Chemical Liquid Engines can have one or more propellants combined to produce the chemical reaction. There are several examples of monopropellants. Some such as the Hydrazine (N2H4 ISP around 220) are commonly used on satellites and spacecraft. Others like Hydrogen Peroxide (H2O2 ISP around 160 ), Nitrous Oxide (N2O ISP around 180) and assorted blends, are rarely used. The environmental toxicity of N2H4 has seen a new interest in blends of older, non-toxic propellants. Monopropellants have a real advantage in that the mixture ration is nearly always correct, the plumbing is half as complicated, and generally there are half as many tanks. Some of the Blends may have an ISP as high as 300 sec. At this level, meaningful spacecraft propulsion is possible.
Bipropellant combinatins that can be stored in a reasonable environment generally can be one of the following oxidizers burned with one of the following fuels:
Reasonable performance storable Oxidizers: Nitrogen Tetraoxide (N2O4), Hydrogen Peroxide (H2O2), Nitrous Oxide (N2O), ???
Reasonable Performance storable Fuels: Kerosine style fluids (n*CH1.72) Methanol (CH3OH) Ethanol (C2H5OH) Hydrazine/UDMH/MMH+ (N2H4), ???
The major differences are the freezing/boiling point, performance, and relative toxicity. Toxicity can range from 'if you can smell it you are already dead' to 'addictive qualities in a social environment'. The toxicity has a tendency to drive the acquisition and operational cost up considerably.
We have experience in using all these propellants with the exception of N2O4 and/or N2H4 in any way due to toxicity and cost. They are probably ruled out at this stage.