In a turbofan engine, the baseballs that the engine is throwing out are air molecules. The air molecules are already there, so the airplane does not have to carry them around at least. An individual air molecule does not weigh very much, but the engine is throwing a lot of them and it is throwing them at very high speed. Thrust is coming from two components in the turbofan:
- The gas turbine itself - Generally a nozzle is formed at the exhaust end of the gas turbine (not shown in this figure) to generate a high-speed jet of exhaust gas. A typical speed for air molecules exiting the engine is 1,300 mph (2,092 kph).
- The bypass air generated by the fan - This bypass air moves at a slower speed than the exhaust from the turbine, but the fan moves a lot of air.
As you can see, gas turbine engines are quite common. They are also quite complicated, and they stretch the limits of both fluid dynamics and materials sciences. If you want to learn more, one worthwhile place to go would be the library of a university with a good engineering department. Books on the subject tend to be expensive, but two well-known texts include "Aircraft Gas Turbine Engine Technology" and "Elements of Gas Turbine Propulsion."
There is a surprising amount of activity in the home-built gas-turbine arena, and you can find other people interested in the same topic by participating in newsgroups or mailing lists on the subject.
For more information on gas turbine engines and related topics, check out the links on the following page.