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The modern jet engine has completely transformed the way we travel, conduct business and connect with the world. This intricate feet of engineering has helped thrust our worlds together and is set to have a potentially huge impact on the future of air travel.
All jet engines work on the same principle. A large fan located at the front of the engine sucks in a huge amount of air which is then split into two parts. One section directs the air through the core of the engine and the other bypasses the core through ducts that surround the entire engine. The air that bypasses the engine is responsible for a large amount of the force that push the engine forward.
The air that reaches the core is forced into a compressor with causes the air pressure to rise. The compressor is made from a series of blades which are attached to the shaft.
The compressed air is then mixed with fuel and ignited inside the engines combustor. The burning gases then expand and force their way out through the nozzle located at the back of the engine. The process thrusts the engine forward.
Before passing through the nozzle, the hot air passes through another set of blades called the turbine. The rotating turbine causes the compressor to spin.
A Short History
Frank Whittle, a British pilot, designed and patented the first ever turbo jet engine in 1930. However, it was not until 1941 before he saw his first successful flight. The engine had a multistage compressor, combustion chamber, turbine and nozzle similar in construction to the modern day jet engines we use today.
Around the same time as Whittle was perfecting his engine in England, Hans von Ohain was constructing a similar engine in Germany.
In 1939 the gas turbine engine powering the German Heinkel He 178 made the world’s first successful turbojet powered flight.
From these humble origins General Electric went on to build the first American jet engine for the US Army Air Force jet plane.
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A range of lightweight, strong, corrosion resistant and thermally stable materials are vital to the success of any jet engine.
Titanium is one such material which plays a pivotal role in modern jet engine design. Its strength to weight ratio and resistance to extreme heat make it the ideal candidate for aerospace applications.
Over the years titanium has been mixed with other metals, such as nickel and aluminium, to create materials which offer unique characteristics helping engineers develop more efficient engines.
The Fan Blade System
The fan blades must be operational in extreme temperatures and conditions for the entire lifetime of the engine which typically reaches up to 10,000 flights.
The blades rotate around 3300 times every minute with a tip speed of 1730 Km/h. The centripetal force at the base of each blade during rotation reaches 900 Kilonewtons.
They are expected to withstand extreme weather and even impact from birds or other small objects. For this reason it is essential the right material is chosen.
Modern fan blades are made from a single piece of titanium alloy roughly 10kg in mass, 100cm high and 40cm wide.
One method used for manufacturing fan blades uses two blade skins shaped from molten titanium in a hot press. Once shaped the blades are removed and welded to a mate, with a hollow cavity in the centre. This cavity is then filled with a titanium honeycomb structure for increased strength.
Compressing the Air
The compressor blades are designed to provide high pressured air to the combustion chamber. As the pressure increases the temperature of the air also increases.
Compressor blades need to have high strength to deal with the range of stresses that occur during this process. They also need to be corrosion resistant at high temperatures, resistant to deformation and light weight.
Nickel based alloys are typically used as they can retain their strength at temperatures above 700⁰C and have excellent corrosion resistant properties.
Compressor blades are traditionally made by forming wax copies of the blades and then submerging them in a ceramic slurry. After heating until hard and the wax is melted away, molten metal is poured into the cavity left by the wax.
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The Combustion Chamber
Once the air leaves the compressor it reaches the combustor. Here fuel is added and burnt while the air is slowed down to ensure combustion takes place.
During combustion the air can reach temperatures up to 2100⁰C. Most metals melt at around 1600⁰C so to ensure the engine does not melt, the inside is lined with advanced ceramic thermal coatings which offer excellent heat resistance.
These coatings conduct heat 35 times slower than the metals used and are 250 micrometers thick. The resulting reduction in the metal temperature is around 300⁰C.
The combustion chamber is traditional made from a titanium alloy designed to increase its ductility. After heating, the titanium is poured into a series of complex segment moulds. The sections are then welded together and mounted in the engine.
The last stage of the jet engine is the turbine. It is constructed from a series of bladed discs made from a nickel based super alloy, which collects energy from the hot gas leaving the combustor. This energy is used to drive the compressor and the fan.
The turbine experiences extreme conditions with each blade spinning at 10,000 RPM resulting in high levels of centrifugal load.
Temperatures can reach up to 1600⁰C and metals at these temperatures are prone to creep. To avoid this the blades employ advanced cooling systems whereby a thin layer of cool air from the compressor coats their surface. This method is hugely effective and if applied to a blade of ice placed inside a conventional oven, it would remain frozen indefinitely.
Each blade is also coated in an advanced ceramic material designed to shield them from the high temperatures produced.
Pressure from international authorities is forcing the aerospace industry to produce more efficient engines.
Projects run by the U.S Air Force air developing more advanced engines capable of maximizing their efficiency and are known as ‘adaptive-cycle jet engines’. Unlike conventional jet engines that are most efficient at a single point in flight, these new age engines would be able to vary their bypass ratios for a variety of speeds and altitudes.
Other projects include a new research and development facility run by Rolls Royce at Virginia Tech and the University of Virginia. The facilities are focused on developing the next generation of engine for the British Company.
Whatever the future holds for the jet engine it's clear that advancements in materials science will play a pivotal role.
References and Further Reading