Development of Novel Base Bleed Composite Solid Propellant and Igniter for 155 mm Artillery Projectile vis-a-vis Parametric Studies

Abstract

Range extension is an essential and perpetual requirement for gun ammunition while no country in the world can afford to replace the gun system per se. Accordingly, it becomes all the more important to extend the gun range without changing its paraphernalia. When an artillery shell is fired, there are many forces acting on it which affect its range namely wave drag on warhead nose (30%), body drag due to spin and friction (20%) and base drag due to partial vacuum behind projectile (50%). There are many methods to extend the projectile range which may be related to the weapon or the projectile itself. Base bleed unit is one of the devices used to decrease base drag and, consequently, enhance the range. Base bleed decreases the base drag resulting due to vacuum behind the projectile by filling up the wake zone with hot gases to gain ambient pressure produced by combustion of composite propellant grain. The present work is an attempt to develop a novel ammonium perchlorate and hydroxyl-terminated polybutadiene (AP/HTPB) based composite solid propellant (CSP) for Base Bleed (BB) grain by a simplified method together with the development of zirconium and potassium perchlorate-based corresponding igniter for ignition of CSP grain. In order to accomplish the successful production of BB system, development of new CSP compositions together with igniter compositions commensurate with the ballistic and mechanical properties was conducted. In the present research work, efforts have been made to study various CSP compositions at lab scale for evaluation of ballistic parameters and high pressure Closed Vessel technique was employed for the first time to tune the ballistic properties of BB grain. The newly developed CSP and igniter compositions were studied for burning rate, pressure-time data, rate of change of pressure and heat of explosion and were found in fair agreement with existing data. The selected composition for BB grain was also characterized through different analytical techniques including Scanning Electron Microscopy (SEM), Differential Scanning Calorimeter (DSC) and Bomb Calorimeter to study its morphological and thermal cum kinetic parameters. The lab scale developed composition was then produced on pilot scale employing a horizontal twin sigma blade planetary kneading machine (50 litre capacity). To produce BB grains of a specific geometry moulds were designed having inhibitors housed inside mould cavity. This enabled easy casting of CSP, curing and easy extraction of fully inhibited BB grain avoiding the machining and inhibitor application process. Selected igniter composition was filled in igniter cups by hydraulic press. All the safety precautions required during manufacture, filling and formulation of explosives have been ensured to avoid any untoward incident. Functional test of BB grain was done on static test bench and igniters were evaluated several times on igniter testing setup for ignition, flame study and burning time. The complete BB unit was test fired on static test bed. On confirmation of BB unit ignition and burning performance, these units were fixed with 155 mm artillery projectiles (Extended Range Base Bleed projectile ERBB). Modern 155 mm gun, M198 Howitzer, was used for dynamic trials of the complete ERBB projectiles for performance evaluation and consistency. Range enhancement to the tune of 30% in all trials was achieved which stamped the performance and success of BB grain composition and igniter, developed during the ibid studies. In a nutshell, the present research vis-à-vis the development of CSP composition with corresponding igniter composition for BB unit has provided a new insight in the range extension of existing guns. The successful test firing of the pilot scale lots has great potential for plant scale production besides providing useful data in the academic domain in the form of comprehensive work