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the power of jet engines...

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the power of jet engines...

Old 29th Oct 2000, 23:43
  #41 (permalink)  
Lu Zuckerman
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To: SRR99

Please tell me if your books can tell you if it is possible to expell this high velocity gas from a rocket engine or a gas turbine engine that is open at both ends. On a gas turbine engine I am speaking about the combuster can(s) and on the rocket engine I am speaking about the upper end of the engine combuster not being there. In order to expell the gasses, the gasses must be generated in some sort of container that will allow pressure to build up. Once again I don't disagree with what you are saying as it is the expulsion of a large mass of hot gasses that make these engines work. My point is with out the front end of the combustor there would be nothing to allow the creation of the high pressure and high velocity gasses in the first place.

For every action ther is an equal and opposite reaction. The hot gasses flow out and the pressure differential works against the forward wall and is transfered to the attaching structure. Whatever is attached to that structure is caused to move. The amount of work that is done is pressure times surface area. Getting back to the previous posting, because pressure is exerted at 90 degrees to the containing vessel some of the force cancels out the forward motion. All of these elements must be factored in to determine how much useful force is actually developed.

By the way, I was a techrep on the Atlas for three years and I worked as a project engineer on the Saturn SIV-B at Douglas and NASA MSFC for six and a half years, and no I am not a rocket scientist.

------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 29 October 2000).]

[This message has been edited by Lu Zuckerman (edited 29 October 2000).]
 
Old 1st Nov 2000, 01:11
  #42 (permalink)  
alosaurus
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SRR99-Lu is correct.Also,as I said, your hose pipe analogy is inappropriate in this case.With a liquid as flow increases,temperature increases whilst viscosity and resistance decrease.With a gas the reverse is true.
 
Old 1st Nov 2000, 01:54
  #43 (permalink)  
AVPIN
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Jet V Rocket engine explanations seem to be getting very complicated.

Try these analogies - they work for me

JET engine-

Sit on a kids skate board - pick up brick from ground - throw brick backwards - you go forwards. Repeat for continuous motion.
Bricks = AIR, energy used to throw brick = FUEL
Run out of either and you stop.


ROCKET engine-

Sit on the same skateboard, this time with a pile of bricks on your lap - throw bricks backwards one at a time - you go forwards.
In this case, Bricks and fuel need to be treated as one and the same - in a rocket engine, the fuel is both the energy source AND the accelerated mass. (OK-perhaps not in ION propulsion)

Too simple ?
 
Old 5th Nov 2000, 09:54
  #44 (permalink)  
18Wheeler
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FWIW, you can get a gadget that stick onto the window of a car to tell you how much horsepower it has. It's a little thing with a reasonable accurate accelerometer, and you feed in the weight of the car and accelerate away, and after a few seconds it'll tell you how much power you used to accelerate that amount of mass that fast, etc.
It's called a G-Tec, and I popped it up on the window of a 747-200, powered by RR RB-211 D4X's, 53,000 lbs rated for T/O.
Allowing for a little derate, I got a grand total of 72,000 hp!
 
Old 8th Nov 2000, 01:46
  #45 (permalink)  
Old Dog
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Lu, I agree with SRR99 fully. He has stated clearly all I would have said anyway. Force is derived from acceleration of some mass, F= ma, as stated by Newton.

You were a tech rep (read: salesman) with the rocket maker? You would have more credibility if you were the rocket scientist. I bet you did not major in mechanical engineering or applied maths. You knowledge of mechanics were too primitive.

(edited for typos)

[This message has been edited by Old Dog (edited 08 November 2000).]
 
Old 8th Nov 2000, 01:54
  #46 (permalink)  
Old Dog
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Deleted due double posting.

[This message has been edited by Old Dog (edited 08 November 2000).]
 
Old 8th Nov 2000, 02:06
  #47 (permalink)  
Old Dog
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Lu I am sure you did not qualify in mechanical engineering. If indeed you have a degree in engineering, it would be in electronics or something else. Your did not know about F=ma, no surprise.

SRR99 is correct.
 
Old 8th Nov 2000, 16:17
  #48 (permalink)  
SRR99
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I've been having a nap, but I'm back.

Alosaurus, the hosepipe analogy stands. Forget the water if that causes confusion....Consider a hose, full of air.

Lu, I understand part of what you're saying, but to use your own example, a ramjet is indeed open at both ends, and yes "The Jet Engine" by Rolls-Royce does say that a ramjet, which is open at both ends, does work. Let's not compare credentials, because I think that the authors of that book have bigger ones than you, and I don't even want to talk about mine

Read the book for yourself!

Also, while you obviously know quite a bit about rocket engines, you are comparing apples with oranges when you say that the combustor cans develop the force in a turbine. The combustor can/annulus is a very open framework whose purpose is to promote efficient combustion. It is not a pressure vessel. It sits inside the combustor case, which is a pressure vessel, but which is also open at both ends. The compressor in front of it takes the place of the ram effect and subsequent diffusion that occurs in the pure ram jet.

I'm still interested in where the force is actually transmitted. Is there anyone out there?
 
Old 9th Nov 2000, 14:55
  #49 (permalink)  
18Wheeler
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The compressor blades do all the 'pushing'. There's also a fair amount of plain jet thrust (no pun intended) from the combustors, but the turbine soaks up a rather large chunk of all that forward thrust, leaving the left-overs for forward 'push'.
 
Old 9th Nov 2000, 18:27
  #50 (permalink)  
Lu Zuckerman
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To: SRR99

It is true, that a ram jet is open at both ends. However when combustion takes place the hot gasses can't be expelled from both ends as to do so there would be no propulsive force. It is the pressure entering the front end that acts as a forward wall during combustion which provides a "forward wall" to react against. That is why it is called a "ram jet".

This ram effect is the same as the louvers in the pulse jet but in this case, there are no moving parts. There is also a device called a flame holder which assists in maintaing the flame front.

The combustion cycle is not continuous. As each combustion cycle takes place the flame front (pressure pulse) passes down the tube and in doing so reduces the combustion pressure in front of the flame holder and the pressure drops allowing ram air pressure to enter the tube and the whole thing starts over again. The flame holder also maintains a part of the fire providing an ignition source for the next combustion cycle.

Regarding the combuster cans, they are full of holes which it would seem would allow the combustion gasses to escape and flow forward as well as back. What keeps this from happening is the compressed air that is being forced into the combusters by the compressor thus creating a closed container.

If this wall of pressure is ever disturbed due to an aerodynamic problem in the compressor section the flame would shoot out of the front of the engine.

------------------
The Cat
 
Old 9th Nov 2000, 22:03
  #51 (permalink)  
Old Dog
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On a different note: I was in Taiwan a few months ago, and I read in the local news that The Taiwanese military have successfully flight trial their supersonic missiles, powered by maybe ram jet or pulse jet (they didn't say in the non-technical papers). If that was true, that made them (the Taiwanese) the third country in the world to have successfully made a ramjet engine, after the USA and Russian.
 
Old 9th Nov 2000, 22:17
  #52 (permalink)  
Old Dog
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SRR99: while I agree 100% with you, I noted an error in one of your post on 29 October (3rd para):

Speeding gas up drops its pressure, while slowing it down increases it. That is why there are components with names like nozzles and diffusers. Diffusers expand the gas, slow it down, and drop the static pressure. Nozzles tend to do the opposite, and direct the flow in the desired direction.


Was it a typo, or did you really meant that?
 
Old 10th Nov 2000, 00:31
  #53 (permalink)  
AVPIN
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Old Dog wrote:

On a different note: I was in Taiwan a few months ago, and I read in the local news that The Taiwanese military have successfully flight trial their supersonic missiles, powered by maybe ram jet or pulse jet (they didn't say in the non-technical papers). If that was true, that made them (the Taiwanese) the thirdcountry in the world to have successfully made a ramjet engine, after the USA and Russian.

Sorry, can't allow that statement to stand.

What about the BRITISH built ramjets such as the RR/Bristol Thor as used on the Bloodhound SAM back in the 60's ?

Sorry - VERY proud of British achievements

AVPIN

 
Old 10th Nov 2000, 00:49
  #54 (permalink)  
SRR99
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Old Dog,

Yes it was a typo - slowing the gas down raises its pressure. Thanks.

Lu,

I responded to your question about whether a device which is open at both ends can provide forward thrust. I do know how these engines work.

I just find some of your explanatins lack clarity, which is why I refer everyone to the books I mention.
 
Old 13th Nov 2000, 20:59
  #55 (permalink)  
Old Dog
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AVPIN, thanks for the info. I stand corrected.

BTW, is the Thor powered Bloodhound Missile you mentioned still in service ? Just curious.
 

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