Published on Jan 28, 2016
Jet Propulsion is the thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high-velocity liquid or gaseous stream.A simple example of jet propulsion is the motion of an inflated balloon when the air is suddenly discharged.
While the opening is held closed, the air pressure within the balloon is equal in all directions; when the stem is released, the internal pressure is less at the open end than at the opposite end, causing the balloon to dart forward. Not the pressure of the escaping air pushing against the outside atmosphere but the difference between high and low pressures inside the balloon propels it.
An actual jet engine does not operate quite as simply as a balloon, although the basic principle is the same. More important than pressure imbalance is the acceleration due to high velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the gas to flow backward forming the jet. Newton's second law shows that these forces are proportional to the rate at which the momentum of the gas is increased. For a jet engine, this is related to the rate of mass flow multiplied by the rearward-leaving jet velocity.
Newton's third law, which states that every force must have an equal and opposite reaction, shows that the rearward force is balanced by a forward reaction, known as thrust. This thrusting action is similar to the recoil of a gun, which increases as both the mass of the projectile and its muzzle velocity are increased. High-thrust engines, therefore, require both large rates of mass flow and high jet-exit velocities, which can only be achieved by increasing internal engine pressures and by increasing the volume of the gas by means of combustion.
Jet-propulsion devices are used primarily in high-speed, high-altitude aircraft, in missiles, and in spacecraft. The source of power is a high-energy fuel that is burned at intense pressures to produce the large gas volume needed for high jet-exit velocities. The oxidizer required for the combustion may be the oxygen in the air that is drawn into the engine and compressed, or the oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding atmosphere. Engines that depend on the atmosphere for oxygen include turbojets, turbofans, turboprops, ramjets, and pulse jets. Non atmospheric engines are usually called rocket engines.
A turbofan engine is the most modern variation of the basic gas turbine engine. As with other gas turbines, there is a core engine. In the turbofan engine, a fan in the front and an additional turbine at the rear surrounds the core engine. The fan and fan turbine are composed of many blades, like the core compressor and core turbine, and are connected to an additional shaft.
As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two-spool engine (one "spool" for the fan, one "spool" for the core.) Some advanced engines have additional spools for sections of the compressor, which provides for even higher compressor efficiency.
The force produced by a jet engine is expressed in terms of kilograms of thrust. This is a measure of the mass or weight of air moved by an engine times the acceleration of the air as it goes through the engine. Technically, if the aircraft were to stand still and the pressure at the exit plane of the jet engine was the same as the atmospheric pressure, the formula for the jet engine thrust would be:
Weight of air in kilograms per second * velocity
Thrust = ___________________________________________
9.81 (normal acceleration due to gravity, in meter per second 2)
Imagine an aircraft standing still, capable of handling 97.522 kilograms of air per second. Assume the velocity of the exhaust gases to be 1,500 feet per second. The thrust would then be:
Thrust = 97.522 kg of air per second * 457.2 m / s
9.81 m / s 2
= 9.941 * 457.2
Thrust = 4545.025 kg.