China released videos of two sixth-generation aircraft in December, causing a stir both domestically and internationally. Many people are talking about China’s sixth-generation aircraft, equipped with supersonic combustion ramjet engines capable of flying at hypersonic speeds. For example, the video mentioned that the sixth-generation fighter jet has an operational radius of over 10,000 kilometers and a flight speed exceeding Mach 5.
Let’s start by briefly introducing the types of aircraft engines. Generally, they are categorized as turbofan engines, ramjet engines, and scramjet engines. In terms of speed, turbofan engines are suitable for subsonic to below Mach 2 flights, ramjet engines are suitable for Mach 2 to Mach 5 flights, and scramjet engines are suitable for flights exceeding Mach 5.
In terms of mechanical structure, turbofan engines are the most complex, with a compressor and a turbine. On the other hand, ramjet engines and scramjet engines do not have these components, making their mechanical structure simpler, relying primarily on shockwaves generated during high-speed flight to compress air.
Looking at the shape, China’s J-20 fighter jet equipped with three engines, two located under the fuselage, likely featuring cowlings, and one on the back, resembling a DSI air inlet. Some believe that the high supersonic flight capability of the J-20 is due to the engine at the back, which is likely a ramjet engine.
However, from an aerodynamic perspective, this is not feasible. The combustion process has specific requirements regarding temperature and speed. When airspeed exceeds the combustion speed, which is termed the flame propagation speed, combustion cannot occur. Aircraft, when flying, intake air at very high speeds, possibly exceeding supersonic, needing to decelerate to subsonic speeds to achieve adequate combustion conditions.
So, how does one decelerate the airflow to achieve ideal combustion conditions during supersonic flight?
The operating principles of turbofan engines and ramjet engines are different. Turbofan engines rely on a compressor that rotates to compress air. When the exhaust gases are released, they also drive the turbine at the back to drive the compressor at the front. This is a clever mechanical design.
As the aircraft speed increases to two to three times the speed of sound, the engine faces several issues. The rotation of the compressor has its limits. External air moving at speeds such as three to four times the speed of sound makes it challenging for the compressor to effectively compress the air, reducing the efficiency of the compressor and affecting the combustion in the engine.
At speeds of two to three times the speed of sound, numerous shockwaves will affect the engine’s structural integrity. Additionally, due to the excessive speed, the airflow temperature rises, causing structural issues with the turbine and compressor, potentially leading to phenomena like flutter, affecting engine efficiency and causing structural damage.
To sum it up, traditional turbofan engines are suitable for speeds below Mach 2. If they operate above Mach 2, various issues can arise.
At this point, the scramjet engine comes into play. As previously mentioned, compared to turbofan engines, scramjet engines have two additional components—a compressor and a turbine—while ramjet engines do not. This simplifies their structure, but without a compressor, how does one decrease speed?
Scientists use a unique method to solve this, termed shock waves. In air, sound and vibrations propagate at the speed of sound. However, when an aircraft reaches the speed of sound, the propagation speed matches the flight speed, creating a phenomenon where vibrations cannot be transmitted rearward, causing air to accumulate in front of the object, leading to compression and shock waves. Shock waves help reduce speed and compress air, aiding in combustion.
Looking at the American SR-71 Blackbird reconnaissance aircraft, you’ll notice a long conical spike at the front of the engine, known as an inlet spike, designed to generate shock waves. Strictly speaking, the engine used in the Blackbird is a variable cycle engine. When flying at subsonic speeds, it functions as a turbofan engine; at supersonic speeds, it turns into a ramjet engine. Furthermore, the position of the inlet spike on the Blackbird is adjustable. This introduces another concept—the Mach angle.
As mentioned earlier, shockwave angles vary at different speeds. In simple terms, the angle between the shockwave’s direction and the flight path is the Mach angle, equal to arcsin(sound speed divided by current speed). Simply put, when the speed is at the speed of sound, the Mach angle is 90 degrees, perpendicular to the flight direction. As the speed increases, the Mach angle decreases; hence, the position of the Blackbird’s inlet spike is adjusted based on speed.
Many viewers may wonder what the airflow behind a shockwave looks like. By simulating fluid dynamics, we see various vortices formed behind the shockwave. Computational fluid dynamics is a complex science, and from this simulation, it’s evident that scramjet engines rely on shock waves for air compression and deceleration.
Having explained the working principles of turbofan and scramjet engines, let’s revisit the J-20. It becomes apparent that the J-20 could not be equipped with a scramjet engine.
Firstly, the geometry doesn’t match. As mentioned, the Blackbird utilizes a scramjet engine with a long inlet spike at the front. However, such a design is absent on the J-20. Another example is the BrahMos hypersonic anti-ship missile jointly developed by Russia and India, which also uses a scramjet engine. The BrahMos missile features a long inlet spike at the front followed by the missile’s body and the inlet.
So, could Chinese scramjet engines have a different form? In late 2024, China revealed significant news, introducing the Supersonic Combustion Ramjet (SCRAMJET) engine named “Jindouyun-400”. Looking at the external design of the Jindouyun-400, there is a long inlet spike at the front of the engine. It’s the most advanced scramjet engine in China currently; however, there’s no evidence of it being installed on the J-20.
Are engine inlet designs limited to this form only? Not necessarily. An exception is the European Meteor air-to-air missile, which uses a solid scramjet engine. The missile features two square inlet openings under the missile body, rather than a conical spike. Scramjet engines control airflow speed using shockwaves, but a conical spike isn’t the sole method to generate shockwaves. Designers may have employed alternative means internally to generate shockwaves for speed control. Without seeing the internal design, it’s challenging to provide further details.
Returning to the J-20, the DSI air inlet at the back is distinct from the Meteor missile’s inlet, highlighting that the DSI inlet is suitable for speeds of up to Mach 2 and isn’t compatible with a scramjet engine.
In summary, from a geometric perspective, the J-20 couldn’t be equipped with a scramjet engine. Furthermore, in terms of engineering efficiency, pairing a scramjet engine with turbofan engines is nearly impractical. It’s considered a foolish design choice. Why? The scramjet engine barely functions at subsonic speeds. Scramjet engines compress gas using shockwaves, which are produced only at supersonic speeds, indicating that the scramjet engine is ineffective at compressing air at subsonic speeds. The gas pressure and speed inside the combustion chamber fail to meet combustion requirements at subsonic speeds, preventing the scramjet engine from igniting.
This combination poses another dilemma. If the aircraft reaches high supersonic speeds, say, Mach 3, at such speeds, the scramjet engine can operate at full capacity while turbofan engines cannot. Turbofan engines dramatically decline in efficiency past Mach 2, causing potential issues like structural instability and flutter. Even with two turbofan engines, their combined power may not surpass that of a single engine in high-speed scenarios.
The outcome is clear—if the aircraft nominally has three engines, in any speed range, the power output is equivalent to just two engines. At subsonic speeds, the scramjet engine is inactive, akin to having only two turbofan engines. If the aircraft attains high supersonic speeds, the scramjet engine works efficiently but the turbofan engines lack power, with their combined output less than that of a single engine. So, why install three engines?
Upon reflection, installing both a scramjet engine and turbofan engines in a fighter jet is an ill-founded design. As a top-level aircraft design bureau in the Chinese Communist Party, Chengdu Aircraft Design and Research Institute would undoubtedly not opt for such a design that aviation experts would scoff at.
There’s only one solution to this dilemma—the Variable Cycle Engine in the United States. At low speeds, it functions as a turbofan engine, transforming into a scramjet engine at high speeds. This serves as the technological path for the sixth-generation fighter aircraft in the U.S., analogous to the technology path of the Blackbird reconnaissance aircraft.
So, why did the J-20 have three engines? Two reasons stand out. Firstly, Chinese aircraft engine performance is still developing. To improve payload capacity and armament load, the J-20 currently relies on stacking three engines to achieve its goals. Secondly, to increase the aircraft’s power generation capabilities, aligning with the demands of future fighter aircraft over the next three to four decades. Future fighter aircraft may integrate laser weapons, various electronic warfare equipment, high-power radar sensors, necessitating more electrical power, which two engines cannot supply, hence the choice of three engines.
Today, we offered a brief overview of scramjet and turbofan engine operation principles. Engineering-wise, the J-20 couldn’t house a scramjet engine nor achieve high supersonic capabilities. Its top speed would not exceed Mach 2, and the use of three turbofan engines is due to inadequate engine thrust and to meet future power requirements.
At this point, some viewers may take issue and say, “Zhou Ziding, you’re spreading misinformation again.” I don’t have a background in aviation; my expertise lies in mechanical engineering, with degrees from Shanghai Jiao Tong University and Carnegie Mellon University. I’ve studied fluid dynamics, conducted CFD simulations using Fluent and OpenFOAM, and written simple code for numerical calculations.
I recognize that among the audience are aerodynamics experts. If there are inaccuracies in what I’ve shared today, feel free to comment or email me. Despite differing viewpoints, we can engage in scientific and engineering discussions.
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