Turbojet Engine

Turbojet engines are the first jet engines used in aviation and are propulsion systems that operate using gas turbine technology. Their fundamental operating principle relies on compressing airflow, mixing it with fuel, and igniting it to produce high-velocity exhaust gases. When these exhaust gases are expelled rearward from the engine, they propel the engine forward in accordance with Newton’s action-reaction principle. Turbojet engines are powerful thrust systems that enable aircraft to fly efficiently at high speeds and high altitudes.

Historical Development

The history of turbojet engines extends back to the early 20th century. While the earliest principles date back to an evolution spanning from ancient times to the present, the foundational development of turbojet engines was particularly established in the early 1930s.

• 1930s – Initial Research: British engineer Frank Whittle and German engineer Hans von Ohain independently began working on jet engines. Whittle obtained a patent for a turbojet engine in 1930 and later used it in the Gloster E28/39 aircraft in England in 1939. During the same period, Hans von Ohain from Germany achieved the world’s first successful jet flight in 1939 with the Heinkel He-178 aircraft.
• World War II: Jet engines rapidly advanced for use in military aircraft. In 1941, Germany flew the Messerschmitt Me 262, the world’s first operational jet-powered combat aircraft. These developments solidified the importance of turbojet engines in military aviation.
• Subsequent Developments: In the 1950s, turbojet engines began to be used in commercial aircraft. However, their efficiency decreased significantly at low speeds. Nevertheless, turbojet engines provided high speed and performance suitable for military aircraft and supersonic flight. From the 1960s onward, the development of turbofan engines led turbojet engines to gradually be replaced by more efficient propulsion systems.

Components of Turbojet Engines

Turbojet Engine Schematic

^{Subaşı Hilal, Eren Türkoğlu, Kerim Yazıcı, Sefer Çelik, and Özge Erdoğan. PID Control of the Radial Fan of a Turbojet Engine. Master's thesis, Karabük University, 2019. Advisor: Dr. Öğr. Üyesi Cihan Mızrak.}

A turbojet engine consists fundamentally of five main components:

Inlet: This section draws air from the atmosphere into the engine. The design of the inlet varies depending on whether the aircraft is flying subsonic or supersonic. The inlet is a important component that significantly affects engine efficiency and performance.

Turbojet Engine Inlet (Source: Freepik/jannoon028)

Compressor: Air is compressed in the compressor section of the engine. Compressors are typically either axial-flow or centrifugal type. Compression increases air pressure and temperature. This stage determines the engine’s efficiency and power output.

Combustion Chamber: The compressed air is mixed with fuel and ignited here. As a result, high-temperature, high-pressure gases are produced. These high-energy gases are directed toward the engine’s turbine section.

Turbojet Engine Combustion Chamber (Source: DefenceTurk.net)

Turbine: The turbine uses the high-energy gases from the combustion chamber to drive the compressor and other components. The turbine enables the engine to sustain its work cycle.

Turbojet Engine Turbine Section (Source: Levtems)

Exhaust Nozzle: Gases passing through the turbine exit through the nozzle, gaining additional velocity and pushing against the atmosphere to propel the engine forward. The nozzle optimizes gas speed and pressure to achieve maximum thrust.

Turbojet Engine Exhaust Nozzle Section (

Operating Principle of Turbojet Engines

Turbojet engines first compress incoming air, then mix it with fuel and ignite it. The resulting gases are directed through a turbine to generate energy. The turbine provides the necessary energy to sustain engine operation. Finally, the combustion gases are expelled at high speed through the nozzle, generating thrust.

The fundamental principle of the engine is that the high-speed combustion of the fuel-air mixture produces energy that accelerates gases rearward. This rearward thrust propels the aircraft forward.

Types of Turbojet Engines

Turbojet engines are classified into several types, each with distinct applications and performance characteristics:

1. Turbojet: This is the basic type of jet engine. Air is compressed, mixed with fuel, and ignited to produce thrust. Turbojet engines are inefficient at subsonic speeds and are typically used in military aircraft and cruise missiles.
2. Turbofan: Similar to a turbojet engine but features a large fan at the front. This fan generates additional thrust and improves engine efficiency. Turbofan engines, which are more efficient at subsonic speeds, are widely used in commercial aircraft.
3. Turboprop: This engine type uses a propeller instead of a jet nozzle. It operates on the same fundamental principle as a turbojet engine but uses exhaust gases to drive a propeller. It is commonly used in small aircraft and short-range flights.
4. Ramjet: A ramjet is a simple jet engine with no moving parts. Air enters the engine due to the aircraft’s speed, is compressed, mixed with fuel, and ignited. However, this engine type requires a minimum speed to begin operation and is only efficient at high speeds.
5. Scramjet: A variant of the ramjet, the scramjet compresses and ignites air at speeds exceeding the speed of sound. These engines are suitable for hypersonic flight and achieve extremely high velocities. NASA’s X-43A, powered by a scramjet engine, reached very high speeds.

Advantages and Disadvantages of Turbojet Engines

Advantages

1. High Speed and Performance: Turbojet engines operate effectively at high speeds and are ideal for supersonic flight. They are a preferred choice for military aircraft and some high-speed commercial aircraft.
2. Simplified Design: The structure of turbojet engines is simpler compared to more complex engine types. This simplifies maintenance and repair processes.
3. High Efficiency at High Speeds: At high flight speeds, turbojet engines are highly efficient because thrust is generated by accelerating exhaust gases to high velocities.

Disadvantages

1. Low Efficiency at Low Speeds: Turbojet engines become inefficient at low speeds, which is why most civilian aircraft prefer the more efficient turbofan engines.
2. High Noise Levels: Turbojet engines can produce high noise levels, especially at low speeds. This poses a significant environmental concern in civil aviation.
3. High Fuel Consumption: Turbojet engines consume more fuel, particularly at low speeds, increasing operational costs.
4. Long Takeoff Distance: Turbojet engines require longer takeoff distances compared to other engine types. This can be problematic at airports with short runways.

Development of Turbojet Engines in Türkiye

In line with its goals of technological independence and advancement in the defense industry, Türkiye has made several significant strides in recent years. The development of turbojet engines has emerged as a key milestone in this process. The “Development of Turbojet Engines Project,” carried out by the Scientific and Technological Research Council of Türkiye and the Undersecretariat for Defense Industries, has marked a major achievement in Türkiye’s efforts to develop indigenous jet engines.

KTJ-3200 Turbojet Engine (Source: Defence Turkey)

Kale Arge and the National Turbojet Engine: KTJ-3200

The KTJ-3200, Türkiye’s first indigenous turbojet engine, was developed by Kale Jet Engines and has been deployed in domestic missile systems. Designed to power the ATMACA and SOM missiles, the engine is manufactured entirely using domestic resources. The development of the KTJ-3200 represents a critical step for Türkiye’s defense industry. Its local production has reduced the country’s dependence on foreign suppliers and provided a vital safeguard against various export restrictions.