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, 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.
Read more about this topic: Monopropellant
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