On December 27, the South China Morning Post reported an explosive piece of news: Chinese hypersonic engine scientists claim to have provided an unprecedented solution to hypersonic propulsion. They have developed a revolutionary power system that combines rotating detonation and inclined detonation engines, solving the long-standing “thrust trap” problem that has troubled scientists in the field of hypersonic technology!
How is the Rotating Detonation + Inclined Detonation Engine achieved?
According to the South China Morning Post, this “revolutionary” air-breathing engine can accelerate an aircraft from zero to 16 times the speed of sound, reaching altitudes of about 30 kilometers above sea level. At this speed, the flight from Shanghai in eastern China to San Francisco on the west coast of the United States takes less than two hours.
The report also states that this engine combines the previously unprecedented rotating detonation engine and inclined detonation engine. According to information from the South China Morning Post, the author found an article titled “Study on Inclined Detonation Combustion Characteristics in Circular Combustion Chambers” in the domestic journal “Propulsion Technology.” The authors of the paper are Zhang Yining, Jin Hong, Tu Shengjia, Zhou Lin, and Teng Honghui. The introduction of the paper directly provides the basic structure of this engine:
“The inclined detonation combustion organization form induced by a circular platform suitable for axisymmetric annular channels is verified by numerical simulation for the feasibility of inclined detonation combustion induced by a circular platform at low Mach number flight conditions. The ignition and stabilization characteristics of the inclined detonation wave induced by a circular platform are studied from two aspects: geometry and incoming flow parameters. The results show that, in the Ma7 flight condition, compared with the conical and slant detonation, the circular platform in the annular combustion chamber can simultaneously achieve the accelerated ignition and stable stabilization of the inclined detonation wave.”
Combining the structural description and illustrations provided in the paper, it can be observed that this is a “series-connected” detonation engine. The front half is a rotating detonation engine, operating in rotating detonation mode up to Mach 7. After reaching 7 Mach, the front rotating detonation engine stops working, and the rear part activates the inclined detonation stationary detonation mode using a “circular platform” as a ramjet nozzle. Based on the inclined detonation data, this mode can achieve a maximum working limit speed of 14 to 17 Mach, demonstrating significant potential!
From the figure, it can be seen that this is an axisymmetric hypersonic engine, with the front half being rotating detonation and the rear half being inclined detonation. It is unclear which genius designed the structure to integrate these two engines. Previously, no one had attempted such integration because the two engines have completely different structures. How they are combined remains a mystery.
Detonation Combustion and Rotating Detonation vs. Inclined Detonation
Detonation combustion is essentially an explosion, different from the continuous combustion of conventional jet engines. Detonation combustion occurs in the form of pulsed explosions, igniting the fuel-air mixture at explosive concentrations, with shock waves propagating at supersonic speeds, reaching speeds above 5-6 Mach, achieving complete combustion and extremely high efficiency, with publicly available data reaching over 80% efficiency.
The burning efficiency of continuous combustion turbofan engines is already impressive at 45-50%, achieved in turbofan mode. However, the highest speeds are significantly reduced. For example, the high-bypass-ratio turbofan with the highest combustion efficiency achieves only high subsonic speeds, making it more suitable for commercial airliners.
In contrast, detonation combustion is different. This combustion mode is completely different from continuous combustion, making it easy to achieve high efficiency. However, there is a challenge: the difficulty of increasing the frequency of detonations. This is a crucial issue for improving the thrust-to-weight ratio of detonation engines. For example, the popular PDE (Pulse Detonation Engine) mode can achieve a frequency of at most 80-100 Hz, or 80-100 detonations per second. Although the efficiency is high, the thrust-to-weight ratio is poor, limiting its use to unmanned drones or large toys. Multiple-tube detonation often increases weight, reducing the thrust-to-weight ratio.
Rotating detonation is a cyclic detonation combustion mode within a circular space. One or more intakes form a detonation zone. The rotating detonation engine operates by igniting different detonation zones of the mixed fuel at different time points in the annular combustion chamber, creating a continuous detonation pattern around the annular combustion chamber. This method can also achieve self-sustained detonation combustion without the need for continuous ignition, as required in multiple-tube detonation.
According to publicly available reports, current tests of detonation engines can achieve frequencies of around 10 kHz, with maximum thrust approaching three tons and a maximum duration of 4-5 minutes. This is close to practical application, and rotating detonation is the most practical of the three detonation engine modes.
The inclined detonation engine structure is somewhat similar to the intake duct of a scramjet engine. However, the inclination angle is greater, and the shock waves formed by the compressed airflow after the inclined intake duct turn cause a stationary detonation process. Each ignition of the fuel-mixture results in a small explosion. Because the shock waves affect the compression wave, it changes the shock waves. However, with the exhaust, the shock waves return to the pre-explosion state, consistently appearing in the same position. This is why it is called a stationary detonation inclined detonation engine.
Although both are called detonation engines, the structural differences between the two are significant. Furthermore, a rotating detonation engine can start at zero speed, while an inclined detonation engine cannot. Previously, inclined detonation engines were tested separately, but combining the two engines is undoubtedly a revolutionary structure. The question is how they are integrated.
How is Rotating Detonation + Inclined Detonation Implemented?
Many may think of the combination of the Hypersonic Turbojet and Scramjet (TBCC) mode and the Supersonic Combustion Ramjet (SCRJ) mode. In theory, these two modes can be combined; both use shock waves for compression. The difference is that TBCC has subsonic airflow in the combustion chamber, while SCRJ has supersonic airflow. Adjusting the intake duct structure can achieve this.
However, rotating detonation is not supersonic combustion ramjet, as there are qualitative differences. Rotating detonation uses an injection intake mode that can achieve natural intake without the main moving parts, using a narrow slit structure. This injection structure is more complex than the intake duct of the hypersonic turbojet and scramjet.
The premise of the rotating detonation + inclined detonation engine combination is likely to use the rotating detonation engine’s intake duct and combustion chamber as the inclined detonation engine’s intake duct. At least, the airflow must reach the rear half of the combined engine smoothly. Therefore, the injection intake structure of the rotating detonation must be designed to contract and hide, allowing the airflow passage to be used by the inclined detonation. Since the paper does not specify the exact structure, this is only a speculative guess.
Compared to Lockheed’s latest engine, what are the differences between the two?
On December 14, GE Aviation demonstrated a combined engine called the Detonation Mode Rotating Detonation and Ramjet (DMRJ), integrating turbojet, hypersonic turbojet, supersonic combustion ramjet, and rotating detonation. The highest flight speed can exceed 5 Mach, and it has been successfully tested.
Currently, there is a popular saying that after the United States breaks through in a certain technology, China will announce a similar breakthrough shortly afterward. For example, the announcement of the breakthrough in Sino-U.S. hypersonic engine technology came within a month. This is an interesting observation, and people naturally compare the two to see which one is more advanced!
I have carefully studied the DMRJ combination engine. Its structure is quite complex, essentially a “parallel” engine with a “three-channel” structure sharing the intake duct. The intake duct is divided into three channels, and the engine has four operating modes. The ramjet channels can work simultaneously in hypersonic turbojet and supersonic combustion ramjet modes. The engine’s intake duct has a movable valve, directing the airflow to the corresponding channel at different speeds. However, this engine is still a variant of the turbofan engine and is a turbojet-based hypersonic turbojet and supersonic combustion ramjet combination (TBCC).
Compared to the Rotating Detonation + Inclined Detonation mode, it’s not a direct comparison because they are on different tracks. For example, the Rotating Detonation takes time to accelerate as it starts in an injection self-feeding mode. The acceleration is relatively slow until the detonation engine gradually becomes optimal. On the other hand, GE’s DMRJ has the advantage that the turbofan engine can quickly start and provide normal takeoff thrust, making it advantageous for takeoff.
However, the problem with DMRJ is its complex structure and limited top speed. Even with a hypersonic turbojet using hydrogen fuel, achieving speeds beyond 10 Mach becomes challenging. In contrast, Rotating Detonation + Inclined Detonation does not have this issue, with a maximum speed reaching 14-17 Mach, offering a much broader speed range than DMRJ.
NASA recently announced the Rotating Detonation Rocket Engine (RDRE), another type of rotating detonation engine. It operates as a rocket using oxygen pipes instead of an air channel, making it capable of working in a vacuum. This is not challenging for a rotating detonation engine, and the publicly available data suggests it is quite advanced.
However, this engine belongs to a single structure and is limited to space applications. Its comparability with the technology of the engine discussed in this article is almost non-existent. Nevertheless, based on the current publicly available data, NASA’s RDRE is in the “vanguard” because China has not released similar detailed data for comparison. Let’s tentatively consider NASA at the forefront in the RDRE field.
Tengyun Project: Is the Engine Finally Stable? Which One is More Suitable?
The Tengyun Project aims to develop a two-stage-to-orbit aerospace plane, a reusable structure that can repeatedly perform space launch missions. The flight process is as follows:
1. The first-stage aircraft takes off horizontally from the ground, carrying the second stage. It accelerates and climbs in the atmosphere.
2. When reaching an altitude of 30 to 40 kilometers, the first and second stages separate. The first-stage aircraft returns horizontally and lands at the airport.
3. The second stage continues to climb into low Earth orbit, completes the transport mission, re-enters the atmosphere, and waits for reuse.
The second stage of this aircraft is likely a rocket as a propulsion system. The key is the first stage, which needs to deliver the second stage to an altitude of about 30 kilometers at a speed of at least 7 times the speed of sound. Then, it detaches from the first stage and ignites the rocket to fly into a low Earth orbit.
Currently, the considered power methods are TBCC or TRRE. The former is a turbofan engine plus hypersonic turbojet, which is more challenging to implement. The TBCC project with a turbofan engine plus supersonic combustion ramjet with subsonic airflow has relatively lower implementation difficulty. The United States is making good progress in this area, but the difficulty of implementing a turbofan engine plus supersonic combustion ramjet is high, with little progress.
TRRE is a rocket injection supersonic combustion ramjet engine. The basic principle is to start the rocket at zero speed, then start the hypersonic turbojet or supersonic combustion ramjet after reaching supersonic speed. The rocket is also restarted at speeds of 4-5 times the speed of sound to ensure a smooth transition when the “thrust trap” occurs and to maintain stable engine operation during acceleration or deceleration. This engine is the most suitable for engineering application status.
Rotating Detonation + Inclined Detonation is also very suitable for this type of aerospace vehicle. Moreover, it has no rotating parts and requires only simple maintenance for reuse. The appeal is substantial, and the main challenge now is how to maximize the thrust and extend the stable working time of this engine.
