Jet Propulsion and Types of Jet Engines
In this article you learn what is jet propulsion and types of jet engines its construction and working principle.
The term propulsion means moving forward. A jet propulsion engine is a form of the internal combustion engine in which hot gases are produced by combustion of fuel. The jet propulsion engine is in the form of the internal combustion engine, therefore, the hot gases produced by the combustion of fuel is with the compressed air.
It works on Newton’s third law, the moving body is propelled in the opposite direction to the jet. It is most commonly used in the jet engine but is also the most common means of spacecraft propulsion. This jet using for propelling the vehicle in which the system is mounted, such systems are known as the jet propulsion system.
The hot gases produce are discharge through a nozzle in the form of a jet with a very high velocity. Which produces a required thrust in opposite direction known as a propulsive force, which is responsible for moving the vehicle.
For running the propulsion engines, the Oxygen is required. The atmospheric air is used by the engine up to a certain height above the ground level. But beyond that limit, there is no air, in such case, the propulsion engine takes the oxygen from the oxygen carrier along with the engine.
The jet propulsive engine may divide into two distinct types,
- That which uses atmospheric air as its oxidant.
- That which carries oxygen for its own requirement. (This type of propulsive engine is called rocket motor).
In both the cases, the fuel burns inside a combustion chamber and the hot gases thus produced are discharged at a high velocity through a nozzle. As a result, thrust is produced a direction opposite to the direction of discharge. This reactive force moves the propulsive unit at a very high velocity.
Types of Jet Engines
Following are the types of jet engines.
- Ram Jet Engine
- Turbo Jet Engine
- Rocket Motor
1. Ram Jet Engine
In these types of jet engines, the ramjet is the simplest of all the propulsive units. Ramjet requires neither a compressor nor turbine. The term ramjet is short of ram-air compression. The ramjet is the simplest form of a jet engine ’cause it has no moving parts’.
It consists of only three main components such as a diffuser, combustion chamber, and the expansion nozzle. The pressure of the incoming air increases to a high value by the diffuser.
This kind of engine is required a hollow tube into which fuel is injected, mixed with air, and burnt to produce thrust, as illustrated below. The ramjet only works when it is already moving fast enough that the incoming air is compressed simply by being forced into the engine.
This behaviour is called the ram effect because when a volume of air is forced into a small space at high enough speeds, it is compressed to a higher pressure.
This compression is completed in the diffuser section where the incoming air is squeezed into a small area and compressed to a high enough pressure that it can be burned with fuel.
Once the fuel injects the mixture is fed into a combustion chamber where the fuel-air mixture is ignited to produce a high-speed exhaust. Since a ramjet must already be travelling at high speeds before it will start working, a ramjet-powered aircraft is unable of taking off from a runway under its own power.
That is the advantage of the turbojet, which is a member of the gas turbine family of engines. A turbojet operates much like a ramjet except that it does not trust purely on the motion of the engine to compress the incoming air flow.
Instead, the turbojet contains some rotating machinery that compresses incoming air and allows the engine to function during takeoff and at slow speeds.
1.1 Working Principle
At the start, the ramjet is moved and accelerated to about 500km/h by some booster device with maybe a small turbo-jet unit.
After high speed is attained in the ramjet, air enters into the diffuser with the speed of the ramjet. The function of the diffuser is to convert the KE of entering air into its pressure energy.
The fuel is now injected through the fuel injection nozzle and ignited by means of a spark plug. The hot gases are produced, now expand in the expansion chamber and ultimately leave the propulsive unit through the expansion nozzle.
As the exhaust gases pass through the nozzle, its pressure energy is converted into KE and the gases leave the nozzle with very high velocity producing a reactive force or trust in the opposite direction.
Ramjet engine works with efficiency only if its speed is supersonic. For this reason, this engine is not used in aircraft, It is used in the missile.
1.2 Advantages of Ramjet Engine
- It is lighter in weight.
- It has no internal moving parts.
- The wide variety of fuels can be used.
- Easy to maintenance.
- It has a low cost.
- High efficiency at supersonic speed.
- It is simple in construction.
1.3 Disadvantages of Ramjet Engine
- It cannot be started from rest without any booster.
- They have to be accelerated to a certain flight velocity by some launching device.
- It is always equipped with a small turbo-jet which helps in starting the ramjet.
2. Turbojet Engines
This types of jet engines take its oxygen supply from the surrounding atmosphere. The turbojet engine contains a diffuser at the front end, an expansion nozzle (also called propelling nozzle) at the rear end. The energy producer which consists of a combustion chamber, a fuel pump, a gas turbine, and an air compressor.
The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where i is accelerated to high speed to provide propulsion.
2.1 Working Principle
Atmospheric air enters into the diffuser with the same velocity as that of the propulsion unit. The function of the diffuser is to convert the KE of entering the air into pressure energy of it.
High-pressure air now enters into the compressor where its pressure is further increased and its temperature also increases correspondingly. The high pressure and high temperature compressed air now enters into circular combustion in which liquid fuel is injected through a ring by means of a pump.
In the combustion chamber, the fuel burns at constant pressure by coming in contact with hot compressed air. Hot gases thus produced in the combustion chamber, now enter into a gas turbine in which it expands producing power.
It is with this power that the compressor and the fuel pump are operated. The exhaust gases from the turbine now pass into the atmosphere through the expansion nozzle.
As the exhaust gases pass through the nozzle, its pressure energy is converted into KE, and the gases leave the nozzle with a very high velocity producing a reactive force or thrust in the opposite direction. This thrust carries the propulsive unit forward.
To start the unit, a starting device such as the electric motor is required. Turbojet engines are used in aircraft.
2.2 Advantages of Turbo-jet
- Simple in construction.
- Free from vibrations.
- Minimum over-hauling.
- Higher rates of climb.
- Low specific weight.
- Speeds as high as 3000kmph are possible.
- Uninterrupted and smooth power supply.
2.3 Disadvantages of Turbo-jet
- It has low efficiency.
- Relatively shorter life.
- Not efficient in performance below 550kmph speed.
- It is noisier.
- Expensive materials.
- Low takeoff thrust.
3. Rocket Motor
The rocket motor is a type of turbo-jet propulsion engine which carries its own supply of oxygen. It is used in outer space there is no trace of air. It consists of a cylinder in which fuel tank, oxidizer tank, combustion chamber turbine and expansion nozzle are incorporated.
A rocket motor is reaction engines. The principle of it is that the fuel contained within the body of the rocket us rapidly expanded and expelled out of the tail cone nozzle of the rocket.
This reaction then causes thrust and propels the rocket forward. This is an example of one of the Isaac Newton’s fundamental laws, “For every action, there is an equal and opposite reaction”.
Rocket thrust is the reaction force produced by deported particles at high velocity from a nozzle opening. These deported particles may be solid, liquid, or gaseous. The engines ability to produce thrust will suffer only so long as the supply or particle, or working fluid holds out.
Expulsion of material is the abstract of the thrust production and without material to expel no thrust can be produced, regardless of how much energy is available.
There are two main categories of rocket engines
- Liquid Rockets
- Solid Rockets
3.1 Liquid Rockets
In a liquid rocket, the fuel and the source of oxygen necessary for combustion are stored separately and pumped into the combustion chamber of the nozzle where burning occurs.
Hot exhaust gas is produced which is used to propel the rocket. A fuel and an oxidizer are pumped into a combustion chamber. A reaction takes place it expands propelling the rocket forward. The expanding gas is then forced through a nozzle that makes them accelerate to a higher velocity.
The fuel and oxidizer may be fuel to the combustion chamber by pump or by pressure in the tanks. Propellant flow rates must be extremely large for high-thrust engines. Liquid rocket tends to the heavier and more complex because of the pumps used to move the fuel and oxidizer.
And we usually load the fuel and oxidizer into the rocket just before launch. with a liquid rocket, we can stop the thrust by turning off the flow of fuel or oxidizer.
3.2 Solid Rockets
In a solid rocket, the fuel and oxidizer are mixed together into a solid propellant which is packed into a solid cylinder. Under normal temperature conditions, the propellant does not burn but the propellant will burn when exposed to an external source of heat.
Some type of igniter is used to initiate the burning of a solid motor at the end of the propellant facing the nozzle. As the propellant burns, hot exhaust gases are produced in the cylinder which is used to propel the rocket, and a ‘flame front’ is produced which moves into the propellant.
Once the burning starts, it will proceed until all the propellant is burned. But with a solid rocket, to stop the engine casing is must be destroyed. A solid rocket is much easy to handle and can withstand for a year before firing.
4. Multi-stage Rockets
These depend on the number of stages, the rocket may be divided into
- Single stage rocket, and
- Multi-stage rocket
4.1 Single Stage Rocket
In a single stage rocket, one rocket motor is used.
4.2 Multi-stage rocket
In a multi-stage rocket, more than one rocket motor is used. In this stage, thrust is multiplied by each stage so that the final velocity of this rocket is scary or tremendous. Multi-stage rocket is used for launching space vehicles.
4.3 Application of Rockets
- It is used in the spaceships.
- It is used in the long-range artillery.
- Used for making the lethal weapons.
- Used in fighter planes.
- For satellites.
- In signalling.
5. Fuels used in Jet Propulsion
Light distillate types of fuels are commonly used in jet propulsion plants in order to overcome limitations of combustion systems and varying climatic conditions. They must possess relatively very high caloric value, high density, non-corrosive, low freezing point and the least risk of fire.
A few examples are Aviation, Kerosene, paraffin, ether, amines, alcohol, natural, gas etc. The hydrocarbon fuels are generally available in the present area, a mixture of paraffin, iso-paraffins, olefins, naphthalene, aromatics, etc.
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