In liquid rocket propellants the heat to be converted to work is most frequently.obtained as a result of the chemical reaction of combustion, i.e., a turbulent reaction from the oxidation of one-substance by another. The oxidizing substance is called oxidizer, and the oxidized substance is called fuel. In other heat engines using liquid propellant, including ram-jet engines, the vehicles carries only the
fuel while the oxidizer (air) is taken from the surrounding medium. Vehicles driven by liquid propellant rocket engines (LPRE) carried both the fuel and the oxidizer. Hence the propellant of, an LPRE is understood to be the combination (propellant composition) of substances participating in the chemical reaction releasing heat.
The substances entering into a propellant composition are called components. Bipropellants consist of two separately stored fluids (components) of which one is the oxidizer (liquid oxygen, nitric acid, liquid fluorine, etc.) and the other the fuel (kerosene, alcohol, etc.). Heat can be obtained, however, not only from the chemical reaction of combustion but also from such other chemical reactions as, e.g., the decomposition of certain chemical compounds, and others. When using the decomposition reaction to release heat in the LPRE we can obtain a propellant consisting only of one component and which is called a monopropellant. Another method for creating monopropellants is to prepare mixtures of fuel and oxidizer which under normal conditions do not ignite spontaneously and can therefore be carried in a single container. Frequently various admixtures are introduced in the propellant components, e.g., to create better conditions for the flow of the reaction, or to slow it down or stop it all together. In the former case they are called catalyzers, and in the latter inhibitors. Depending on whether the propellant starts burning with or without an external flame, they are classified as hypergolic and anergolic propellants.
The choice of the propellant for the LPRE depends on the purpose of the engine and of the rocket itself, and also on the state at which rocket technolog finds itself at that time. To understand these questions we must acquaint ourselves with the design of the engine and study some general problems relating to the use of rocket propellants LPRE's.