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History of the Jet Engine – Part 3

Assembly

When inspection of the components is complete, assembly of the engine can begin. Most engines today are built from 'modules'; you can think of them as building blocks. Rolls-Royce's biggest range of passenger engines, the RB. 211 series, consists of up to eleven modules, or blocks. Smaller engines are likely to have fewer.

A module is a working section of the engine, such as a high-pressure turbine, a compressor or a combustion chamber. Each module is constructed in a different part of the factory - or by another company. When the modules are ready they are taken to the part of the factory where engines are assembled. They are then checked and slotted into place, which is a fairly simple operation. The engine is either built vertically or horizontally in a big gantry which can turn the engine over so that work can be carried out on both sides.

When all the modules have been bolted together the supporting components (known as the 'peripherals') such as oil and hydraulic pipes, generators and electrical wiring, are then added, and the engine is ready for testing.

Once it has been proved that the engine can meet all the requirements, it is transported to the aircraft manufacturer, either by air - in a big cargo plane - or by ship. Low-loading lorries then carry the engine to the aircraft factory. As it is worth at least two million dollars and weighs up to six tons it must be handled very carefully.

Engines are attached to the aircraft's wings by means of pylons, which are designed to carry the weight of the engines and to transfer their thrust to the airframe. Fuel passes through the pylons to the engines from tanks in the wing, while electrical and hydraulic power is transferred by wires and pipes through the pylons to the aircraft.

If you watch an aircraft as it taxies along the runway you will notice that the engines are 'nodding'. They must be allowed to swing slightly - otherwise their weight would be too heavy for the wing to bear. The pylon is designed to allow the engine to 'nod' while holding it firmly in place.

The Rolls-Royce 535 engine consists of seven modules. Each module is a pre-balanced unit, and can be replaced with a new or repaired unit without the necessity of matching it to the rest of the engine. The seven modules are: Fan, Intermediate-pressure compressor, IP module, High-pressure system (comprising HP compressor, combustor and HP turbine), Intermediate- and low-pressure turbine, Gearbox, Fan casing. When installing the fan on a big jet engine for the Rolls-Royce RB.211. The fan is one of the 'modules' of the engine.

Module build

Stage 1: The fan case module 07 is bolted to a specially designed table called a build-stool, and the gearbox 06 is attached.

Stage 2: Module 03 is lowered into the fan case and secured.

Stage 3: Intermediate-pressure compressor module 02 is added.

Stage 4: Fan module 01 is attached.

Stage 5: The engine is now placed in a horizontal build-stand, and the high-pressure system module 04 is bolted on.

Stage 6: The intermediate and low-pressure turbine module is joined to module 04.

Stage 7: The exhaust cone and fan spinner are attached, and the engine is made ready for dispatch.

(see image below) The big jet engines which power aircraft such as the Boeing 747 are joined to the wing by pylons, which also carry fuel lines; hydraulic and electrical services; and control-cables. Pylons have a very demanding job, and are manufactured from solid metal for maximum strength. But they must be designed to allow the engine to 'nod' to relieve stress on the wing when the aircraft is flying through turbulence or taxiing along the runway.

i.imgur.com_nsergg0.jpg

Engines are fitted with electrical wiring looms, and hydraulic pipe work. They are mounted vertically, with the fan module uppermost. Cleanliness is essential in a jet engine factory. Floors are polished, and small components are kept in sealed plastic bags until needed. Fitting the 'hot section' of the engine, comprising the turbines and combustor, to the fan and compressor section.

Made by automation

All the parts of a jet engine are produced to a very high accuracy and must fit together perfectly if they are to be reliable, safe and long-lasting. Only very skilled workmen with many years' experience can operate the machines which cut the metal parts to shape. Today, some of the work is already being carried out by computers, which soon will be involved in every stage of jet engine manufacture, from the original selection of the metal to the final shaping of the parts. Computers are even helping out with the actual design of the engine. They have huge memories which can store all of the information needed to work out the best shape for a particular part. They can then 'draw' the part on a special television screen. The computer can also turn the drawing so that the designer is able to look at it from any angle. All of the dimensions needed to enable it to be cut to shape (engineers call it 'machining') are added automatically.

On the factory floor, robots take care of many of the operations which were previously carried out by skilled men. The robots are not like the ones in science fiction films. It is performing a welding operation which would normally take a man around four hours to complete. The robot will do the job in one hour, and can then proceed to the next job immediately.

There are many other advantages in giving jobs like this to robots. To begin with, they don't mind being put into an unpleasant place to work. Sparks, heat, and dangerous chemicals are no problem: the robot simply goes on working, accurately and quickly. Robots are also very strong, and can make light work of carrying heavy castings, such as turbine discs, from one machine to another, ready for the next operation to be carried out.

Of course, robots have to be told what to do, and the most common technique is to feed into them a special paper tape with a series of holes punched through it. The robot can 'read' the information on the tape by noting the position of the holes, which correspond to the exact requirements of the engineer who designed the part. The tape can be used again and again to instruct any robots in the factory which are free to carry out a particular job.

In jet engine factories of the future, punched tapes will not be necessary. Instead, all of the robot cutting machines will be connected directly to one big computer, which will carry the information needed to instruct the machines to cut the shapes that the designer wants. So when an engine is being built, the computer will know when each machine is ready for its particular job. It will then automatically order the materials from stock and will distribute them to the machines. By the use of these methods jet engines will be built in a much shorter time than they are today, which will make them less expensive.

Flux is metered precisely through the needle and deposited ort the compressor drum as it rotates.

In an automated disc manufacturing system, the raw material store, process area, and computer-controlled machine tools, are served by a robot truck system.

In an automatic robot production line for putting the finishing touches to turbine blades, there are eight pairs of machines in this production line. Each pair, or 'cell', is served by a single robot. There is a queue of blades waiting to be picked up for work to be carried out. The robot will pick up each one and place it in the machine. While one blade is being trimmed another will be removed from the opposite machine and replaced on the conveyor belt. It will then be carried to the next cell.

Performance testing

The safety of the passengers in an aircraft depends on the reliability of the engines, particularly at take-off, when the engines are all operating at full power and when every component is working under full pressure. Before an engine manufacturing company can sell its engines it must therefore prove to the airlines, aircraft builders, and safety authorities that they are reliable and safe. When this has been proved to the satisfaction of everyone concerned, the engine is given its 'ticket' (certificate of airworthiness) and is allowed to go into service.

An engine is not considered airworthy until all the parts from which it is made have passed tests which prove that their design meets the official requirements. One of the most important of these tests concerns the big fan at the front of the engine.

When an aircraft is accelerating down the runway for take-off, the fan provides about three quarters of the total power produced by the engine. It spins round 6000 times per minute, during which time the fan blades are under enormous strain. At take-off, there is a danger that the engine might suck in an object, such as a bird, just when the aircraft needs all its power. The first components that the object will encounter are the spinning fan blades, so they must be extremely strong in order not to break.

The way in which a fan is tested is by simulating the conditions that are likely to prevail when it is working at full power on the aircraft. So it is mounted in a special frame, and turned by a motor at full speed. Dead birds weighing up to 4lbs (1.8 kgs) are then fired at the engine from a special 'gun'. The fan must be able to withstand the impact of the largest bird without failing in a way that would endanger the aircraft and passengers in flight. This is quite an achievement, since the bird and the fan meet at a combined speed of over 1000 miles (1609 km) per hour.

In order to test the ability of the fan casing to retain a broken fan blade, a blade is deliberately detached at its root by an explosive charge while the fan is running at full power. To achieve this, engineers place an explosive bolt at the root of one of the blades, and detonate it at precisely the correct moment, causing the blade to fly off with tremendous energy. The casing is wrapped in Kevlar to withstand the shock.

This is just one of the many tests that have to be carried out on various components of the engine to prove their safe and efficient performance. When each of the separate components has been tested, the entire engine then has to prove its performance in a test cell. There are several test cells in a factory, each specially designed for the different engines that the company makes. Some of them are huge, so that they can deal with the most powerful engines.

The engines are mounted on a gantry. They are then connected to a fuel supply and wired up so that measurements can be taken of the thrust, fuel consumption, and all the other functions that must be tested. Test engineers watch the engine through a thick glass window in the wall of the test cell, or on a TV screen.

Jet engines are also tested in a special chamber which simulates the low-pressure conditions outside an aircraft when it is flying at high altitude.

Once the engineers are satisfied that the engine is safe to fly, it is often mounted on a specially converted aircraft for testing in flight. This will finally prove that it is fit to meet the needs of the airlines that are interested in buying it.

For checking the quality of the alloy in a turbine blade, a TV screen is used for a microscope's view of the metal, so that the scientist can look for imperfections. Scientists use light to make sure that the size and shape of a component are within the correct dimensions. The technique is called holography. It uses the interference between two beams of light, one of which is reflected off the object being checked, to produce these patterns. The patterns can be deciphered by skilled engineers.

The fan casing of a big jet engine is wrapped around with a Kevlar bandage. The bandage prevents the possibility of a broken fan blade escaping through the casing. Kevlar is an extremely tough, light material, and its use saves 200lbs (9lkgs) in weight over the traditional metal containment ring.

When an engine is mounted in a test cell, the fan is visible at the end of the inlet duct. The duct is carefully shaped so that engineers can calculate the amount of air used by the engine. The engine (while it is running) is closely monitored by hundreds of electronic probes and watched by TV cameras. Using an outdoor test stand, a 50,000 lb thrust General Electric CF6-50 jet engine is tested for endurance. It will be run for many hours to ensure that it is reliable enough for long-distance flight.

All kinds of jet engines

Jet engines are built in many shapes and sizes. The smallest are about the size of a standard loaf of bread, and are used to power long-range missiles and light target aircraft. The biggest and most powerful are the turbofans used on 'wide-bodied' aircraft such as the Boeing 747 jumbo jet. They weigh up to six tons.

As well as powering aircraft, jet engines are used for many other purposes - such as on certain naval vessels. For example, a standard A-class Royal Navy destroyer has four gas turbines. Two of them are similar to those fitted to Concorde and are switched on for high-speed dash and maneuvering. The other two are smaller, and are used for cruising.

In remote parts of the world, gas turbines are used for driving generators to provide electricity. Sometimes they are used at normal coal or oil-burning stations as back-up during a breakdown or during maintenance, or to provide peak-load power.

One of the most unusual jet engines in use today is the Rolls-Royce Pegasus, developed specially for the Harrier, which is the only single-engined vertical take-off fighter in the world. This amazingly versatile aircraft often puts in an appearance at air shows, when it demonstrates its ability to hover, fly backwards and sideways, and to take off and land vertically.

The Pegasus engine is equipped with four nozzles - two at the front and two at the rear. The rear nozzles take hot air from the exhaust. The front nozzles are fed by cool air from the fan. In a normal engine the fan would pass this air back along the nacelle to provide only forward thrust. But for vertical take-off the Harrier also needs downward thrust. So the air from the fan and the exhaust is passed through rotatable nozzles which can be pointed both downwards and backwards.

The Harrier's Pegasus engines have nozzles which can be rotated, enabling the plane to take off vertically, and then fly horizontally like a normal aircraft.

(see image below) A racing car, named 'Project Thrust', is powered by a jet engine from a fighter aircraft. 'Thrust' was the first vehicle to break the sound barrier on land.

i.imgur.com_i8ypm4z.jpg

Jet engines for helicopters are very small. In a twin-engined Dauphin helicopter, made by Aerospatiale of France, the engines are mounted side by side just behind the rotor.

Facts and figures

Fastest military jet aircraft: Mikoyan. Mig-25 'Foxbat' Mach 3.2; 2,110mph/ 3,394kmph. (USSR).

Most powerful passenger jet engine: GE CF6-80C2, run at over 62,000lb/ 28,180kg thrust in 1983. (USA).

Most powerful military jet engine: Tumanski turbojet powering Mig-25 'Foxbat'. Thrust 30:, 864lb/14,000kg. (USSR,).

Fastest passenger jet aircraft: BAe/Aerospatiale Concorde. Cruising speed is Mach 2.2; 1,450mph/2330kmph. (UK/France).

Biggest production jet engine: General Electric TF39 powering Lockheed C-5A Galaxy military airlifter. (USA). Diameter 8.3ft (2.54 meters). Length 15.7ft (4.7 meters). Thrust 41,100lb/18,680kg.

Smallest production jet engine: Williams WR2-6 turbojet powering small target aircraft. (USA). Diameter 10.8in (274mm). Length 22.3in (522mm). Thrust 125lb/57kg.

World's only production Vertical take-off and landing engine: Rolls-Royce Pegasus Mk103. Thrust 22,000lb/ 10,000kg.(UK).

Most common passenger jet engine: Pratt & Whitney JT8D powering Boeing 727, Boeing 737-200: McDonnell Douglas DC-9 etc. Thrust 15,000-20,000 lb. Over 13 000 engines produced (USA).

Important dates

120BC: Hero demonstrates principle of jet reaction.

AD 1937: World's first turbojet, designed by Frank Whittle (UK), is tested.

1939: First turbojet flight, German Heinkel HE178 (engine was Heinkel HE536).

1941: First British jet engine to power an aircraft, Whittle turbojet Gloster E28139.

1948: First turbojet-powered aircraft to break the sound barrier, de Havilland D.H.108 (UK), powered by de Havilland Ghost turbojet.

1949: First pure jet aircraft into commercial service, de Havilland Comet, powered by Rolls-Royce Avon turbojets (UK).

1955: First use of reheat to increase thrust of a turbojet (UK).

1960: First V/Stol engine tested, Rolls-Royce Pegasus powering P.1127 (UK).

1969: First supersonic passenger aircraft flies - Concorde (UK/France) powered by four Rolls-Royce/Snecma Olympus 593 turbojets, 30,500 lb thrust each.

1969: First Boeing 747 Jumbo Jet flies, powered by four high bypass ratio Pratt & Whitney JT9D turbofans, each producing 43 ,500 lbs thrust (USA).

1983: First test of General Electric CF6-80C2 at 62,000 lbs thrust – world’s most powerful passenger jet engine (USA).

1997: The first supersonic car, ThrustSSC, boosted by two turbofans accomplishes the land speed record to 1,228 km/h.

2003: GE90-115B earned the FAR 33 certification; presently holds the world record for engine (fan) size and thrust for a gas turbine powered engine at 128 inches and 127,900 lbs of thrust, respectively.

2003: Concorde is resigned from service.

2004: Hyper-X is the first scramjet to maintain altitude

2005: Hyper-X is the first air-breathing (scram) jet to achieve Mach 10

Who makes jet engines?

Program manager: Every different type of engine manufactured by a company comes under the watchful eye of the program manager, who makes sure that all aspects of its production and performance go according to plan.

Designers: Responsible for looking after all the different aspects of engine design. The Chief Designer is in charge of a team which creates ideas for making and improving every part of a jet engine so that it works as efficiently as possible without being too costly.

Production manager: Heads the team which looks after the building of the engine. He has to ensure that all of the thousands of components are ready at the right time so that the engines are delivered on schedule.

Inspectors: Every stage of engine manufacture has to be checked by qualified inspectors responsible for ensuring that all parts are of the right quality.

Salesmen: Responsible for selling the engine to aircraft manufacturers and airlines.

Engineers: All the different stages of engine manufacture are carried out by highly qualified engineers whose skills range from cutting, shaping and forming, to the testing of parts.

Computer programmers: Modern jet engines are all built with the help of computers, which can simplify many of the manufacturing tasks. Computer programmers write the instructions telling the computers what to do.

Back to Part 2

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