What describes a straight gas turbine engine that gets thrust by accelerating a mass of air through the engine?

• A. Turbojet
• B. Turbofan
• C. Turboprop
• D. Accessory section

Answer: (A)

This is a turbojet. In a turbojet, thrust comes from accelerating air that travels through the engine core and out the exhaust at high speed. Air enters, is compressed, fuel is burned, and the high-energy gases expand through the turbine and then out the nozzle. The momentum change of that exhaust jet pushes the aircraft forward. There’s little or no bypass air around the core, so most of the thrust comes from speeding air through the engine itself.

By comparison, a turbofan uses a large forward fan to move a lot of air around the core (bypass air) in addition to the core flow, producing thrust mainly from the bypass stream with a lower exhaust velocity. A turboprop drives a propeller, getting thrust primarily from the propeller’s action, not by accelerating air through the engine core. The accessory section, meanwhile, supports engine systems and doesn’t generate thrust.