The easiest way to demonstrate thrust is to blow up a balloon and let it go. It will fly all over the place. The most common thought as to why the balloon is propelled forward is that the air is flowing very rapidly from the the narrow neck of the balloon. Some people will extrapolate that the ballon example can be applied to a jet engine. If they did, they would be wrong. An example of this error would be to have them explain how a rocket engine can work in outer space where there is no field of air to react against. The real thrust comes from the pressure exerted on the forward wall and side walls of the combuster. Each can or on an annular can represents a rocket engine. The exhaust from the combuster flows through a turbine which drives the rest of the engine (Fan and Compressor). There is a pressure differential between the forward wall of the can and the rear end of the can which resultst in a pressure differential. The pressure on the forward wall of the can is much greater than the pressure at the rear of the can. Since the combuster can(s) are attached to the engine and the engine is attached to the wing this pressure differential forces the airplane forward. The fan acts as a propeller and it too is attached to the engine and it provides an added force moving the aircraft forward just like a propeller on a conventional aircraft. Use the formulae to calculate the thrust assuming you have all the figures, but if you want to explain a jet or, a rocket engine to your friend use this.
Here is a little gem. An Atlas rocket engine that generates 150,000 pounds of thrust only has an internal pressure of about 600 PSIA. This pressure working against the surface area of the forward or, upper wall of the engine and the wall of the exit nozzle is what creats the force. Do the math. The forward or upper wall has a surface area of 250 square inches or there abouts. Also the higher the rocket goes until they get up to the top of the atmosphere the faster the gas flows out of the nozzle creating an even higher differential increasing the thrust over the level generated at sea level which is somewhat lower than the rated 150,000 pounds.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]