A fuel containing a metal of high heat of combustion as a principal constituent. High propellant 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 lower atomic-weight metals in the upper left-hand corner of the periodic table. The preferred metals include lithium, beryllium, boron, carbon, magnesium, and aluminum.
The metal with the highest heat release for all applications is beryllium. However, beryllium is extremely poisonous and prohibitively expensive ($150/lb!), so other metals are used. Boron shows essentially equivalent performance for air-breathing engines. Lithium, boron, and aluminum work almost as well as beryllium in rockets (aluminum is used in the SRB's of the Space Shuttle). Aluminum, magnesium, and carbon enjoy the advantage of being relatively low cost, and carbon enjoys 100% gaseous exhaust products, which adds to the working fluid mass, and reduces two-phase flow losses and entropy-induced endothermic decomposition reactions that plague performance in the other metals.