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Introduction Propulsion is the science of designing an engine to propel a vehicle forward or up. For aviation, propulsion is generally broken into 2 categories: air-breathing propulsion for airplanes and rocket propulsion for spacecraft. Both work on the principle of pushing high velocity exhaust gases out the back end (reaction thrust principle), but they differ in one significant detail. An air-breathing engine uses the air stream in which the airplane is flying to augment the propulsive abilities of the engine so it can carry less fuel. A rocket engine travels in space where there is no air, and therefore it must carry all its fuel internally. An air-breathing engine will have both an inlet and an exit, while the rocket will be closed in the front and only have an exit. In general, an air-breathing engine will get more thrust for less fuel than a rocket. The forces of flight (lift, drag, weight, and thrust) were discussed in the Fundamentals chapter. Thrust is the forward facing force generated by the engines of the airplane. The air flows into the engine at roughly the flight speed of the airplane, and it exits the engine flowing much hotter and faster. The thrust is computed using the rate of mass flowing through the engine times the difference between the high velocity of the exhaust gases and the original velocity of the air into the inlet. 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. If the opening of the balloon is pointed to the side and allowed to open, the high pressure air inside escapes at high velocity to the lower pressure air outside. The air flow is to the side, but the balloon experiences a push, or reaction thrust, in the opposite direction.
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