chemical rocket generally uses a fuel source and an oxidizer .there are three primary THINGS that make up a rocket and with it, rocket fire the combustion chamber the throat and the nozzle. this combustion chamber, throat, and nozzle create thrust by the Newtonian principle that to every action there is an equal and opposite reaction. gases leaving the rocket engine are pushing downward, which in turn forces the vehicle upward. the engine is using the mass and velocity of the fuel it burns to create thrust. Even though the fuel starts as a liquid and the combustion turns to gas the and mass of fuel will Stay the same. it is the massive accelerations of this transformation from liquid to gas that causes the momentum