AU4800285A - Gun flash suppressants - Google Patents

Description

GUN FLASH SUPRESSANTS

INTRODUCTION

The suppression of muzzle flash has long been of concern to propellant formulators and systems engineers. The, important, undesirable aspects of muzzle flash such as signature, temporary loss of night vision by gun crews, significantly increased muzzle blast and reduced gun performance are well known. More recently muzzle flash has been implicated in loss of aircraft due to compressor stall and flame-out .

It is generally assumed that gun muzzle flash usually consists of three components: a weak "primary" flash due to visible emission from hot product gas molecules; a strong but brief "intermediate" flash cause by recompression of the product gases on passing through the shock front; a strong and long-lived

"secondary" flash corresponding to rapid combustion of fuel-rich muzzle gases in the atmosphere. Secondary flash is the most important and it is the suppression of this phenomenon that is one of the objects of this invention.

PRIOR ART

According to known prior art methods several mechanical and chemical methods have been used to alter the flash characteristics of a particular weapon. Mechanical flash suppressors consist of conical or slotted nozzles which are designed to interfere with the formation of the shock structure in the plume. The most widely used form of flash suppression is accomplished by the addition of alkali metal salts, usually potassium, to the propellant during processsing. The mechanism of flash suppression by these materials is reasonably well understood. They are able to form compounds which act as catalysts in a free radical chain-breaking mechanism which interferes with hydrogen-oxygen combustion and have been referred to by A. Cohen and L. Decker, 16th JANNAF Combustion Meeting, Monterey, California, September 1979 p. 691. A problem with alkali metal salt flash suppressants is that they usually reduce the impetus of the propellant, cause smoke and, when used in aircraft cannon, are the prime cause of aircraft skin erosion and fouling. The alkali metal salts can also foul, corrode or otherwise degrade the performance of aircraft engines.

THE OBJECT OF THE INVENTION

The object of this invention is to provide a method of and means of flash suppression which can be readily applied and will not give rise to significant skin erosion or fouling, will not materially reduce propulsion and which produces minimal smoke.

Recently the flash suppressant properties of a series of ammonium salts has been reported, see A.J. Bracuti, L.A. Bottei, R. Davis, in Journal of Ballistics, 7, 1609 (1983). These compounds appear to have advantages over the traditional alkali metal salts if they could be shown to be effective secondary flash suppressants. They would produce less smoke, may reduce or eliminate loss in propellant performance and they should alleviate the aircraft skin erosion problems encountered with potassium salts. For this reason research was undertaken to show under what conditions, if any, ammonium salts could function as secondary flash suppressants in a weapon system. The propellant used in the experiments of this invention were a variation of standard single base powder which was formulated in accordance with Australian Specification DEF (AUST) 5609. Batches were manufactured with the omission of all alkali metal salts and were deterred with dinitrotoluene. Some batches were manufactured with the candidate ammonium salts incorporated directly.

As an example the flash suppressants were added to the charge in several ways. All suppressant salts were analytical reagent grade and powdered in a mortar and pestle then dried before being used. Suppressant salt discs were 6mm in diameter and contained either 100 or 200 milligram of material. The discs were pressed in a Manesty Pelleting Press under sufficient pressure to form hard pellets. They were stored in a dessicator until ready for loading. They were placed on top of the propellant charge prior to crimping. Suppressant salt grains were formed by mixing 1:1 w/w salt and hydroxyethyl cellulose (Hoechst TYLOSE H4000) with sufficient water to form a thick gelatinous paste. The paste was then extruded through a glass hyperdermic syringe with the needle removed. The resulting strands were dried and cut to length with a mechanical propellant cutter. The dried, finished grains had nominal diameters of 1.0mm and a length of 1.5mm, comparable dimensions to those of propellant grains. These flash suppressant salt grains were loaded on top of the propellant charge. It was not possible to process ammonium sulphate or potassium carbonate by this method as these salts appeared to be incompatible with the hydroxyethyl cellulose. The suppressant salts were also used in powder form similar to the way salts were incorporated in the experiments of Bracuti et al referred to earlier herein. In this case the appropriate weight of each salt was wrapped in a small piece of polymer film and loaded on top of the propellant charge.

RESULTS OF FLASH ASSESSMENT

The experiments were carried out in four sessions, the results of which are summarized in Tables 1 - 4 which form part of this specification. The first session was primarily to assess the secondary flash produced by the unsuppressed single-base propellant. Three propellant batches were produced, one deterrent coated in the same way as standard AR 2206 and two others processed as uncoated base grains. The base grain batches were of differing web sizes produced by dies having diameters of 0.050" and 0.055". As Table 1 shows the coated propellant and the smaller web base grain flashed consistently at all charge weights. The larger web base grain did not flash because it was found that the propellant grains did not burn completely. The coated propellant was used in the remaining two experiments as the "base-line" propellant for which flash was to be suppressed.

The next two experiments assessed the flash suppressant properties of eight ammonium and potassium salts, several of which had been studied by Bracuti et al above in a vented bomb. As Table 2 shows, ammonium chloride was the only ammonium salt to show any flash suppressant activity in the second experiment. All of the potassium salts showed the expected elimination of secondary flash. The third experiment attempted to reproduce the results of Bracuti et al by using powdered salts placed in front of the charge. The efficacies of the different forms of incorporation (powder and grains) were also compared. It is clear from Table 3 that the results of Bracuti et al could not be duplicated in this calibre. The incorporation of ammonium salts as powders appeared to be almost completely ineffective in reducing or eliminating secondary muzzle flash. The incorporation of lesser amounts of flash suppressant material in the form of grains was found, however, to be quite effective (300 mg. of suppressant salt grains contained 150 mg, of salt). The results obtained by Bracuti et al appear to be quite systems-dependent or invalid.

The final experiment investigated the efficacy of ammonium chloride and ammonium nitrate salts when mixed into the propellant at processing stage in the proportion of 6% weight for weight (W/W). As table 4 indicates, both compounds were very effective at eliminating muzzle flash. The ammonium nitrate was found to be superior to ammonium chloride in propellant compatibility and ballistic performance. No attempt was made to incorporate ammonium carbonate or bicarbonate salts into this particular propellant due to extreme incompatibility.

From these experiments we were able to show that ammonium chloride and ammonium nitrate were effective in reducing or eliminating muzzle flash if incorporated in the charge in the form of grains or mixed into the propellant at processing in proportions not exceeding 12% W/W. Suppressant salts incorporated in this way appeared to burn completely and cleanly as there was no evidence of fouling, unburnt propellant and the smoke produced was no worse than that of a service round. In addition comparison of ballistic data showed that ammonium nitrate, in particular, enhanced the ballistic performance when added to the charge.

CONCLUSION

The experiments to assess the effectiveness of several ammonium salts as muzzle flash suppressants have failed to reproduce the results of Bracuti et al. It was found that ammonium chloride and ammonium nitrate gave good secondary flash suppression only when incorporated in the charge in the form of grains similar in shape to those of the propellant, or when incorporated directly into the propellant.

Thus the basis of this invention is the use of ammonium salts in a proportion not exceeding 12% W/W, excepting the carbonate and bicarbonate salts, either incorporated directly into a granular gun propellant used for any gun, or in the form of inert grains containing the salts which are added to the propellant charge, the percentage limitation ensuring that the ammonium salt does not materially effect the propellant charge but acts simply as a flash suppressant.

Claims (7)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A gun propellant incorporating a flash suppressant characterised by the incorporation into a granular propellant charge of an ammonium salt in the proportion of not more than 12 percent of the ammonium salt W/W.

2. A gun propellant according to claim 1 characterised in that the said ammonium salt is incorporated in the said charge in the form of inert grains containing the said salt and added to the said charge.

3. A gun propellant according to claim 2 characterised in that the inert grains containing the ammonium salt are similar in shape to the grains of the said propellant charge.

4. A gun propellant according to claim 1 characterised in that the ammonium salt is incorporated in the grains of propellant charge.

5. A gun propellant according to claim 4 in which the ammonium salt is incorporated in the said propellant grains during manufacture in the proportion of about 6 percent weight for weight of ammonium salt to propellant,

6. A gun propellant according to any one of claims 1 to 5 in which the said flash suppressant is ammonium nitrate.

7. A gun propellant according to any one of claims 1 to 5 in which the said flash suppressant is ammonium chloride. AMENDED CLAIMS

[received by the International Bureau on 28 January 1986 (28.01.86); original claim 1 amended; other claims unchanged (1 page)]

1. A gun propellant incorporating a flash suppressant characterised by the incorporation into a granular propellant charge, as a flash suppressant, of an ammonium salt in the proportion of not more than 12 percent of the ammonium salt W/W.

2. A gun propellant according to claim 1 characterised in that the said ammonium salt is incorporated in the said charge in the form of inert grains containing the said salt and added to the said charge.

3. A gun propellant according to claim 2 characterised in that the inert grains containing the ammonium salt are similar in shape to the grains of the said propellant charge.

4. A gun propellant according to claim 1 characterised in that the ammonium salt is incorporated in the grains of propellant charge.

5. A gun propellant according to claim 4 in which the ammonium salt is incorporated in the said propellant grains during manufacture in the proportion of about 6 percent weight for weight of ammonium salt to propellant.

6. A gun propellant according to any one of claims 1 to 5 in which the said flash suppressant is ammonium nitrate.

7. A gun propellant according to any one of claims 1 to 5 in which the said flash suppressant is ammonium chloride.