The propulsion system for the acceleration of projectiles is based on a multi-perforated grain propellant and is composed from nitrocellulose, a crystalline energy carrier of a nitramine type and an inert plasticizing additive. The number of perforations is 2 to 6, preferably 4. The propellant grains can have round or polygonic profiles, depending on the number of perforations. The preferred grain geometry is cubic with a rectangular grain profile. The nitramine compound contains a structural element of the general chemical structure formula R—N—NO2, where R is a residual group. The nitramine compound is present in a concentration in the range from 0 to 35% by mass, in particular in the range from 5 to 25% by mass. The nitramine compound is preferably RDX. The inert plasticizing additive is a water-insoluble polyoxo compound, if necessary in combination with a substance containing carboxyl groups. In layers near the surface an increased concentration can be present. The inert plasticizing additive is present in a concentration of 0 to 10% by mass, preferred 0 to 5% by mass.

EMBODIMENTS AND EXAMPLES

It is evident that, when using e.g. five perforations, the grain profile can either be round or pentagonal. Three perforations would imply a triangular grain profile, six perforations would accordingly have a hexagonal profile. Of course, the grain profile can, independent from the number of perforations, always be round according toFIG. 1b.

A unique feature if the multi-perforated grain geometry is the fact that there are two different web sizes, namely the inner web defined as the average distance between the perforations in the center of the grain, and the outer web being the average distance between the perforation and the grain surface. This unique feature might enhance the thermal conversion and be the cause for the observed high ballistic performance of propellants with the new grain geometry.

As noted before, a key element of the invention is the surprising finding that 4 perforations in the grain greatly enhance the interior ballistic output of a nitroglycerine-free high-performing propellant, meaning higher muzzle velocity and lower gas peak pressure for a given charge mass. This means that the high ballistic performance level of generic nitroglycerine-free ECL formulations, which have been proven in numerous medium caliber applications, can for the first time be adapted towards small caliber applications and thereby compete with conventional nitroglycerine-containing in-service propellants. Another surprising finding of the invention lies in the fact that the peak port pressure levels are on average approximately 100 bar higher over the whole temperature range as compared to the conventional 1 perforated grain geometry for small caliber uses.

It was a surprising finding that the rise from 1 perforation to 4 perforations leads to the observed performance increase without changes of the generic ECL propelling formulation. In Table 1 the interior ballistic results of a conventional 1-perforated ECL propellant is compared to a cubic 4-perforated ECL propellant of similar composition. For the 4-perforated type the velocity measured 24 meters in front of the muzzle is 24 m/s higher at a 61 bar lower pressure level. The temperature coefficients for velocity and pressure are very similar for both propellant types. The significantly higher thermal conversion of +8% of the 4 perforated type indicates that the new grain geometry is obviously affecting the burning behavior in a positive way and makes these new propellants more suited for the small caliber systems. This leads to an increase of energy conversion from the energy content of the propellant to muzzle energy of the projectile.

What is even more surprising was the finding that the geometry change yields to a significant improvement of the critical parameter peak port pressure, which goes up by approximately 100 bar as compared to the conventional 1-perforated ECL propellant. An example is given in Table 2.

Another surprising element of the invention is the finding that the bulk density of the grain propellant improves by switching from the conventional cylindrical grain geometry to a cubic geometry. This can be seen in Table 1, where the cubic 4-perforated type allows for a higher charge mass as compared to the 1-perforated cylindrical type. It was found that this effect does not occur for unfinished propellant grains, probably due to their rough surfaces. However, after finishing and glazing, the propellant surface is very shiny and smooth. This might allow the individual propellant grains to align their surfaces. This effect reduces the empty volume in the propellant bed and leads to a higher packaging density.

Table 3 illustrates how the bulk density of nitroglycerine-free grain propellants with the same generic formulation is affected by the grain shape. Two surprising findings can be seen: Firstly, for 4-perforated grain geometry, the cubic grain shape leads to higher bulk density as compared to conventional cylindrical shape. Additionally, for cylindrical grain shapes, 4 perforations lead to higher bulk density as compared to only 1 perforation.

The diameter of the 4-perforated grains according to the invention, measured as true diameter for the cylindrical grains (FIG. 1b) or as side length for the cubic grains (FIG. 1c), is determined by the targeted application, e.g. the projectile caliber and weight. For the envisioned small caliber applications it lies between 0.5 to 5 mm, preferably 0.5 to 2 mm. The length of the propellant grain is typically 0.5 to 5 times the size of the grain diameter, preferably 0.5 to 2.5 times. The dimension of the perforations in the grain must be large enough that the flame front can penetrate throughout the whole channel and take use of the surface area during the burning cycle of the propellant, but not too big in order to prevent excessive empty volume and therefore lower bulk density. Typically the diameters of the perforations are between 0.03 to 0.3 mm, preferably between 0.05 to 0.2 mm. For most applications the diameters of the perforations are of similar size, but for certain applications different diameters might be used on purpose on the same grain.

The propellant formulation and the coating parameters are basically the same as described in EP 1857429 (A1). The propulsion system contains nitrocellulose as the base material as well as a crystalline energy carrier on a nitramine base. Additionally, it contains one or a plurality of inert plasticizers, which can be localized in the grain matrix and/or in an increased concentration in the zones near the grain surface.

A further great advantage of the propulsion system according to the invention is the surprising finding that the velocity drop towards extreme cold firing temperatures and the velocity rise towards extreme hot firing temperatures can be tuned by the amount of deterrent applied into the zones near the grain surface. This is shown in Table 4, which shows the velocity slopes towards extreme hot (+70° C.) and extreme cold (−54° C.) firing temperatures for four different 4-perforated propellants with different deterrent concentrations. The correlations with the slopes of the corresponsive gas pressure values are lower. This effect can be used to optimize the temperature characteristics of the propellant for a specific application, meaning to minimize the changes of ballistic output (velocity and to a lesser degree gas pressure) by changing the powder bed temperature over a wide temperature range.

In a horizontal kneader with approx. 30 liters volume were placed 14.2 kg of nitrocellulose (13.25% N, wetted with approx. 25% ethanol and 3% water), 5.0 kg of hexogen with an average particle size of 6.8 micrometer, 240 g Akardite-2 and 300 g potassium sulfate as the key components, together with 20 kg of a mixture of ethanol and diethyl ether.

Kneading was allowed to proceed for 90 minutes total time. For the last 40 minutes an air stream was blowing through the kneader for partial solvent removal. Afterwards the dough was extruded through a die according toFIG. 1cwith 1.3 mm side length and 0.15 mm pin diameter. After extrusion, the grains were pre-dried, cut and bathed for solvent removal. Then the grain propellant was transferred into a sweety barrel heated to 60° C. and treated with a solution of 800 g of a low molecular weight deterrent dissolved in dilute ethanol according EP 1857429 (A1). Glazing was done using 40 g graphite.

The dough composition was exactly the same as in Example 1. The only difference was the dye form, which in this case was 4-perforated with a round shape according to Figure lb having a diameter of 1.3 mm and a pin diameter of 0.15 mm. After extrusion, the following processes, including cutting, bathing, surface treatment with deterrent, and glazing with graphite, were exactly the same as in Example 1.

The dough composition was basically the same as in Example 1 with the exception that only 4.0 kg of hexogen and 400 g of low molecular weight deterrent was used, which was compensated with 15.6kg nitrocellulose. The dough was extruded through a conventional 1-perforated round dye according to Figure la with a diameter of 1.1 mm and a pin diameter of 0.20 mm.