SpudFiles

im trying to make a small ram jet to put on a knex car to make it simply go forwards, i have made the jet and have an air source connected to it so that i can have air running through at high speed, i tested it with a small amount of meths in the jet but when i let the air through it blows the flame out, does anyone know how to stop this from happening so i can make it work properly? the car will move with just the air going through but i want it to be a ram jet not an air jet

I remember reading that ram jets are only effective at very high velocity, like bullet speeds+.
A pulse jet would be the way to go with a project like this.

You stated you have made the ram-jet already?(diagram/pics )

Sounds like you'd need to use flammable fuel, or maybe oxygen sprayed in from a 8 gram co2 type cartridge for micro-welding?

If it was a pulse jet, maybe it could use a set-up like the co2 cartridge powered cars they used to build in shop class; where the co2 cart. was inserted in a hole drilled in the wooden car. A string was yanked and a board with nails flipped up to poke a hole through each cartridge. then the line of cars raced up a long hallway. 8)

Do you have a flame catcher in the jet?
If not add one, it will prevent the flame from going out.
Ram jets not only need a high velocities of air passing through them. They also need to be "warmed up" This aids the combustion of fuels.

For intrest could you also add a picture of the jet?

i will go and take pictures of it now, so they will be posted shortly.

Thunderlord your right ram jets are only effective at high speeds because the high speed air goes onto the jet, that is why i have an air source connected to the front so i can have air going through it at high speeds.

No i dont have a flame catcher, the jet is in the prototype stages at the moment, so im still learning about how to make it properly

The blowgun is held open with tape so that i can connect to the compressor and have air going through at high speed, i am fully aware that it looks extremely simple but up until yesterday i didnt even no what a ram jet was



The blowgun has a small nozzle on it to increase air speed




Im going to drill another hole in the front of the jet to have a pipe connected to the fuel tank, and i will do what spitfire said and make a flame catcher for it

Errr, that is an awesome use of knex.
Looks like the way the nozzle is at the top it will flip over if it goes forward.

Maybe you could mount the on top of the wheels and make it a knex cannon/ flame thing. That's a huge jet nozzle for something that size. 8)

i figured it would be a good thing to use because of how light it is.

hmm that sounds like a pretty dam good idea i will probably do that later, maybe ill go see how big a flame i can make with it

the pump is not exactly great as it doesnt have a high air flow, but its still a pretty cool flame

http://s256.photobucket.com/albums/hh17 ... 104mp4.flv[/youtube]

Uhh... Has anyone told him yet?

Sorry, I like the optimism but the fact is there is just too much wrong with it to ever be a viable idea. K'nex=not heat resistant. I myself play with the stuff all the time or, at least, used to. Also bascically, you need to stop using tape... Tape doesn't take a flame well either. None of that will hold well together at all assuming the engine works. Those wheels break at around 80mph. I know, I've tried. Now onto the engine. The air intake is not large enough to obtain self propulsion. Also, you will need to consider that the current setup you have now won't compress incoming air at all. You need to obtain supersonic nozzle and intake velocities while maintaining subsonic compression chamber flows. It's one of the fundamentals of RAM jet design. My final words, good enthusiasm, not so good design and materials. Ditch the plastic and go onto metal.

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

You might want to look into valveless pulse jet designs. I used to look them up a lot, because I was interested in doing it for a school project, but I don't know any of the sites anymore. Sorry I couldn't be of more help in that area. You might try propane for a fuel, but that would require you to scale up a bit. Metal mounts secured to a wooden chassis should be OK.

I will try and find as much info as i can about pulse jets and ram jets, then i can work out which would be most effective/easy to make for what i want. I intend to scale up anyway, so maybe i can keep the can as the combustion chamber then add a nozzle onto the end of that to make it bigger and also increase thrust

Something like this?

Good heavens...that's one big Zippo lighter he's got there...

I thought I would never see the day where knex vehicles are powered with ramjet engines made from tin cans.

A ramjet is not going to work, its designed to function at supersonic speeds.
A pulsejet or just some kind of rocket can be done.