Eaton Char-Lynn Motor – Ramjet Motor Precedent: Motor J-58

        Eaton Char-lynn Motor      : “The difficulty of ramjeting is like igniting a match in a tornado?”

　　There are many kinds of aero motors, and at present, there is an motor that is the most difficult and tempting to the greatest extent that the major aviation countries are chasing each other. It is a ramjet. What are the advantages of this motor compared to other motors? What is the difficulty?

　　Principle of ramjet

　　Most of the current fighters use turbofan motors.

　　The basic principle of a turbofan motor is to compress the air through the front fan and compressor, and then mix the fuel in the combustion chamber to provide thrust. Passenger aircraft or transport aircraft use high bypass ratio turbofan motors, while low bypass ratio turbofan motors are used on fighter aircraft.

　　The turbofan motor is the mainstream of the current aero motor. However, the pursuit of mankind is endless, and the performance of the turbofan motor is basically the limit. It is very difficult to make progress, and the cost is huge.

　　To this end, while the major aviation powers are focusing on turbofan motors, they are also pursuing more efficient motors. How can we improve efficiency?

　　Very simple, let’s look at the turbofan motor. In order to get the high-pressure dense air, there are fans and compressors at the front end of the motor. If the two are removed, the high-pressure air can be obtained. The efficiency must not be said. How to achieve it?

　　Use speed to achieve!

　　It is not difficult to understand that the faster the aircraft flies, the faster the oncoming air will be. When the speed of sound is several times faster, the air will naturally be compressed. Such high-pressure air enters the combustion chamber of the motor directly, mixes with fuel, and violently fuels it and ejects it at a high speed. This is the principle of a ramjet.

　　Obviously, the ramjet directly removed the fan and compressor in the turbofan motor, and the rear turbine was also removed. This structure is perfect. There is almost no moving part inside. After the high-speed airflow enters the motor from the front inlet, the change of the cross-sectional area of ​​the motor duct is used to reduce the speed of the high-speed airflow, thereby increasing the gas pressure, and then the compressed gas enters the combustion chamber, and the fuel After mixing, it burns and is sprayed backward to provide thrust.

　　Seeing this, many people may wonder why the high-speed airflow is decelerated. After the airflow speed is reduced, the pressure should be reduced.

　　Contrary to what everyone thinks, according to Bernoulli’s principle, the higher the airflow speed, the smaller the pressure.

　　Compared to other conventional jet motors, the ramjet is lighter in weight and simple in construction. It is highly efficient at several times supersonic flight and is greatly pressed against other motors. For this reason, it is commonly used on various missiles. For example, China’s C301 shore-to-ship missiles and the former Soviet Union’s Sam-6 air defense missiles, as well as the “Meteor active radar-guided over-the-horizon combat air-to-air missiles” jointly developed by many European countries.

　　When the C301 shore-to-ship missile is launched, the solid booster first ignites, causing the missile to take off, accelerate, and climb. When the speed reaches the speed required for the ramjet motor to start, the ramjet ignites and continues to fly forward.

　　Developed by multinational group MBDA, the Meteor air-to-air missile delivers superior performance and the ability to strike long-range air targets in high-interference environments. In the future, it will be equipped with the Royal Air Force, the German Air Force, the Spanish Air Force, the Italian Air Force’s typhoon fighters, and the British and Italian equipped F-35 fighters. Its assembled ramjet motor allows it to speed more than Mach 4.

　　Difficulties and weaknesses of ramjet motors

　　Although the ramjet motor is very efficient in some cases and its structure is very light, its weakness is also outstanding. From the above introduction of the missile, it can be seen that the ramjet motor needs to reach a certain speed before it can be opened. At low speeds, there is no high-speed airflow in front to compress the air, and the ramjet will fail. This is also why the U.S. hypersonic (also called supersonic) aircraft X-43A is usually tied to the Pegasus rocket during flight, and then transported to the high altitude by the B-52 stratum bomber.

　　The X-43A made a total of three test flights. In the last test flight, the X-43A made an ultra-high-speed flight close to 9.8 Mach (approximately 11,200 km/h) after the last short sprint, flying over the surface. 35 kilometers high. This flight is also the last time the NB-52B mother machine served NASA for 40 years.

　　It should be noted that the motor used in the X-43A is a “super-burning ramjet”, which is an upgraded version of the “ramjet motor”. The difference between the two is that when the supersonic airflow enters the ramjet motor, it needs to be decelerated to the subsonic speed through appropriate diversion before entering the combustion chamber, that is, the airflow inside the ramjet combustion chamber is subsonic. The intake of the scramjet motor can be maintained at a supersonic speed, so that a higher flight speed can be achieved. In terms of difficulty, the scramjet is clearly higher than the ramjet.

　　Say the weakness of the ramjet, let’s take a look at its difficulties. It has a lot of difficulties, and one difficulty that is easily imagined by everyone is: How to achieve stable combustion?

　　The ramjet motor is better. After all, the combustion chamber is in the subsonic state, and the scramjet motor is difficult. The airflow in the combustion chamber is supersonic, which brings a problem. The airflow stays in the combustion chamber for a very short time. In the millisecond level, in this case, it is so difficult to successfully ignite and achieve stable combustion, so that someone is more like this difficulty than “igniting a match in a tornado.”

　　Not insurmountable

　　It is said that the weaknesses and difficulties of the ramjet motor seem to be difficult to solve, but it is not entirely true. Technology has been advancing on how to achieve successful ignition and sustained steady combustion, and one day it will mature. The weakness of the ramjet motor, which requires a high speed to start, seems to be difficult to solve. In fact, there are precedents. This is the motor J-58 of the Blackbird SR-71 reconnaissance aircraft.

　　The J-58 can be classified as a hybrid jet motor and is essentially a combination of a ramjet motor and a turbojet motor. When the blackbird reached a certain speed, its ramjet began to work, eventually pushing the blackbird to a speed of more than Mach 3.

　　Blackbird aircraft designer Kelly Johnson once joked to Pratt & Whitney’s friends that the motor provided only 17% of the thrust required for flight, while our own intake and nozzles provided the rest.

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