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Beginner's Guide to Propulsion


Welcome to the Beginner's Guide to Propulsion

image of jet engine What is propulsion? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. On airplanes, thrust is usually generated through some application of Newton's third law of action and reaction. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine.

A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. Different propulsion systems generate thrust in slightly different ways. We will discuss four principal propulsion systems: the propeller, the turbine (or jet) engine, the ramjet, and the rocket.

Why are there different types of engines? If we think about Newton's first law of motion, we realize that an airplane propulsion system must serve two purposes. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In fact, the greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.

Some aircraft, like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners.

Some aircraft, like fighter planes or experimental high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Military aircraft typically employ afterburning turbojets. Future hypersonic aircraft will employ some type of ramjet or rocket propulsion.

The site was prepared at NASA Glenn by the Learning Technologies Project (LTP) to provide background information on basic propulsion for secondary math and science teachers. The pages were originally prepared as teaching aids to support EngineSim, an interactive educational computer program that allows students to design and test jet engines on a personal computer. Other slides were prepared to support LTP videoconferencing workshops (http://www.grc.nasa.gov/WWW/K-12/CoE/Coemain.html) for teachers and students. EngineSim is currently available as a JAVA applet which runs in your browser. The program can also be downloaded to your machine.

We have intentionally organized this site to mirror the unstructured nature of the world wide web. There are many pages here connected to one another through hyperlinks. You can then navigate through the links based on your own interest and inquiry. However, if you prefer a more structured approach, you can also take one of our Guided Tours through the site. Each tour provides a sequence of pages dealing with some aspect of propulsion.

NOTICE --- The site has recently been modified to support Section 508 of the Rehabilitation Act. Many of the pages contain mathematical equations which have been produced graphically and which are too long or complex to provide in an "ALT" tag. For these pages, we have retained the (non-compliant) graphical page and have provided a separate (compliant) text only page which contains all of the information of the original page. The two pages are connected through hyperlinks.


Beginner's Guide to Aerodynamics
Beginner's Guide to Propulsion
Beginner's Guide to Model Rockets
Beginner's Guide to Kites
Beginner's Guide to Aeronautics

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byTom Benson
Please send suggestions/corrections to: benson@grc.nasa.gov



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