In this blog post, we’ll explore the evolution of aircraft propulsion systems—from humanity’s longing for flight to piston engines, propellers, turbojets, turboprops, turbofans, and ramjets—and examine how this progression led to space propulsion technology.
The fact that people have constantly strived to reach the sky can even be found in Greek mythology. The story of Icarus, who flew through the sky to escape the Labyrinth but ultimately lost his life after flying too close to the sun, illustrates both the longing for flight and the frustration that comes with it. Humankind has sought countless methods to fly through the skies. As understanding of mechanics broadened following Newton’s ‘Principia’, people discovered what was needed to make an object fly; one of the answers was that sufficient thrust—that is, the force that propels an object forward—was required.
Following the Industrial Revolution, when humans began relying on mechanical power rather than human strength, the invention of the steam engine ushered in an era where power far exceeding human capacity could be harnessed. As technology advanced, more sophisticated internal combustion engines emerged, and as their efficiency and power output gradually improved, so did our understanding of and applications for transportation. This led humanity to take on the challenge of the skies once again. In 1903, during the first powered flight, the Wright brothers successfully flew by using a water-cooled, four-stroke, four-cylinder internal combustion engine to drive two propellers. The engine used at the time produced approximately 12 horsepower, and the propeller played a key role in converting the engine’s power into thrust.
Although the number of propeller-driven aircraft using these reciprocating engines increased, their limitations began to surface over time. One of the biggest problems was that, since thrust generation relied on the propeller, propeller efficiency decreased as flight speed increased, and compressibility effects and shock waves occurring at the blade tips led to a significant decline in performance. Therefore, the development of high-speed aircraft required a new propulsion system to replace propeller propulsion. In fact, while propellers are highly efficient at low speeds, their performance limitations become apparent as speed increases, making them unsuitable for high-speed aircraft.
Consequently, research was conducted to develop a new propulsion system for high-speed flight. Jet propulsion—which generates thrust by rapidly expelling combustion gases backward—emerged, but initially, there were limitations in effectively converting the energy from fuel combustion into thrust. To address this, a design was adopted that compresses external air to supply high-pressure air to the combustion chamber. Today, the basic components of a turbojet engine consist of a compressor that compresses air, a combustion chamber where fuel is burned, a turbine that drives the compressor and the turbine itself, and a nozzle that rapidly expels high-temperature, high-pressure gas to generate thrust. The turbojet engine is a representative early form of such gas turbine propulsion systems.
Following the development of the turbojet, a new concept emerged: rather than converting all the energy of the combustion gases into high-speed exhaust gases to generate thrust, a significant portion of that energy would be used to drive a turbine, which would then transfer that energy to the propeller as rotational force. This method of driving a propeller with a gas turbine is known as the turboprop engine. In a turboprop, the gas turbine’s turbine rotates the propeller via a reduction gear and drive shaft, and the propeller generates the majority of the total thrust. Consequently, the speed and thrust of the jet exhaust are relatively low, and the system primarily relies on the propeller accelerating a large volume of air with a relatively small change in speed. For this reason, turboprops demonstrate high efficiency at low and medium-low speeds and are still used today in small passenger aircraft and transport planes. On the other hand, as flight speed increases, the propeller’s efficiency decreases, making it unsuitable for high-speed aircraft.
So, is it possible to create an engine that retains the advantages of a propeller while also being suitable for high-speed flight? The turbofan engine was developed based on this concept. Like a turboprop, a turbofan utilizes the energy from a gas turbine, but instead of a propeller, it uses a fan at the front of the engine and is designed so that a portion of the air passing through the fan bypasses the engine’s core and flows along the outer perimeter. The air that bypasses the engine’s core in this manner is called bypass air. A turbofan with a high bypass ratio can accelerate a large volume of air with a relatively small change in speed, thereby achieving high thrust efficiency and fuel efficiency. Conversely, turbofans with a low bypass ratio are used in military aircraft and other applications that prioritize high-speed flight and high acceleration performance. Today, most large commercial airliners use turbofan engines, which offer high thrust and fuel efficiency, and turbofans with various bypass ratios are employed depending on the aircraft’s mission and speed.
Another propulsion system capable of operating efficiently in supersonic environments is the ramjet engine. Unlike turbojets or turbofans, ramjets do not use rotating compressors or turbines; instead, they utilize the aircraft’s forward speed to compress the air entering the engine. Their structure is very simple, consisting essentially of an air intake, a combustion chamber where fuel is burned, and a nozzle that accelerates the combustion gases to generate thrust. As the aircraft moves quickly, the speed of the air at the intake decreases, causing the pressure to rise. Fuel is then injected into this compressed air to ignite it, and the resulting hot gases are accelerated through the nozzle. Consequently, a ramjet cannot generate sufficient thrust on its own while stationary; it must first be accelerated to a certain speed or higher by another propulsion system. Due to these characteristics, ramjets are well-suited for high-speed flight, and research has been conducted on scramjets, which maintain the combustion process itself at supersonic speeds in even higher velocity ranges.
Subsequently, research on propulsion technology gradually expanded beyond aviation to include rocket-based space propulsion, and humanity finally broke free from the Earth’s surface to venture into Earth orbit and beyond into deep space. One of the future goals is to develop propulsion technologies capable of efficiently traveling greater distances within the solar system and carrying out long-duration space missions. In this process, ion propulsion has established itself as one of the key technologies. An ion engine uses electrical energy to ionize propellant and then accelerates the ions at very high speeds using an electric field to generate thrust. Although the thrust itself is very small compared to chemical propulsion systems, it has the advantage of using propellant extremely efficiently. Therefore, it is better suited for spacecraft that require continuous acceleration over long periods rather than launch vehicles that need to generate high thrust in a short time. In fact, ion propulsion is no longer merely a next-generation technology of the future; it has been used in NASA’s Deep Space 1 and Dawn spacecraft, playing a crucial role in the exploration of asteroids and dwarf planets. In particular, the Dawn spacecraft used ion propulsion to explore the asteroid Vesta and the dwarf planet Ceres in succession, and electric propulsion, which utilizes this high specific impulse, is being utilized as an important option for long-distance space missions. However, due to its low thrust, continued research will be necessary in areas such as power supply, thrust enhancement, and propellant efficiency improvement.
