History of the Jet Engine – Part 1

What is a jet engine?

If you visit a busy airport, have a look at the engines mounted on the aircraft. You will see them hanging under the wings, or fixed to the rear of the fuselage. There may be two, three, or even four engines, depending on the type of airplane. A Boeing 747 jumbo jet, for example, needs four engines - which between them produce enough power to light a medium-sized town.

Once this monster has taken to the air, the jets will be running continuously for ten hours or more. On a ten-hour journey they could burn 50,000 gallons (227,305 liters) of fuel - enough to drive a car around the world sixty times! So what exactly are jet engines? If you look in at the front of one all you will see is what seems to be a big fan. There is even less to see at the other end, because the cowling around the engine extends for quite a long way beyond the rotating parts inside. From a distance, all you can see is a hole, blackened by exhaust smoke.

(see image below) A Rolls-Royce RB. 211. Four of these jet engines power a Boeing 747. You can see part of the fan sitting back in the engine cowling shell. The engineer has opened a section of the cowling to inspect the engine before take-off.


The job of a jet engine is to produce a very powerful stream of air which will push the aircraft forward. Huge quantities of air enter at the front, pass through the rotating machinery inside, and are expelled at high speed from the exhaust at the rear. It works something like a garden sprinkler, which spins round when water is forced through the nozzles. This is known as 'jet reaction', because as the water leaves the nozzles it 'reacts' or presses against them. You can test jet reaction yourself by holding the end of a hosepipe and turning on the tap. As soon as the water comes out you will feel a force pushing the hosepipe nozzle towards you.

Instead of water, a jet engine uses air to provide the reaction. But before it can be made to do any work, the air has to be squeezed. The machinery in the engine is therefore designed to squeeze the air as much as possible before letting it escape out of the exhaust. This machinery is known as the gas turbine. A jet engine consists of the gas turbine, the casings that surround it, and other equipment, such as the thrust reverser, which helps to slow the aircraft down when it lands.

Jet engines were first used to power aircraft about forty years ago, towards the end of the Second World War. Before that, forward thrust was always provided by propellers, turned by a piston engine. When airplanes first took to the air at the beginning of this century, their engines were little more than copies of those used in cars. But they were soon developed, so that by the 1940s aircraft were reaching speeds of up to 400 mph (644 kmph)which was as fast as a piston-powered airplane could fly. Then came the jet engine which, fortunately, was invented in time to take over from the piston engine, pushing aircraft to higher speeds than anybody had thought possible. Nowadays, jet-powered aircraft can travel way beyond 400 mph (644 kmph). Concorde, with her four jet engines each providing 28,000 pounds (12,700 kgs) of thrust, can reach 1250 mph (2011 kmph) - which is about twice the speed of sound.

This garden sprinkler is made to spin in the direction of the arrow by the reaction of water escaping from the nozzles. The more water pressure there is, the faster the sprinkler whirls. This same principle of reaction is applied to create the thrust in a jet engine, but instead of water it uses air.

Aircraft are powered either by propeller-driven engines or by jet engines. Propellers push back a large mass of air relatively slowly, while jets force back a smaller mass, but at a much faster rate. Jets therefore work more efficiently on high speed aircraft, while propellers are better suited to slower types.

How a jet engine works

The jet engine job is to force much air out of its exhaust duct as possible so that it will be pushed forwards - just as a toy balloon flies aero the room when the air rushes out of it.

The way in which a jet engine works is really quite simple. Think of the propeller used to power a toy airplane. Its blades are twisted into curves which direct the air towards the rear of the airplane. The faster the propeller spins, the more air there is flowing back to push the airplane forwards.

Inside a jet engine this pushing job is carried out by a compressor, which is really a series of 'propellers', one behind the other, all mounted on the same shaft. But the blades of a compressor are much shorter than those of a propeller, and there are many more of them. Also, compressors spin round much faster than propellers, pushing enormous quantities of air back along the engine and compressing it as it goes.

Most of the air forced back by the compressor is used to push the aircraft forwards. But some of it is mixed with fuel. It then becomes a vapor, which is burned in a combustion chamber or combustor. This makes it expand very rapidly, and the only escape for it is through an opening in the rear of the combustion chamber. It rushes out of here with tremendous energy, and carries on through the exhaust duct. It then joins the air from the compressor and helps to push the aircraft forwards.

Finally there is the turbine, which extracts from the gas-stream the power needed to drive the compressor. It behaves just like a windmill which turns when blown by the wind. It is mounted on the same shaft as the compressor, and is blown round by the hot, high-pressure air rushing out of the combustion chamber. The turbine is one of the most difficult parts of a jet engine to make. It has to work for many hours while running in a jet of very hot gases, so it has to be immensely strong, as well as light in weight.

The power of a jet engine is measured in pounds of thrust. This may seem odd, because pounds are usually a measurement of weight, not power - but in fact it makes sense, for when we say that a jet engine produces 20,000 lbs (9072 kgs) of thrust, we mean that it is powerful enough to exert a forward force of 20,000 lbs. For example, when you push a 2 lb bag of sugar along a table you are producing a force of 2lbs. The engines of big passenger jets can each exert up to 60,000 lbs (27,216 kgs) of thrust, which is about twenty-five times the weight of a family car.

The principle of jet propulsion is seen when an inflated balloon is released. The balloon rushes across the room, propelled by the reaction of air rushing from the nozzle. In a jet aircraft the air reacts against the machinery, pushing the engine, and the aircraft, forward.

Inside a jet engine there are hundreds of small blades, like this high-pressure turbine blade. The turbine turns the compressor, which provides the flow of air needed to drive the aircraft forward. Turbine blades sit in an extremely hot jet of gas, produced by the combustor, and have to be designed to withstand the high temperature.

Comparison of jet and piston engines. Jet and piston engines work on a similar principle. Each operates in four stages: intake, compression, combustion, and exhaust. In a jet engine, however, combustion of fuel is continuous, whereas in a piston engine, combustion occurs only once in every revolution.

Airfuel Intake - Jet: Air is sucked into the intake by the compressor. Piston: As the piston travels down, a mixture of air and fuel is drawn through the inlet valve.

Compression - Jet: The compressor pushes air towards the combustor, each row of blades squeezing it a little more. Piston: With both valves closed, the piston rises, compressing the air as it does so.

Combustion - Jet: Air is forced into the combustor, fuel is added, and a continuous torch of burning gases is formed. An igniter starts off the combustion process. Piston: With the piston at the top of its movement, a timed spark ignites the fuel/air mixture. This explodes, forcing the piston down. The piston is connected to the rear wheels via a crankshaft and gearbox.

Exhaust - Jet: The hot combustion gases expand through the turbine and into the exhaust duct, pushing the engine forwards. The energy extracted from the exhaust by the turbine is used to drive the compressor. Piston: The piston rises, pushing waste gases out through the open exhaust valve. The gases are discharged through the exhaust pipe.

Inside a jet engine

A jet engine is extremely complex, consisting of around 20,000 parts. This Rolls-Royce RB.211-535 is designed in three sections, with each section mounted on a separate shaft. It is therefore called a three-shaft engine. The low-pressure (LP) section consists of the fan and the low-pressure turbine which drives it, while the intermediate section comprises the six-stage intermediate-pressure (IP) compressor and IP turbine. The air is finally squeezed in the six-stage high-pressure (HP) compressor, which is driven by the HP turbine. After passing through the compressors the air is burned in the combustor before expanding through the turbine and out into the exhaust duct. Three-shaft engines are built only by Rolls-Royce. Jet engines built by other manufacturer have two shafts; in these engines the fan and IP compressor form one unit, which is driven by the LP turbine.

Building a jet engine

It takes about two years to build a modern jet engine - longer than it takes to build the aircraft that it will power. A major jet engine manufacturer employs up to 40,000 people. At least half of them are engineers working on the design and looking after every stage of production and every component, from the smallest compressor blade to the finished engine.

Because a jet engine has to work so hard and because it has to be very reliable, each of its components must be made to an extremely high standard. In the type of engine which powers the Boeing 747 jumbo jets, there are almost 25,000 parts. Each part has to be designed to the highest possible standard by skilled engineers. It can take up to five years to design them all.

Making the parts and then assembling them involves a great deal of planning. Some of the parts may be produced by small engineering or electronics companies operating a long way from the main engine factory. For example, fan blades for American engines are produced by a company who specialize in the different techniques required. Turbine blades for some American engines are made in England, while castings for a British engine may be German-made.

Every single part carries its own number and individual record card. As it passes through the factory all the different processes that it goes through are recorded on the card, so that even the smallest detail of its manufacture is known. If a fault occurs during the engine's life, investigators can check back to the records and find out if the problem was anything to do with the way in which a component was made.

Computers play an increasingly important part in helping engineers to design jet engines. Computer can produce a three-dimensional outline of a turbine blade. The operator can view the blade from any angle and ask the computer to print out details of the design, along with all the dimensions needed. Often the computer forms part of a production network which includes robots.

The traditional way of designing jet-engine components is by producing engineering drawings. Several drawings may be needed for each part, which means that for an entire jet engine many thousands have to be created. Engineers will have to discuss the design of a turbine blade. Nowadays drawings are usually produced automatically by computers, working from dimensions supplied by a design engineer. They can be made in a fraction of the time that it took to draw them by hand.

The three biggest companies, Rolls-Royce, Pratt & Whitney and General Electric, have several plants in different locations. Rolls-Royce, for example, build their passenger engines at Derby in the Midlands and their military engines at Bristol in the West Country.

Some engines are produced by the manufacturers of several countries working together. Satellite links make possible the transfer of drawings and information, each manufacturer working on a separate part of the engine.

One such engine is the CFM56, a very successful 22,000 lb turbofan for medium-sized aircraft. The CFM56 fan, and the low-pressure turbine that drives it, are built by the French company Snecma, while the 'heart' of the engine, the high-pressure compressor and turbine, is the responsibility of General Electric, in America.

Complete CFM56s are assembled in both France and the GSA. In each case the parts are flown across the Atlantic and carried to their final destination by road. Computers keep track of progress and the information is flashed from one country to another by satellite.

Blades have to be made for the big fan at the front of a Rolls-Royce jet engine. The titanium blade skin will then be removed from the hot press. The skin will be joined to its mate, forming a hollow blade. For strength, the cavity between the skins is filled with a honeycomb core of titanium.

The many parts of the jet engine are tested in different ways by the most advanced equipment. Ultrasonic testing of a fan blade are used to detect flaws in the metal.

The compressor

A big jet engine gulps down around two and a half thousand tons of air for every hour that it is working - which means that during a 6 1/2-hour flight from London to New York it needs more than seventeen thousand tons of air. All of this air is pumped by compressors, which are the 'heart' of a jet engine. They work like big, powerful pumps which thoroughly squeeze the air sucked in through the engine-intake. The more they squeeze the air, the more efficiently the engine will work.

Compressors look rather like porcupines. They have many rows of blades attached to the outside of a drum, which is tapered from front to back. The long blades are at the front, the short ones at the back. Each row of blades turns in the same way as a simple electric fan, and each passes air to the row behind. As the air moves back along the compressor it has less and less space to occupy, because of the tapering, so it becomes squeezed. A modern jet-engine compressor can squeeze the air into a space about twenty times smaller than the opening at the intake. It is therefore said to have a compression ratio of 20:1.

On most modern jet engines there is an especially big set of blades at the front of the engine. They are mounted on a different shaft from that which turns the main compressor and they operate like a powerful fan, blowing air around the outside of the engine as well as through the compressor itself. The fan improves the engine's performance, particularly during low speeds at take-off and climb.

When the air passing through the compressor has been squeezed, it is burned in the combustor. It then expands very quickly, rushing out of the exhaust in a jet, and helping to push the aircraft forwards.

With so much work to do, compressors have to be very strong. Used in aircraft, they must also be light. To make something strong and light, special materials have to be used (this of course applies to all the components of a jet engine).

A jet engine compressor consists of a rotor and a stator. The stator blades are attached to the inside of the compressor drum and do not rotate. Air passing through the engine first meets a row of big stator blades, called guide-vanes. These direct the air at the first row of rotor blades.

An example of a high-pressure compressor rotor from a modern jet engine is the Pratt & Whitney PW2037. The engine powers the 180-seat Boeing 757, and produces 37,600 lbs of thrust. The compressor is made up of 12 stages, each stage consisting of a number of blades. Driven by the high-pressure turbine, it rotates up to 12,000 times a minute. The blades get shorter towards the back of the compressor, where the air is squeezed as tightly as possible before it enters the combustion chamber.

The turbine

Of all the parts of the engine, turbines have the most exacting job to do. They work on the same principle as a windmill, though a windmill has only a few sails, while turbines needs up to one hundred blades. They are placed in a powerful jet of hot, high-pressure gas which rushes out of the combustion chamber at a temperature of up to 1200°C (2544°F) - hot enough to make the turbines glow red as they spin round. They can make as many as 20,000 revolutions per minute.

Turbines are constructed so that they take enough energy from the hot gas-jet to turn the compressor, which is fixed to the opposite end of the main engine shaft. The rest of the energy in the gas-jet passes out of the exhaust and is used to push the aircraft forwards.

The blades of a turbine are fixed to a large wheel, or disc, in such a way that they can be removed separately for repair. A row of blades is called a stage. There is nearly always more than one stage. In big jet engines up to seven turbine stages may be used. They are usually divided into two sections - the high-pressure turbine and the low-pressure turbine. The high-pressure turbine - consisting of the two stages nearest to the combustor - uses the energy which it extracts from the exhaust jet to drive the high-pressure part of the compressor. The low-pressure turbine - which is connected to a shaft passing through the centre of the engine - drives the low-pressure compressor. This compressor is the part which often has a big fan at the front of it.

The low-pressure turbine may also be used for other jobs. For example, on jet engines which power helicopters it would be joined to a shaft which turns the rotor. It can also be used for turning aircraft propellers or ships' screws.

(see image below) A low-pressure turbine. This extracts enough energy from the hot gases rushing through the engine to drive the big fan at the front. There are five stages, and You can see that the blades have rings around the outside. The rings are designed to prevent the gases escaping as they pass through the turbine into the exhaust duct. The turbine blades are attached to discs, fixed to the shaft in the center.


Continue to Part 2

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