Rocket engines are reaction engines, in that they throw mass, generally in the form of a hightemperature and high-pressure gas, in one direction to create a reactionary thrust in the opposite direction. The physics of this reaction is simply F=ma, or force equals mass times acceleration. The mass comes from the fuel that the rocket engine burns. The burning process accelerates the mass down through the nozzle of the rocket engine, causing force to push the rocket skyward. The kinetic energy of the combustion products causes the thrust. At this point, the science and art of rocket-engine design come into play.
