1. The viscosity of the uncured mixture prior to casting was 34 Kps at 124.degree. F. The cured product exhibited the properties given below.

From the foregoing description it should be evident that the present invention provides a propellant composition capable of achieving the objectives listed above. The compositions have a relatively high low pressure deflagration limit. Also by using acrylates having an oxygen-containing functional group, the fluidity of the uncured composition can be substantially reduced and mixing of the ingredients thereby facilitated.

It is of course to be understood that the foregoing Examples are intended to be illustrative only and that numerous changes can be made in the ingredients, proportions and conditions set forth therein without departing from the scope of the invention as set forth in the appended claims.