Definition
An engine that produces thrust by accelerating a mass of gas rearward, with the forward force on the engine being the equal and opposite reaction to that rearward flow. Turbojets, turbofans, ramjets, and rockets are all reaction engines.
Plain English
An engine that pushes itself forward by throwing gas backward. The harder and faster it pushes the gas back, the harder the engine is pushed forward.
Context Anchor
Seen in powerplant discussions of jet engines, turbine engines, and other engines that produce thrust directly instead of turning a propeller.
Derivation
From Newton's third law: for every action there is an equal and opposite reaction. The engine is named for the reaction force it produces, not for the action of expelling gas.
Why Pilots Care
This is the basic operating principle behind all jet and rocket propulsion used in turbine-powered aircraft.
Analogy
Step off a small boat onto a dock and the boat slides backward. You pushed the boat back; the boat pushed you forward. A reaction engine does the same thing continuously, using hot gas instead of your foot.
Grounding Statement
Picture hot gas being driven out the back of the engine while the aircraft is pushed forward.
Intuition Check
Reaction does not mean the engine is reacting emotionally or automatically. Here it means the forward push that results from forcing gas or material backward.
Example Sentence 1
The turbojet is a reaction engine that develops thrust by drawing in air, heating it, and expelling it rearward at high velocity.
Example Sentence 2
Technicians inspect the turbine blades that are critical to efficient operation of a reaction engine.