Energy Output of Ball Propellant vs Extruded Propellant

Fri, 02/09/2018 - 08:46 -- rdm

From a ballistic viewpoint the best propellant grain size and form is that which, with the smallest charge mass, will impart the required velocity to a projectile without exceeding the maximum pressure of the weapon system. A propellant composition and configuration that burns too fast (generates gas at a too rapid rate) creates a destructive pressure spike that usually has a very short duration. Using a propellant that burns too slowly produces poor energy transfer and leaves a lot of unburned propellant slivers (slivers are small sections of the propellant that have not burnt out completely).

For maximum energy transfer to the projectile which results in maximum muzzle velocity, the propellant should burn out just before the projectile leaves the barrel. Typically only approximately 32% of the chemical energy of a propellant is transferred to the projectile as kinetic energy with heat losses to the combustion gasses and to the gun and projectile as the major causes of energy loss.

For specific applications, both S321 and S335 have been optimised to address the above.

Factors that influence barrel erosion (barrel life) are amongst others:

 

·        Propellant composition, Specific energy and flame temperature

 

·        Pressure in the weapon

 

·        Muzzle exit velocity

 

·        Mechanical erosion (from the projectile)

 

·        Surface area and burn rate of the propellant

 

·        Coating of the propellant

While S335 is a single base propellant extruded (consisting almost entirely from Nitrocellulose,); S321 is a double based spherical propellant. In both cases, additives are included in the propellant formulation to manipulate burning characteristics, improve amongst others process ability, mechanical properties, shelf life and to reduce flash.

The specific energy of S321 and S335 differ with only 1 J/g (1018.7 versus 1019.7 J/g) – so the available energy of both are basically the same.

Contrary to what one would expect, the flame temperature of the S321 double base ball powder is slightly lower than the single base S335 (2928 K versus 3000 K) and from their thermochemical properties there should be no meaningful difference in their erosion properties.

 

The charge mass of both propellants is optimised for the same muzzle velocity and therefore the S335 with the lower charge mass would be the cheaper option with everything else being more or less the same. The only meaningful effect on erosion would be it there is a real difference in the maximum pressure generated for the same muzzle velocity.

 

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