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.
Step 1: Gather 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.
-Small diameter copper or steel tube for fuel line.
-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.
-Angle grinder or other metal cutting saw.
-Grinding wheel (or a hand file).
-Lathe, helpful but not absolutely necessary.
-Dremmel tool or other small cutter/grinder. Optional but very helpful.
Step 2: Make the combustion chamber
-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
-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.
-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
-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
-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
-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
-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.
-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
-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
-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.