How Scramjets Work?
Hypersonic Vehicle
On September 7, 2020 India’s Defence Research and Development Organisation (DRDO) successfully flight tested its indigenously developed Hypersonic Technology Demonstrator Vehicle. With this successful test flight India has now become one of the four countries in the world to possess Hypersonic Technology. Before India, only USA, Russia and China were the 3 countries in the world to successfully demonstrate the hypersonic capabilities.
Hypersonic speed was achievable only after the development of scramjet engines. A SCRAMJET is a supersonic combustion jet engine which can generally attain a speed of more than Mach 5. The theoretical calculations suggest that SCRAMJET engines can attain up to Mach 12 to Mach 24. However, the fastest SCRAMJET vehicle the NASA X-43a, which was designed by NASA was able to attain a speed of 9.6 Mach back in 2004.
In this article I am going to give you brief idea about SCRAMJET engines.
Working of Jet Engines
Scramjet engines are basically designed based on combustion technology of normal jet engines in which combustion process occurs at a much higher speed to generate high thrust to attain speed greater than Mach 5. Jet engines are used all over the world for multiple purpose, basically to achieve high speed. Jet engines are used in jet planes, fighter jets, missiles, drones and also in high speed cars and power plants.
A jet engine basically consists of the following components – a fan in the front, compressor, combustion chamber, gas turbine and exhaust nozzle.
The working mechanism of a jet engine is as follows –
1. 1. A normal jet engine cannot achieve a speed more than Mach 1 (In very few case Mach 3). A fan at the front of the engine sucks air from the atmosphere to supply required air for burning the fuel. The air enters through the inlet of the engine. The air is slowed down by about 60% when passes through inlet.
2. 2. The air entered into the engine through the inlet is then compressed further and slowed down by a compressor. Due to this high compression the temperature of the gas increases.
3. 3. When this compressed highly heated gas enter the combustion chamber, the liquid fuel squirted into the combustion from fuel tank is then mixed with the compressed gas. A spark plug then ignites the mixture. The mixture is brunt at a temperature of nearly 900°C.
4. 4. The mixture expands spontaneously, creating a high-speed jet. This jet of gas is then enters the turbine chamber to spin the turbine blades like windmill. As the turbine starts to rotate faster the gas loses its kinetic energy significantly.
The turbine blades are connected to a long axle which is connected compressor and the fan. When the turbine blade spins the compressor and the fan also starts spinning.
5. 5. The jet of hot exhaust gases then enters the exhaust nozzle to exit the engine. When the gas enters the nozzle, due to the tapering design of the nozzle, the gas is squeezed dramatically and exits through the opening end of exit nozzle at a very high speed (about twice the speed of cool air entering the inlet from atmosphere). Following the law of conservation of momentum, the engines moves forward at a very high speed.
Photo Credit: NASA
The major drawback of traditional jet engines is that they consist of a large no of moving parts like, fan, turbine, compressor and other mechanical components like, spark plugs, cooling system, fuel control unit etc. Due to the presence of these components, the jet engine becomes very heavy.
The mechanism of the compressor is to slow down the speed of incoming air so that flame does not blow out. To make sure flame does not blow out compressor is introduced. The compressor limits the speed of incoming air to subsonic level, so the exit jet speed also limited to subsonic levels. Thus, the jet engine can only generate a limited amount of thrust for moving the engine.
Ramjet Engine
Ramjet engines are jet engines in which the moving parts such as, compressor and turbines are eliminated. Speed of less than Mach 5, i.e. supersonic speed is achievable by ramjet engines. Ramjet engines utilizes the incoming natural supersonic air from the atmosphere to combust fuel.
The main parts of a ramjet engines are: the inlet, the combustor and the nozzle. The different parts and the working of the engine can be understood with the help of the figure below.
When any object moves through atmosphere at a very high speed, a very high-pressure zone is created upstream and air from the atmosphere can enter at a very high speed through any opening of the object. The ramjet engines utilize this technique. Ramjets require initial assistance for take-off until it attains a specific speed, so that the air entering the inlet reaches a supersonic speed.
The inlet is designed to reduce the supersonic speed of the incoming cool air into subsonic level. At first oblique shock waves slow down incoming air and then a final normal shock wave limits the air speed to subsonic level. The inlet is divergent in shape to maintain the constant subsonic flow of air. The air slowed down and heated by inlet then enters the combustion chamber. The combustor air mixed with fuel and burnt creating a very high temperature and pressure. The air moving at such high speed can easily blowout of the nozzle and required thrust may not be achieved. To prevent any air blowout, flame holders are installed in the combustor. The flame holders are specially designed to temporarily trap the incoming air so that it can react fully with the fuel in the combustion chamber before being expelled.
Scramjet Engine
Ramjet and scramjet engines basically follow the same working principle. Like ramjets, scramjets also don’t have any moving parts like compressor or rotor. The question here is how can scramjets achieve hypersonic speed? The answer lies in the inlet geometry of the scramjets. In ramjets the divergent inlet geometry reduces the incoming air speed to subsonic level however in scramjets the incoming air speed never reduced to subsonic levels.
The combustion chamber of scram jets is also modified to control the burning of this supersonic air. Specially designed exhaust nozzle of scramjets can generate higher Mach number for the exhaust jet. Thus, much greater thrust is generated by scramjet engines which makes possible for the scramjets to achieve hypersonic speed.
References
1) NASA(1)
2)NASA(2)
3) Hypersonic Speed Through Scramjet Technology, Kevin Dirscherl et. al., University of Colorado
nice info
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