In this blog post, we’ll examine the evolution of aircraft propulsion technology—from propellers to jet engines and on to ramjet engines—and explore the structure, operating principles, advantages, and limitations of ramjet engines, which utilize the ram effect.
The Evolution of Propulsion Technology Toward Faster Flight
Ever since the Wright brothers’ first powered flight in 1903—during which they flew approximately 36.5 meters in 12 seconds—humankind has continued to research ways to fly even faster. The Wright brothers’ first flight took place on December 17, 1903, in Kitty Hawk, North Carolina, USA; the aircraft, piloted by Orville Wright, flew approximately 36 meters in 12 seconds. In July 1909, Louis Blériot (1872–1936) of France crossed the English Channel in an aircraft he built himself, and by the 1910s—fueled by the influence of World War I—the design and performance of aircraft advanced rapidly. However, until the 1930s, aircraft propulsion relied primarily on piston engines and propellers. In the 1930s, Frank Whittle (1907–1996) of Britain and Hans von Ohain (1911–1998) of Germany began developing jet engines independently of one another. Whittle began designing turbojet engines in 1928, and von Ohain’s engine was used in 1939 for the flight of the Heinkel He 178, the world’s first pure jet aircraft. The jet engine developed by Whittle in the United Kingdom successfully completed its first flight test in 1941.
Like a propeller, a jet engine generates thrust by pushing air backward. However, a jet engine compresses air, combusts fuel, and then expels high-temperature, high-pressure gas backward at high speed, thereby generating much more powerful thrust than a propeller. In propeller-driven aircraft, as the speed increases, so does the relative airspeed at the tips of the propeller blades. When the speed at the propeller tips approaches the speed of sound, efficiency drops significantly due to compressibility effects and shock waves, while noise and structural stress also increase. In contrast, since jet engines are not directly subject to these limitations imposed by propeller rotational speed, they have evolved into a propulsion method suitable for supersonic flight.
In propeller-driven aircraft, the speed of sound has long been considered a major barrier that was difficult to overcome. One of the reasons for this is shock waves. When an aircraft travels at a speed below the speed of sound, the sound waves generated by the aircraft can propagate forward; however, as the aircraft’s speed approaches the speed of sound, the pressure waves generated around the aircraft overlap, forming powerful shock waves. The Mach number is a measure of an object’s speed relative to the speed of sound; for example, if an airplane flies at Mach 3, it means it is traveling at approximately three times the speed of sound in its environment. When an aircraft approaches or exceeds the speed of sound and flies at supersonic speeds, a shock wave is formed; as this shock wave travels to the ground, it can produce a loud sound known as a sonic boom. Under certain atmospheric conditions, water vapor may condense around the shock wave, creating a band-like phenomenon; however, this does not necessarily indicate a sonic boom itself. In an aircraft flying at supersonic speeds, shock waves form around the aircraft, and the shock waves spreading toward the rear of the aircraft form a cone-shaped structure called a Mach cone. A sonic boom does not occur only at the exact moment an aircraft breaks the sound barrier; it can occur whenever the shock waves formed during supersonic flight reach the ground.
How do jet engines and ramjet engines differ?
Jet engines essentially generate thrust based on Newton’s third law of motion—the principle of action and reaction—and changes in momentum. A change in momentum occurs between the air entering the engine and the high-temperature exhaust gases exiting the rear, and the resulting reaction creates the thrust that propels the aircraft forward. Since momentum is defined as the product of mass and velocity, the engine generates significant thrust by appropriately compressing the air, burning fuel to increase the energy of the gases, and then ejecting them backward at the highest possible velocity. The velocity of the exhaust gases ejected through the nozzle increases significantly due to the energy gained during combustion and the engine’s internal structure. In a typical turbojet engine, the compressor located at the air intake compresses the air, and the compressed air enters the combustion chamber, where it is burned together with fuel. During combustion, the gas—expanded to high temperatures and pressures—passes through the turbine, causing it to rotate, and transmits the power required for the compressor via a shaft connected to the turbine on the same axis. The remaining energy is then used to expel the gas through the nozzle at high speed, generating thrust.
However, unlike conventional jet engines, ramjet engines do not have a compressor. This is because they utilize the ram effect—in which the aircraft or flying vehicle’s high forward speed forces air into the engine—to perform the function of a compressor. The ram effect refers to the phenomenon in which air is drawn into the engine using the vehicle’s forward speed, and the air’s velocity is reduced while its pressure is increased within the intake and diffuser. In particular, in ramjet engines flying at supersonic speeds, shock waves are generated inside the intake; these shock waves, combined with the shape of the intake, are used to reduce the velocity of the incoming air and increase its pressure. The shock cone or intake at the front of the ramjet engine properly forms shock waves and controls the airflow to generate the compression necessary for combustion. Since there is no compressor, there is also no need for a turbine to drive the compressor, as in a typical turbojet engine. Ultimately, the greatest advantage of a ramjet engine is that its structure is simpler and lighter than that of a conventional turbojet engine. Ramjet engines do not use rotating machinery such as compressors and turbines; instead, they utilize the aircraft’s forward speed itself to compress the air. NASA also explains that because ramjet engines lack compressors, they are lighter and have a simpler structure than turbojets.
Due to the nature of the ram effect, the speed of air entering at supersonic speeds decreases to subsonic levels as it passes through the intake and diffuser before entering the combustion chamber. This occurs because the supersonic incoming air experiences a shock wave inside the intake, causing its speed to decrease and its pressure to increase. In the ramjet engine’s combustion chamber, the air—now slowed to subsonic speeds—is mixed with fuel and combusted. The energy generated during combustion produces high-temperature, high-pressure gas, which is then accelerated to high speeds again as it passes through the nozzle and is expelled from the engine. Therefore, a ramjet engine can deliver high performance when the aircraft has attained sufficient speed. Typical ramjets are suitable for high-speed ranges of approximately Mach 3 to Mach 6; if the speed becomes excessively high, shock waves generated at the inlet and aerodynamic losses increase, leading to a drop in efficiency. NASA explains that typical ramjets operate in the Mach 3 to 6 range, and performance can drop sharply if the speed significantly exceeds Mach 5 due to losses caused by shock waves.
Where are ramjet engines used?
Ramjet engines have a simple structure and can be built relatively small and lightweight, making them well-suited for high-speed vehicles such as missiles. However, because ramjet engines rely on the vehicle’s forward velocity to compress air, they cannot generate thrust from a standstill and have very limited performance at low speeds. Therefore, aircraft using ramjet engines must be accelerated to a speed at which the ramjet can generate sufficient thrust with the aid of other propulsion systems. Since ramjet engines mounted on missiles often require supersonic flight, they are typically launched from an aircraft or use a separate booster to achieve initial velocity before the ramjet engine is activated. NASA also explains that, since ramjets cannot generate thrust from a standstill, the vehicle must be accelerated to a speed at which the ramjet can operate using another propulsion system.
Ramjet engines have been used particularly in supersonic missiles due to their simple structure and ability to generate thrust efficiently at high speeds. A prime example is the MBDA Meteor air-to-air missile, operated by several European countries, including the United Kingdom. The Meteor is designed to provide continuous thrust using ramjet propulsion until it reaches its target and, as of 2026, remains integrated into and operational on fighter jets in several European countries. Another prime example is the BrahMos hypersonic cruise missile, known to be a joint development between India and Russia, which uses a liquid-fuel ramjet engine after gaining initial velocity from a booster. According to official BrahMos data, the missile is first accelerated to hypersonic speeds by a solid-fuel booster, which then separates, after which the liquid-fuel ramjet provides propulsion during the cruise phase.
The Evolution of Jet Engines Toward Faster Flight
The propellers used in early aircraft had limitations in increasing speed due to compressibility effects and shock waves generated when rotating at high speeds; jet engines played a crucial role in overcoming these limitations through a new approach. While the basic concept of generating thrust—pushing air backward—links jet engines to propellers, jet engines have evolved to effectively generate thrust even at high speeds by compressing air, combusting fuel, and expelling high-speed exhaust gases. Subsequently, jet engine technology established itself as a crucial propulsion technology that made supersonic flight possible and evolved into the ramjet engine, which actively utilizes the air compression effects that occur in the supersonic range. Since ramjet engines eliminate rotating machinery—such as compressors and the turbines that drive them—and instead use the aircraft’s forward speed to compress air, they can be constructed with a much simpler and lighter structure than jet engines. However, because they rely on the ram effect, they have the drawback of being unable to generate efficient thrust until sufficient speed is reached. This limitation can be overcome in missile applications by either launching the missile directly from an aircraft or using a separate booster to accelerate it to sufficient speed before engaging the ramjet engine. Meanwhile, jet engines have evolved into various types—including turbojets, turbofans, and turboprops—in addition to ramjet engines. Recently, research has also focused on scramjet technology, which builds upon the combustion principles of ramjets to enable hypersonic flight. While a conventional ramjet decelerates air to subsonic speeds in the combustion chamber before combusting it, a scramjet maintains a supersonic airflow throughout the combustion process. NASA explains that scramjets are being researched as technology for hypersonic flight at Mach 5 or higher, and notes that the X-43A successfully demonstrated scramjet propulsion in 2004. Although nearly 90 years have passed since the advent of the jet engine, aircraft propulsion technology continues to evolve, and the development of new propulsion technologies for faster and more efficient flight is ongoing.
