The potential energetic performance of fuel ingredients for the use in the solid fuel ramjet is presented in terms of the enthalpy of combustion per unit mass and per unit volume of the fuel. Combustion phenomena and combustion efficiency are briefly discussed, but are not accounted for in evaluating the theoretical fuel performance. Practical considerations rule out the use of beryllium, which is extremely toxic, and metal hydrides (especially boranes) which are both toxic and great fire hazards. Compared with hydrocarbons, some metals and metal compounds have somewhat higher energy per unit mass, but up to about three times higher energy per unit volume. Boron has the highest energy density (energy per unit volume) of all elements, while among all other fuel candidates only boron compounds exhibit similar potential performance. Other possible high performance fuel candidates are boron carbide, which is the least expensive source fo boron, A1B12 (alloy) with energy density slightly exceeding that of elemental boron, and the high borides of magnesium and silicon
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