An aircraft propulsion system comprises an engine and a propeller or a propulsive nozzle which converts motion from an engine and generates thrust. NASA defines the propulsion as a machine that produces thrust to push an object forward. The thrust is generated when the engine is accelerated by gas or any other working fluid which makes the engine produce the force. The thrust generated by the propulsion system, first of all, is responsible for drag balance of the aircraft when it is cruising. Secondly, thrust from the propulsion system must exceed the drag of the aeroplane for the aeroplane to accelerate.
Aircraft propulsion has a wide variety of systems available. The choice depends on the speed or Mach number required and the role for which the aircraft is to be used. Take a look at the chart of civil transport applications for the various propulsion systems described: