JET PROPULSION

Jet propulsion

Introduction

Jet propulsion is said to be the process at which thrust produced by passing a jet of matter in the opposite direction to the direction of motion. In this Newton's third law applied, the moving body is propelled in the opposite direction to the jet. Now a days it is commonly use for the spacecraft propulsion.

Jet propulsion revolutionized the science of flight by dramatically increasing possible speeds and altitudes, hence enabling space exploration. The term jet propulsion refers to the action produced by a reactor to the ejection of matter.

Principle jet propulsion.

There are many everyday examples of jet propulsion. A blown-up toy balloon with its neck closed shows no tendency to move because the air inside is pressing equally in all directions. If the neck is opened suddenly, the balloon shoots away. The escaping air relieves pressure at the neck, and there is a reaction from the air opposite the neck. It is not the air rushing out of the neck and pushing against the outside air, however, that drives the balloon ahead. It is the air pushing against the inside front wall of the balloon that propels it forward. In fact, a jet would operate more efficiently in a vacuum because there would be no air to obstruct the escaping gases.

Type’s jet propulsion.

There are two general types of jet propulsion

• 1.     Air-breathing engine.
• 2.     Non air-breathing engine.

Air-breathing engines use oxygen from the atmosphere in the combustion of fuel. While Non air-breathing engines carry an oxygen supply. They can be used both in the atmosphere and in outer space.

Air-breathing engine are further sub divided into four types which are,

o   Turbojet.

o   Turboprop.

o   Ramjet.

o   Pulse-jet.

Air-breathing engine

Turbojet.

The most widely used air-breathing engine is the turbojet. After the air is drawn into the engine through an inlet, its pressure is first increased by a component called a compressor. The air then enters the combustion chamber, where it is burned with fuel to increase its temperature. The hot, high-pressure gas then expands through a wheel-like device called a turbine. Where it produces power. The turbine is connected to the compressor by a shaft, and the power output of the turbine drives the compressor. At the turbine outlet the hot-gas pressure is still above that of the surroundings, and the final expansion takes place through an exhaust nozzle where the speed of the exhaust gas is increased. It is the final high-velocity jet that produces the thrust to push the plane through the air. Although in concept a jet engine is much simpler than a reciprocating engine that turns a propeller, the actual design for efficient operation is complex, and large jet engines are extremely costly.

Turboprop.

In turboprop engines a conventional aircraft propeller is usually mounte

d in front of the jet engine and in one type of engine is driven by a second, or free, turbine. This is located behind the turbine that is driving the compressor. In other designs the power is obtained by additional stages on the main turbine. Since turbine speeds are much higher than propeller speed, a reduction gear is required between the turbine and the propeller. About 90 percent of the energy in the hot gases is absorbed in the turbine, and only about 10 percent remains to increase the speed of the exhaust jet. Accordingly, only a very small portion of the overall thrust is produced by the jet; most of it comes from the propeller.

Ramjet.

The air into which an engine rushes at high flight speeds is partially compressed by the so-called ram effect. If the speed is high enough, this compression can be sufficient to operate an engine with neither a compressor nor a turbine. A ramjet has been called a flying stovepipe because it is open at both ends and has only fuel nozzles in the middle. A straight stovepipe, however, would not work; a ramjet must have a properly shaped inlet diffuser that produces low-velocity, high-pressure air at the combustion section, and it must also have a properly shaped exhaust nozzle to increase the speed of flow.

Pulse-jet.

A pulse-jet is similar to a ramjet except that a series of spring-loaded, shutter-type valves is located ahead of the combustion section. In a pulse-jet the combustion is intermittent or pulsing rather than continuous as in a ramjet. Air is admitted through the valves, and combustion begins. This increases the pressure and closes the valves, preventing back flow through the inlet. As the gases expand through the rear nozzle to produce thrust, the pressure in the combustion section drops to the point where the valves open again to admit fresh air. This cycle is then repeated.

Non air-breathing engine

Rocket engines carry both fuel and oxidizer on board, and they are therefore not dependent on the surrounding atmosphere for the needed supply of oxygen. Accordingly, they provide the primary means of propulsion in outer space. Rockets are usually classified by the type of fuel burned; solid-propellant rockets carry a solid mixture of fuel and oxidizer. This mixture is similar to gunpowder and burns completely after ignition. The burning generates a large volume of high-pressure gas in the combustion section. This gas is then expanded into a high-velocity jet as it leaves the exhaust nozzle. The burning rate is controlled by shaping the solid fuel in such a fashion that the combustion gases are released at a nearly uniform rate. The control of the thrust, however, is limited, making solid-propellant rockets only suitable for the first, or takeoff, stage of space rockets. Better control can be obtained in liquid-propellant rockets. In these, both fuel and oxidizer are stored in separate tanks and are then pumped in a carefully metered fashion into the combustion chamber. There they are atomized, mixed, and burned. Because liquid-propellant rockets can be restarted and fully adjusted, they have become the primary propulsion systems in space programs.