Instructables

Turbojet Engine (how NOT to)

This is not finnished because it won't work. You could make it work though according to rajat1sharma by making a bigger multistage compressor, or mabey with a big centrifugal compressor. Sorry to anyone who started building this, you will have to do your own problem solving if you want it to run.

The concept of a jet engine is basically a rocket that uses air as the oxidizer instead of something that's already stored on board. There are many types of jet engines, but I will focus on turbojets. The purpose of a turbojet is to push a lot of air out the back of itself at high speed to provide forward thrust. It does this by combining the air with a fuel like propane or kerosene in a combustion chamber and igniting it to cause it to expand and rush out the rear of the engine. The heated, high pressure gasses flowing out spin a turbine at the back of the engine which provides the power for the compressor. The compressor is a fan at the front of the engine that forces air into the combustion chamber to be mixed with fuel.

This instructable will show you how to build a small turbojet from scratch. This design could be scaled up to be used on anything once you have practiced with a small one.
 
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Step 1: Gather Materials

Picture of Gather Materials
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Materials:
-Stainless steel water bottle, the mouth should be narrower than the main part.
-Steel tube that fits snugly in mouth of water bottle (1 3/4" diameter on mine). Make sure it has no weld line or a flat weld line on the inside. 
- ~ 1/32" thick stainless steel pan or tray.
-1/2" diameter steel rod.
-1/16" steel wire and 1/32" steel wire.
-Bearings that fit over the rod and can handle high RPMs and heat.
-1/2" diameter bolt (x 3) with fitting nut (x 3).
-Spark plug with short threads ( x 3) with fitting nut (x 3) and washers (x 3). 9/16" with 18 threads per inch works for the nut.
-Propane cylinder and valve.
-Piezoelectric igniter.
-Small diameter copper or steel tube for fuel line.
-JB Weld.
-Electric motor for starter (optional).
     -Battery and switch for the motor.

-Note: If possible, use stainless steel in place of regular carbon steel on every steel part. I used mild steel on some parts because it is easier to get and cheaper. Do not use any aluminum, plastic, or glue, only use steel.


Tools:
-Angle grinder or other metal cutting saw.
-Grinding wheel (or a hand file).
-Drill.
-Welder.
-Lathe, helpful but not absolutely necessary.
-Tin snips.
-Vice.
-Dremmel tool or other small cutter/grinder. Optional but very helpful.

Step 2: Make the combustion chamber

-Cut the top of the water bottle off just below the threads. If a little of the threading is left, flatten it with pliers.

-Drill 3, 1/2 inch holes equally spaced around the side of the bottle about 1.5 inches to 2 inches in from the bottom. These will accept the fuel inlets. I drilled the holes in 3 steps with 2 smaller drill bits and then the final 1/2 inch bit. After the holes are drilled you may notice that the rim of each hole is bent in. If it is bend it straight with a screwdriver and file out any leftover chunks that the drill just bent out of the hole instead of cutting.

-Then drill 3 identical holes another 1.5 inches beyond those first 3 holes. These are for the spark plugs.

-Drill or cut a hole in the center of the bottom of the bottle to the same diameter as the steel tube. This is for the steel tube to go through. Check that the tube fits closely in this hole and can pass all the way through both ends of the bottle.

Step 3: Fuel and ignition systems

Fuel:

-Drill a 1/4" diameter hole through each of the three 1/2" bolts end to end.

-Grind the heads of the bolts down to about 1/8" or 3/16" thick. Push the bolts through the holes in the combustion chamber from the inside and tighten nuts onto them from the outside.

-Cut three, 3" segments of the 1/4" steel tube. These will attach to the copper tubes at one end and to the bolts with holes through them at the other end.  You can either thin down the first inch of one end of the tubes so the copper tube can fit tightly over it, or get a small tube that fits tightly over the copper and steel tubes and join them inside it.

-Give part of these tubes a slightly oval cross section by squeezing them with pliers a little way back from the end and force them into to holes in the bolts. The deformation will cause tension and hold it in place. You can optionally braze this connection for strength and a better seal.

-Mark the copper tube and cut it to make three equally sized pieces about one to two feet long. Fix the ends where the tube was cut so they are round.

-Attach the tubes to the propane cylinder valve. How you do this will depend on what your propane valve looks like. Mine has a short brass nipple screwed onto it with a nut at the end, and the tubes are JB Welded into the nut.

-After combustion chamber section is mounted on the center tube: JB Weld the other end of the copper tubes onto the ends of the steel tubes. Cut the length of the copper tubes down to improve fuel flow before doing this. One or two feet can be enough length, but don't make it to short to reach.

Ignition:

-Grind the nuts that fit on the spark plugs down to 1/4" thick. Put a washer over the threaded end of the spark plugs, put the threads through the hole in the combustion chamber, and screw a nut on from the inside.

-Connect one lead of your piezo igniter to the front of the combustion chamber (or wait until the end and connect it to the front of the center tube) and the other lead to the top connection point on the spark plugs. Using only one piezoelectric igniter you will only get a spark at one of the spark plugs. To get a spark at all three plugs you will need a separate igniter for each one.

Step 4: Make the Rotor Shaft

-Mark the length of the steel rotor shaft.  Mark it 1 inch shorter than the center tube (the center tube should be 2" longer than the combustion chamber) and put marks on the shaft where the center tube will be cut. Cut it at the length you marked. Grind the last inch or so to a point.

-Shave the middle part of the shaft that will be between the bearings down to 1/4" to take off some weight. Then drill a 1/4 inch diameter hole about 1 inch deep into the end of the shaft in the front.

-Put the rest of the 1/2 inch diameter steel rod in the lathe and take a 3/4 inch long section at the end of it down to just under 1/4 inch in diameter. This will be a pin to slide into the hole in the front of the shaft. Then cut the rod off 2 inches behind the pin you just made and smooth off the end. If you are using an electric starter, drill a hole into the end of this piece that will accept the motor's rotor shaft. If not then you can grind the front of this piece to a point.

Step 5: Make the center tube

-Mark the tube at the important points. The first is 1 inch back from the front, this marks the front of the combustion chamber. The next mark is 2 inches back for the holes that the air from the compressor will pass through. Make a line all the way around the tube 3 inches back from the front where the tube will be cut. Make marks with the same spacing coming in from the other end. Make sure there is 1 inch of clearance on either side of the hole drilling marks.

-Cut the tube at the lines and clean up the edges.

-Drill 3/8 inch diameter holes around the front and rear sections at the marks for the air/ flames to pass through. I drilled 4 in the front and 8 in the back because there should be a lot more pressure flowing out the back than in the front.

Step 6: Cut out the turbine and compressor blades

-Measure the inside diameter of your steel tube. The diameter of the blades should be at least 1/8 inch less than that so there will be 1/16 inch clearance around them.

-Draw 3 circles with a diameter 1/8 inch less than the inside diameter of the tube on the stainless steel pan.

-Roughly cut the circles with an angle grinder or other metal cutting saw, then clean up the edges with a grinding wheel. Check that it fits in the steel tube with a little clearance.

-Drill a 1/4 inch hole in the center of each circle. This will let you connect the blades to the rotor shaft.

-Draw a circle that is the diameter of the rotor shaft in each circle. This will mark the furthest in you should cut the blades. Then draw 4 to 8 blades and cut them. I used the corners of a 1/2 inch hex nut to mark the blade spacing.

-Now bend the blades with pliers. Bend them at about 30 to 45 degrees and give the blades some curvature. The compressor blades should be steeper than the turbine blades. Bend the turbine blades the same direction as the compressor blades, but we will mount them backwards.

- Check if each blade fits cleanly in the tube. If it doesn't, fix it with the grinding wheel.

Step 7: Assemble center tube, rotor shaft, and blades

Turbines and Rotor Shaft

-Put the bearings into the ends of the center tube. Slide the rotor shaft through the bearings and use either a steel collar or a small tack weld to block toe rotor shaft from sliding back and forth in the bearings. If you opt for the tack weld, avoid fusing the bearings and the shaft. Just make a tab on the shaft. Fusing them causes tension and resistance in the bearings.

-Do a final size check on the turbine and compressor blades, then start mounting them. You can save yourself some work by JB Welding the compressor blade into place but that won't work on the turbines at the rear. It will be too hot at the turbine end.

-Drill two to four holes spaced evenly around the turbine blades that match the diameter of your thicker steel wire. Drill holes with the same spacing 1/2" to 3/4" in from the back end of the rotor shaft, and on the pointed part that caps the back end of the rotor shaft.

-Cut pieces of wire 1/2" longer than the front to back spacing of the holes in the rotor shaft. The number of these you will need will be the same as thee number of holes you drilled in each turbine blade. Bend the last 1/4" at each end of the wires at a 90 degree angle to form a thick and wide staple.

-Feed these pieces of wire through the holes in the turbine blades with the blades on the middle spacer-segment of the rotor shaft. Then slide the three parts of the back end of the rotor shaft together and put the ends of the pins into their matching holes.

-To hold the staples in place wrap the thinner steel wire two or three times around the front and back sections of the staples. Twist the ends of the thin wire until the wraps around the staples are very tight.

Center Tube

-Hold the engine straight up and down with the front facing up. Slide the front end-piece of the center tube over the compressor and onto the middle piece of the tube. Make sure the holes drilled in it are on the side closest to the center of the engine. Carefully position the front end-piece so the compressor blades don't scrape the inside of it and give it one tack weld. Check that it is still aligned correctly and tack it in 2 or 3 more places. Then grind the tack welds down so they are flush with the tube.

-Repeat this for the back end-piece.

Step 8: Attach the combustion chamber

-Lay a thick bead with a welder all the way around the front end-piece 1" back from the front. Then grind the back of this so it forms a thick wall for the front of the combustion chamber to butt up against.

-Slide the comb. chamber over the center tube from the back until it is pressed against the weld bead. You could try to braze this spot, but it will be hard to not burn through the thin combustion chamber.

-Attach the back of the combustion chamber to the center tube. There are a couple ways to do this.

       -Wrap some thick steel wire over the area where the back of the comb. chamber meets the center tube and braze for a good seal. I don't think you can melt bronze with a propane torch so this shouldn't melt, but it could.

       -Make a collar out of a piece of steel tube that fits over the center tube, slide it on and weld it into place. This method could also be used in the front instead of a weld bead.

Step 9: Electric starter

You don't have to have an electric starter, but if you choose not to put one on you will have to blow air into the front with an air compressor or something when starting it.

-Choose a motor. It should be big enough to turn the rotor shaft on your engine fairly fast. The compressor has to be spinning fast enough to make all of the hot gasses from the combustion chamber flow the right way. Make sure it turns without too much resistance when the power is disconnected so it doesn't stop the engine.

-Drill a hole about 1 inch deep to the same diameter as the electric motor's spindle into the front piece of the rotor shaft. Then grind the same end of that piece to a cone.

-If the spindle on your electric motor doesn't stick out far enough to mount easily into the hole in the rotor, you will have to extend it. Find a nail or something that is the same size as the hole and join it to to spindle on the electric motor with a collar/ sleeve. The collar can be made by drilling a hole through a piece of aluminum or steel rod.

note: I did not choose to have this on mine.
ron3421 year ago
Not yet for pics, I'm still putting the materials together and drawing it out - its to be used at the end of a long string of piped water to power a drill to put down a geothermal well - think a 100' of 1 1/4" polyprop pipe ending in maybe a 2' piece of 3" pvc pipe with a row of axial blades turning on a shaft inside with water pumped in at maybe 50 psi/ 50 gpm at one end with the blades driving a 4" dia earth bit at the outlet of the 3" pipe - this is all a maybe! but hell I now have an easy way to make a bunch of blades. Jets have static blades that deflect the flow back to a better angle for each seceding row of rotary blades but I'm hoping I don't have to do that. I'll send a pic when I get there
Jaycub (author)  ron3421 year ago
A geothermal well, what is that exactly? One that is deep enough to never freeze or something? I am just hoping you are not actually trying to put a geothermal power plant in your yard! But if you do then that is very ambitious of you.

You will need to have some stator blades to hold the axel, so why not put them between each turbine blade?
ron3421 year ago
Hello Jaycub and congratulations on a great attempt at an axial flow jet. One of the comments added that its really difficult to come up with a working axial compressor.
I think you did well with the tools you had - learning to tig weld would be a great help to you.
I had been looking for a week for a simple way to build an axial water turbine short of CNC machining - the blades had me stumped til I came across your instructible - it gave me your idea on how to make the blades simply - so thanks!!
Jaycub (author)  ron3421 year ago
Cool, I'm glad there was some benefit to this other than just what I learned. Do you have pictures of your water turbine?

I can tig weld but it has been a couple years now since I've done it. But since this instructable I purchased a cheap flux wire welder, and I have an oxyhydrogen torch partway made.
brendonl2 years ago
You have done an excellent job, it looks like you you put a quite decent amount of work into this bit of engineering. You have got some inventors potential :)
marksteamnz3 years ago
. Your design unfortunately has zero chance of working. Even if the compressor stage was perfectly designed and the combustor correctly dimensioned you can not get enough pressure rise in the compressor to over come the drag of the bearings etc. Axial Gas turbines need multiple compressor blades stacked with stators and the blades need to be aerodynamically perfect and unfortunately this gets worse the smaller the turbine is.

Thumbs up for trying but by your lack of responses recently I guess you have found it won't spin up.
Don't give up, just take a step back. You need to go to a gas turbine forum and talk to experienced builders and perhaps start with a bigger engine with a centrifugal compressor and an axial exhaust turbine. Get your eye in. and slowly sneak up on a pure axial, it's a monster of a mission
Jaycub (author)  marksteamnz3 years ago
I didn't realy expect it to work, at least not very well. I just wanted something that would look cool to submit to the epilog challenge and the makerbot contest. Thanks for the advice.
Hi Jaycub
I am agree with marksteamnz as i tried the project and found it not working. to get it working i increase the size of compressor so that more intake air can flow into combustion chamber. and this worked. also i opened the ball bearing covers and washed the excess grease which make them run smoother.
Jaycub (author)  rajat1sharma3 years ago
You actually got it running with a bigger compressor?
Yes it worked and runs fine.. but now trying to add a turbo prop to make cooler.
You need to clarify. You can not have got this design to run with a bigger axial fan compressor. With out many compressor stages of vastly improved sophistication the design will not run. As far as I can determine no amateur builder had ever got a home made axial turbine to run certainly not in the small sizes this instructable refers to. If you have used a centrifugal fan that is quite possible but that is not what is under discussion. Pictures or a youtube video would help clear up the confusion
Jaycub (author)  rajat1sharma3 years ago
Awesome! I'll have to re-work mine to have a better compressor. Can you post a video on youtube or some pictures?

Is there anything else you changed from what I show on this intructable that I would need to change to make mine work?
_Scratch_3 years ago
Argh! Don't cut the tubing with tin snips! Use a pipe cutting tool, the one that clamps down and spins around.
Hello Jaycub.. Great work ... your way of design is very simple and will definately runs.. I am waiting for your last steps.. I have already build till the step discribed..

Please get well soon and help us to finish this project.
Jaycub (author)  rajat1sharma3 years ago
Thanks. I can't guarantee it will work but I think it will. The only steps left are to braze the back of the combustion chamber to the center tube, and attatch the fuel lines ans igniter wires. You could be done before me! Be carefull if you test it.

I am already well but it is spring break so I can't use the high school's welders for a week.
I got most of the parts to build the engine. It sure would be nice to get the plans on how to finish it.
Jaycub (author)  fixitmankeith3 years ago
I'm working on it.
rimar20003 years ago
Video? This work deserves it.
Jaycub (author)  rimar20003 years ago
There will be a video by the end of the week if everything works out as planned.
OK, thanks. Please, then send me a PM.
Hi nice job, is there more? As how it is the tube attached to the pipe ? Could the blades be silver soldered in place? All of that wire does not look like it will be balanced to well.
Jaycub (author)  fixitmankeith3 years ago
This is not quite finnished, I had to post it like this to enter it in the epilog challenge. I have pretty sick since Friday. This instructable should be finnished within the next few days.
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