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Monopropellants are chemicals or mixtures of chemicals which can be stored in a single container with some degree of safety. While stable under defined storage conditions, they react very rapidly under certain other conditions to produce a large volume of energetic (hot) gasses for the performance of mechanical work. Although solid deflagrants such as nitrocellulose, the most commonly used propellant in firearms, and ammonium perchlorate/aluminum, widely used in military and spacecraft boosters, are technically monopropellants, the term is usually reserved for liquids in engineering literature. These can either be a single chemical that can be made to decompose exothermically, or a mixture of chemicals (generally a fuel and an oxidizer) that can be made to react with one another to release energy.
The most common use of monopropellants is in low-impulse rocket motors, such as reaction control thrusters, the usual propellant being hydrazine, or its variants (such as dimethylhydrazine) and occasionally hydrogen peroxide (specifically, high test peroxide), both of which are generally decomposed by exposure to a platinide catalyst bed (often pre-heated to accelerate the reaction) to produce the desired jet of hot gas and thus thrust. Monopropellants are also used in some air-independent propulsion systems (AIP) to "fuel" reciprocating or turbine engines in environments where free oxygen is unavailable. This application is almost exclusively military, the prime example being a subset of torpedoes where high speed and/or long range is required, and the emission of (copious) exhaust gas bubbles not a consideration. 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". The most recent models of the British Spearfish torpedo (see Torpedo and the references therein for further discussion) are powered by an admixture of this with hydrogen ammonium perchlorate (HAP), a solution that, excluding future developments, would appear to be very nearly optimal. A potential future use for monopopellants not directly related to propulsion is in compact, high-intensity powerplants for aquatic or exoatmosperic environments.
Research in brief
Much work was done in the US in the 1950s and 1960s to attempt to find better and more energetic monopropellants. For the most part, researchers came to the conclusion that any single substance that contained enough energy to compete with bipropellants would be too unstable to handle safely under practical conditions. With new materials, control systems and requirements for high-performance thrusters, engineers are currently re-examining this assumption. Many partially nitrated alcohol esters are suitable for use as monopropellants. Trimethylene glycol dinitrate is isomeric with PGDN, and produced as a fractional byproduct in all but the most exacting laboratory conditions; the marginally lower specific gravity (and thus energy density) of this compound argues against its use, but the minor differences in chemistry may prove useful in the future. The related dinitrodiglycol, more properly termed diethyl glycol dinitrate in modern notation, was widely used in World War 2 Germany, both alone as a liquid monopropellant and colloidal with nitrocellulose as a solid propellant. The otherwise desirable characteristics of this compound; it is quite stable, easy to manufacture, and has a very high energy density; are marred by a high freeze point (-11.5 deg. C) and pronounced thermal expansion, both being problematic in spacecraft. Dinitrochlorohydrin and tetranitrodiglycerin are also likely candidates, though no current use is known. The polynitrates of long chain and aromatic hydrocarbons are invariably room temperature solids, but many are soluble in simple alcohols or ethers in high proportion, and may be useful in this state.
There is an entire chapter on the history of monopropellant development in the book Ignition! An Informal History of Liquid Rocket Propellants (ISBN 0-8135-0725-1) by John D. Clark, first published in 1972.
The book "Germany's Secret Weapons In World War Two" by Roger Ford (ISBN 0-7603-0847-0 c.2000) contains some useful information on the surprising diversity of fuels and propellants employed by wartime Germany.
"The Chemistry Of Powder And Explosives" by Tenney L. Davis is an outstanding, if outdated, source of information on a great many aspects of high enthalapy compounds. (This work originally published by MIT Press, 1943, as a textbook. Subsidy republication as late as 1995 by Pyrotek Inc., an amateur rocketry supply house. No catalog data given in this edition. Current publication status unknown.).