Motor Jets!

Jets that defy normal classification

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Chadly33
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Motor Jets!

Post by Chadly33 » Mon Aug 11, 2008 6:07 am

I have been doing a lot of reading of late about motor jets, which are basically an afterburning ducted fan. Yet they seem to be yet another forgotten engine that initially showed a lot of potential but was lost in the great jet engine race! Any way I will be building one shortly, as soon as I finish playing around with the go-kart and all the other smaller engines I have built but have yet to run. I was thinking initially to use a compressor unit I have at work from an Allison turbine but then started to look at the out of time Allison wheels we seem to be accuring in the workshop. Might as well put them to good use!
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Re: Motor Jets!

Post by Zippiot » Mon Aug 11, 2008 9:13 pm

Ive always wondered
afterburning ducted fan VS ramjet behind ducted fan?
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Chadly33
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Re: Motor Jets!

Post by Chadly33 » Wed Aug 13, 2008 1:10 am

That is what I was wondering pretty much! I was going to use a ducted fan unit with the tapered thrust tube and mate that up to the front of a subsonic ramjet with a variable exhaust nozzle. Most of the units I have seen have poorly designed combustion chambers and I think this is where we could make the real difference!
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Viv
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Re: Motor Jets!

Post by Viv » Wed Aug 13, 2008 1:22 am

I think your right Chaders but I am assuming that Briggs and Stratton wont have any thing in their catalogue to turn that compressor ;-)

How many horse power do you think it will take to spin that sucker up the right amount?

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Chadly33
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Re: Motor Jets!

Post by Chadly33 » Wed Aug 13, 2008 3:08 am

Damn Briggs and Stratton ! lol Really I dont think it will take that much to spin it up to the same rpm as it works on an Allison turbine! I will have to look through our maitenance books etc. and work out what gear ratio the gear box transfer from the starter to the compressor ratio is and then see if I can gear my motor to match! I have a 33cc brushcutter motor that is ported and polished running a tuned pipe on an RC airboat I built a while back which spins up a custom ducted fan I built from a '900' series bell 47 cooling fan, it put out plenty of grunt(thrust) I will probably try to use this for the project! I gave up on the air boat because it kept drawing the surface water up into the fan unit! So I guess it really Sucks and Blows!lol
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Re: Motor Jets!

Post by larry cottrill » Wed Aug 13, 2008 12:06 pm

Chadly -

That's a fabulous idea; as you probably know, I've always liked the notion of motorjets. I think it needs to be looked at as a "ramjet" application more than as an "afterburner" setup, mechanically speaking. In basic theory, there's little difference other than the entrance temperature & density.

Don't forget that you'll probably need to allow some room (basically length) for a "flow straightener" to take any residual spin out of the air stream coming off your "fan". My basic picture of it would include a nicely formed diffuser, but for that to work, you've got to have your flow pretty much straight-in, without spin and with no significant turbulence.

Good luck with this! You're working at a lot bigger scale than I would have imagined for a first try at a motorjet. I'd still love to try a really small one, with one of the nice little DC electric fans that are so popular with modelers now.

L Cottrill

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Re: Motor Jets!

Post by Mike Everman » Wed Aug 13, 2008 9:14 pm

Chadly33 wrote:...I dont think it will take that much to spin it up to the same rpm as it works on an Allison turbine!...
I think you underestimate the amount of horsepower it takes to spin a compressor many tens of thousands of rpm... Just gearing a motor up to that level is a major engineering hurdle.
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Chadly33
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Re: Motor Jets!

Post by Chadly33 » Mon Sep 29, 2008 2:27 am

Ok! this is my first try at a motor jet but using the vortex CC!
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Re: Motor Jets!

Post by Chadly33 » Mon Sep 29, 2008 9:17 am

Some short vids of my first runs ! http://www.youtube.com/watch?v=9CH9iBCkrCA :Dhttp://www.youtube.com/watch?v=800fF0keCWc 8) http://www.youtube.com/watch?v=EoffrfRiAOs :twisted: These are all in (jet) mode. I will upload another vid that is in (burner) mode that shows the actual flame vortex, it didnt show up on the vid from directly behind the engine but you can see the hollow vortex tube going right through the engine when watching it in person. What did show up is the vortex when I moved to the side! Looks pretty cool to me!http://www.youtube.com/watch?v=P8I0rrIeDY8 8)
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Re: Motor Jets!

Post by Sebastian LFT » Mon Sep 29, 2008 1:19 pm

That's pretty cool...Could you get your hands on some pyrex and mount a window on it??

When I first read your post on this project my thoughts went back to this video. http://www.youtube.com/watch?v=QRl0jzdAY6U In the beginning there's a nicely formed vortex just before pulsation begins. Where'd you get those funky blowers??
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Chadly33
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Re: Motor Jets!

Post by Chadly33 » Mon Sep 29, 2008 2:03 pm

Thanks! 8) The air blowers are $16 raft inflators from the local camping and outdoor shop, far from optimised as compressor units. I actually melted one just before I ran the engine in (burner) mode due to blast back in the engine!lol Funny thing is when run in jet mode when I turn the fuel up the exhaust nozzle on the CC is obviously too small as it builds up to much pressure and blasts back through the engine. Then when run in (burner) mode as I call it (lit from the rear instead of using the spark plug) which is best to observe the vortex and I turn the fuel down it then also blasts back through the engine because the exhaust nozzle is too small and the engine is trying to revert back to that similar to a pulse jet. :? However the two fuel settings that cause the blast back or detonation between the two different modes is vastly different. I must say that I am learning a great deal from this little experiment and am glad for trying it! The only downfall I can see is that when looking through the vortex from behind the engine it is like looking into one of those Sci-Fi wormholes you see in a movie and I just seem to want to get closer all the time!lol :lol:
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Re: Motor Jets!

Post by Viv » Mon Sep 29, 2008 2:06 pm

Hey Chadders you are the god of wacky jets for sure with that last video, that is so awesome ;-)

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Zippiot
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Re: Motor Jets!

Post by Zippiot » Mon Sep 29, 2008 3:52 pm

Thats just AMAZING

Is there any type of flamholder?
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Chadly33
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Re: Motor Jets!

Post by Chadly33 » Mon Sep 29, 2008 9:01 pm

Thanks Viv!lol Hmmmm? A wacky jet bible, now theres a thought! :lol: Cheers, Zip. ! No there aint no flame holder needed. For once I did a bit of research and concluded that one really isnt needed as the vortex acts as a flame holder! Funny thing is it retains the flame, provides better mixing and increases the core temp above what would normally be achieved while also setting up a boundary layer helping to cool the engine casing somewhat. Well thats my theory anyway, based on what I have found out. It would be very interesting though to get an actual reading inside the vortex tube if it could be done without destablising it.
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Re: Motor Jets!

Post by racketmotorman » Tue Sep 30, 2008 5:50 am

Hi Chad

If I may add a few points to the discussion ..................

Assuming your blowers will be producing less than 1 psi of total pressure , a combo of static and dynamic components .

That assumed 1psi total when fed into a cylindrical tube and fuel burnt will expand the air , that expansion/velocity increase requires a static pressure drop with the result that you will experience thermal choking of the tube long before you can burn all the air and gain full expansion .

By having a jetnozzle on the "tube" , it'll require another static pressure drop for the hot gases to pass thru the nozzle , something has to give , and the result will be a loss in mass flow .

You need to remove the nozzle to regain mass flow and you need to have the tube made into "diffusing" taper of ~14 degrees to slow the air so that the thermal choking is reduced and more fuel can be burnt to raise temperatures and thermodynamics of the limited energy available

The current setup has too many things wrong with it :-(

Cheers
John

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