Storage of rocket motors loaded with composite solid propellant for long periods may change the propellant properties, thus causing failure and affecting the safety during launch. In this study, an accelerated aging assay was carried out, in order to predict the useful lifetime and to evaluate variations on the propellant properties with time by means of thermal analysis (TG/DSC). The aging temperatures used were 65°C, and samples were withdrawn after 3 months. Aging was also carried out at room temperature. There was significant variation in the activation energy of the solid propellant samples thermal decomposition in the two kinetic methods used – Ozawa or model-free isoconversional method and Kissinger method – during the aging period. There was significant decrease of enthalpy of aged propellant enthalpy causing changes in ballistics parameters of the solid propellant grain affecting the rocket's performance.
TopIntroduction
Rocket engines are widely used in military missions, mainly for being the driving force of military artifacts of conventional use, including weapons and auxiliary devices. Currently, the peaceful intent and growing diplomatic relations between the UN countries make storage for long periods of time and readiness of use increasingly crucial for its proper functioning and safe operation. The high costs of acquisition and maintenance of solid propellant based weapons require the prediction and understanding of the useful life by the operators and manufacturers.
The solid composite propellant used in rocket motors is basically constituted by a polymeric matrix (MM 10% on average), rich in carbon and hydrogen, which serves as a binder and gas generator; an inorganic salt as the oxidant (70% average MM), rich in oxygen; and a ballistic auxiliary, generally a metallic additive, with the most commonly used is aluminum powder (20% on average MM). Once produced, the solid propellant grain installed in the engine shall attain appropriate ballistic characteristics and pre-defined mechanical behavior, which entails the use of raw materials and specific manufacturing processes for the propellant formulation design function motor. In addition, the proper storage, transport and handling is also of utmost importance for its proper functioning and safe operation (Sutton & Biblarz, 2010).
The chemistry composition of solid propellant changes with time, this is known as propellant aging. The parameters affected by aging are the chemical and the mechanical properties of the grain, compromising thus the ballistic properties of the motor. This phenomenon is critical to determine the window of use of use rocket engines. Among the parameters that affect aging, can be mentioned temperature, mechanical stress, environmental conditions (e.g., humidity) and even the contact with other organic materials (Kubota, 1984).
Studies conducted in order to obtain more accurate information about the propellant grain deteriorating conditions are an important issue in rocket engine applications, they allow the determination of the service life of each component. Due to the large surface area and its chemical potential energy, the composite propellants are subject to various tensions during handling, storage and operation, having consequences in ballistic results and in mechanical and chemical properties (Davenas, 2003).
The solid propellant composite is a complex and stable mixture of oxidizing and reducing ingredients. After ignition, the reagents react with each other, burning in a homogeneous, continuous and controlled manner, forming gas molecules at very high temperatures and pressures. The binder or polymeric matrix, most commonly used consists of a polyurethane elastomer obtained from hydroxylated polybutadiene
(Figure 1), commercially known as HTPB, which is classified as a liquid prepolymer with molar mass of the order of 2800 g.mol-1 containing terminal hydroxyl groups and reactive groups.
The HTPB is obtained by polymerization of butadiene (Figure 2), initiated by hydrogen peroxide using an alcohol as diluent (Vilar, 2005). A major advantage of the use of hydroxylated polybutadiene is the lack of secondary reactions and by-products during curing or forming of the propellant grain. The curing process consists of reacting the hydroxyl HTPB with compounds containing isocyanate groups, forming a polyurethane polymeric matrix.
Figure 1. Structural form of hydroxylated polybutadiene (HTPB)
Figure 2. Obtaining reaction of hydroxylated polybutadiene (HTPB)
The rocket engines loaded with solid propellants have a predetermined useful life, in which the security, the mechanical parameters and the ballistics can be guaranteed during launching. The aging of the propellant, despite being a slow and gradual phenomenon, can hamper adequate performance.