A new series of composite propellant compositions, based on ammonium perchlorate, hydroxy-terminated polybutadiene and having metallic fuel as aluminium powder, have been developed. Pressure cast-cum-cured compositions have also been developed with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and a mixture of both curatives, i.e., TDI and IPDI, respectively, to study their effect on processibility, mechanical and ballistic properties of the compositions. The data indicate that the compositions based on bicurative have a pot life of 7 -8 h, viscosity build up is from 13280 poise to 14080 poise after 4 h, and the smooth processibility of the slurry is enhanced. Further, the mechanical properties are in the range 12.2 kg/cm 2 , 40.2 kg/cm 2 , and 40.2 per cent for tensile strength, E-modulus and elongation, respectively, and burn rate is almost the same, i.e., 16 ±0.5 mm/s. . INTRODUCTIONHydroxy-terminated polybutadiene (HTPB)-based solid rocket propellants are currently being used in space as well as in different ongoing missile programmes 1 . The low viscosity and low specific gravity along with high fuel value of HTPB makes it attractive to enable higher solid loading. The HTPB pre-polymer used for propellant slurry mixing is cured with a difunctional isocyanate curative. The urethane linkages -NH COO-formed by the -NCO with -OH reaction is chemically stable and is able to impart flexibility to the cured binder and makes the polymer an ideal elastomer 2 . A number of diisocyanates, like toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HMDI), diphenylmethane diisocyanate (MDI) and 4,4-dicyclohexylmethane diisocyanate (H 12 MDI) have been reported and used for HTPB Defence Science Journal, Vol. 57, No. 5, September 2007, pp. 669-675 2007, DESIDOC 669 system 3,4 . However, the most commonly used curative for the HTPB pre-polymer is TDI. The reactivity of TDI is comparatively more than other curatives except MDI and due to this, the pot life of TDIcured propellant is only in the order of 4-5 h. IPDI is another curative which is preferred to TDI mainly on merits of its slow reactivity, resulting in much extended pot life, i.e., 15-18 h as well as low toxicity [5][6][7] .Also, there is increasing trend to substitute TDI with other curatives in composite propellant based on HTPB. For structural integrity of grains, IPDI may not be a suitable curator for a casebonded motor. The thermal stresses in propellant grains are higher as the curing temperature is generally 70 o C. The curing temperature of a composition based on TDI is less as the reactivity is higher compared to IPDI. Owing to this, a new curative system has been conceived and utilised as bicurative

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[a] 1IntroductionComposite propellants are composed of an elastomeric polymeric binder,i nw hich solid particles, such as oxidizer, fuel, and additivesa re incorporated.R ockets and missiles are propelled by composite propellants.T hey basically contain an oxidizer,m ainly ammonium perchlorate (AP), ab inder such as hydroxyl terminated polybutadiene (HTPB), and am etallic fuel like aluminum powder along with certain process aids as well as ballistic modifiers [1]. The burning rate of composite propellants is considered to be one of the most important properties governing the ballistic performance of solid rocket motors,w hich in turn dependsm ostly on particle characteristics of oxidizers, burning rate catalystsa nd metallic fuel [2].T oa chieve the desired burning rate of composite propellant formulations, the amount of oxidizer,p article size of oxidizer,a nd burning rate modifiers are used as variables. AP is used in composite propellantf ormulationsa st he major ingredient (oxidizer) [3] in different size fractions to enhance the burning rate. However,f ine and superfine particle fractions of AP increase the burning rate as well as the propellant slurry viscosity thus casting of slurry becomes difficult [4].Therefore, to cope with suchp roblems, burning rate modifiersa re preferred. The burning rate modifiers are transition metal oxides (TMOs) or their complexes. The burning rate modifiers added in small quantities affect the burning rate by lowering the thermald ecomposition temperature of AP and binder [5].T he effect of transition metal oxides on thermal decomposition of ammonium perchlorate has been studieda nd reviewed in literature [5,6].M any researchersh ave studied in detail the effect of TMOss uch as Fe 2 O 3 ,C uO, Co 2 O 3 ,N iO, and Cr 2 O 3 over ballistic properties of propellant mainly on burning rate behavior and found substantial increase in burning rate [7,8] [16].A lso, the number of nanoscale catalyst, Abstract:D ifferentp ropellant compositions were prepared by incorporatingn ano-sizedc obalt oxide from 0.25 %t o 1% in HTPB/AP/Al-based composite propellant formulations with 86 %s olid loading. The effects on viscosity buildup, thermal, mechanical and ballistic properties were studied. The findings revealed that by increasing the percentage of nano-Co 3 O 4 in the composition,t he end of mix viscosity,t he modulus and the tensile strength increased, whereast he elongation decreased accordingly.T he thermal property data envisaged ar eduction in the decomposition temperature of ammonium perchlorate( AP) as well as formulations based on AP.T he ballistic property data revealed an enhanced burning rate from 6.11mms À1 (reference composition) to 8.99 mm s À1 at 6.86 MPa and am arginal increase in pressure exponent from 0.35 (reference composition) to 0.42 with 1% nano-cobalt oxide.

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Fluorocarbon polymers are used to enhance thermal stability and electrostatic protection of composite propellant compositions. A precipitation technique has been developed to coat ammonium perchlorate (AP) using a copolymer of hexafluoropropylene and vinylidene fluoride (HFP‐VF) with the help of solvent‐counter solvent method. The coated AP has been used to prepare propellant compositions in different ratio based on hydroxyl terminated polybutadiene (HTPB), aluminium powder along with uncoated AP and studied for viscosity build‐up and visco‐elastic behaviour as well as mechanical, ballistic, thermal and sensitivity properties keeping 86% solid loading. The data on viscosity build‐up indicate that as the percentage of viton coated AP increases end of mix viscosity and viscosity build‐up increase accordingly. The mechanical properties data reveal that tensile strength and percentage elongation are found in increasing order. The burn rate of the composition also increases on higher percentage of HFP‐VF coated AP. The thermal stability of composition increases as the percentage of HFP‐VF coated AP increases. The data on sensitivity indicate that impact sensitivity decreases on increasing the percentage of HFP‐VF coated AP while no change is observed in friction sensitivity value.