View Full Version : Jet Engines 101
Dave Jackson
24th Jun 2002, 19:10
Found a good site on the jet engine, for anyone who might be interested.
Engines 101 (http://www.ueet.nasa.gov/Engines101.html)
Lu Zuckerman
25th Jun 2002, 01:57
Discounting turbofans if you read the text accompanying the animated illustration and compare it with the engine explanation provided by Rolls Royce you will find a basic disagreement. The Rolls Royce explanation uses a balloon analogy, which is correct, but the animated illustration (text) is incorrect because it leads the uninitiated reader to believe the jet exhaust is pushing against the surrounding air mass. If this were true, rockets would not work in outer space.
:confused:
The balloon analogy is wrong anyway - Lu is right - that's the way rockets work.
Phil
Lu Zuckerman
25th Jun 2002, 20:22
To: Paco
Actually the balloon analogy is correct. If you look closely at the illustration you will note the forward inner surface of the balloon is indicated as being the reaction point. On a turbojet engine the forward surfaces of the combuster can(s) is the surface that provides the reaction to move the engine forward with the engine being attached to the airframe it moves also.
:confused:
Sorry, I don't see how it can be. I haven't seen the illustration, but a jet works by accelerating a thin hot stream of air backwards, so it comes from somewhere and goes somewhere. This does not happen with the balloon which is self contained.
Phil
Taff Missed
26th Jun 2002, 11:50
Oh Christ. Here we go again.
Taff
Flash2001
26th Jun 2002, 21:34
"All jet engines, which are often called, gas turbines, work on the principle that a tremendous thrust is required to drive the plane forward."
Has anyone, including the author, actually read this sentence? Since when did need become a principle of physics?
Nick Lappos
27th Jun 2002, 23:39
Wow, here we go!
The engine101 web site is great, and not misleading at all. nowhere does it infer that the jet exhaust pushes against anything. It is quite accurate and precise.
The balloon is just like a rocket, in that a stored energy source propells mass backwards, which reacts against the chamber to push the vehicle forward. A balloon stores the energy in its elastic walls, a rocket in the chemistry of its fuel.
A jet does suck in air and heats it to toss it out the back, so it is different in that respect. Were a jet to have to carry all its oxygen, it would have a vastly reduced payload. For a 767 with 100,000 pounds of fuel, if the typical 15 to 1 mixture ratio is met, it must carry 1.5 million pounds of air for the journey (or 300,000 pounds of pure oxygen). This would put a big dent in its payload, like all of it!