Smoke projectile charge and process for its manufacture

This invention relates to a shell filling comprising smoke pots which are stacked one above the other in the firing direction in the shell casing and which consist of a closed metal housing and, accommodated therein, a smoke charge based on hexachloroethane, zinc oxide and metal powder, and to a process for producing smoke pots for shell fillings of this type.

BACKGROUND 
Smoke pots containing smoke charges based on hexachloroethane, zinc oxide 
and metal powder are used for a variety of different types of ammunition. 
In general, the smoke pots are not very highly compressed. However, 
certain types of ammunition which have recently appeared are exposed to 
such high accelerations on firing that conventional smoke pots of this 
type cannot be used, because tests have shown that the housings of the 
smoke pots are deformed by the high acceleration on firing to such an 
extent that the smoke pots can no longer be ejected from the shell casing. 
This is understandable when it is considered that, for an acceleration on 
firing of, for example, 18,000 g, considerable forces act on the housing 
of the smoke pots and particularly on the housing of the lowermost smoke 
pot. In addition, however, it has been found that the reaction time of 
smoke pots such as these is no longer reproducible either. This is 
explained by the fact that the reaction velocity is dependent upon the 
compression of the smoke charge and uncontrollable compression effects 
occur at the acceleration levels referred to above. 
Accordingly, the object of the present invention is to provide a smoke 
shell filling and a process for its production, which is able to withstand 
extremely high accelerations on firing, for example of the order of 18,000 
g, and which is comparable with conventional smoke shells for 
comparatively low accelerations on firing in regard to its smoke effect, 
stability in storage and manufacturing costs, but superior in regard to 
its reaction time. 
According to the invention, this object is achieved by virtue of the fact 
that the smoke pots lie one on top of the other in complete surface 
contact, by virtue of the fact that the components of each pot housing 
consist of the same material, the load-bearing components being equal in 
strength, and by virtue of the fact that the smoke changes completely fill 
the housing and are self-supporting in themselves. In this way, an 
uninterrupted structure is obtained both for the stack of pots as a whole 
and for the individual smoke pots, the self-supporting smoke charges 
making a considerable contribution towards stability. 
Shell fillings of the type in question normally consist of smoke pots of 
which the housings consist of two coaxial tubes which between them form an 
annular chamber for the smoke charge, the annular chamber being covered on 
both sides by annular covers. According to the present invention, the 
smoke pot housings are preferably designed in such a way that, at both 
ends, the outer tube comprises a projecting annular rim of reduced wall 
thickness, the annular rim is set back in relation to the annular chamber 
to form an annular shoulder in alignment with the end face of the inner 
tube and is formed with a screwthread on that surface which faces the 
annular chamber, and the annular covers are provided on their outer edge 
with a screwthread corresponding to the screwthread on the annular rim of 
the outer tube and are screwed into the outer tube with such 
countersinking that the surfaces of the covers are flush with the end 
faces of the annular rims and the inner surfaces of the covers rest, on 
the one hand, on the annular shoulders of the outer tube and, on the other 
hand, on the end faces of the inner tube. In order to show self-supporting 
properties, the smoke charge which completely fills the annular chamber is 
highly compressed, preferably under a pressure of 1300 kp/cm.sup.2. 
The production of the smoke pots according to the invention with a highly 
compressed and, hence, self-supporting smoke charge has revealed a problem 
which was not previously known in smoke charges of the same composition, 
namely that the highly compressed smoke charges show low stability in 
storage which, in the extreme case, can result in complete failure of the 
ammunition. According to the invention, these difficulties are obviated by 
a process for producing the smoke pots with highly compressed smoke 
charges, wherein the zinc oxide is calcined at at least 900.degree. C. 
before being mixed with the other components of the smoke charge. For 
reasons which will be explained hereinafter, this treatment makes the 
smoke charges extremely stable in storage, in other words their stability 
in storage is at least equivalent to that of non-compressed or lightly 
compressed smoke charges.

According to FIG. 1, the smoke pot comprises an outer tube 10 provided at 
both ends with an annular rim 11a, 11b of reduced wall thickness. The 
references 12a, 12b denote the annular shoulder formed by the reduced wall 
thickness of the annular rim. Disposed coaxially inside the outer tube 10 
is an inner tube 13 of which the edges 14a, 14b align with the annular 
shoulders 12a, 12b. The references 15a and 15b denote two annular covers 
which are formed on their outer edges with screwthreads which correspond 
with screwthreads formed on the inner surfaces of the annular rims 11a, 
11b. A retaining tube 16 fits into the inner tube 13, being flanged into 
recesses in the annular covers 15a , 15b to surround them at both ends. 
The annular chamber between the outer tube 10 and the inner tube 13 is 
filled with a smoke charge 17 which communicates through openings 18 in 
the inner tube 13 and the retaining tube 16 with the interior 19 of these 
tubes, the so-called degassing channel. 
In the production process, one of the annular covers, for example the 
annular cover 15b, is first of all screwed into the outer tube 10 until it 
rests on the annular shoulder 12b of the outer tube and the end face 14b 
of the inner tube 13. The retaining tube 16 is then flanged and the 
projecting part of the outer tube and, optionally, of the retaining tube 
is squared off with the screwed-in cover 15b. In this way, a bearing 
surface is formed in the vertical plane, corresponding to the entire wall 
thickness of the outer tube 10 and the inner tube 13. A pressure acting on 
the smoke pot is thus uniformly absorbed by the entire supporting surface. 
The smoke charge 17 is pressed into this supporting structure, consisting 
of the outer tube 10, the inner tube 13 and the annular cover 15b, under a 
pressure of 1300 kp/cm.sup.2 exactly to the level of the annular shoulder 
12a of the outer tube 10 and the end face 14a of the inner tube 13. The 
projecting, compressed charge is optionally further compressed and squared 
off to the necessary level. Finally, the annular cover 15a is screwed into 
the outer tube 10 until, like the annular cover 15b, it rests on the 
annular shoulder 12a and the end face 14a, after which the retaining tube 
16 is flanged and the projecting part of the outer tube 10 is squared off 
with the screwed-in cover 15a. 
In this way, the smoke charge forms a substantially self-supporting element 
of the smoke pot and enables the wall thicknesses of the housing to be 
kept comparatively thin. For the same volume of the smoke pot, it is 
possible to introduce more smoke charge than in the case of smoke pots 
where the metal housing is the sole supporting element. For reasons of 
specific strength, weight and cost, highstrength aluminium is the most 
suitable material for the housing. 
It is of course necessary to incorporate an ignition system in the smoke 
pot. The smoke charge and the ignition system are compressed together to 
form a uniformly compressed block under such a high pressure that no 
further deformation can occur under a load of 18,000 g. FIGS. 2 and 3 show 
one example of an ignition system of this type which consists of three 
ignition tubes 20a, 20b and 20c which are embedded in the smoke charge. 
The number of ignition tubes, their size, the structure of the ignition 
charge and its position in the smoke charge play an important part. On the 
one hand, they have to extend as far as possible into the smoke charge 17 
to ensure satisfactory heat transfer and hence safe initiation of the 
smoke-forming reaction, whilst on the other hand they have to be situated 
so closely to the degassing channel 19 that an outwardly directed opening 
is formed in time to allow the gaseous active smoke ingredient to escape. 
At the same time, this degassing opening forms the necessary space for the 
actual smoke-forming reaction, which is known to occur in the gas phase, 
to take place. The formation in good time of an opening to the degassing 
channel and, hence, the creation of the empty space are important because 
otherwise the housing could either be explosively split open by the gas 
pressure generated during the reaction, thereby terminating the 
smoke-forming reaction, or alternatively the reaction of the smoke might 
not start at all on account of the missing gas space. It has been found 
that the optimum distance from the ignition cartridges 20 to the degassing 
channel 19 amounts to between 5 and 10 mm and preferably to 7 mm. 
The composition of the ignition charge of the ignition cartridges must be 
such that, on the one hand, the thermal energy is sufficient to initiate 
the smoke charge 17 but, on the other hand, is not so intense as to cause 
explosive initiation of the smoke-forming reaction, in which case the 
housing would no longer withstand the sudden increase in gas pressure. 
Accordingly, the reaction temperature of the ignition charge must lie in a 
certain temperature range. In addition, the ignition charge must flux very 
quickly on burning up to ensure that it is not forced outwards under the 
effect of centrifugal force (rotation 16,000 rpm), as a result of which it 
could again result in overviolent initiation of the smoke charge 17. 
According to the invention, this problem is solved by using a reaction 
mixture of Si/Pb.sub.3 O.sub.4 in a ratio of about 30:70. 
As already mentioned, it has been found in practice that highly compressed 
smoke charges based on hexachloroethane, zinc oxide and metal powder, of 
the type that have to be used here, give rise to difficulties which were 
previously unknown in smoke charges of the same composition. This is 
because, when these smoke charges are highly compressed, they show poor 
stability in storage which, in the extreme case, can result in complete 
failure of the ammunition. This instability is attributable to the water 
content of the smoke charge, this water content being much more harmful to 
highly compressed smoke charges than to the conventional smoke charges of 
comparatively low compression. Accordingly, an attempt was initially made 
to eliminate the instability of the highly compressed smoke charge by 
drying the final smoke mixture at temperatures above 100.degree. C. 
However, the desired result was not obtained because, during its further 
processing (compression, filling), the smoke charge reabsorbs most of its 
original water content. It was not possible, even by further processing 
the dried smoke charges in air-conditioned atmospheres, reliably to 
eliminate the instability of the smoke charge in storage. 
Exhaustive tests have now shown that the stability of a smoke charge in 
storage is primarily determined by its zinc chloride content. Part of this 
chloride is actually introduced into the smoke mixture as an impurity of 
this zinc oxide, and part is formed during pressing of the smoke charge. 
Zinc chloride is highly hygroscopic and, hence, is responsible for the 
fact that, even when produced from predried chemicals, smoke charges very 
quickly absorb moisture from the air and, accordingly, cannot be 
effectively processed under high pressures. 
Now, the process according to the invention seeks to eliminate the presence 
of and the formation of zinc chloride during production of the smoke 
charge. According to the invention, this result is achieved by calcining 
the zinc oxide at at least 900.degree. C. before it is mixed with the 
other components of the smoke charge. This calcining treatment has a 
two-fold effect. Firstly, the zinc oxide contaminated by hygrosopic zinc 
chloride is purified through volatilisation of the zinc chloride, so that 
it loses it hygroscopic effect. Secondly, no more chloride is formed 
during pressing of a dry charge. Accordingly, the compressed smoke charge 
also shows no further tendency to absorb water. Thus, it was not possible 
to detect any chloride, for example in a smoke charge produced with 
calcined zinc oxide, even after storage for 5 months. By contrast, a smoke 
charge of the same recipe, produced from non-calcined zinc oxide, was 
found to contain 1.3% of zinc chloride after only 2 days. In consistency 
with these findings, there had been no change in the reaction time of the 
first, calcined charge over the 5-month storage period, whereas in the 
case of the second, non-calcined smoke charge, the reaction could no 
longer be initiated after only a few weeks. Accordingly, it is possible in 
the process according to the invention to start with the known basic 
components in their standard commercial-grade form. In general, there is 
no need to pretreat the hexachloroethane and the metal powder because 
standard commercial-grade hexachloroethane and metal powder are 
substantially anhydrous and free from hygroscopic impurities. By contrast, 
the zinc oxide contains highly hygroscopic impurities, above all zinc 
chloride and zinc sulphate. These impurities are now removed by heating 
the zinc oxide to a temperature above 900.degree. C. (calcination). After 
this calcining treatment, the three components of the smoke charge may be 
mixed together in the usual way. The mixture is then compressed under a 
pressure of at least 1300 kp/cm.sup.2. Tests have shown that the smoke 
charges produced in this way are extremely stable in storage. In addition, 
the process according to the invention affords the possibility of using 
zinc oxide which, as a result of incorrect or poor storage, no longer 
appears suitable for normal mixtures. Similarly, even extremely moist zinc 
oxide can be made re-useable by calcination before use. 
Instead of using standard commercial-grade zinc oxide, it is also possible 
to use zinc oxide which ab initio is free from hygroscopic impurities 
providing it has the necessary reactivity. In this case, there is no need 
for the calcining treatment. 
Smoke charges based on hexachloroethane and zinc oxide additionally contain 
a proportion of metal powder as reaction accelerator. Of the powders 
normally used for this purpose, aluminium powder is particularly suitable 
in the present case because the housing material also consists of 
aluminium. This standardisation precludes the formation of local elements 
and, hence, reciprocal corrosive destruction. 
The formation of chloride is also a measure of the passivation of a smoke 
charge. By virtue of the fact that it does not occur in cases where 
calcined charges are used, the proportion of metal powder can be reduced 
by half in comparison with charges that are not completely dry. Since the 
aluminium not only contributes towards smoke formation, but also increases 
the residue, the smoke yield is also considerably increased by the present 
invention. 
In one example of embodiment where the primary object was to obtain the 
longest possible smoking times, a smoking time of 3.5 minutes was obtained 
for a filling weight of 2.2 kg using the following recipe: 
47.25% of ZnO (calcined) 
47.25% of hexachloroethane 
5.50% of aluminium powder. 
If non-calcined zinc oxide were to be used, an aluminium powder content of 
at most 10 to 11% would be required to obtain the same smoking time.