question about jet engine

[kevin90]

OK I have question about the way jet engine works. I am only 16 and haven't got into any of this stuff in hs yet. my buddy and I were having a disscusion about the way a jet and rockets work and got stump by something. I looked up online how they both work and heard of people refferring to a jet engine "pushing against air/atmosphere" and they have to be in a fluid to work. My question is say a jet engine had an atmosphere on one side(intake) and space on the other would it still work? Would there be force to push, since there is no air to push against?

I nkow its a stupid question, but would really like to know.

Thanks

kevin

[Pengwuino]

Well if that situation were at all possible... I believeit would still work because its not really pushing on the atmosphere, its pushing the gases it just got from its intake out the back from the continuous explosion being produced.

[Danger]

A jet is essentially a rocket that doesn't carry its own oxidizer. The reaction thrust (equal and opposite reaction, remember) of the exhaust gas pushes forward on the engine the same way. Nothing is required for it to push against outside.

[kevin90]

thanks guys....I thought that newtons 3rd law would apply, but wasn't sure since someone said it had to push against air.

[Danger]

You're welcome, Kevin. And welcome to PF as well. This is the most reliable science-oriented site there is, so if anybody else feeds you suspicious theories, run them past us to find out what's what. You have world-class experts in all areas on hand. (I'm not one of them, by the way.)

[Pengwuino]

Me neither...

So don't take our word for it :P

[Mk]

Here's (http://powerlabs.org/turbine.htm) an interesting site about a kid building a jet engine.

oh I forgot...

DON'T TRY IT AT HOME!!

[Danger]

Thanks, Mk. I don't have time to read it now, but it looks very interesting. I've thought about doing that for a few years, but never got around to it.

[simple123]

Another way of looking at this situation is to imagine yourself sitting on a chair on a frictionless surface. Then you attempt to throw something away from yourself and the chair you're sitting on..lets say, your shoe for instance...you would slightly move in the opposite direction in which you threw the coin. This happens becuase when you threw the coin, your hand is pushing on the coin and the coin is in turn pushing your hand and thus the chair backwards...this basic idea also applies to the jet engine situation you were refering to

[Mk]

Thanks, Mk. I don't have time to read it now, but it looks very interesting. I've thought about doing that for a few years, but never got around to it.
Well you are very welcome to thank me Danger.

[McQueen]

My question is say a jet engine had an atmosphere on one side(intake) and space on the other would it still work? Would there be force to push, since there is no air to push against?
No it wouldn't.

[FredGarvin]

No it wouldn't.
Do you care to elaborate on your answer?

The scenario is a tough mental exercise. I will say that if you could overcome the obvious physical hurdles, i.e. combustion would be supported/sustainable, the engine running without an appreciable back pressure on the outlet and others, then I would say that the engine would indeed produce thrust.

A jet is also referred to as a "reaction engine." That means that the thrust produced is due to the reaction forces. Where do those reaction forces come from? They DO NOT come from the jet "pushing" on the outside atmosphere behind it. The reaction comes from what is happening inside the engine. In a nutshell, the engine itself works on the airstream to accelerate the air out the back end of the engine. Simply stated, the engine applies a force in the direction of the back of the engine to accelerate the air out. Newton's 3rd law kicks in. If the engine is "pushing" the air out (remember, it is being accelerated) then there must be a corresponding force in the opposite direction, i.e. thrust.

So again, if the inlet atmosphere makes it's way out the tail and is accelerated, then the engine will produce thrust.

[Art]

No it wouldn't.I guess we can never manouvre a rocket in space then. :biggrin: