The most common liquid rocket fuels for launch vehicles are hydrogen, kerosene, and a type of hydrazine mixture. Each fuel has advantages and disadvantages. Hydrogen, used for Delta IV, is the highest performing fuel (i.e., most efficient) and a good coolant for combustion chambers; however, its low density (more than 10 times lower than kerosene) results in large propellant tanks and its 20 Kelvin liquid temperature presents operability challenges. Kerosene, used for Atlas V, can be stored at room temperature, costs much less than hydrogen or hydrazine, and is easy to pump; on the other hand, it offers the lowest performance of the three fuels. The hydrazine mixture, used on Titan rockets, is relatively easy to store and has a performance similar to kerosene; however, it is lowest on the operability scale because it's highly toxic and expensive.
Liquid oxygen and nitrogen tetroxide are common oxidizers. Liquid oxygen is typically used with kerosene and hydrogen fuels. This cryogenic fluid cannot be stored for long periods in current systems but is relatively inexpensive. Nitrogen tetroxide, used with the hydrazine mixture, is storable but expensive and toxic. Hydrogen peroxide has been reconsidered recently as a rocket oxidizer because its ease of storage and low toxicity may improve operability; however, it offers lower performance and has strict cleanliness requirements.
Clearly, the best fuels and oxidizers would be inexpensive, easy to store, highly efficient, relatively dense, easy to pump, and provide good cooling capabilities. Not all of these characteristics can be achieved at once; however, Air Force researchers are seeking to develop new fuels, including high performing synthetic kerosenes and nontoxic storables.