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rocket propellant[′räk·ət prə‚pel·ənt]
a substance or combination of substances used as a source of energy and as a source of the working fluid for a rocket engine. Rocket propellants must satisfy a number of basic requirements. They must have a high specific impulse (the thrust developed in expending 1 kg of fuel per sec) and a high density, and the components must be in the required state of aggregation under operating conditions. Propellants must also be stable, safe to handle, nontoxic, and compatible with the rocket’s structural materials. In addition, sources of raw materials for the propellant must be available.
Both chemical and nonchemical propellants exist. With chemical propellants, the required energy is released as a result of chemical reactions, and the gaseous products thus formed serve as the working fluid; that is, upon expansion in the rocket engine nozzle, the gaseous products provide for the conversion of the thermal energy of the chemical reactions into the kinetic energy of a jet flowing from the nozzle of the rocket engine. With nonchemical propellants, the energy of intranuclear transformations or electric energy (as in nuclear or electric rockets) is imparted to a special substance that constitutes the working fluid or the source of the working fluid. The specific impulse of nonchemical propellants depends on the thermodynamic properties and permissible operating temperature of the working fluid and on the energy expended in creating the thrust. In principle, nonchemical propellants may be superior to chemical propellants on the basis of specific impulse.
Most existing rocket engines operate on chemical propellants. The principal energy coefficient (specific impulse) is determined by the amount of heat released in oxidation, decomposition, or recombination (the calorific value) and by the chemical composition of the reaction products. The latter dictates the degree of conversion of thermal energy into kinetic jet energy (the specific impulse increases with decreasing molecular weight).
Chemical propellants are classified as monopropellants, bipropellants, tripropellants, and multipropellants, depending on the number of separately stored components. Depending on the state of aggregation, propellants may be classified as liquid, solid, hybrid, pseudoliquid, or jellied propellants, including thixo-tropic propellants (thickened, gel-like propellants whose viscosity decreases sharply with a pressure gradient). The state of aggregation determines the engine’s design, characteristics, and area of efficient use. Liquid and solid propellants are the most commonly used.
Under operating conditions, all the components of a liquid propellant are located in tanks on board the rocket and are fed separately into the combustion chamber using pumps or the pressure of a compressed gas. Liquid propellants must satisfy several specific requirements. They must remain liquid over the widest possible temperature range, and at least one of the propellant components must be suitable for cooling the engine; that is, the component must have a high thermal stability, a high boiling point, and a high specific heat. A high-performance pressurizing gas must be obtainable from the principal components (oxidizer and fuel). The propellant components must have a minimal viscosity, and the viscosity must not vary greatly with temperature. Bipropellants, consisting of an oxidizer and a fuel (see Table 1), are the most commonly used liquid propellants.
Various additives, introduced into the propellant either in the form of solutions or suspensions or as a third component, may be added to improve engine characteristics. These may include metals, for example, Be, Al, B, and their hydrides, used to improve the specific impulse, components used to obtain a pressurizing gas if the principal components are not suitable for this purpose, corrosion inhibitors, stabilizers, ignition promotors, and substances that lower the freezing point. An oxidizer and a fuel that react upon contact in the liquid state and cause ignition of the mixture constitute a hypergolic propellant. The use of such propellants simplifies the design of the rocket engines and is the simplest way to provide for engine restarts. Space rocketry is based on the use of highly efficient liquid propellants.
Liquid monopropellants, such as hydrogen peroxide and hydrazine, release energy upon decomposition and may be used for auxiliary engines and for obtaining the pressurizing gas needed to drive the turbopump assembly.
Solid propellants are homogeneous mixtures of components—double-base propellants—or heterogeneous compositions cast to shape—composite propellants. Composite propellants may consist of an organic fuel-binder (for example, rubber, polyurethane, or polyester or epoxy resins), a solid oxidizer (usually ammonium perchlorate, although potassium perchlorate and ammonium nitrate are used as well), and additives for various purposes (for example, Al, Mg, Be, and B powders for improving energy characteristics). The fuel-binder facilitates the formation of a uniform grain and determines the propellant’s physicochemical properties and the method by which the grain is formed. The major specific requirements for solid propellants include uniformity in the distribution of the components and, as a result, constant physicochemical and energy properties in the grain. Other major requirements are a constant, uniform combustion in the rocket’s combustion chamber and mechanical properties that ensure efficient engine operation under overload conditions, varying temperatures, and vibration.
Solid propellants are inferior to liquid propellants as regards specific impulse because chemical incompatibility sometimes prevents the use of high-energy components in the composition of solid propellants.
In hybrid propellants, the components are present in different states of aggregation, for example, a liquid oxidizer and a solid fuel or a solid oxidizer and a liquid fuel. All the components of liquid and solid propellants may be used as components of hybrid propellants. On the basis of specific impulse, these propellants occupy an intermediate position between liquid and solid propellants.
REFERENCESSarner, S. Khimiia raketnykh topliv. Moscow, 1969. (Translated from English.)
Termodinamicheskie i teplofizicheskie svoistva produktov sgoraniia: Spravochnik, fasc. 1–8. Edited by Academician V. P. Glushko. Moscow, 1971–74.
Kosmonavtika, 2nd ed. Edited by Academician V. P. Glushko. Moscow, 1970. (Malen’kaia entsiklopediia.)
|Table 1. Major characteristics of some possible high-efficiency liquid bipropellants at the optimum ratio of components1|
|Oxidizer||Fuel||Fuel density2 (g/cm3)||Temperature in combustion chamber (°K)||Specific impulse in a vacuum3(sec)|
|1Pressure in the combustion chamber is 10 meganewtons/m2 (100 kg-force/cm2); pressure at the nozzle section is 0.1 meganewton/m2 (1 kg-force/cm2) 2The calculated value is the ratio of the total mass of the propellant components (oxidizer and fuel) to the volume of the components 3Specific impulse of the engine at an ambient pressure equal to zero|
|Liquid oxygen||Liquid hydrogen||0.3155||3250||428|
|Liquid fluorine||Liquid hydrogen||0.621||4707||449|