Types Of Propulsion Systems and Their Applications – I

Hello Defence Lovers, this article will discuss the types of propulsion systems in layman's terms as some concepts can be too advanced for those not familiar with this field. Propulsion refers to the act of propelling or pushing something forwards. This article will be restricted to jet propulsion, i.e. propulsion for objects that fly in the atmosphere. Propulsion on aircraft or flying vehicles UAVs is responsible for producing the thrust that keeps the plane moving forward. Missiles generally produce thrust via rocket motors that come under the category of rocket propulsion. The engine accelerates a working fluid, and the reaction to this acceleration produces a force on the engine propelling forward. These engines are broadly classified into shaft and jet engines. Shaft Engines Shaft engines are usually driven by reciprocating engines or by turbine engines. The power plants are independent of the propeller. The shaft is mechanically coupled to the air propeller. The propeller is usually ahead of the engine, and propulsion is achieved by traction, i.e. the shaft transmits torsion and tensile stresses. These engines power 90% of aircraft during the Second World War. However, since the 1960s, jet engines have been widely used. DRDO has developed the Wankel Rotary combustion engine (WRCE). It has been considered for powering light aircraft at us given its advantages. It is a relatively small size and correspondingly high specific output. It has integrated a rotary liquid-cooled engine with the Nishant and Panchi UAV and conducted several flights successfully. DRDO Wankel Engine, Image Source - DRDO Jet Engines A jet engine is a propulsion device that produces thrust solely by a high-speed gas exhaust from inside the engine. The term jet engine is usually restricted to air-breathing engines, mainly those with mechanical compression. They are further classified according to the mode by which they compress air. Jet Engines With Dynamical Compression Ramjet Ramjet operates by fuel combustion in a stream of air compressed by the aircraft's forward speed, contrary to a typical jet engine in which the compressor section compresses the airflow through the ramjet engine. Subsonic ramjet propelled vehicles operate from Mach 3 to Mach 6. The solid fuel ducted ramjet (SFDR) is a solid fuel-based ramjet engine. It derives its oxygen from the atmosphere instead of carrying an oxidizer, thus increasing its range. This engine will power the Astra mark 3, which is based on the SFDR technology. It may also be used for future surface-to-air missiles for airspace denial. Even the Brahmos missile incorporates a liquid ramjet engine in its second stage which takes the missile to Mach 3. Ramjet Engine, Image Source - Springer Link Scramjet A scramjet engine (Supersonic Combustion Ramjet) is a ramjet engine in which the airflow through the engine remains supersonic. Scramjet-powered vehicles can operate at speeds up to Mach 15. Scramjet engines cannot operate between Mach 3 and Mach 6. Hence they are paired with another kind of engine for low-speed operation. Scramjets operate between Mach 5 and Mach 15 for speeds greater than Mach 15 rocket engines are used. The Indian Space Research Organisation (ISRO) has developed and successfully tested a scramjet engine. Scramjet Engine, Image Source - NASA Jet Engines With Mechanical Compression Examples of these engines include the simple turbojet and its various derivatives. These include the turbofan, the turboprop, and the turboshaft jet engines. Turbojet Nozzle exit is usually supersonic in turbo jets but subsonic in other jet engines. The flow's cross-sectional area decreases downstream along the compressor stages and increases along turbine stages. These were used on the HAL Ajeet and the HAL Marut. Turbojet Engine, Image Source - Aerospace Engineering Blog Turbofan A turbofan engine can either be a high bypass or a low bypass turbofan engine. In simple terms, the bypass ratio is the ratio of the air flowing around the core section to the air that actually goes into the core section, i.e., for an engine with a bypass ratio of 10:1, 10 units of air will bypass the core section. Low bypass turbofans are used in jet engines. The Kaveri engine is a turbofan engine. Other examples include the C-17 and the IL-76. High bypass engines are primarily used in civilian aviation, where efficiency is critical. In fighter jet applications, these turbofan engines are coupled with an afterburner to increase the engine’s thrust, where raw power is required. The P-8I Poseidon makes use of a high bypass turbofan engine. Turbofan Engine, Image Source - ResearchGate Turboprop These are gas turbine engines that drive a propellor. These are used on the RTA (Regional Transport Aircraft), the upcoming C295, AN-32 and the C-130J Super Hercules. Turboprop engines also power a majority of our operational and upcoming UAVs. Rustom 2 with Turboprop Engine, Image Source - WION NewsTurboprop Engine, Image Source - GE Aviation Turboshaft These are used in helicopter applications. One example is the HAL Shakti engine that powers most of our helicopters today, including the HAL LCH and the ALH Dhruv. The turboshaft engines have replaced the age-old reciprocating engines due to increased reliability and their decreased weight. HAL Shakti, Image Source - HAL Overall, this was a rudimentary overview of the different engines used to power aircraft and air-based platforms in service with the Indian Armed Forces. The second part of this article will be about Rocket propulsion which will elaborate on the propulsion used by rockets and various missile systems in service with the Indian Armed Forces today.