Monopropellant

Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with bipropellants that release energy through the chemical reaction between an oxidizer and a fuel. While stable under defined storage conditions, monopropellants decompose very rapidly under certain other conditions to produce a large volume of energetic (hot) gases for the performance of mechanical work. Although solid such as nitrocellulose, the most commonly used propellant in firearms, could be thought of as monopropellants, the term is usually reserved for liquids in engineering literature.

The most common use of monopropellants is in low-impulse rocket motors, such as reaction control thrusters, the usual propellant being hydrazine which is generally decomposed by exposure to an iridiumcatalyst bed (the hydrazine is pre-heated to keep the reactant liquid). This decomposition produces the desired jet of hot gas and thus thrust. Hydrogen peroxide has been used as power source for propellant tank pumps in rockets like the German WWII V-2 and the American Redstone. The hydrogen peroxide is passed through a platinum catalyst mesh, or comes in contact with manganese dioxide impregnated ceramic beads, or Z-Stoff permanganate solution is co-injected, which causes hydrogen peroxide to decompose into hot steam and oxygen.

Monopropellants are also used in some air-independent propulsion systems (AIP) to "fuel" reciprocating or turbine engines in environments where free oxygen is unavailable. Weapons intended primarily for combat between nuclear-powered submarines generally fall into this category. The most commonly used propellant in this case is stabilized propylene glycol dinitrate (PGDN), often referred to as "Otto fuel". A potential future use for monopropellants not directly related to propulsion is in compact, high-intensity powerplants for aquatic or exoatmospheric environments.

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