How a Jet Engine Works

When talking about jet engines, people usually mean gas turbine engines, like those used in airplanes. However, in reality, a jet engine can also refer to a rocket engine or a ship’s water jet. It’s all about the principle of motion. If it uses a jet stream, it’s a jet engine.

But this article will mainly focus on aviation turbojet engines or gas turbine engines. What are they, and how do they work?

What Is a Jet Engine?

There are many types of jet engines, and some prototypes even exist in nature. So, understanding how they work is quite simple.

A jet engine is an engine that uses the principle of jet propulsion.

In simple terms, to move forward, something must be pushed backward. For example, a squid or cuttlefish does this. These animals take in water and then quickly expel it in the opposite direction of where they want to move. The secret to motion lies in the word “quickly.”

The Principle of Jet Propulsion

The law of conservation of momentum states that in a closed system, the total momentum remains constant. In the case of a squid and water, this means that by expelling water backward, the squid must move forward. The speed of movement will be greater the larger the mass of water expelled and the faster it’s expelled.

Squid — a living jet engine

You can demonstrate the law of jet propulsion with a person. Just stand on skates or rollerblades (to reduce friction), pick up something heavy, and throw it to one side. As a result, the person will move in the opposite direction. The heavier the object and/or the faster it’s thrown, the farther and faster you’ll move.

It’s no coincidence that momentum is also called “quantity of motion.” Momentum is mass multiplied by velocity. A simple formula:

P = mV

In our case, the law of conservation of momentum is:

m₁V₁ = m₂V₂ or m₁V₁ – m₂V₂ = 0

So, a jet engine works by expelling a working medium (gas or liquid) in one direction to move in the opposite direction. Since the mass of the engine and the vehicle it powers is greater than the mass of the working medium, a very high speed is needed.

Otherwise, it won’t work. The mass of the working medium can’t be greater than the engine’s mass, or you’d end up with a very strange vehicle. Thus, the main task of a jet engine is to ensure the highest possible speed of the jet stream and the highest possible mass flow rate.

Mass matters too — the more air or water the engine processes, the better. After all, momentum is velocity multiplied by mass.

The principle of a jet engine is simple, as even primitive creatures like squids use it. But this simplicity doesn’t mean creating the first turbojet engine was easy.

How a Jet Engine Works

A turbojet engine:

1. Draws in surrounding air
2. Compresses it
3. Mixes it with fuel
4. Ignites the mixture
5. Expels it at high speed

A rocket engine works the same way, except it doesn’t need surrounding air.

The key difference between a rocket and an airplane lies in this: a rocket (with a rocket engine) doesn’t need air, while an aviation jet engine does.

Why does an airplane engine need air? First, air contains oxygen needed for combustion. Second, there’s plenty of air around, so you don’t need to carry it with you. You can rely on the fact that the jet stream, which creates motion, will contain a large proportion of air.

That’s why we fly on airplanes with turbojet engines, not rocket engines. It’s simply more efficient. An airplane doesn’t carry the working medium for the engine, like a rocket does, it takes as much as it needs from the surrounding atmosphere.

That’s how a jet engine works, in simple terms. But a simple explanation doesn’t mean inventing and creating a turbojet engine was easy. In contrast, a primitive rocket engine is simple to create — China did it before the Common Era.

So, a jet engine must create thrust, which requires taking in large volumes of air and expelling it at high speed in the direction opposite to the airplane’s flight.

Thus, one of the key characteristics of such an engine is the compression ratio, which shows how many times the pressure before and after the compressor differs.

Structure of a Gas Turbine Engine

A gas turbine engine typically consists of the following parts:

1. Air intake or inlet device
2. Compressor (single-stage or multi-stage, with or without a fan)
3. Combustion chamber
4. Turbine
5. Nozzle

4.1 Afterburner chamber between the turbine and nozzle (modern passenger airplanes don’t have this, it was used long ago only in the Concorde and Tu-144). Additional fuel is injected there to sharply increase thrust.

Compressor

The compressor is the part of a jet engine that compresses air. In simple terms, thanks to the compressor, the engine can “suck in” surrounding air through the intake, compress it, and deliver it to the combustion chamber.

Combustion Chamber

After the compressor comes the combustion chamber. This is where compressed air is mixed with fuel and ignited. This is needed to:

1. Increase pressure
2. Increase velocity
3. Increase temperature—an undesirable side effect, but unavoidable

The combustion chamber’s task is to “extract” the energy created by the burning mixture and convert it into the velocity of the exhaust stream, as well as to rotate the turbine.

Turbine

The turbine, located right after the combustion chamber, is hit by hot gas from the combustion chamber. “Hot” can mean temperatures around 2000 degrees Celsius.

The turbine rotates the compressor, which compresses the air, and the fan, if it’s a turbofan engine. In general, the turbine is not a mandatory component in a jet engine’s design.

If there were an efficient way to rotate the compressor without a turbine, it would be better. Such an engine would be more efficient and cheaper, as turbine blades endure the greatest loads, making the turbine and its blades a very complex and expensive mechanism.

Moreover, energy spent rotating the turbine could be used to create jet thrust.

An engine without a turbine, where the compressor is driven by an external source, is called a motor-compressor engine. Early in jet engine development, such designs were proposed and even used, but they didn’t gain traction.

Thus, the classic jet engine consists of three main parts: compressor, combustion chamber, and turbine.

However, there’s one type of engine without a turbine or compressor—a ramjet engine. In such an engine, air is compressed due to the speed of movement. This means the ramjet engine needs to be accelerated to a high speed to work.

After the turbine is the nozzle, whose task is to reduce the speed of the exhaust gases to an optimal value, usually subsonic. This is because supersonic speed leads to the formation of shock waves.

In subsonic passenger airplanes, the nozzle is just a pipe. But in supersonic airplanes, the nozzle is made controllable. Additionally, in supersonic airplanes, an afterburner chamber is placed between the turbine and nozzle, where extra fuel is injected when needed to increase engine thrust.

The above describes, in simple terms, the design of a gas turbine engine with an axial compressor, the most common type. But the simplicity of the description and principle shouldn’t be misleading. A gas turbine engine is a highly complex mechanism to develop and manufacture.

Only five countries in the world can create competitive jet engines. The civilian engine market is dominated by four major manufacturers:

• CFM International (France and USA) — 39%
• Pratt & Whitney (USA) — 35%
• General Electric (USA) — 14%
• Rolls-Royce (UK) — 12%

Countries like China, India, Turkey, or South Korea are only at the beginning of this path, developing engines for their military aircraft, and it’s impossible to know how high-quality or not their products are.

Despite the complexity, a gas turbine engine has a very high efficiency, theoretically up to 60%. For comparison, the maximum theoretical efficiency of a piston engine is 40%. In simple terms, such an engine is very efficient. But most importantly, it provides significant thrust and an excellent thrust-to-weight ratio.

A piston engine loses out because as its power increases, its weight grows too much (due to the square-cube law). In simple terms, further increasing its size is inefficient, as it becomes too heavy to lift into the air.

The First Jet Engine

Who Invented the Jet Engine?

This is a debated question, best split into two or even three parts:

1. Who came up with the idea of a jet engine?
2. Who first created a jet engine?
3. Who first managed to bring an engine to flight tests?

The first patent for something resembling a turbojet engine was obtained in 1921 by French engineer Maxime Guillaume. But it was just a turbine drawing and nothing more. So, Guillaume can’t be considered the inventor of the gas turbine engine.

So, who invented the jet engine?

The first working engine was demonstrated by British engineer Frank Whittle. That’s why he’s credited as the inventor of the jet engine in history.

Despite the fact that the first airplane with a jet engine took off not in the UK but in Germany. In 1939, Germany saw the first flight of an airplane powered by an engine created by another engineer, Hans von Ohain.

Whittle patented his invention in 1930, while Ohain did so only in 1936. Thus, Frank Whittle is considered both the inventor of the gas turbine engine and the first to create a working prototype.

Frank Whittle’s first jet engine

The first serial jet aircraft in the UK, the Gloster Meteor with the W1 engine, took off on May 15, 1943, 13 years after Whittle filed his patent.

Creating a turbojet engine took so long primarily because its inventor struggled to find investors or interest the British government.

Whittle’s invention gained attention only after the start of World War II.

In his jet engine design, Whittle found solutions to several engineering problems. These solutions were so successful that they are still used today.

Problems of the First Gas Turbine Engine

Here are a few examples of challenges and their ingenious solutions:

• How to prevent the combustion chamber from burning or melting? After all, the combustion temperature in the first jet engine reached 700 degrees Celsius. In modern engines, temperatures are even higher, from 1500 to 2000 degrees Celsius.

Whittle devised a brilliant solution: the mixture burns inside a tube of cold air. The combustion chamber was designed with multiple holes through which air entered, enveloping the chamber from the inside.

Thus, the flame doesn’t contact the combustion chamber walls. Although this chamber was invented by Whittle in the 1930s, it’s still used in modern engines.

• How to ensure the mixture ignites at all? After all, the airflow speed at the combustion chamber inlet was 90 meters per second, or 324 km/h. How do you ignite combustion in such a “wind”?

Frank Whittle solved this problem too. He installed special blades that swirled the air to create a region of reverse flow. In simple terms, a pocket of air where the flow swirls, creating a “calm zone” like the eye of a hurricane. This solution is still used by engineers today.

The third problem and its ingenious engineering solution:

• How to prevent the turbine blades and the turbine itself, rotating with gas at 700 degrees, from melting or deforming? Making the blades and turbine disc thicker? But then the engine would be too heavy.

The British engineer didn’t weigh down the design, he created a turbine disc cooling system to keep it cool.

The same is done in modern gas turbine engines. In addition to cooling the disc, the turbine blades are also cooled, with special channels inside them for airflow.

It’s also worth noting that the turbine rotated at 17,000 revolutions per minute. Besides overheating, it also faced mechanical stress and would inevitably break. So, the turbine blades in Whittle’s engine were replaceable, allowing a broken blade to be swapped without replacing the entire turbine. This is still done today.

The first jet engine in history was very similar to modern ones, except for one key detail: Whittle’s engine had a centrifugal compressor, not an axial one like in modern engines.

Axial compressors are used because it’s easier to increase pressure with them—just add more discs. A centrifugal compressor can achieve a compression ratio of up to 4, while a modern axial compressor can reach 40!

However, all early jet engines used centrifugal compressors because engineers didn’t yet know how to calculate airflow through multiple compressor stages or deal with harmful effects like compressor surge.

So, a gentler, though less efficient, solution was chosen. After all, centrifugal superchargers were already widely used in piston engine designs, and their construction, advantages, and disadvantages were well-studied.

Hans von Ohain’s engine also had a centrifugal compressor.

He could also be called the inventor of the jet engine, as his aircraft flew earlier, suggesting his design was better. But no, it’s known that von Ohain saw Whittle’s patent, as patents aren’t secret documents.

Work on creating a jet engine in Germany progressed faster than in the UK because it started later, on the eve of a new war, and Germany spared no resources for weapon development.

Heinkel, where Ohain worked, provided all the resources Europe’s largest aviation company could offer. But in return, they demanded that the jet engine lift an aircraft into the air within a year. Such haste couldn’t end well…

The First Jet Airplanes

1. The first airplane with a jet engine was the German Heinkel 178, which made its first flight in August 1939.
2. In the UK, an airplane with Whittle’s engine took off only in May 1941, it was the experimental Gloster E.28/39.
3. In the USA, the first jet fighter was the F-80 Shooting Star, which flew in 1944. Notably, it used a licensed copy of the British Rolls-Royce Derwent engine designed by Frank Whittle.
4. The first serial jet airplane was the Messerschmitt Me 262, equipped with two serial Jumo-004B jet engines. A distinctive feature of the Jumo-004 was its axial compressor, like in modern aviation engines.

But why wasn’t Ohain’s engine used in serial German airplanes? Why was the Junkers engine chosen for the Me 262 fighter and the Arado Ar 234?

By the way, BMW also designed a gas turbine engine, the BMW 003, but it also lost the competition to Junkers.

Perhaps because Ohain’s engine proved unsuccessful. There’s no mention anywhere of how long it could operate or its reliability. The history of Soviet jet airplanes hints at this…

The first Soviet jet fighter was the MiG-9, equipped with the R-10 engine, a copy of the German Jumo-004. It made its first flight in 1946. Later, the USSR preferred British engines.

The mass-produced and effective MiG-15 fighter was equipped with a copy of the British Rolls-Royce Nene engine. The Soviet government bought several of these from the British, but in the Soviet Union, they were called RD-45. Essentially, they “honestly” stole the engine from the British (without paying for a license) because it was the best at the time.

The first passenger jet airplane was the British De Havilland Comet. It made its first flight in 1949 and began commercial flights in 1952. Early models used engines with centrifugal compressors, like Whittle’s, but these were later replaced with axial compressor engines from Rolls-Royce.

Types of Jet Engines

• GTE — Gas Turbine Engine. A jet engine where the turbine is driven by a gas stream and rotates the compressor.
• TJE — Turbojet Engine. An engine where thrust is generated by a stream of hot gases.
• TPE — Turboprop Engine. Unlike a turbojet, thrust is created by a propeller.
• TSE — Turboshaft Engine. An engine where the energy of the jet stream is used to rotate a shaft. Such engines are used in helicopters.
• TFJE — Turbofan Jet Engine. A jet engine with two circuits: an inner one with hot gas and an outer one with cold air.
• TFE — Turbofan Engine. The same as a dual-circuit engine, but the outer circuit with cold air is larger than the hot one. Thus, what’s called a “low-pressure compressor” in a dual-circuit engine is called a “fan” in a turbofan engine. These engines are more efficient and economical, so modern passenger aviation uses dual-circuit engines with high bypass ratios and fans. The fan creates most of the engine’s thrust.
• RJE — Ramjet Engine. An engine without a compressor or turbine. The gas is compressed not by a compressor but by the incoming airflow. This means the engine needs a very high initial speed to work. Ramjet engines are most effective at supersonic and hypersonic speeds. The world’s first cruise missile, the German V-1, had a pulsating ramjet engine because it was cheaper and simpler than a piston engine.

The Largest Jet Engine in the World

The largest gas turbine engine in history is the GE90 from General Electric. It’s also the most powerful turbofan engine in the world. Its characteristics and size are truly impressive:

• Diameter — 4.2 meters*
• Fan diameter — 3.3 meters
• Maximum thrust — 532,800 Newtons**
• Compression ratio — 1:40
• Compressor: fan, 3 low-pressure stages, 10 high-pressure stages
• Turbine: 2 low-pressure stages, 6 high-pressure stages
• Combustion chamber temperature — 1300 degrees Celsius

*This is larger than the fuselage diameter of an airplane like the Boeing 737 — 3.76 meters.

**Or 53.2 tons. The record thrust for this engine is 61 tons.

The principle of a jet engine is simple. Even primitive creatures like squids use it. But this simplicity doesn’t mean building a turbojet engine was easy. It remains a highly complex task, despite the simple operating principle. Implementing it correctly and efficiently is very challenging.