The present invention relates to a propulsion engine mounted on a spacecraft. In particular, the present invention relates to a propulsion engine with which a propellant based on a new concept is selected and loaded for a simple and inexpensive small satellite.　The present invention provides a method for forming a new propellant from a mixture, i.e., a cocktail of a plurality of polar liquids, it being possible to actively manage the boiling point and melting point of the propellant by adjusting vapor pressure in the temperature ranges of a pre-launch propellant-filling work environment and an in-orbit operating environment. This mixed propellant leads to ensuring work safety because: (1) the polar liquids are mutually soluble with extremely high solubility; and at the environment temperatures, (2) the propellant is stored and transported while kept in a liquid phase, i.e., (3) freezing is avoided, and (4) boiling is avoided. In addition, due to the use of the mixed propellant, (5) the propellant can be completely vaporized without leaving any residue at a practically low temperature, and it is therefore possible to (6) maintain mutual concentration ratios of the constituent polar materials irrespective of propellant consumption.