The rocket engine works on both liquid and solid fuel or combination of both. Liquid rocket engines further subdivided into two types mono-propellant and bi-propellant systems.Mono-propellant engines operate either as a simple cold gas system or apply a catalyst for an exothermal decomposition of the propellant, such as hydrazine (N2H4) or laughing gas (N2O).
Generally, these type of engines are only in use for low thrust satellite propulsion systems.Typical bi-propellant engines use either earth storable propellants, generally combinations of nitric acid or its anhydride with derivates of hydrazine, asymmetric di-methyl-hydrazine or mono-methyl-hydrazine (MMH), mixtures of storable and cryogenic propellants, liquid oxygen and kerosene or fully cryogenic, liquid oxygen and liquid hydrogen.
Although rocket engines show large differences depending on mission profile and staging of the launcher it is possibly useful to separate them into four major classes.
2. main stage
3. upper stage engines