Aircraft generally derive their propulsion from fuel-fed heat engines whose power is fed to a propulsor. The propulsor accelerates a stream of air through the engine (as in the case of turbojets, turbofans, and ramjets) or around the aircraft (as in the case of helicopter rotors and propeller rotors) in a direction rearward of the flight direction. The integrated pressure forces impacting on the surfaces of the propulsive machinery required to accelerate the propulsive stream ultimately react on the aircraft to propel it in the flight direction. This is an application of Newton's second and third laws: The force required to accelerate a mass flow is proportional to the quantity of the mass flow multiplied by the rate of its acceleration, and for every action (of accelerating the mass flow) there is an equal and opposite reaction (on the engine and ultimately on the aircraft).