Jet propulsion Engines - The Secret Behind Airplanes Thrust

Jet propulsion accounts for the thrust of airplanes as they are high up in the sky. Its mode of operation is similar to the release of an inflated balloon. The pressure inside the balloon is pushing in all directions. It is also "jetting" out from the mouth of the balloon. The end of the balloon opposite to the mouth is not open. This creates an imbalance and causes the balloon to move in the direction away from the open mouth. Jet engines work in a similar fashion.

There are several types of jet engines and these various types though working on the same principles have varying configuration we must know. The types of jet engines includes

• Ramjet
• Pulsejet
• Turbojet
• Turbofan
• Turboprop

 The last three are the most widely used.

The ramjet

This is as simple a jet engine as can be found. Air enters the inlet and is compressed by ramming effect of the being squeezed into a smaller space due to airflow tunnel configuration. The weakness of this engine is that the air coming in the inlet must be traveling at a very high speed (supersonic) for good efficiency. A ramjet does not work well at low speeds. This is simply not practical for most flying situations.

This raises the pressure of the air. As the air arrives at the combustion chamber, fuel is added and an electric spark is generated. This causes a controlled explosion that raises the temperature and the pressure of the air tremendously. The hot, high-pressure air "jets" out the nozzle of the engine providing the forward thrust. This seems so simple, why would anyone want a more complex engine?

The pulse jet

This jet engine type solves the problem of requiring supersonic speeds. It works well at a lower speed and with a little help, can get started when it is standing still. They are very noisy and inefficient. They are the gas guzzlers of the aviation world.The hot, high-pressure gas then "jets" out the exhaust nozzle. The cycle of air in, doors closed, air out, then repeat, is where this engine gets its name. Pulse jets are not widely used for two reasons.

It is much like the ramjet, except that it has doors that close the inlet. When the doors are open, the air flows in and is compressed. The doors then close, forming a chamber in which the fuel is ignited.

The turbojet

This was the first really useful jet engine to be built. The air flows into the engine through the inlet. The design of the inlet makes the air slow down and also raises the pressure. The air then goes through the compressor where sets of blades compress the air even more, greatly raising the pressure. The air then enters the combustion chamber where the fuel is added and ignited. The very hot, high-pressure air rushes past the turbine blades making them spin very fast. The turbine blades are connected back to the compressor blades by a shaft. The turbine blades take some of the energy from the air and returns it to the compressor. The hot, high pressure air that gets past the turbine, "jets" out the exhaust nozzle thrusting the engine forward.


To increase the thrust available, a device called an afterburner is sometimes built into the engine. Fuel is dumped into the hot exhaust gas exiting the nozzle causing another controlled explosion. This makes the air even hotter which adds more energy to it, thereby increasing the thrust. This is not an efficient thing to do however, and is only done for brief periods when extra thrust is needed, for example, on takeoff or when a burst of speed is needed during a dog fight, or when an extra push is needed to reach supersonic speed. You may have seen movies with high performance jets, like the F-14. If you watch one of these jets from the back, and the pilot turns on the afterburners you will hear a burst of noise and see an orange glow around the outlet of the engines. The airplane will then shoot up into the sky.

The turbofan

This jet engine is a refinement to the turbojet that results in a more efficient engine. A large set of fan blades is set right in the front of the inlet. The fan works much like a propeller, thrusting the engine forward, pushing a large amount of air backwards. As the air is pushed back by the fan some of it goes into the engine and some bypasses the engine. The engine that sits behind the fan is basically a turbojet. The air that goes into this engine receives the same treatment as air that goes through the turbojet. The turbine in a turbofan drives the fan as well as the compressor. The air that "jets" out the back of this engine has less thrust than air that exits a turbojet, but that decrease in thrust is made up for by the thrust generated by the fan. A turbofan engine actually is more efficient than a turbojet and is quieter as well. Afterburners can be fitted to a turbofan if required.

The turboprop

This jet engine is essentially a turbofan engine where the fan is replaced by a propeller. The propeller is placed outside of the inlet. A gearbox is introduced which controls the spinning of the shaft, enabling speed control for the propeller. This is the most efficient means of propulsion, however it is limited in forward speed. Because the propeller is out in the free stream air, not mounted in the inlet (where the air speed is reduced) the propeller has to rotate at faster speeds. The speed of the propeller approaches the speed of sound well before the airplane itself. As the airplane approaches the speed of sound, drag greatly increases. So the speed of the airplane must be kept well below the speed of sound to prevent the tips of the propeller from going too fast.

If you want to fly at moderate speeds efficiently, then turboprops are a good choice. If you want to fly fast, but subsonically, then turbofans are a good choice. If you want to fly supersonically then a turbofan with afterburner is a good idea. If you want to fly slowly and only have a small budget or a small airplane, then a piston engine is a good choice.

Reference source: virtualskies.arc.nasa.gov