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ram jet knex car

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Unread postAuthor: THUNDERLORD » Wed May 28, 2008 11:11 am

cool video link you added yesterday. I think it would be cool to see a working knex that shoots flames even with a title like knex ramjet posted on youtube :D

I was thinking you should trigger it remotely for safety/coolness. :roll:

BTW the Germans did a lot of pulsejet engineering. Here's a link on that:
http://www.youtube.com/watch?v=bZ-97pxT3LU&feature=related 8)
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Unread postAuthor: starman » Wed May 28, 2008 11:37 am

THUNDERLORD wrote:BTW the Germans did a lot of pulsejet engineering. Here's a link on that:
http://www.youtube.com/watch?v=bZ-97pxT3LU&feature=related 8)


Didn't see any pulsejet video, although it was a pretty cool song/video matchup.
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Unread postAuthor: rcman50166 » Wed May 28, 2008 12:01 pm

trollhameran wrote:As i said In an earlier post, i didnt know a single thing about ram jets until yesterday, so your useful tips are much appreciated, I will be attaching an air source to the intake so that I can have air going through at high speeds and I will further research intake designs, but from what I have read so far I think it should work with a subsonic intake as i dont think a small car like this will go over the speed of sound, please correct me if i am wrong about the intake as you seem to know alot more about it than me. Also is methylated spirits an ok fuel to use? i'm not sure if it burns quick enough. I will be rebuilding the car soon, once i have more knowledge of the design elements, this was really just a quick thing i whacked together to test the things i have learned about ram jets so far, but the next model will hopefully be much stronger and more accurately put together


Don't get me wrong, you can make a ramjet work at subsonic speeds, but it's incredibly inefficient, kinda like starting a gasoline engine. A ramjet works best when the air flow into the engine is supersonic. At ignition the air flow needs to be reduced to at least subsonic. The lower velocities in the ignition chamber the more powerful and efficient the engine becomes. For the fuel, I would use kerosene. The stuff comes in bottles for outdoor torches. Kerosene is the same stuff used in jet engines. However, it doesn't light very well if its cold. Before making an engine I would play around with different ideas for ignition. I've got the idea for a chamber that significantly reduces air flow using a coffee can and a bit of steel tubing. It's not designed for prolonged use but rather a proof of concept model. The same design is used in homemade jet engines. It is almost a standard (if there was one) for homemade engines.

Here is a link showing (mostly) how to set up a flame tube or combustion chamber for small homemade jet engines.
http://www.badbros.net/jetbike2.html
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Unread postAuthor: Spitfire » Wed May 28, 2008 12:31 pm

here is a basic design of a ram jet engine. It has been drawn up useing info that i have got over the last 3 years while working on my own one.
A flame cather reduces the speed of the airflow enough to sustain combustion. Parafin as a fuel also works well, most turbine helicopters run on the stuff.
As rcman pointed out the engine needs to be "warmed up" enough so that the fuel combusts automaticaly when reaching the combustion chamber.

I have made one out of spray paint cans, it in itself is not a ram jet, but works on the principles of a number of jet engines.

Note: the flame catcher fuel lines and ignition must be moved forward, to the point that the flame catcher and fuel lines are just in front of the curve of the engine.
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Unread postAuthor: TurboSuper » Wed May 28, 2008 12:34 pm

I gotta ask: How do you plan on steering it?
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Unread postAuthor: brplatz » Wed May 28, 2008 6:18 pm

or do both! pulse jet and a ram jet

http://www.youtube.com/watch?v=b0KJwa5iWTY
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Unread postAuthor: Fnord » Wed May 28, 2008 8:39 pm

Try researching pressure jets.
It's basically just a steel pipe and an open nosecone with a coiled copper/steel tube inside it. The fuel (propane) is run through the coil and directed down the length of the larger pipe. As it burns, it heats the coil up, increasing the local speed of sound in the propane enough that it goes supersonic out of the injector. These don't produce much thrust, but you don't need much to push knex around.
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Unread postAuthor: rp181 » Wed May 28, 2008 9:49 pm

I did this on accident:

I had a about a ft. of 3/4in steel pipe, there was a hacksaw mark 1/4in of a down, i was going to cut it. there was some crap on the inside, so i used a propane torch and stuck the end about 1/2in from the end on full blast. The flame instantly went out, but the flame stayed at the end of the pipe (the end was also hammered, so it was rectangular). Pretty cool, propane went in and drew air in.
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Unread postAuthor: Lentamentalisk » Wed May 28, 2008 10:16 pm

starman wrote:
THUNDERLORD wrote:BTW the Germans did a lot of pulsejet engineering. Here's a link on that:
http://www.youtube.com/watch?v=bZ-97pxT3LU&feature=related 8)


Didn't see any pulsejet video, although it was a pretty cool song/video matchup.


LOL you just got Reich Rolled
Thats even worse than Rick Rolled!

edit:
Pipes and flames are tons of fun
Here I lit the propane coming out of my 20lb tank on fire, then directed it down a 10' piece of aluminium pipe. I put it in from the top end, as it was at an angle. The flame got sucked down to the bottom of the pipe, and then flamed out the bottom. When I cut the propane, the flame was carried back up the pipe, and burst into flames at the top again
http://s229.photobucket.com/albums/ee58 ... CT4287.flv
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Unread postAuthor: THUNDERLORD » Wed May 28, 2008 11:00 pm

@Lentamentalisk, that looked dangerous. Imagine if the pipe fell off the stand and landed pointed at you. Was cool though. :wink:
Yeah, I was just trying to add humor to a post that's already humorous (but enjoyable too).
Seriously, I always think off the buzzbomb when I see pulse jets. :twisted:

Also it was funny when trollhammeran was like I'll post some pics of my ram-jet and I clicked on the reply right away to see it.
(sort of like bein' reich rolled) :lol: 8)
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Unread postAuthor: trollhameran » Thu May 29, 2008 2:09 pm

thanks for everyones help on this project, i am still going to work on it as a type of ram jet but with some changes made to it.

Could somebody please post a front view of the flame catcher as im not entirely sure what it should look like.

I was thinking, if i had a relatively large chamber with compressed air in it and had that at the front to provide the flow of air, could i partly fill the chamber with fuel first so that i have the mix of fuel and air i want both in the same chamber, which would make it easier to inject the fuel into the jet, and then have both the air and fuel going through the flame catcher, into the ignition, and then being forced out the back at a high speed by making a nozzle that will increase the speed as it goes out the back.
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Unread postAuthor: Spitfire » Thu May 29, 2008 2:26 pm

You can use a ordanary household sieve as a flame catcher. It worked for me.
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Unread postAuthor: trollhameran » Thu May 29, 2008 3:02 pm

Ill have to go and buy one in a day or two, its probably going to be on hold until the weekend/early next week as im busy fitting out my workshop at the moment
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