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History of the Jet Engine – Part 2

How compressor and turbine discs are made

If you tie a small piece of wood to the end of a length of string and whirl it round your head, you will feel the wood pulling away from you, even if you whirl it quite slowly. The same force acts on the blades of compressors and turbines spinning at some 20,000 revolutions per minute: but because they spin so fast, the pull is much bigger. In fact, the force wrenching at the blades on their mountings can at full power reach several tons. To stand up to this force, the blade mountings, or roots, have to be extremely strong, and very carefully made.

The disc to which the compressor and turbine blades of all large jet engines are fixed looks like a big wheel with teeth. As it has to carry the load of all the blades pulling away at once, it is a heavy, solid structure. It must never break, for if it did it would cause enormous damage to the engine.

When discs are made, they must be as perfect as possible. The smallest crack or imperfection in the metal can become the starting place for a fracture, which will grow as the disc spins. They must also be light, since they make up most of the weight of a jet engine.

The traditional method of making compressor and turbine discs is by forging the metal into shape. First a large block of metal is roughly cut, or machined, to size, then it is heated until very hot, and stamped into the right shape. Heating the metal not only makes it softer and easier to force into shape, but also helps to remove faults which could lead to cracks.

The latest method of manufacturing turbine and compressor discs ensures that the metal is almost completely free from imperfections. It is called 'powder metallurgy' and is now being used in the manufacture of some jet engines. In order to produce discs by this method, a very fine powder is made by pouring molten metal on to a fast-spinning turn-table. When the molten metal hits the surface, it breaks into millions of fine droplets which are flung from the edge of the table. The moment they leave the table, the droplets cool and solidify, forming a very fine metal powder. The cooling process happens extremely quickly, with the temperature dropping by about 1000°C (2120°F) in half a second. During this half-second the molecules within the metal powder are 'frozen' in their positions so quickly that the metal does not have time to pick up impurities.

The roots of a turbine or compressor blade are shaped so that they lock into the 'teeth' of the disc which carries them. Because of its shape this is known as a 'fir tree' root. The blades are slotted into place around the disc and secured with a bolt. They are free to rock slightly in their mountings, to help spread the enormous load which each root carries.

(see image below) This is what a disc looks like before it is finally shaped. The example here is from a high-pressure turbine. It has been pressed into shape by forging. The metal is as near perfect as possible, to ensure maximum strength. After forging, the disc is mounted on to a giant cutting machine and cut automatically to exactly the correct dimensions for the engine; within a few thousandths of an inch.

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Next, the powder is compressed, under hundreds of tons of pressure, into a shape very similar to that of the turbine or compressor disc. To make the disc ready for the engine, only a small amount of tidying up needs to be done on a large cutting machine: a process which is quick and inexpensive.

The forged disc is then machined again; the roots that will carry the blades are cut; and the finished product is dispatched to be carefully inspected.

Today, much of the manufacturing work is controlled by computers. They help to design the disc, making all the calculations needed to ensure that it is as strong and as light as possible. They also control the machines which do the cutting.

Making powdered metal turbine discs. A forming case is filled with metal powder, and put into a vacuum (1). The case is vibrated, so that the powder when shaken down becomes tightly packed (2). The vacuum ensures that there are no air 'voids' within. Next the case is sealed (3) and the metal powder is subjected to extremely high pressure - about 2,000 pounds per square inch (4). It is also heated, so that the metal particle become fused together, forming the disc which can then be removed from the forming case and machined to its final shape.

For hard-facing a high-pressure turbine disc, the disc teeth need to have a very hard surface to cope with the wear and tear of carrying turbine blades. The operator watches the spraying of a special metal on to the spinning turbine disc. Under intensive heat the hard-facing metal fuses with that of the disc.

How compressor and turbine blades are made

One of the ways most frequently used for manufacturing the blades of modern compressors and turbines is by casting: a method first used by the Chinese about 2000 years ago for making statues. You can think of casting as being something like making a jelly in a mould. The liquid is poured into it, cools, and sets to the shape of the mould.

Before it is cast, the metal has to be heated up to a very high temperature so that it melts. It can then be poured into the mould, which is the shape of the compressor or turbine blades. Generally, several different types of metal are mixed together, forming what is called an alloy.

The moulds are made of a ceramic material, similar to that used for making cups and saucers. The molten mixture is poured into the moulds in a casting furnace, and allowed to cool. The ceramic is then broken away, revealing the freshly cast blades. These are then cut to their final shape by very accurate cutting machines. After the blades have been made, they are all carefully tested.

Compressor blades have to withstand the powerful pressure from the enormous quantities of air pumped back down the engine. They also have to withstand damage caused by objects sucked in at the intake. So they must be tough. They must also be light in weight; and they become very hot, particularly at the end nearest to the combustor, where the pressure is highest. The alloy used for these blades, therefore, is one which gives them the best combination of strength and lightness.

Turbine blades are cast in a different way from compressor blades. If you take a piece of balsa wood like that used in a model aircraft, and try to break it, you will find that it is stronger in one direction than it is in the other. This is because the wood has a 'grain'. With turbine blades, the grain in the metal is made to lie in the correct direction, which is along the blade. This is achieved while the blade cools after it has been cast. The process is called directional solidifying.

For turbines, special alloys have been developed so that they can cope with temperatures of up to 1300°C. They are based on nickel, and have small amounts of aluminum, titanium, and other metals designed to retain the blade's strength while it is in operation.

To reduce the enormous heat created by the gas-jets, the turbine blades are cooled by air passing through a maze of tiny holes within them. The holes, or cooling passages, are arranged so that the blades are cooled in exactly the right places. Cooling enables the blades to operate in gas streams hotter than the melting point of the metal from which they are made.

Jet engine turbine blades work at very high temperatures, sometimes becoming red hot. They usually have to be cooled by air blown via the compressor into the root of each blade. The network of cooling passages within the blade, which is very complex, ensures that the blade skin is prevented from burning. The inner passages are formed during the casting process. The hundreds of tiny holes connecting the passages to the blade surface are drilled either by a small laser beam, or by spark erosion. This is a technique that uses a carefully controlled spark to eat away the metal.

Making turbine blades using the 'lost wax' process, wax copies of the blade are created by pouring wax into a metal mould; allowing it to set; and removing the mould. The wax blades are then mounted on a 'tree' to form a cluster. The cluster in the picture contains eight wax turbine blades, ready for dipping into a ceramic slurry.

(see image below) A cluster of wax blades being removed from the ceramic slurry. The blades are dipped into the slurry several times, building up a ceramic coating about 1/4-inch thick. Next, the cluster is heated to a temperature of about 1000°C, which harden the ceramic coat and melts the wax. Molten metal is then poured into the ceramic shell and allowed to harden special ovens. The ceramic coat is broken away, leaving the turbine blade, which is now ready accurately machined to its final shape. The turbine blade is hardened under very carefully controlled conditions. This is an automatic, computer-controlled oven for making directionally solidified blades.

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Fuel control and the combustion chamber

Before they can do any work, all engines need fuel. Jet engines burn a type of fuel which is very similar to paraffin. It is called Jet-A. On a long flight lasting seven or eight hours, the four engines of a jumbo jet will gulp down 30,000 gallons (136,383 liters) of it.

The Jet-A is stored in the wings of the aircraft and is fed to the engines by high-speed pumps. When it reaches the engines it passes through a fuel control system which accurately monitors the fuel before pumping it into the combustion chamber, where it is burned, releasing its energy to drive the turbines and to push the aircraft forwards.

Before the fuel can burn it has to be mixed with exactly the right quantity of air. This also applies to a car engine, which uses a carburetor to do the mixing. In a jet engine the mixing is carried out in the combustion chamber. If the mixture is to be completely burned, about fifteen times more air than fuel is needed. Extra air, not needed for burning fuel, passes around the combustor to prevent it from overheating.

The job of the fuel control system is to make sure that the correct amount of fuel is pumped into the combustion chamber throughout an aircraft's flight - from engine starting, through full power take-off, to cruise. In order to do this, it has to know how much air is passing through the engine, so that it can adjust the mixture correctly.

The amount of air needed by the engine changes constantly, depending on the aircraft's speed and height, and on the outside temperature. (The density of air at 30,000 feet, 9,144 meters, is much lower than it is on the ground.) So a fuel control system is very complicated. It is a kind of 'brain' which constantly looks after the fuel needs of the engine. Its sensors (devices which 'sense out') measure air pressure and temperature, and a number of other factors, such as engine speed. On some very up-to-date jet engines the 'thinking' is done by a computer, which makes life easier for pilots, flight engineers and maintenance men.

When the fuel reaches the combustion chamber it is forced under great pressure through very small nozzles which break the liquid into tiny droplets. It then mixes more easily with the air which rushes through the combustion chamber.

The combustion chamber is wrapped around the middle of the engine, between the compressor and the turbines. The compressor pumps a lot of air back down the engine. Some of the air is channeled into the combustion

The combustion chamber, or combustor, is responsible for converting the fuel energy into power to make the turbines spin. Fuel and air are sprayed through nozzles, at very high pressure, into the chamber. The mixture is ignited, forming a continuous flame which rushes backwards. More air is introduced through holes in the side of the combustion chamber to prevent the walls from becoming too hot, and from burning. The escaping gases are directed at the turbine by the nozzle guide-vanes.

In a partly assembled combustion chamber, the fuel spray nozzles will fit into the large holes in the rear. The holes in the walls are for cooling. chamber, where it is thoroughly mixed with the fuel droplets before being burned.

You can think of the combustion chamber as a kind of blow torch, which when the fuel is burned produces enough heat to warm 17,000 standard-size, eight-room domestic dwellings. During the burning process the mixture of fuel and air expands very rapidly and is forced under enormous -pressure out of the chamber. The gas-jet is aimed at the turbines, blowing them round so that they can turn the fan and compressors which push the aircraft forwards.

Combustion chambers have to mix the air and fuel thoroughly in a very short distance, so they are a complicated shape. And since they work for many hours at high temperatures they have to be made of very special metals. Sometimes a metal called titanium is used. This comes from an ore found in certain rocks. It is a very hard substance, and as it is difficult to mould into the right shapes it is only used in aircraft when there is a particularly tough job to do.

Combustion chambers are produced in several sections, which are welded together before they are mounted on the engine. The sections are made by heating up the titanium until it is soft. It is then pushed into a mould under great pressure.

The exhaust system

The exhaust is one of the few systems of a jet engine that you can actually see in action. Its job is to pass the gases rushing through the engine out into the atmosphere. Without it, the engine would perform very badly. By getting the shape of the exhaust duct right, the designers can improve the performance of the rest of the engine.

Aircraft flying at less than the speed of sound need exhaust systems with nozzles that are tapered towards the end. Aircraft flying beyond the speed of sound need nozzles which open out at the end, but which at slower speeds can be tapered.

When you are next at an airport, look at any passenger aircraft, and you will see that the engines are housed in nacelles which become narrower towards the rear end. These nacelles form the wall of the outer exhaust duct, which carries the air pushed back by the big fan at the front of the engine. The inner exhaust duct, which is narrower and extends beyond the outer duct, carries the air pumped back by the core, or heart, of the engine.

Between the outer and inner exhaust ducts is the thrust reverser, the mechanism which blocks off the air driven back by the fan, thus forcing it in the opposite direction. On landing, the reverse thrust acts as an extra brake and helps to slow the aircraft down.

When you are landing in a passenger aircraft you can hear the increase in engine noise as the wheels touch the ground. This is the moment when the pilot selects reverse thrust, opening the throttle to full power to slow the aircraft down in as short a time as possible.

In a modern fighter engine like the 23,000 lb-thrust Pratt & Whitney F100 which powers F-15s and F-16s. Fuel is being pumped into the exhaust system and ignited, greatly increasing the thrust of the engine. The nozzle at the rear is wide open. When reheat is reduced, or switched off, the nozzle contracts, reducing the duct area.

In an experimental system for reducing the noise made by a civil jet-engine exhaust, the serrated ring is designed to mix the high-speed air - which is produced in the core - with the slower air pushed back by the fan. The slower the final exhaust jet, the less noise it makes. You can test this yourself by blowing through various sizes of tube.

Aircraft such jet fighters are equipped with adjustable nozzles which are automatically tapered at low speeds, and opened out when maximum power is needed for take-off and supersonic flight. The nozzles are also fitted with afterburners, which are used to increase the power of the engine for take-off, climb, and supersonic flight. In military aircraft, they are also switched on for combat, when the extra power is sometimes needed to out fly the enemy. If you are at an air show watching a jet fighter taking off, look carefully at the exhaust as the pilot begins his take-off run. You should see the nozzle open out at the end, and a red glow as the afterburner lights up. All modern supersonic aircraft are equipped with afterburners.

The afterburner is a simple mechanism which consists of a ring of fuel nozzles set into the exhaust duct, just behind the turbines. Air from the turbines rushes through the nozzles, is mixed with the fuel, and ignited, turning the exhaust duct into a kind of very powerful blow torch. By means of the afterburner, engine power can be doubled. It can only be used for short periods, because it gulps down a great deal of fuel, but it is a very practical way of increasing power without having to make the engine bigger. For the engines of high speed aircraft, which must be as slim as possible, this is very important.

The exhaust systems of jet engines create enormous heat, particularly if an afterburner is fitted, when the temperature at the nozzle can reach 1500°C (3180°F). This is hot enough to melt most metals, but not titanium, which is therefore often used for parts of the exhaust system which have to withstand great heat, such as the nozzle.

At present, exhaust systems on passenger aircraft are manufactured from aluminum alloy, and lined with special material designed to absorb the noise made by the engine's exhaust jet and its rotating parts.

Some of the latest engines use composites for the outer parts of the exhaust duct and engine nacelle. Composites consist of man-made fibers held together by resin. A very successful composite material is Fiberglass, which has many everyday uses. Even lighter and stronger than Fiberglass is Kevlar, which is now being used to build complete light aircraft. As it is so strong it is also used to surround the big fan at the front of the engine to prevent broken blades from escaping through the nacelle.

Reversing the thrust of a jet engine helps to slow the aircraft after touchdown. Only the air of the fan is reversed, since this provides most of the engine's thrust. Large doors block the fan duct, forcing the air to escape through a grille in the outer casing. The grille is angled so that it directs the escaping air forwards.

Sound-absorbing acoustic panels are used in civil jet engines to reduce fan noise. The tips of a fan blade can exceed the speed of sound, making them one of the noisiest parts of a modern turbofan. The acoustic panels are made of a special lightweight material which is wrapped around the inside of the fan duct.

Continue to Part 3

Science | Engineering | Science in Society | Sciences | Technology


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