1.3 THEORY OF JET PROPULSION

 

 THEORY OF JET PROPULSION    

1.3.1 General: Jet propulsion is the method of producing propulsive force in a device by the reaction of an accelerating mass of air (or gas) expelled out through a nozzle in the form of a jet. The generated propulsive force is used to propel the device (or the aircraft) forward in the air.

 

1.3.2 Basic Principle: Jet propulsion is a practical application of Sir Isaac Newton's 3^RD Law of Motion which states that: "For every action, there is an equal and opposite reaction."

For aircraft propulsion, the 'body' is atmospheric air that is accelerated as it passes through the engine. The force applied to the air giving this acceleration (or changing momentum) has an equal effect in the opposite direction onto the engine. The effect by the accelerating air coming out of the engine through its propelling nozzle in the form of a jet is the ‘jet reaction’ which is conventionally termed as the ‘thrust’.

Jet reaction is an internal phenomenon and does not result from the pressure of the jet acting on the atmosphere as shown in balloon example, Figure 1.4, depicting a non-mathematical or mechanical approach of justifying jet-propulsion. A turbo-jet engine could be considered as such an arrangement as the compressor and combustion chamber sections having high pressure air acting on all surfaces, this pressure being dropped through the exhaust pipe, hence, unbalance pressure forcing the engine forward internally similar to the toy balloon. 

1.3.3 Operating Principle: To have jet propulsion based on Newton's Third Law, jet-engines are designed for producing high-velocity gases at the jet-nozzle. To achieve this, a jet-engine first compresses air. Heat is then added to the compressed air in the combustion chamber by burning fuel to produce hot expanding gases that rush towards the rear of the engine and finally escapes through jet-nozzle in a form of high-velocity ‘kinetic jet’. 

All kinds of jet engines, like turbo-jets, ram-jets, pulse-jets etc are designed for the sole purpose of producing high-velocity gases at the jet-nozzle so that reaction forces come into play as a result of jet-reaction. But, propulsive force is also possible by propellers and fans. The basic principle is same, that is, accelerating or changing momentum of air. So, these are also called prop-jets and fan-jets, similar to the turbo-jets. 

Figure 1.4: Toy-balloon experiment (explaining jet-propulsion as in internal phenomena)