The shelf life of a composite solid propellant is one of the critical aspects for the usage of solid propellants. To assess the ageing behavior of the composite solid propellant, the activation energy is a key parameter. The activation energy is determined by analysis of visco-elastic response of the composite solid propellant when subjected to sinusoidal excitation. In the present study, dynamic mechanical analyzer was used to characterize six different types of propellants based on hydroxyl terminated polybutadiene, aluminium, ammonium perchlorate cured with toluene diisocyanate having burning rates varying from 5 mm/s to 25 mm/s at 7000 kPa. Each propellant sample was given a multi-frequency strain of 0.01 percent at three discrete frequencies (3.5 Hz, 11 Hz, 35 Hz) in the temperature range -80 °C to + 80 °C. It was observed that all the propellants have shown two relaxation events (α-and β-transition) in the temperature range -80 °C to +80 °C. The α-transition was observed between -66 °C and -51 °C and β-transition between 7 °C and 44 °C for the propellants studied. The activation energy for both transitions was determined by Arrhenius plot from dynamic properties measured at different frequencies and also by time temperature superposition principle using Williams-Landel-Ferry and Arrhenius temperature dependence equations. The data reveal that the activation energy corresponding to α-transition varies from 90 kJ/mol to 125 kJ/ mol for R-value between 0.7 to 0.9 while for β-transition the values are from 75 kJ/mol to 92 kJ/mol. The activation energy corresponding to β-transition may be used to predict the useful life of solid propellant.

[a] 1IntroductionSolid propellants are most commonly used in strategic,t actical, and rocketry system to deliver the requiredt hrust to the missiles or rockets due to their simple design, ease of operation, safety,reliability,long shelf life, and comparatively lower cost. Composite solid rocket propellants are ah eterogeneous mixture of solid particles of oxidizer crystals and metallic fuel dispersedi napolymeric binder matrix. Hydroxylt erminated polybutadiene( HTPB) based composite solid propellants are used in many operationalp ropulsion systemsd ue to their better performance and established processingt echnology.G enerally,t he percentage of solid particles in composites olid propellant varies from 80-90 %a nd liquid binder nearly 10-20 %d epending on ballistic and structural requirements. The mechanical behavior of solid propellants is mainly influenced by the polymeric nature of binder,c uring agent,a nd binder-filler interaction [1,2].The mechanical properties of solid propellantsd epend on intrinsic and extrinsic variables. The intrinsic variables also termed as constitutional variablesa re due to cross linking, branching,c rystallinity,p lasticization, quality and quantity of fillers, etc. The extrinsic variables also known as environmental variablesa re due to variation in temperature, time, strain rate, pressure, %r elativeh umidity( RH), mode of deformation, etc. [3,4] Eachs olid propellant grain is considered to be unique by virtue of its manufacturing processing and its subsequent loading history.H ence, ap articular solid propellant rocket motor,w hich has been deployed in field conditionsm ay have different mechanical properties from as ample propellant, whichi ss tored in the magazine under controlled environment [ 5].The mechanical properties of propellants, which govern the structural integrity of as olid propellant rocket motor (SPRM) are:ae longation [%],m odulus[ MPa] and tensile strength [ MPa],a nd evaluated by universal testing machine (UTM). The propellant cartons are cast along with SPRM and stored in amagazine. The propellant in SPRM experiences varioust ypes of loadings duringh andling, transportation, deployment storage, and environmental conditions over the period of time. The mechanical properties evaluated from the propellant carton are considered to be the mechanicalp roperties of the propellant in ar ocket motor,i s not truly representative [6][7][8].H ence an ew method is required to evaluate the mechanical properties of propellant in rocket motor,w hich experiences varioust ypes of loading over the period. Penetrometer-an ondestructive method is used for evaluation of mechanical properties of solid pro- Abstract:I ti se ssential to evaluate the mechanical properties of propellants in as olid propellant rocket motor (SPRM) for structural integritya nd its performance evaluation before the flightt est. Conventionally,u ni-axial tensile testing on an universal testing machine (UTM) is used to evaluate the mechanical properties of solid propellant carton which is cast along w...

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