Monopropellant

Monopropellants are propellants consisting of chemicals that do not require an oxidizer to release their stored chemical energy. While stable under defined storage conditions, they decompose very rapidly under certain other conditions to produce a large volume of energetic (hot) gases for the performance of mechanical work. Although solid deflagrants such as nitrocellulose, the most commonly used propellant in firearms, and ammonium perchlorate/aluminum/synthetic rubber, widely used in military and spacecraft boosters, could be thought of as monopropellants, the term is usually reserved for liquids in engineering literature. Monopropellants release their energy through exothermic chemical decomposition. The molecular bond energy of the monopellant 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.

Uses
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 iridium catalyst bed (Hydrazine is pre-heated to keep reactant liquid) to produce the desired jet of hot gas and thus thrust. Hydrogen peroxide has been used for main thrusters and 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 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" or 1,3-propanediol 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 diethylene 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.

Hydrazine, ethylene oxide, hydrogen peroxide, (especially in its German World War II form as T-Stoff) and nitromethane are common rocket monopropellants. As noted the specific impulse of monopropellants is lower than bipropellants and can be found with the Air Force Chemical Equilibrium Specific Impulse Code tool.

Newer monopropellants include nitrous oxide fuel blends. Nitrous oxide may be blended with a range of different fuels and emulsifiers and used as a rocket monopropellant.

Nitrous oxide monopropellants decompose into oxygen-rich air, and they require no separate supply of oxidizer such as the liquid oxygen used in many high performance rocket engines today.