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Jet enigne idea

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Goody34

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Feb 1, 2005
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This may or may not have been thought of by someone allready but here goes, tell me what you think

I attatched a rough schematic of the design. It is basically a combo between a pressure jet and a turbo jet.

Here is my theory behind it:

1.The engine is started with either a spark plug or bbq ignitor or somthing of that nature.

2. The gas (not sure what yet) travels through the tube and is heated by the allready burning engine. As a result of this heating the fuel will come out more turbulantly (hopefuly)

3. The small amount of thrust that is produced by the pressure jet portion exits out the rear of the jet causing the rear fan to spin. The rear fan is connected to the forward fan by a rod and the spinning of the rear fan will cause the forward fan to also spin. The spinning of the forward fan will draw more air into the combustion chamber.

Walla! a jet engine.......

To achieve a higher compression more fans could be added just before the combuster.

I was thinking of making the outer shell out of Foam and fiberglass

I was also thinking i would make the blades out of fiberglass or carbon fiber.

The crankshaft would be aluminium and the fuel tube copper.


What do you guys think

How far fetched is my idea?
 

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cgwendling

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Howdy, and welcome!:p:

Now as to your design,
1. The temps caused by burning fuel will exceed all of the materials you suggested. Short answer, it will burn or melt.
2. The way you have it pictured most of the expanding gasses will come out the big end not the small end.

I don't know but I think you misunderstand the reason for the big ducted fan blades that are on most commercial jets. They provide bypass thrust mostly, the real work to keep the exhaust coming out the back are the compressor blades behind the big one. These are designed not to allow the expanding, pressurised, hot gasses from coming out the front and they provide compressed combustion air thus forcing the gasses to expand out the rear where the rear blades keep the whole thing turning. The assembly must handle high pressure and high temps.

There are a bunch of sites on web that explain this much better than I.

Just type (how a jet engine works ) in your favorite search engine.
I hope this helps and doesn't keep you from exploring this further.:D
 

wsimpso1

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+1 on what cgwendling said.

You just described an axial flow turbojet. In most of that type, there are several compressor rotor blade sets attached to the shaft and with stator blade sets in between each of the rotors.

Then there are burner stages.

Then the turbine section consisting of rotor and stator blades extracts sufficient power to run the compressor. The energy remaining propels the airplane (or car or boat, etc)

Variations:

You can put on a turbine section that removes as much of the energy from the stream as possible, gear it down to a reasonable speed and run a propellor, and that makes it an axial flow turbo prop, which is good for high speed;

Build your turboprop, but skip the gear box and propellor and it is a axial flow turboshaft engine suitable for helocopters, hydroplanes, etc;

Make the compressor and or turbine stages centrifugal designs and you have a centrifugal flow jet (Whittle's first jet engines were this type), turbo prop or turbo shaft (centrifugal types are common for turning shafts and propellors);

Another variation is to place a large fan on the front, run its shaft through the center of the main shaft and connect another turbine on the back end of the engine, and now it is a turbo fan, which gives increased thrust at low airspeeds and greatly increased mission fuel economy.

One of the common threads in all of this is that the higher the temperature from where you are taking your energy, the more efficient the engine becomes - this is Thermodynamics... That means that the hotter you can run the hot section (burners and exhaust side turbines)
the more fuel efficient the system is. It turns out that steel is not good enough for jets and other turbine systems, and all sorts of super-alloys with much higher creep points have been developed to allow higher temperatures, which disallows most composites. In fact, even the compressor blade sets run to prohibitive temps for most composities. And with that, hot section is the thing that needs to be periodically inspected and rebuilt...

So, yeah, somebody else has thought of this before... Keep up the creative direction, you may actually succeed in coming up with something new.

Billski
 

Goody34

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Wow thanks for the quick responses guys!

Why would the gasses come out the large end? I know the picture doesnt look like it but i have the holes on the tubing pointed towards the rear?

I figured that the fiberglass wouldnt hold up to the extreme temperatures. I know GE uses carbon fiber blades on there fans but they are titanium tipped.

I figured i wasnt the first to come up with this. My goal is just to make some sort of working turbo-jet type engine that is cost effective and doesnt have a big old car turbo attatched to it.

By the way im not completley clueless about the subject, i am a freshman aerospace engineering student, so im only about 94.6% clueless.

Thanks Guys
Brad Goodman
 

wsimpso1

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The direction the nozzles point has little to do with it. The big mass being handled is the air being pumped through. What they actually do is build the burners so that the flame won't travel forward past a certain point, but if the air flow is too low, the whole thing quits working (sometimes with a bang and fire coming out of both ends) and coasts down.

To make these big turbines start, they crank them up to 30 to 40% of rated speed, and then turn on fuel and spark. The air is going through them at a pretty big rate, and so the hot stuff goes out the back. If the fuel and spark are turned on at too low a speed (and air flow) the fire comes out both ends and doesn't run sustainably.

Big old car turbo? Hmmm. A big turbocharger with a burn bottle between the compressor and turbine and a way to run it up to speed for starting actually makes a pretty simple jet, if kind of thirsty...

Yeah, some of the big fans are carbon composites with high temp resin systems, but they run with very little temperature rise. The high pressure compressor stages gain quite a bit of heat (just pump up a bicycle tire and then grab the bottom end of the pump for an example of what happens when you compress air), and then the fuel flow is set low enough that the hot section can run as long as it is supposed to. The best super alooys we know of still require that we run at relatively lean fuel-air mixtures.

It is time for you to go read some articles on engine theory, thermodynamics, etc.

Billski
 

Goody34

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shouldnt what i have pictured theoretically work as a ram jet without needing a leaf blower though? I have done some reading on jet engine theory, nothing ever seems to mention how the engine starts spinning in the first place.

Back to the thing about the flame coming out the front, isnt the coiling i have effectivley a Gluhareff Pressure Jet? Although inefficent isnt there pressure jets out there creating around 50 or so pounds of thrust?

Im not saying what your saying is wrong im just trying to better understand and i feel the best way to do that is to question.


And for those of you who are wondering dont worry i dont plan on using this (that is if i ever got it working) in a way that could kill me such as on an airplane or somthing of the like. I just want a simple jet engine to monkey with.
 

Georden

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Originally posted by Goody34
I just want a simple jet engine to monkey with.
If that is the case definately look into building one of those turbocharger based engines. I've been wanting to build one for a long time now. There is a guy who built a jet powered go-kart with one, i'm sure if you search google you will come across his site.
 

wsimpso1

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Goody34,

The thing that all jets have in common is they need significant air flow rates to get them to start.

Conventional starting of turbines is to run them up with a starter motor, although they can be started with ram air - in flight restarts are an emergency procedure in jet aircraft operation and it does the same thing as the leaf blower in RC models with jets - it spins up the turbines and gets the air flow high enough to make the thing run.

Ramjets need to be moving through the air to fire and run.

Pulse jets are an exception - they have the shutter valves to ensure that flow is primarily in one direction, but they have a fairly narrow range of thrust and a very narrow range of operating frequency. Fuel flows per unit thrust are huge in these guys.

If you want to play with simple, you need some theory (engine theory, thermodynamics, and fuel handling systems) and and then the turbo charger based systems should work pretty well. Most likely a practical centrifugal flow single burner jet would be made from not "some big old car" but would be the parts for a big diesel engine.

Billski
 

wally

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southwest TN.
Originally posted by wsimpso1
Goody34,

thing as the leaf blower in RC models with jets - it spins up the turbines and gets the air flow high enough to make the thing run.

Billski
I went to a big fly-in in Cape Girardeau, MO for R/C jets a couple of months ago. There were at least 50 of them.

They now have developed the little jet engines to the point of having built-in electric start. They spool up and light off like the big ones. They do use an external propane tank for the starting fuel but after it comes up to idle speed, they run on kerosene just like big jets! They do add a little oil for lubrication to the fuel. Most of the engines were about the size of a small coffee can. I even saw some awesome smoke systems on some of the planes.

They have a little computer that monitors the rpm, e.g.t., and throttle position and it controls a little fuel pump. I think! They are really incredible.
One company makes about 6 sizes up to about 50 lbs thrust. Be sure your Visa can stand about $3500 before you order.

The planes look Real too!
Wally
 

Goody34

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Feb 1, 2005
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thats amazing that they can pack that kind of technology and power into such a small package
 

Raptor

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Slightly off topic, what ever happened to the (Davis) cold-jet. (The scale T-33 looking thing) that used a small-block Chevy to power the compressor.

Paul
 

StRaNgEdAyS

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Oct 20, 2003
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Northern NSW Australia
Goody, If I've got it right, you want to combine the operations of the pulsejet with turbojet like augmentation.
Pulse jets take a specifically sized combustion and exhaust sections. the sizes of them are determined to provide a specific resonance required for operations. the length and diameter of the tailpipe are the most critical of these.
The biggest thing that will effect the operation of a pulse jet is restriction at either end can prevent operation.
While the blades at the rear will briefly turn the ones at the front, I'm afraid the back pressure will disrupt the resonant flow of the exhaust and the incoming pressure will work against the intake valves.
 

Captain_John

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Feb 3, 2003
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KPYM
This guy has one working!

on you tube.

:gig: CJ
 
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