Building a 3 LB thermojet from plans

[dwayne247]

Hey everyone
This is my first post here and the first time im building a valveless jet engine. i got the plans from this forum, i think from a post named "jims little 3LB thermojet". its a valveless thermojet engine that should give about 2-3 LBs of thrust ( as said in the plans ) and it seems good to start of with.

Im gonna start working on it this week and i'll keep uploading photos of my progress. Im gonna try my best to stick to the plans.

I bought stainless steel pipes today with internal diameters of 3", 1.25", 1". I got another two with internal diameters of about 1.2cm (~.47") and 1.5cm (~0.6")
But im not really happy with the 3" and 1.25" pipes because the metal is just too thick but i'm not gonna make too much of a fuss about it cause this engine is going to be static anyway. (although i hope to make lighter ones in the future.. cause i hope they may fly one day )

ok now my first doubt is about the intake.. according to the plans the engine has two intakes of .5" diameter, so i was wondering if i should go for the 1.2cm tube or the 1.5cm tube.
Secondly, i was also wondering about the length of the combustion chamber. The plans require it to be 3" but would it be ok if i increased that to maybe half an inch more or reduce it half an inch or maybe a whole inch. would it be worth giving a shot?.
And lastly does the closed end of the chamber have to be slightly rounded? or will a flat surface suffice?

Anyways im really excited about this.. i've always wanted to build a jet engine since i was in the 8th. i used to cut out the bottom parts of cheese tins and make small cuts around it and twist the metal to resemble compressors of a turbo jet engine. but all i could do was look at it
Now when i read about valveless pulsejet engines and how it needs NO moving parts.. my heart skipped a beat. and no matter what im building one

and im uploading the plans that im using just so you would know what exactly im building

[Eric]

Build everything as close to the dimensions as possible, and you will get an engine that will put out over 3lbs thrust.

The intakes can vary in size. The flare you put on the intake helps them draw in air easier. A .47" intake with a really good flare will breath better than a .60" intake with little to no flare.

The end cap should be domed, only from a metallurgical standpoint, to reduce stress on the weld joints.

Its easy to cut a circle out of stainless, and make it domed by hitting it with a hammer, over top of something like a sand bag, or pile of leather work gloves, Or pounding downward over a trailer ball hitch mount.

[dwayne247]

oh ok thanks Eric
and what about a .6" with a good flare?

[dwayne247]

i got the funnel thing from the 3" to the 1.5" pipe done. i got it cut from a hollow oval shape steel ball. Im gonna over to a friends place and drill the intake holes into it today evening. i hope it goes well.

[Ghrey]

Welcome to an addictive and loud hobby. You have a nice start.

If you have not already, I strongly recommend reading the safety information here viewtopic.php?f=3&t=4950.

Cheers.

[dwayne247]

yeh ok i will check that out.

This is a little off but, i've been reading and searching alot about the "acoustics" of a pulse jet engine but i just cant understand what is meant by "properly exploiting the acoustics of a pulse jet to increase efficiency/thrust".
And how do you know if your engine is acoustically proper or how do you build something that has these acoustic properties?.
And do harmonics play a part in the combustion chamber as well as the exhaust pipe?.

[dwayne247]

Another thing is, does the combustion chamber and exhaust pipe act as two separate pipes? As in the CC acts as a pipe with a one closed and one open end, whereas the exhaust pipe acts a pipe open at both ends. Or do both together act as ONE pipe closed at only one end?

[dwayne247]

i got the tailpipe welded to the cone today.. and i would have got the CC welded on to but.. i realized they were just brazing it instead of gas welding it with steel!
now i have to travel nearly 12-14 Kms tomorrow to get to where all the gas welding shops are :\

Im hoping the brazing wont melt when the engine gets really hot. Cause if it does.. i would be very annoyed -.-

[metiz]

they probably will

[dwayne247]

they probably will

probably?:\ lol

Anyway, I have a 1/8" O.D brass tubes for the fuel injectors (although plans require stainless steel), but i was wondering what O.D of tubes i will need to connect them to, and what type of welding do i use to hold them together.
I made a diagram of what im talking about.
I figured i'll use 1/4" for the green tubes which branch out into the 1/8" tubes.
I don't know what O.D tube i am going to use for the blue tube. should i stick with the 1/4" dia or use a 3/8" dia?
And how will i connect the blue pipe to the propane hose? Can push the brass tube into the propane hose and use an adjustable metal ring around the hose that i can tighten?

[nickwilson]

Dewane what i think is for the first question it doesnt matter i would stick with a single piece, Also you can flair the tube or use a flange and just adapt it up to what you need, Or you could use a piece of rubber hose to connect them with hose clamps. For the injectors you could sod-er them but if they are to close to the intake's it will melt the sod-er and blow it all over your exhaust, Or you could JB weld it or i think you said it was brass you could braise it. But for your t piece JB weld and sod-er will melt from the exhaust heat probably. If you need help for your injectors there is alot of stuff on here the more you now the easier it is.

[Jim Berquist]

What Happened?