Studies on combustion of solid composite propellants condensed mixtures.

Abstract

Combustion of solid composite propellant is characterised by its rapidity and the highly complex nature of the various physico chemical phenomena taking place both in the flame zone and in the solid phase of the combustion regime. The reactions taking place in the flame zone are known as gas phase reactions, and those in the solid phase as condensed phase reactions. The two schools of thought-one emphasising the role played by the gas phase reactions (giving less importance to condensed phase reactions) and the other highlighting the significance of condensed phase reactions (giving less attention to the gas phase reactions)-differ due to the difficulty in actually measuring the relative contributions of each phase. However, recent investigations have shown that at least about 30% of the overall combustion is derived from reactions taking place in the condensed phase. Thus, since a significant contribution to sustaining combustion comes from condensed phase reactions, it is necessary to understand the factors that influence and control these reactions. These factors may be physical parameters or the nature of chemical reactions occurring at the burning surface. Physical factors include thermal conductivity, heat capacity, and the nature of the layer surrounding the burning surface (whether molten or inert). Chemical factors include whether the reactions are endothermic or exothermic, formation of eutectics, and interactions of solid products with reacting species at the surface. A clearer picture of the burning surface can be obtained in the presence of additives, since they bring about specific effects. Additives may melt, form intermediate complexes with oxidizers, or form eutectics. Incorporation of additives enables understanding of combustion in different environments. An important aspect of solid propellant combustion is combustion instability, which adversely affects rocket performance. Several causes of unstable burning have been identified. One method to overcome instability is through modification of the thermal conductivity of the oxidizer using a coating (encapsulation) technique. The materials used for coating are polymers capable of releasing the oxidizer particles under specific triggering conditions such as pressure, heat, or mechanical impact. Because polymers have low thermal conductivity and decompose at characteristic temperatures, the oxidizer decomposition is influenced by the polymer coating. Although the coating technique is primarily intended to overcome instability, it also helps study the surface during combustion. Different polymers release oxidizer particles at different stages of combustion. In particular, polymers that decompose at temperatures lower than the surface temperature of the propellant release the oxidizer earlier, providing insight into the release mechanism. Encapsulation also helps overcome incompatibility problems between energetic oxidizers and polymeric binders. Many new oxidizers contain reactive groups capable of reacting with binders, which can degrade the propellant. Encapsulation prevents such reactions and improves storage and manufacture reliability.

The present work aims to understand aspects of solid propellant combustion in the presence of additives that melt and burn rapidly. It also presents preliminary studies on encapsulation of solid propellant oxidizers, since much of the existing literature is patented. The behaviour of encapsulated oxidizers during decomposition and combustion has been examined.

Chapter 1 A general introduction to solid propellant combustion is provided, emphasising the role of surface reactions. Details of oxidizer decomposition and binder decomposition available in the literature are discussed. Theories of composite propellant combustion are outlined. The encapsulation technique and its applications-particularly in composite propellant manufacture and combustion-are reviewed. The scope of the work, namely the effects of additives and encapsulation, is presented.

Chapter 2 This chapter describes the experimental methods used. Details of materials for casting, curing, cutting, inhibiting, and measuring burning rate of propellants are given. Preparation of condensed mixtures by incorporating additives into oxidizers through pelleting is described. Experimental details of Thermogravimetry (TG) and Differential Thermal Analysis (DTA) used to study decomposition of propellant components are provided. Details of the co acervation encapsulation technique for coating oxidizer particles are also presented. Methods for testing coating quality-solubility, melting behaviour, and Scanning Electron Microscopy (SEM)-are described.

Chapter 3 This chapter presents the results on the effects of additives and encapsulation.

Section A: Effect of melting additives such as LiClO ·3NH and Mg(ClO ) ·7NH on decomposition and combustion of AP. Section B: Effect of a fast burning additive, trimethylammonium perchlorate, on decomposition and combustion of KP and KP PU propellant. The behaviour is explained by the early formation of KCl. Section C: Effect of polystyrene coating on oxidizers such as AP, lithium perchlorate, ammonium nitrate, and aluminium perchlorate trihydrazinate. Coating desensitises decomposition and protects hygroscopic oxidizers. Section D: Effect of various polymer coatings (polystyrene, cellulose acetate, novolak rosin, PMMA) on AP decomposition, showing that the thermal and chemical nature of the polymer strongly influences oxidizer behaviour.

Appendix Contains results of AP decomposition in highly pre compressed pellets. Pre compression caused desensitization in the high temperature region (>500 °C), contrary to earlier reports of sensitization. This effect is attributed to higher density and poor heat transfer.