The principal equation of rocket dynamics is here derived from the first formulation of the second basic Newton’s law of motion, where no assumptions concerning the constancy of mass are made. In this way the following has been avoided: first, the application of the theorem on the conservation of the quantity of motion, and second, the application of the principle of reaction or the theorem on the impact of perfectly non-elastic be bodies, both of which cannot taken into account without the assumption of solid contact.