How A Jet Engine Works
Today's jet engines represent an incredible journey in aviation and technological discovery. Sir Frank Whittle created the first turbojet engine in 1930 and subsequently introduced the world to an invention that would revolutionize the way humans travel the world.
Over eight decades ago it seemed impossible to travel across continents at high speed but today no one thinks twice about it. Most of the time we don’t worry about how a jet engine works, as long as it keeps us up in the sky and safely en route to our destination - but if you’re feeling a little curious we’ve got the answers for you.
We asked Guy Ellis, author of Britain’s Jet AgeFrom the Meteor to the Sea Vixen (Amberley Publishing 2016) and Britain’s Jet Age From the Javelin to the VC10 (Amberley Publishing 2016), to help us explain just how a jet engine works.
Sir Isaac Newton’s Third Law of Motion
Fun at any age is to inflate a balloon and then release it to make it perform a crazy, uncontrolled flight. As we all know, it moves in the opposite direction to the outflow of air, which is an example of thrust in its rawest form
A jet engine operates on a similar principle. Air flows through the engine via the intake at the front, is compressed and then forced into combustion chambers, where fuel is injected and the resulting mixture ignited. The gases formed expand and are then released from the rear of the combustion chamber. This backward movement of heated-expanded air exerts an equal and opposite force in propelling the aircraft forward – known as ‘thrust’. As the gases are expelled, they pass through the turbine, a set of fan-like blades rotating a shaft that, in its turn, rotates the compressor, which, again, brings in a new supply of air.
The first gas-turbine engine was patented and built by John Barber of Nottingham in 1792. This was a chain-driven engine with a compressor, combustion chamber and turbine, powered by wood, coal and other inflammable substances. However, the heat-resistant materials necessary to generate enough power to compress the air and gases, and to have sufficient power to effectively drive the chain and any attached devices, were simply not available. These same limitations meant that Frenchman Maxime Guillaume was unable to build his multi-stage axial-flow turbojet, patented in May 1921. In a 1926 paper, An Aerodynamic Theory of Turbine Design, Alan Arnold Griffith laid out the founding principles of a turboprop engine using an axial compressor fitted with aerofoil shaped blades creating more efficient airflow.
By that same year, Frank Whittle had developed his theory of using gas turbines to power aircraft at speeds and altitudes that could not be achieved by piston engines, which he patented in 1930. His patent and papers were carefully studied in Germany by Dr Hans von Ohain who registered his patent for a turbojet in 1936. Working for Ernst Heinkel, in a secret private venture, he developed a jet engine which flew for the first time on 27 August 1939 in a He 178. Captain Erich Warstiz made the first jet aircraft flight which was marked by the loss of the engine shortly after take-off, when a bird was sucked into the intake – the world’s first jet–bird strike.
Largely ignored by the Reichsluftfahrtministerium, the development of the jet was hampered by the demand to produce ‘normal’ aircraft. A less-developed ducted-fan jet system was used to power the Italian-built Caproni-Campini CC2 which became the world’s second jet aircraft to fly in 1940.
Jets at work
Whittle’s centrifugal compressor-driven turbojet engine forces the incoming air outwards into compression chambers, where the air is pressurised, ignited, heated and ejected at high speed through the blades of a turbine. This reverse thrust pushes the aircraft forward and drives the compressor, drawing in more air. These types of engines were robust and relatively easy to manufacture contrary to the axial-flow engine which, while they provide higher pressure ratios and therefore more thrust, require far more advanced metals and more sophisticated manufacturing methods. The axial engine compresses air through a set of fans on the same axis and does not force air into compartments surrounding the main chamber.
Apart from the turbojet, both axial and centrifugal, there are four other main types of jet engine. The first is the turboprop. Here, exhaust gases drive a propeller which provides better efficiency at speeds below 500 mph, typically applicable to light aircraft and smaller short-haul passenger aircraft. With development, higher speeds have been achieved; consequently, propellers are smaller in diameter, with multiple scimitar-shaped blades that increase the airflow into the compressor.
Secondly, most modern airliners are equipped with turbofan engines where a very large fan at the front sucks in air, but most of that air flows around the engine rather than through the compressor, pushing additional air out of the rear of the engine without increasing fuel consumption. This has the added advantage of quieter engines.
Thirdly, for helicopters specifically the turboshaft engine was developed which drives the rotor blades at a different speed to that of the compressor. As the rotor blade provides lift and directional control, it is important that it maintains a constant speed even while extra power is provided by the engine.
Fourthly, the ramjet is a simple engine with no moving parts. Here forward speed rams air into the front of the engine and so requires an assisted take-off, such as an air launch or a catapult. Thrust is then dependent on increasing the forward speed. These engines are used in high-speed, short-burst environments such as missiles.
Jet engines are lighter, have fewer moving parts, provide greater power and are more fuel efficient than piston engines. The Supermarine Spitfire had on average a top speed of 367 mph; the first British jet, the Gloster E28/39, produced 850 lbs of forward thrust and a top speed of 466 mph, whereas the Concorde could produce 38 050 lbs of thrust per engine with a top speed of 1 334 mph.