There is an Amway article on the rotary knock engine on US media 1945, which puts forward a point of view. The rotary knock engine is the ideal power for the future fighter . It seamlessly switches from zero speed to hypersonic , which is better than any engine at present! So is it really as the US media thinks?
Rotary knock engine: What kind of power is it?
This article on US media 1945 stated that the proposed and developed knock engine has been a long time ago, but it has not entered the practical stage. However, the breakthrough in rotary knocking in recent years has hope that this power will become the power of the next generation of fighter jets! What is the difference between
knock engine and ordinary engines?
knock engine can be traced back to the 1950s Emeritus professor of aerospace engineering at the University of Michigan Emeritus of Aerospace Engineering. The principle of this engine is different from conventional combustion. The process that occurs in the combustion chamber of the knock engine is an explosion.
Combustion and explosion are both oxidants and fuel react, but the speed of the two chemical reactions is different. Ordinary combustion is a slow process, while combustion in the combustion chamber of a jet engine is relatively fast, but the propagation speed between fuel particles is still lower than the speed of sound. In a general sense, the scale of combustion chamber is basically only a few dozen centimeters, and even if it is less than the speed of sound, it is completed instantly.
But explosion is not the concept, because the explosion detonation wave speed is supersonic . For example, the detonation wave speed of a gas explosion can easily exceed 1400 meters/sec, which is about 4 times the speed of sound. Therefore, the "combustion" process in the detonation engine is quite fast.
This also leads to two completely different concepts: isopressurized combustion and isopressurized combustion. The combustion chamber of the jet or rocket engine belongs to isopressurized combustion. What is the concept? It will automatically maintain a certain pressure, and if the combustion chamber pressure is too high, it will be discharged from the tail nozzle to maintain a roughly constant pressure.
Therefore, the rocket engine has a contracting throat to maintain high pressure, which will allow the combustion chamber of subsonic to still discharge supersonic airflow. This can be seen from the Mach ring of the diffused engine tail nozzle. Please pay attention to the tail flame of the jet engine taking off under the force.
, and knock combustion is a constant combustion. Even if the detonation wave is detonated in the open space, it will still be transmitted at supersonic speed, and the pressure will not decrease in the open space. Therefore, it can be approximately regarded as a constant combustion (explosion). The speed is naturally supersonic, so it is much better than conventional jet engines and rocket engines!
and the fuel mixing ratio of rocket combustion and knock combustion is completely different. Let’s take the common mixing ratio of oxyacetylene flames as an example. Generally, the mixing ratio of oxygen and acetylene gas when gas cuts is greater than 1.2, that is, the concentration of acetylene is about 46% (please note that at this time, oxygen-rich is still burning, and the acetylene concentration is insufficient), but the explosion concentration range is between .3%-72.3%.
Simply understand that the acetylene consumption ratio that maintains the operation of the knock engine can be as low as 5% of the rocket combustion concentration. Of course, it may actually be larger, but the explosion process is much more costly than the fuel in the combustion process. This is a certain fact. This principle also makes the knock engine more "fuel-efficient".
Rotary knock: better than multi-tube knock
Why is such an excellent knock engine not popular? The reason is actually very simple. Although the fuel efficiency of the knock engine is very high, the thrust-to-weight ratio is too poor. This parameter is affected by the "explosion" frequency of the knock engine, and the ignition speed of the general pulsed knock engine is affected by the knock cycle process.
knock engine is actually somewhat similar to piston engine . Both are explosion-driven engines. For example, the limit speed of a family car-grade 4-cylinder piston engine is about 4,000 rpm. If you turn 2 revolutions at a time, you will have an explosion cycle in total. There will be a total of 2,000 explosions. According to four cylinders, each cylinder works 500 times, 60 seconds per minute, and a little more than 8 times per second!
That is to say, engines like piston engines that have forced inhalation and exhaust processes have an explosion frequency limit of about 8HZ. Of course, the level of racing will be higher. Even if it increases by 2.5 times, it is only 20 times. The threshold of the pulse detonation engine is 80HZ. The higher the thrust-weight ratio, the larger the thrust-weight ratio. The internal combustion engine has a cylinder forcing inhalation and exhaust, while the pulse detonation engine is naturally aspirated exhaust. Therefore, the most troublesome thing about knocking engines is to increase the frequency, but there are also ways to solve it. There are several ways to solve it:
- 1, multi-tube knocking engine;
- 2, rotating knocking engine;
multi-tube knocking means that a single-tube knocking frequency is not enough. So a few knocking tubes are added to the possibility of higher knocking frequency. At present, multiple-tube knocking can reach a frequency of 80HZ, but unfortunately, this only reaches the threshold of knocking engine. Although some aircraft have used multi-tube knocking engines as power and have been tested for flight, the thrust-weight ratio of is still insufficient.
Another type is a rotary knock engine. Although increasing the tube can increase the frequency, each tube is dead weight and requires high-frequency ignition. Rotary knock is an annular combustion chamber. The ring propagation of the detonation waves occurs in the annular combustion chamber. This frequency is affected by the structural design of the annular combustion chamber, and the frequency is much higher than that of the multi-tube knock engine. The characteristics of
rotary knock engine are very excellent and have met some commercial requirements at present, but the application scope is still limited to power as small aircraft, such as drone , etc., and it is likely to be extended to weapons such as cruise missile in the future. In the future, large and stable power can only be expanded to manned aircraft.
In addition, another direction of knock engine is to combine with modern jet engines. For example, multi-tube knock or rotary knock engine is arranged on the outer duct of turbofan engine , which greatly increases the thrust-to-weight ratio. Combined knock engines with turbofan engines are also an important direction of current research.
supercombustion scramble, variable cycle rocket or rotary knock, which one is better?
For the power of aircraft, the important direction for countries around the world to break through is hypersonic aspiration engine. As for the technology you are currently mastering, in addition to rockets, it mainly focuses on the following research directions:
- 1, scramjet engine ;
- 2, transformed cycle rocket engine;
- 3, rotary knock engine;
These three engines can all reach hypersonic speed, but the characteristics of the three engines are very different. Compare from several dimensions:
- 1, structural complexity;
- 2, zero-speed start-up performance;
- 3, maximum speed that can be achieved;
- 4, whether it can be switched to rocket mode (working outside the atmosphere);
- 5, fuel saving angle consideration;
From the perspective of structural complexity, the structure of the scramjet and knock engine is very simple, both have no moving parts or only few moving parts. The structure of the variable cycle rocket engine (similar to the type of Saber-type pre-cooled engine in the UK) is the most complex, including cooling components, turbine components, gas turbines, coolant compressors, and rocket engines. From a structural perspective, the variable cycle rocket engine is the most complex.
British Sabres pre-cooled engine
ram engine
zero speed start, an overburning scram engine cannot do it because this engine needs to be ignited at 4 to 5 times the speed of sound, so its zero speed start performance is the worst, and it can only be combined with a turbine with a TBCC mode or a rocket combination RBCC and other modes.
variable cycle rocket and rotary knock engine can both start at zero speed with the assistance of certain peripheral components. For carriers, one engine must be easier to maintain than multiple combination engines. Under this condition, the scramjet engine is completely defeated.
can reach the maximum speed, and the knock engine may be slightly worse. According to current data, its speed is likely to not exceed Mach 5 to 7 (1 Mach is 1 times the speed of sound), while the limit of scramjet engines when using carbon-hydrogen fuel is around Mach 7, while using hydrogen fuel can reach Mach 1 or more. There is basically no limit to
variable cycle rocket, because it can work in full rocket mode, it can also work outside the atmosphere, which is the most advantageous when working across the atmosphere.
rotary knock engine can also work in rocket mode, but due to its combustion chamber structure problem, its thrust-to-weight ratio is compared with the flow rate and the variable circulation rocket. Under this condition, the variable circulation rocket engine is better.
The final competition is fuel-efficient performance. As mentioned above, the isopressurized rocket and conventional jet engines cannot compare with the rotary knock engines that burn, such engines are naturally fuel-efficient.
The report on US media 1945 is actually very simple, and it is roughly mentioned that the rotary knock engine is excellent. This article has expanded a lot of content to make a rough comparison. Basically, it can be confirmed that a variable circulation rocket engine can be used across the atmosphere, which is very excellent, but the structure is too complex. The scramjet engine has good speed, but it cannot start at zero speed, and can only be used in the atmosphere. The rotary knock engine is between the two. As a future fighter engine, it does have potential, especially when combined with other types of turbofan engines, it is more promising.
extended reading: What is the principle of a variable cycle rocket engine?
engines are a bit out of the norm. From the front, it looks like a jet engine, but from the back it looks like a rocket engine! It is indeed a combination of two engines, and the basic composition is roughly as follows: adjustable (can be turned off in rocket mode) intake , heat exchanger , air compressor turbine, pre-burning turbine, helium compressor and rocket engine.
SABRE Turbospout Rocket Variable Cycle Engine
Main working principle: cool the inhaled airflow to reduce the volume and then compress it through the turbine to enter the rocket combustion chamber as an oxidant, and the fuel entering the rocket engine is discharged from the tail nozzle to provide power. The cooling is required because the air will expand by heat after compression, which will seriously reduce the engine air intake efficiency. Therefore, cooling air is the most important step of this engine.
helium cooling and compression process: the pre-combustion chamber of the rocket core turbo pump is started, providing power to the helium compressor. After being compressed, these helium returns to the heat exchanger to cool the air flow entering the engine;
turbomachine compression process: the pre-combustion chamber of the rocket core is started, and power to the turbine. This secondary compresses the cooling air and injects high-pressure gas into the combustion chamber of the rocket engine as an oxidant.
low-altitude suction working process: The rocket's pre-combustion turbine starts, driving the turbine to inhale air, and at the same time starts compressor , starts to cool the inhaled air, then compress and send it into the rocket engine and ignite the fuel mixed in. It is ignited and discharged from the tail nozzle, providing strong thrust
high-altitude rocket working process: After reaching a height of 30~40 kilometers, the oxygen content in the atmosphere is not enough to support combustion, the engine air intake is closed, the turbine stops, and the turbine pump driven by the pre-combustion chamber pumps fuel and oxidant with all its strength, and the engine is in a pure rocket mode state.
It is an ideal power for aerospace planes, but it is not suitable as a fighter engine, because the engine size of this structure is difficult to shrink and cannot be stuffed into the current fighter. It must use low-temperature fuel , which is too difficult for fighters.
#Telebrity Creation Challenge#
