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Publisher Summary Rocket propulsion is based on the principle that a vehicle can gain momentum by expelling mass stored in the vehicle itself. Therefore, rocket propulsion differs from the air-breathing propulsion of aircraft jet engines, where most of the mass involved in the propulsion is atmospheric air. The source of energy in rocket propulsion is almost always the chemical energy released by the combustion of a fuel with an oxidizer. The basic principles that govern the properties of the gas after combustion as it expands in the nozzle are the same for all liquid-propellant and most solid-propellant motors. For a class of small thrusters, chemical energy is released by the decomposition of a single propellant, referred to as a “monopropellant,” most often hydrazine. Ion or colloid engines, thermal and magnetoplasmadynamic arc jets, resisto-jets, and related devices use electrical energy for their principal energy source. In space, this energy may be derived from solar cells or conversion from the thermal energy produced by radioisotope sources or by nuclear reactors. Electrical propulsion can have a far larger specific impulse, albeit at the cost of a much smaller thrust, than chemical propulsion. The most common liquid propellant systems use a separate oxidizer and fuel. Such propellants are referred to as bipropellants. The propellant tanks of launch vehicles are often designed as integral parts of the vehicle structure. The principal function of the engine control is to ensure the correct flow rates of the propellants.

The present understanding of spray combustion in rocket engine, gas turbine, Diesel engine and industrial furnace applications is reviewed. In some cases, spray combustion can be modeled by ignoring the details of spray evaporation and treating the system as a gaseous diffusion flame; however, in many circumstances, this simplification is not adequate and turbulent two-phase flow must be considered. The behavior of individual droplets is a necessary component of two-phase models and recent work on transient droplet evaporation, ignition and combustion is considered, along with a discusssion of important simplifying assumptions involved with modeling these processes. Methods of modeling spray evaporation and combustion processes are also discussed including: one-dimensional models for rocket engine and prevaporized combustion systems, lumped zone models (utilizing well-stirred reactor and plug flow regions) for gas turbine and furnace systems, locally homogeneous turbulent models, and two-phase models. The review highlights the need for improved injector characterization methods, more information of droplet transport characteristics in turbulent flow and continued development of more complete two-phase turbulent models.

Publisher Summary The application of solid propellants to rocket motor applications represents one specific example of using a change in the chemical state of matter taking place under controlled conditions for practical gain. Other examples include the combustion of coal and wood for a wide variety of applications, the ablation of material for heat shields, the use of packed catalytic reactors and of fluidized-bed reactors, and the explosive combustion of dust particles. One unique characteristic of solid rocket propellants that distinguishes them from all other solid fuels is the incorporation of sufficient reducing and oxidizing components within the solid phase to effect the transformation to gaseous products. Composite propellants are made by embedding a finely divided solid oxidizing agent in a plastic, resinous, or elastomeric matrix. The matrix material usually provides the fuel for the combustion reaction, although solid reducing agents are sometimes included. A third type of propellant, the composite modified-double-base propellant, represents a combination of the other two types. These propellants are made from mixtures of nitroglycerine and nitrocellulose or similar materials, but with crystalline oxidizers, such as ammonium perchlorate also included in the matrix. Development of a theoretical analysis of the combustion mechanism of double-base propellants gave birth to the model of the combustion zone. Because of the decomposition of the solid ingredients, the initial exothermic decomposition reactions could be analyzed as a single reaction occurring at the propellant surface. The initial products of decomposition vaporize and undergo partial oxidation reactions. This gives rise to a boiling porous reaction zone, commonly called the “fizz reaction” zone, which can be visually observed on the propellant surface.

Liquid propellant droplet vaporization temperature and radius histories in subcritical, critical and supercritical high pressure and high temperature gaseous environments