In this blog post, we’ll explore the principles and characteristics of the ramjet engine, how it differs from conventional jet engines, and the evolution of supersonic flight technology.

The Principles of Jet Engines That Made Supersonic Flight Possible

The speed of sound is approximately 1,235 km/h, which is referred to as “Mach 1.” Generally, a bullet travels at a speed of about Mach 2. So, can you believe that there are aircraft on Earth that fly much faster than sound or a bullet? The “SR-71” reconnaissance aircraft, developed in the United States in the 1960s, was capable of flying at speeds exceeding Mach 3. Better known as the “Blackbird,” this aircraft is famous as a quintessential supersonic aircraft that utilized its overwhelming speed during numerous tense situations throughout the Cold War between the United States and the Soviet Union. So, what makes such high-speed flight possible? While the aircraft’s materials and shape also play a role, the most crucial factor is the engine.
Aircraft use jet engines, but the jet engines installed on the “Blackbird” have a different structure from those used in commercial airliners. Thanks to these differences, the “Blackbird” was able to reach speeds far faster than those of commercial airliners.
Commercial airliners use “turbojet engines.” The force that propels an airplane forward is called thrust, and turbojet engines generate thrust using five key components: the diffuser, compressor, combustion chamber, turbine, and nozzle. The diffuser serves as the inlet to the jet engine, drawing air into the engine. Next, the compressor and combustion chamber sequentially compress and burn the air that has entered the engine. The turbine is what supplies the power to drive the compressor. The turbine derives its power from the high-temperature, high-pressure gases produced by combustion in the combustion chamber. After passing through the turbine, the air rapidly expands as it flows through the nozzle. This process of air expansion is the key principle behind the generation of thrust.
When the pressure confining the air decreases, the air expands, and during this process, pressure energy is converted into kinetic energy. This kinetic energy is the source of thrust. Furthermore, the high energy generated during combustion increases the kinetic energy of the air, thereby maximizing thrust. However, to ensure stable combustion, the air’s speed must be sufficiently lower than Mach 1. This is because it is difficult to maintain stable ignition and combustion in air flowing too rapidly. Therefore, air entering at high speed through the diffuser is compressed as it passes through the compressor, and its speed is reduced at the same time. This is precisely why turbojet engines are not suitable for ultra-high-speed flight at Mach 3 or higher.
Looking at the structure of the turbojet engine described in the text, the diffuser, compressor, combustion chamber, turbine, and nozzle are arranged sequentially. While this structure is effective in subsonic or early supersonic ranges, it reveals its limitations at speeds of Mach 3 or higher.

How does a ramjet engine enable supersonic flight?

When airspeed reaches around Mach 3, it becomes difficult to reduce the airspeed to the desired level using a compressor alone; moreover, the shock waves generated by the high-speed inflow of air place a significant strain on the compressor. Therefore, at speeds of Mach 3 or higher, a new method of compressing air without using a compressor is required. The engine that utilizes this very principle is the “ramjet engine.”
Ramjet engines have no compressor or turbine. Their core components consist only of a diffuser, a combustion chamber, and a nozzle. Consequently, while turbojet engines feature a compressor fan that resembles a fan, ramjet engines have a relatively hollow internal structure. So how does a ramjet engine compress air?
A ramjet engine compresses air solely through the shape of its diffuser. Shock waves are generated when air moves at high speeds. While turbojet engines are limited in supersonic flight due to these shock waves, ramjet engines actively utilize them to enable supersonic flight.
If you pass very quickly between people, your shoulders may bump into each other, causing a jolt. A similar phenomenon occurs with objects moving at supersonic speeds. When an object passes through the air at a very high speed, not only the object but also the air itself is subjected to a significant force. The air transmits this force outward at the speed of sound—that is, Mach 1. Under normal circumstances, this force disperses naturally, but if the object moves faster than this, the force cannot disperse sufficiently and accumulates in front of the object. This is what forms a shock wave.
The shape of the shock wave varies depending on the design of the diffuser. However, regardless of its shape, the energy that the shock wave cannot dissipate is directly transferred to the air entering the engine. The air, subjected to this powerful force, is naturally compressed. A key point is that the compressive force generated by the shock wave is much greater than that produced by the compressor. Therefore, by accurately calculating the degree of compression caused by each shock wave and designing the diffuser to achieve the desired compression ratio, supersonic flight using shock waves becomes possible.
Of course, this design process is extremely complex and challenging. However, it can be fully realized through precise aerodynamic calculations, and in fact, ramjet engines have been the subject of research and practical application in various fields.

Limitations and the Future of Ramjet Engines

However, ramjet engines have a critical drawback. Since the process of generating thrust begins only after a shock wave forms, they generate almost no thrust on their own until supersonic speeds are reached. In other words, an aircraft cannot take off using a ramjet engine alone. Therefore, aircraft equipped with ramjet engines must have a separate auxiliary engine for takeoff and initial acceleration. In such cases, turbojet or turbofan engines are typically used as auxiliary engines, and a configuration combining these concepts is known as a turbo-ramjet engine. The J58 engine installed on the “SR-71” is a prime example of this design, operating like a turbojet at low speeds and functioning similarly to a ramjet at high speeds.
It is clear that the ramjet engine is a crucial technology that has enabled supersonic flight at Mach 3 or higher. However, since it cannot be used independently and its design and development are extremely challenging, it remains a field requiring ongoing research. Currently, “scramjet” technology—which enables combustion without fully decelerating the air below the speed of sound—is being actively researched and is attracting attention as a core technology for hypersonic flight. Furthermore, development of next-generation hypersonic aircraft, such as the “SR-72,” is continuing, though they have not yet reached the operational deployment stage. If this research continues to advance, ramjet and scramjet technologies are expected to play a crucial role in the development of even faster aircraft in the future. The advent of the ramjet engine was a major turning point that brought humanity one step closer to realizing the dream of hypersonic flight.