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Introduction Propulsion is the science of designing an engine to propel (push) a vehicle forward or up. For aviation there are two types of propulsion: air breathing and rocket propulsion. (This does not include propeller driven aircraft.) They both work on the principle of pushing high velocity (speed) exhaust gases out the back end (reaction thrust principle), but they are different in one important way. An air-breathing engine uses the airstream it is in to help with propulsion. This saves fuel, also. A rocket engine is made for traveling in space, where there is no air, so it must carry all of its fuel (propulsive power) internally. An air-breathing engine will have an inlet and an outlet, while the rocket will be closed in the front and only have an exit (outlet). In general, an air-breathing engine will use less fuel than a rocket for the same thrust (push). The forces of flight (lift, drag, weight, and thrust) were discussed in the Fundamentals chapter. Thrust is generated by the engines of the airplane. The air flows into the engine at the same speed as the flight speed of the airplane, and exits the engine flowing much hotter and faster. The amount of thrust can be computed knowing how fast the air is moving as it enters and leaves the aircraft. The exhaust gases flow out the back of the engine, causing a reaction force on the airplane, pushing it forward. This concept is called the reaction thrust principle.
An easy way to demonstrate this principle is to take a balloon and blow
it up, filling it with high pressure air. Point the opening down and
let go. As the the high pressure air inside the balloon escapes
(reaction thrust) it pushes the balloon up.
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