HE function of a rocket combustion chamber is to generate thrust by converting the propellant chemical energy into high kinetic energy hot gases. Accordingly, walls of a rocket engine can experience temperatures as high as 3,000°C, withstanding pressures up to 100 atm. Rocket combustion chambers can be classified by the cooling method which they employ. Passively cooled rocket engines are based on the use of ablative materials, generally polymeric composites. With this technique, the heat produced by the exothermic chemical reaction is expended in a material phase change, i.e. ablative cooling occurs when the heat flux changes the state of the surface substrate by melting, sublimation or by thermal degradation. As a result, a layer of relatively cool gases covers the combustion chamber inner wall surface, providing protection against hot combustion gases. Furthermore, film cooling technique can produce additional protection from excessive heat, by introducing a thin film of propellant above the chamber surface. Finally, this technique even allow to extend rocket combustion chamber lifetime, improving protection against oxidizing agents. Polymeric ablative composites are composed by a continuous phase, the matrix and, a discontinuous phase, which is constituted by the reinforcements and functional fillers, both phases can actively participate to the ablation process. Most polymeric ablators are based on thermosetting resins and, in this group of polymers, phenolic resins are undoubtedly the most employed and important matrices. In fact, besides their exceptional thermal stability, they possess excellent features such as: high dimensional stability and thermal insulation. These properties can be directly related to their high crosslink density and their chemical structure. Furthermore, the high crosslinking density of phenolic matrices improves the mechanical performance of the char, leading a better protection of the inner layers of the ablator.
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