Metal-base fuel

A fuel containing a metal of high heat of combustion as a principal constituent. High propellent performance in either a rocket or an air-breathing engine is obtained when the heat of combustion of the fuel is high. Chemically, high heats of combustion are attained by the oxidation of the low-atomic-weight metals in the upper left-hand corner of the periodic table. The generally preferred candidates are lithium, beryllium, boron, carbon, magnesium, and aluminum.

The metallized additive can be used in either a liquid or solid propellent. When the pure metal is added to liquid fuels, an emulsifying or gelling agent is employed which maintains the particles in uniform suspension. When used in composite solid rocket propellents, the metal powder is usually mixed with the oxidizer and unpolymerized fuel, and the propellant is then processed in the usual way.

Early compounding of metallized propellents employed the free metal itself. As a result of extensive research in metalloorganic compounds, however, several classes of metallic compounds have been employed. Two major reasons for the use of such compounds are that the solubility of the metal in the fuel can be realized, resulting in a homogeneous propellant, and performance higher than that for the pure metal can be obtained in some cases.

The major classes of metallic compounds of interest as high-performance propellents include the hydrides, amides, and hydrocarbons. Additional classes include mixtures either of two metals or of two chemical groups, such as an amine hydrocarbon.

Most metallic fuels are costly and many, because of particle-size requirements or synthesis in a specific compound, are in limited supply. The combustion gases all produce smoky exhausts which may be objectionable in use. Engine development problems are also increased because of the appearance of smoke and deposits in the engine.