Step 3: Figuring the size of the combustion chamber
The combustion chamber works by allowing compressed air coming from the turbo's compressor to be mixed with fuel and burned. The hot gasses then escape through the rear of the combustion chamber to move through the turbine stage of the turbo where the turbine extracts power from the moving gasses and converts them to rotational shaft energy. This rotating shaft then powers the compressor attached to the other end to bring in more air to make the process continue. Any additional energy left in the hot gasses as they pass the turbine create thrust. Simple enough, but actually a bit complicated to build and get it right.
The combustion chamber is made from a large piece of tubular steel with caps on both ends. Inside of the combustion chamber is a flametube. This flametube is made of another smaller piece of tubing which runs the length of the combustion chamber and has many holes drilled in it. The holes allow the compressed air to pass through in certain ratios which are beneficial for 3 steps. Step one is mixing the air and fuel. The combustion process also begins here. Step to is to provide air for the completion of combustion, and step three is to supply cooling air to lower the temperatures before the airstream comes into contact with the turbine blades.
To calculate the flametube dimensions, you double the diameter of the inducer of your turbocharger, and this will give you the diameter of the flametube. Multiply the diameter of the inducer of the turbo x 6, and this will give you the length of the flametube. Again, the inducer of the turbo is the part of the compressor blades that can be seen from the front of the turbo with the covers (or housings) on. While a compressor wheel in a turbo may be 5 or 6 inches in diameter, the inducer will be considerably smaller. The inducer of the turbos I like to use (ST-50 and VT-50 models) is 3 inches in diameter, so the flame tube dimensions would be 6 inches in diameter by 18 inches in length. This is of course a recomended starting point, and can be fudged a little. I wanted a slightly smaller combustion chamber so I decided to use a 5 inch diamter flametube with a 10 inch length. I chose the 5 inch diameter flametube primarily because the tubing is easy to aquire as diesel truck exhaust pipe. The 10 inch length was figured because the engine will be going into the small motorcycle frame of the mini jet bike eventually.
With the size of the flame tube calculated, you can then find the size of the combustion chamber. Since the flametube will fit inside of the combustion chamber, the combustion chamber housing will have to be a larger diameter. A recomended starting point is to have a minimum 1 inch space around the flametube, and the length should be the same as the flametube. I chose an 8 inch diameter combustion chamber housing, because it fits the need for the airspace and it is a commonly available size in steel tubing. With the 5 inch diameter flametube, I will have a 1.5 inch gap between the flamtube and the combustion chamber housing. Try to use steel tubing instead of pipe when possible. The difference between 8 inch tubing and 8 inch pipe would be that the tubing would be measured at 8 inches outside diameter and you then select the thickness of the "wall" you need. I chose a 1/8th inch wall thickness for my engine. 8 inch steel pipe would have an inside dimension of roughly 8 inches and the wall thickness is determined by a schedule or strength number such as "schedule 40" or "schedule 80" Steel pipe tends to be much thicker in the "wall" than tubing, and can add considerably to the overall weight of the engine.
Now that you have the rough dimensions you will be using for your jet engine, you can proceed to putting it together with the caps on the ends and the fuel injectors. All of these parts combine to form the complete combustion chamber.