Aerospace - Introduction to Propulsion

A: Afterburning involves injecting additional fuel into the exhaust stream of a jet engine after the turbine section, combusting this fuel to increase the temperature and velocity of the exhaust gases. This process produces a significant boost in thrust, which is especially valuable for supersonic flight and combat aircraft. However, afterburners are very fuel-inefficient and increase the thermal stress on engine components, which limits their duration of use. For a comprehensive explanation, consult https://aviation.stackexchange.com/questions/17079/what-is-the-purpose-of-an-afterburner.

A: Specific impulse (Isp) is a measure of the efficiency of rocket engines, defined as the thrust produced per unit weight flow of propellant consumed (usually seconds). It essentially indicates how effectively a rocket uses its propellant: higher Isp means more thrust for the same amount of propellant, resulting in improved performance and longer mission durations. Specific impulse is crucial when designing propulsion systems for space missions where fuel mass is a critical constraint. For further reading, visit https://www.grc.nasa.gov/www/k-12/rocket/IandI.html.

A: A ramjet engine has no moving parts like compressors or turbines; it relies on the engine's forward speed to compress incoming air through ram pressure before combustion. Due to this, ramjets only operate efficiently at supersonic speeds (typically above Mach 2). Turbojets, on the other hand, use compressors and turbines to actively compress air and can operate across a wider range of speeds, including subsonic. Ramjets are often used in missiles and experimental high-speed aircraft. More information is available at https://www.grc.nasa.gov/www/k-12/airplane/ramjet.html.

A: Jet propulsion is based on Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. In a jet engine, air is taken in, compressed, mixed with fuel, and ignited. The high-speed exhaust gases are expelled out of the back of the engine, generating thrust that propels the aircraft forward. For a detailed explanation, you can refer to NASA's beginner guide on jet propulsion: https://www.grc.nasa.gov/www/k-12/airplane/propth.html.

A: Propulsion systems for space vehicles differ significantly from those for aircraft because space vehicles operate in a vacuum and must carry both fuel and oxidizer onboard. Challenges include the need for extremely high specific impulse to maximize efficiency, thermal management in the absence of atmospheric cooling, and handling microgravity conditions. Unlike jet engines that rely on atmospheric oxygen, rocket engines need to be self-contained. Additionally, weight and reliability are critical factors. For an in-depth discussion, see https://www.nasa.gov/directorates/spacetech/niac/2015_Phase_I_Phase_II/High_Performance_Space_Propulsion.

A: The compressor in a jet engine increases the pressure of the incoming air before it enters the combustion chamber. By compressing the air, the compressor enhances the efficiency of combustion, allowing the fuel to burn more completely and releasing more energy. This increase in pressure is crucial for generating high-velocity exhaust gases that produce thrust. Compressors can be axial or centrifugal types, and modern engines typically use multi-stage axial compressors for better efficiency. For more details, see https://www.grc.nasa.gov/www/k-12/airplane/compress.html.

A: Turbofan engines differ from turbojet engines primarily in the way they generate thrust. A turbojet produces thrust by expelling all the air through the turbine and nozzle, whereas a turbofan uses a large fan at the front that pushes a significant portion of air around the engine core (bypass air). This bypass air generates additional thrust more efficiently and quietly, especially at subsonic speeds. Consequently, turbofans are more fuel-efficient and quieter than turbojets. More information can be found at https://www.grc.nasa.gov/www/k-12/airplane/turbfan.html.