Aeronautic Astronautic Engineering

# Introduction to aircraft propulsion systems This section introduces the fundamental concept of propulsion in flight vehicles, explaining how thrust is generated and its necessity for flight, excluding gliders and sailplanes [4](#page=4). ### 1.1 The necessity of propulsion for flight All flight vehicles require a propulsion system to sustain flight, with the sole exception of gliders and sailplanes. The term "propulsion" itself refers to the creation of a force designed to propel a system forward [4](#page=4) [5](#page=5). ### 1.2 Generating thrust Aircraft propulsion systems, often referred to as air-breathing propulsion systems, typically consist of a mechanical power source (an engine) and a propulsor that exerts force on the air to increase its momentum. This propulsive force can be achieved either through a jet efflux or via the action of a propeller on the air. The thrust generated to propel an aircraft forward is a direct result of increasing the time rate of change of momentum of the fluid as it passes through the propulsion system [5](#page=5). > **Tip:** Understand that "thrust" is the forward force produced by the propulsion system, counteracting drag and enabling flight. #### 1.2.1 Non-air-breathing propulsion In contrast to air-breathing systems, rocket engines are examples of non-air-breathing propulsion systems. These engines generate thrust by expelling high-speed gases, which are the products of fuel and oxidizer combustion, out of a nozzle. For launch vehicles, the thrust produced must be substantial enough to overcome the vehicle's weight, impart the necessary energy to the payload, and ultimately accelerate the payload into orbit [6](#page=6). #### 1.2.2 Underlying physical principles Regardless of the specific type of propulsion system, the fundamental principles governing thrust production must align with Newton's laws of motion and the conservation principles of mass, momentum, and energy [6](#page=6). > **Example:** A propeller pulls air from the front and accelerates it backward. This backward acceleration of air (a change in momentum) results in an equal and opposite forward thrust on the propeller and thus the aircraft, as per Newton's third law. --- # Types of aircraft propulsion systems Aircraft propulsion systems are designed to convert fuel into propulsive force, enabling flight vehicles to overcome drag and achieve desired velocities and altitudes. The selection of a propulsion system is critically dependent on the vehicle's intended speed and operational altitude [12](#page=12). ### 2.1 Fundamental requirements of propulsion systems An aircraft propulsion system must generate sufficient thrust to counteract the aircraft's drag during flight. For acceleration and climb, thrust must exceed drag. Therefore, a margin of excess thrust is essential for takeoff and other performance maneuvers. Similarly, rocket engines on launch vehicles must produce enough thrust to overcome the vehicle's weight and achieve orbital velocity. The performance capabilities of any flight vehicle are largely determined by the thrust generated by its propulsion system and the fuel required to produce it [12](#page=12) [7](#page=7) [8](#page=8). ### 2.2 Categories of propulsion systems Propulsion systems can be broadly categorized based on their operational principles and applications: #### 2.2.1 Propeller-driven engines These systems utilize a propeller, driven by an engine, to generate thrust by accelerating a mass of air backward. ##### 2.2.1.1 Piston engines * **Description:** Reciprocating piston internal combustion engines are commonly used to power low- to moderate-performance airplanes. The propeller is directly connected to the engine's crankshaft [17](#page=17). * **Propeller types:** Propellers can be fixed-pitch for lower-performance aircraft or variable-pitch (constant-speed) for higher-performance aircraft [17](#page=17). * **Advantages:** They are robust, relatively inexpensive, and offer reasonable propulsive efficiency [18](#page=18). * **Applications:** Often found in smaller general aviation aircraft [13](#page=13). * **Example:** MFI-17 Mushshak aircraft, powered by a Textron Lycoming IO-360-A1B6 piston engine [19](#page=19). #### 2.2.2 Turbine engines These engines utilize a turbine driven by hot gas expansion to produce power. They are further classified based on how this power is utilized to generate thrust. ##### 2.2.2.1 Turbojet engines * **Description:** The most basic jet engine, a turbojet produces thrust solely by expelling exhaust gases at high velocity through a nozzle. The turbine drives the compressor, which pressurizes intake air for combustion [10](#page=10) [20](#page=20). * **Thrust generation:** All thrust is derived from the high-velocity exhaust jet [20](#page=20). * **Applications:** Suitable for high-speed flight, including supersonic speeds. Military aircraft often employ them due to the need for rapid acceleration and overcoming drag at high Mach numbers [13](#page=13) [15](#page=15). * **Afterburners:** May include afterburners (reheat) to significantly boost thrust for short durations [16](#page=16). * **Examples:** Lockheed F-104 Starfighter Concorde supersonic transport [21](#page=21) [22](#page=22). ##### 2.2.2.2 Turbofan engines * **Description:** A turbofan engine features a large fan at the front that bypasses a significant portion of intake air around the engine's core. This bypass air, along with the core exhaust, generates thrust [10](#page=10) [23](#page=23). * **Thrust generation:** The fan produces a substantial portion of the thrust (up to 70%), with the remainder from the core's jet thrust. This method of accelerating a larger mass of air at a lower velocity is more efficient than a pure turbojet [23](#page=23) [25](#page=25). * **Bypass Ratio (BPR):** This is a key parameter, defined as the ratio of bypass air mass flow to core air mass flow: $$BPR = \frac{\text{Mass flow rate of bypass stream}}{\text{Mass flow rate of flow through core}}$$ [24](#page=24). Higher BPR values indicate greater efficiency. Modern turbofans typically have BPRs ranging from 8 to 11 [24](#page=24) [25](#page=25). * **Advantages:** Higher thrust-producing efficiency and lower fuel consumption compared to turbojets [23](#page=23) [25](#page=25). * **Applications:** Widely used in commercial airliners and modern military aircraft [14](#page=14) [25](#page=25) [26](#page=26). * **Examples:** General Dynamics F-16 Fighting Falcon Boeing 787 Dreamliner [26](#page=26) [27](#page=27). ##### 2.2.2.3 Turboprop engines * **Description:** A turboprop engine uses its turbine to drive a propeller via a shaft, with nearly all thrust generated by the propeller. Jet thrust from the exhaust is minimal [10](#page=10) [28](#page=28). * **Operation:** Energy from the hot gases drives a turbine and then a shaft connected to a propeller. A gearbox is often used to reduce propeller speed and prevent blade tips from reaching supersonic velocities [28](#page=28) [29](#page=29). * **Efficiency:** Considered a very efficient method for producing propulsive thrust, akin to a high effective bypass ratio [29](#page=29). * **Limitations:** Propeller characteristics limit typical flight Mach numbers to below 0.5 [29](#page=29). * **Applications:** Used in transport aircraft and some military applications [13](#page=13). * **Example:** Lockheed C-130 Hercules [30](#page=30). ##### 2.2.2.4 Turboshaft engines * **Description:** Similar to turboprops, turboshaft engines are designed to deliver almost all their power to a shaft, rather than producing significant jet thrust. The primary difference lies in the power turbine stage, which is optimized for shaft power output [31](#page=31). * **Free Power Turbine:** Many designs feature a mechanically separate power turbine (free power turbine) from the gas generator (compressor turbine), allowing them to rotate at different optimal speeds [32](#page=32). * **Applications:** Primarily used to power helicopter rotors but also found in tanks and ships [10](#page=10) [31](#page=31) [32](#page=32). * **Example:** Mi-171 Helicopter [33](#page=33). #### 2.2.3 Rocket engines * **Description:** Rocket engines are distinct from air-breathing engines as they carry their own oxidizer and do not require atmospheric air for combustion [11](#page=11). * **Operation:** They produce thrust by expelling reaction mass at high velocity. * **Characteristics:** Typically operate at very high thrust levels, pressures, and temperatures, often for relatively short durations [16](#page=16). * **Applications:** Primarily used for launch vehicles and spacecraft [8](#page=8). * **Example:** Liquid Rocket Engine [11](#page=11). ### 2.3 Relationship between propulsion system and operational parameters The choice of propulsion system is directly linked to the aircraft's intended speed and altitude range [12](#page=12): * **Low speeds (general aviation):** Piston engines with propellers are common [13](#page=13). * **Moderate speeds (transport aircraft):** Propeller-engine combinations or turboprops are suitable. Turbofans are also efficient for airliners operating at constant cruise speeds [13](#page=13) [14](#page=14). * **High subsonic to supersonic speeds:** Turbojets are favored for their ability to achieve high Mach numbers [13](#page=13) [15](#page=15). * **Helicopters:** Turboshaft engines are the standard, though some smaller helicopters may use piston engines [14](#page=14). * **Military aircraft:** Require significant excess thrust for maneuverability and high-speed operations, often leading to the use of turbojets with afterburners or high-thrust turbofans [15](#page=15) [16](#page=16). > **Tip:** Understanding the trade-offs between thrust generation efficiency (accelerating large mass at low velocity) and engine complexity is key to appreciating why different aircraft use different propulsion systems. --- # Electric and hybrid propulsion systems This section examines the advancements and challenges in electric and hybrid propulsion systems for aircraft, focusing on their potential to reduce emissions and enhance flexibility. ### 3.1 Electric propulsion systems Electric motors offer several advantages over traditional piston or turboshaft engines used for aircraft propulsion. These benefits include a smaller and more compact design, fewer moving parts, and quieter operation. Furthermore, electric motors produce no direct emissions and have a smaller carbon footprint [34](#page=34). #### 3.1.1 Energy conversion efficiency Electric motors are highly efficient in converting electrical energy into usable power, typically achieving around 60% efficiency. In contrast, conventional engines that utilize fossil fuels convert only about 20% of the fuel's stored energy into useful work [35](#page=35). #### 3.1.2 Challenges in electric propulsion Despite their advantages, electric propulsion systems face significant challenges, primarily concerning energy storage [35](#page=35). * **Energy storage:** Aircraft require a substantial amount of energy for flight, which necessitates large and heavy battery packs. The weight and bulk of these batteries, along with the heavy cables required to connect them to the motors, present a considerable engineering hurdle [35](#page=35). * **Energy density:** Fossil fuels possess a much higher energy density per unit of weight compared to batteries, approximately twenty times greater. This means that for the same weight, fossil fuels can store significantly more energy [35](#page=35). * **Battery life and stress:** The chemical and thermal stresses within batteries, induced by the rapid discharge cycles needed for flight, lead to a relatively short battery life [36](#page=36). * **Cost and replacement:** Battery packs are expensive and require more frequent replacement than internal combustion engines, as their replacement cycle is much shorter than the time between overhauls for conventional engines [36](#page=36). > **Tip:** The current limitations in battery technology, particularly regarding energy density and lifespan, are the primary factors restricting the widespread adoption of fully electric propulsion for larger or longer-range aircraft. ### 3.2 Hybrid propulsion systems Hybrid propulsion systems offer a way to overcome some of the limitations of purely electric aircraft by combining electric power with other engine types. These systems provide greater flexibility and enhanced performance [37](#page=37). #### 3.2.1 Components and benefits A hybrid system integrates an additional engine and generator to supplement the electric motors, providing extra power when necessary. Potential complementary power sources include hydrogen fuel cells, gas turbines, or diesel engines coupled to generators [37](#page=37). The primary benefits of hybrid systems include significantly improved flight range and/or endurance. A common strategy is to use the hybrid system for high-power demands like takeoff and then rely on smaller electric motors for the cruise phase of flight [37](#page=37). > **Example:** A hybrid aircraft could utilize its internal combustion engine and generator to charge batteries during cruise, providing power for a subsequent electric-powered landing or for increased power during climb. Currently, only smaller electric-powered aircraft capable of flights up to approximately one hour are considered feasible [37](#page=37). ### 3.3 Electric Vertical Takeoff and Landing (eVTOL) aircraft Electric Vertical Takeoff and Landing (eVTOL) aircraft represent a significant development in electric aviation, particularly for urban environments. These aircraft are being designed for the transportation of goods and people, functioning as air taxis to enhance urban mobility [38](#page=38). #### 3.3.1 Concepts and technology The AgustaWestland Project Zero is noted as the world's first eVTOL aircraft, employing a tiltrotor concept and serving as a technology demonstrator for all-electric propulsion. Other eVTOL concepts, such as the one depicted, utilize rotors for vertical lift and a propeller for forward thrust. In forward flight, fixed wings generate lift [38](#page=38) [39](#page=39). > **Example:** An eVTOL "Urban-taxi" concept can achieve vertical flight using its rotors and then transition to forward flight, where its wings provide lift for efficient cruising, supported by a forward propeller [39](#page=39). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Propulsion | The act or process of driving or pushing forward; specifically, the creation of a force to propel a system, such as an aircraft, forward. | | Thrust | A reactive force that propels a rocket or spacecraft forward, generated by expelling a jet of fluid or gas in the opposite direction. In aircraft, it's the force that overcomes drag to enable flight and acceleration. | | Air-breathing propulsion system | A propulsion system that uses atmospheric air as an oxidizer for combustion and as a working fluid to generate thrust, typical of jet engines and propeller-driven aircraft. | | Non-air-breathing propulsion system | A propulsion system, such as a rocket engine, that carries its own oxidizer and does not rely on atmospheric air for combustion or thrust generation. | | Propulsor | A device or mechanism that imparts motion to a fluid or a vehicle; in aircraft, this can be a propeller, a jet engine, or a rotor. | | Momentum | The product of the mass of a body and its velocity; the change in momentum of a fluid passing through a propulsion system generates thrust. The equation is $p = mv$, where $p$ is momentum, $m$ is mass, and $v$ is velocity. | | Glider | An unpowered heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its wings and whose forward motion is due to the effect of gravity or to inertia derived from a previous launch. | | Sailplane | A type of glider aircraft that is capable of sustained flight without an engine by using natural air currents to gain altitude. | | Drag | A force that opposes the motion of an object through a fluid (like air or water); for an aircraft, it is the resistance to movement caused by air friction and pressure differences. | | Thrust-to-weight ratio | The ratio of the thrust produced by an engine to the weight of the vehicle it powers. A ratio greater than one is required for a launch vehicle to overcome gravity and ascend. | | Reciprocating engine | An internal combustion engine that converts pressure into rotating motion via reciprocating pistons. This type of engine is often used in older or smaller aircraft, driving a propeller. | | Turbojet engine | A type of jet engine where all the air entering the engine passes through the core, is compressed, mixed with fuel, burned, and then expelled at high velocity through a nozzle to produce thrust. | | Turbofan engine | A jet engine that has a large fan at the front, which bypasses a significant portion of the airflow around the engine's core. This bypass air contributes to thrust and improves efficiency, especially at lower speeds. | | Turboprop engine | A type of jet engine where a gas turbine is used to drive a propeller. Most of the engine's power is used to turn the propeller, with only a small amount of thrust coming from the exhaust. | | Turboshaft engine | A gas turbine engine that is designed to deliver its power to a shaft, rather than producing jet thrust. This shaft power is commonly used to drive helicopter rotors, but can also power other machinery. | | Bypass ratio (BPR) | The ratio of the mass flow rate of air that bypasses the core of a turbofan engine to the mass flow rate of air that passes through the core. A higher BPR generally indicates greater propulsive efficiency. | | Afterburner (Reheat) | A device added to the exhaust of a jet engine to inject additional fuel, which burns with the hot exhaust gases to produce a significant increase in thrust, typically used for short durations like takeoff or combat maneuvers. | | Energy density | The amount of energy stored in a given system or region of space per unit volume or per unit mass. For batteries and fuels, higher energy density means more energy can be stored in a lighter or smaller package. | | eVTOL (Electric Vertical Takeoff and Landing) | Aircraft that use electric power for vertical takeoff and landing. These often incorporate multiple rotors or ducted fans and are being developed for urban air mobility applications. | | Hybrid propulsion system | A propulsion system that combines two or more different power sources or types of propulsion to achieve enhanced performance, efficiency, or flexibility. For aircraft, this often involves electric motors augmented by a traditional engine or fuel cell. |