What Is Aircraft Propulsion?

Aircraft propulsion is the branch of aerospace engineering concerned with the generation of the thrust force required to accelerate an aircraft and sustain flight against aerodynamic drag. All propulsion systems operate by Newton's third law: they accelerate a mass of working fluid rearward, and the reaction to that acceleration produces a forward force on the vehicle. The working fluid may be air drawn from the atmosphere, combustion products exhausted from a rocket nozzle, or air accelerated by a mechanically driven propeller or fan. Each propulsion concept involves a different trade-off among thrust, efficiency, specific fuel consumption, weight, and operating altitude.

The study of aircraft propulsion draws on thermodynamics, fluid mechanics, combustion science, turbomachinery design, and structural analysis, and it spans piston engines from early aviation through to high-bypass turbofan engines that power modern airliners and the rocket motors used in launch vehicles.

Gas Turbine Engines

The gas turbine is the dominant propulsion technology for high-speed and high-altitude aircraft. It operates on the Brayton thermodynamic cycle: a compressor increases the pressure of incoming air, fuel is burned at near-constant pressure in a combustion chamber, and the hot high-pressure gas expands through a turbine that drives the compressor, with the remaining enthalpy available for generating thrust. The turbojet, the simplest gas turbine configuration, exhausts all of the gas through a propulsive nozzle. The turbofan adds a large fan upstream of the core, driven by a low-pressure turbine; a large fraction of the fan airflow bypasses the core entirely and is exhausted at lower velocity, improving propulsive efficiency significantly. High-bypass turbofans with bypass ratios of 10:1 or higher, as used on engines such as the CFM LEAP and Rolls-Royce Trent families, achieve thermal efficiencies and specific fuel consumptions far superior to turbojets at the subsonic cruise speeds of commercial airliners, with propulsion cycle analysis detailed in NASA Glenn Research Center's turbofan engine overview. The turboprop and turboshaft variants extract nearly all of the turbine work to drive a propeller or helicopter rotor rather than a propulsive nozzle.

Propellers and Piston Engines

Propellers convert the torque output of an engine, whether piston or gas turbine, into thrust by accelerating a large mass of air at relatively low velocity. The high propulsive efficiency of propellers at low flight speeds, below roughly 650 km/h, makes them the preferred propulsion choice for light general aviation aircraft, regional turboprops, and unmanned aerial vehicles. Propeller blade design is a complex aerodynamic optimization: each radial section of the blade operates at a different local velocity and must be twisted to maintain an efficient angle of attack across the full span. Variable-pitch propellers allow the blade angle to be adjusted in flight to maintain optimal efficiency across a range of airspeeds and power settings. NASA's Glenn Research Center has documented the fundamental aerodynamics of propulsion systems and provides reference material on the principles governing both propeller and jet engine performance.

Rocket Propulsion

Rocket propulsion differs from air-breathing propulsion in that the oxidizer is carried onboard rather than drawn from the atmosphere, allowing operation at any altitude and in the vacuum of space. Chemical rocket engines use liquid or solid propellants: liquid-propellant engines, such as the RS-25 engines that powered the Space Shuttle main engines and now propel the Space Launch System, pump separate fuel and oxidizer into a combustion chamber, providing throttle control and restart capability. Solid-propellant rockets are simpler and storable but cannot be throttled or extinguished once ignited. The specific impulse (Isp), measured in seconds, is the standard metric of propellant efficiency; liquid hydrogen burned with liquid oxygen achieves Isp values around 450 seconds in vacuum, compared with roughly 270 seconds for solid motors, as characterized in NASA's reference data on rocket propulsion. Electric propulsion systems, including ion thrusters and Hall-effect thrusters, achieve Isp values of 1,500 to 10,000 seconds but produce very low thrust levels, making them suitable for long-duration satellite station-keeping and deep-space missions rather than launch from Earth's surface.

Applications

Aircraft Propulsion has applications in a wide range of disciplines, including:

• Commercial aviation using high-bypass turbofan engines for fuel-efficient long-haul flight
• Military aircraft using low-bypass turbofans and afterburners for high speed and maneuverability
• General aviation and regional transport using piston engines and turboprops
• Launch vehicles using liquid and solid rocket motors to achieve orbital velocity
• Satellite propulsion using electric thrusters for station-keeping and orbit transfer