How Does a Rocket Work?
"Rocket engines generate thrust by putting a gas under pressure. The pressure forces the gas out the end of the rocket... [This] is called exhaust. As it escapes, the exhaust produces thrust according to ... [Issac] Newton's third law of motion [which] states that for every action, there is an equal and opposite reaction. Thus, as the rocket pushes the exhaust backward, the exhaust pushes the rocket forward... The amount of thrust produced by a rocket depends on the momentum of the exhaust—that is, its total amount of motion. The exhaust's momentum equals its mass (amount of matter) multiplied by the speed at which it exits the rocket. The more momentum the exhaust has, the more thrust the rocket produces. Engineers... increase a rocket's thrust by increasing the mass of exhaust it produces... [They also] can increase the thrust by increasing the speed at which the exhaust leaves the rocket." (Heister, Stephen)