Jet engine

Jet engines are reaction engines that generate thrust by expelling a high-velocity jet of heated gases, most commonly derived from air. Although the term may broadly include rocket propulsion or hybrid systems, it is typically used to describe air-breathing internal combustion engines such as turbojets, turbofans, ramjets, pulse jets, and scramjets. These engines operate predominantly on the Brayton cycle, relying on compressing air, mixing it with fuel, combusting it, and expanding the resulting high-energy gases through a turbine and nozzle to produce thrust. Jet engines have become central to modern aviation, powering both civilian and military aircraft across a wide range of speeds and altitudes.

Fundamental Principles and Types of Jet Engines

Jet propulsion is based on Newton’s third law of motion: thrust is produced when mass is expelled in one direction, creating a reactive force in the opposite direction. Air-breathing jet engines draw atmospheric air into a compressor, where it is pressurised. This compressed air is mixed with fuel and ignited in the combustion chamber, generating high-energy gases. These gases expand through a turbine, which powers the compressor, and finally exit through a nozzle to create forward thrust.
Air-breathing jet engines form several key categories:

• Turbojets, which compress air using axial or centrifugal compressors and expel exhaust at high velocity. These were the earliest successful jet engines used on aircraft but are relatively inefficient at subsonic speeds.
• Turbofans, particularly high-bypass turbofans, are the dominant engines in modern subsonic commercial aviation. A large fan at the front accelerates substantial quantities of air around the core engine, improving fuel efficiency and reducing noise.
• Ramjets rely on the aircraft’s forward motion to compress incoming air without mechanical components. They are effective only at high speeds.
• Scramjets operate similarly to ramjets but allow supersonic airflow through the combustion chamber, enabling flight at hypersonic speeds.
• Pulse jets, using intermittent combustion, historically powered early missiles but are comparatively inefficient and noisy.

Technological advancement has significantly enhanced reliability and efficiency. Jetliner engines evolved from the de Havilland Ghost of the 1950s to modern turbofans such as the General Electric GE90, with in-flight shutdown rates falling from around 40 per 100,000 flight hours in the early jet era to fewer than one per 100,000 in the late twentieth century. These improvements enabled long-range twin-engine operations (ETOPS), previously impractical for commercial airlines.

Historical Background and Early Concepts

The conceptual origin of jet propulsion predates modern engineering by many centuries. A rudimentary demonstration is associated with the aeolipile of Hero of Alexandria in Roman Egypt, in which steam escaping from nozzles caused a sphere to rotate. Although little more than a curiosity, it demonstrated the basic principle of reactive thrust. Across various civilisations, devices such as waterwheels and windmills introduced early forms of turbine mechanisms.
Theoretical precedents of jet propulsion have also been linked to Chinese fireworks and early rocketry, technologies that used the reactive force of burning propellant. Historical descriptions, including accounts of a seventeenth-century Ottoman inventor reputed to have experimented with rocket-assisted flight, illustrate the longstanding fascination with reactive propulsion.
By the early twentieth century, engineers understood that conventional propeller-driven aircraft faced performance limitations as blade tips approached the speed of sound, drastically reducing efficiency. The need for propulsion suitable for higher speeds led to renewed interest in gas turbine engines.

Foundations of the Gas Turbine and Early Experimentation

The conceptual basis for the gas turbine stretches back to John Barber, an English engineer, who was granted a patent for a stationary turbine in 1791. The first self-sustaining gas turbine was built in 1903 by the Norwegian engineer Ægidius Elling, although practical aviation applications remained distant due to challenges related to material strength, durability, and compressor design.
A significant milestone occurred in 1921 when Maxime Guillaume filed a patent for an axial-flow turbojet, although the design was ahead of contemporary engineering capabilities. Meanwhile, Alan Arnold Griffith’s influential aerodynamic studies in 1926 laid theoretical groundwork for efficient turbine and compressor systems and inspired further experimental work in Britain.
Parallel developments occurred in a number of countries. Early hybrid engines, such as those of the Caproni Campini N.1 in Italy and the Japanese Tsu-11 intended for kamikaze aircraft, relied on external compressors and lacked the performance necessary for operational viability.

The Emergence of Practical Jet Propulsion

The first practical turbojets were developed independently in Britain and Germany during the 1930s. In Britain, Frank Whittle, then a cadet at RAF College Cranwell, submitted a formal proposal in 1928 outlining a self-contained gas turbine for aircraft propulsion. His subsequent development work led to the first British turbojet-powered aircraft flight in the Gloster E.28/39.
In Spain, notable independent work was carried out by Virgilio Leret Ruiz, who received a patent in 1935 for a jet engine design. Although political events halted further development, the design contributed to broader European investigations into jet propulsion.
In Germany, Hans von Ohain began work in 1935 on a centrifugal-flow turbojet similar in principle to Whittle’s, although developed independently. Collaborating with aircraft industrialist Ernst Heinkel, von Ohain’s team successfully ran an experimental engine by 1937. The Heinkel He 178, powered by von Ohain’s HeS 3, achieved the world’s first jet-powered flight on 27 August 1939.

Wartime Developments and Early Operational Engines

Wartime pressures accelerated jet engine research. In Germany, Anselm Franz at Junkers developed the Jumo 004, the first mass-produced operational axial-flow turbojet. It powered the Messerschmitt Me 262 jet fighter and the Arado Ar 234 jet bomber. Although production and deployment were hampered by material shortages and late-war disruption, these aircraft represented landmark achievements in military aviation.
In Britain, Power Jets Ltd., founded by Frank Whittle, produced engines for the Gloster E.28/39 and subsequently the Gloster Meteor, which entered RAF service in 1944. Both the Meteor and the Me 262 achieved operational status within months of each other, marking the arrival of jet aircraft in combat.
Post-war analysis of German engines and airframes heavily influenced early Cold War development in both the United States and the Soviet Union, particularly in the adoption of axial-flow compressor technology, which remains prevalent in modern jet engines.

Post-war Expansion and Commercial Development

By the 1950s, jet propulsion became nearly universal in military fighter aircraft. Early civil jet airliners, such as the de Havilland Comet and the Avro Canada Jetliner, introduced high-speed passenger travel, although technological refinements were required to ensure safety and efficiency.
The turbofan revolution of the 1960s and 1970s transformed commercial aviation. High-bypass turbofans substantially reduced fuel consumption and noise while improving power output, making them ideal for long-haul subsonic flight. Their efficiency at high altitudes enabled airlines to operate longer routes with fewer stops, supporting the global expansion of commercial air travel.
By the late twentieth century, the development of engines like the GE90 demonstrated remarkable advances in materials, aerodynamics, and digital control systems. These engines offered high thrust, improved fuel burn, and unprecedented levels of reliability, underpinning modern long-range twin-engine aircraft operations.

Applications Across Fields

Jet engines today fulfil a diverse range of applications:

• In aviation, they power commercial jetliners, business jets, military aircraft, and unmanned aerial vehicles.
• In missiles and spaceflight, rocket engines—while not air-breathing—apply similar reactive thrust principles.
• In specialised vehicles, jet engines have been fitted to experimental land speed record cars and high-velocity test vehicles.
• In model rocketry and certain industrial systems, simplified jet or rocket engines are used for controlled propulsion or high-energy outputs.