Well here goes the first parts of the actual Jet build. I must add a warning here.

Jet Engines / Gas Turbines are to be considered VERY DANGEROUS! There are many parts moving at very high speed, high pressures, and EXTREMELY HIGH TEMPERATURES! Do not attempt to build one of these engines without some sort of help from another person that has done so before. There are many good areas of expertise available on the Internet where you can find information on building one of these engines. There is a very real possibility of FIRE, EXPLOSIONS, SERIOUS BURNS, LOSS OF LIMB, or LOSS OF LIFE! Please be extremely careful if you decide to build one of these engines. The author of these pages, and Bad Brothers assume no liability for any injury or damages should you decide to do so. These pages are for educational purposes only, and are not intended as a manual on how to build an engine.

Now, on with the fun part!

Gas Turbine Principle
The first thing you must understand is the basic principal of how a Gas Turbine (otherwise known as Jet) engine works. The process is fairly simple. An amount of air is compressed by the compressor section of the Gas Turbine Engine (GT will be used as an abbreviation throughout as meaning gas turbine engine) The air is then fed into a combustor where fuel is mixed with the air and ignited. The resulting air expands and is forced out of the combustor further into the engine into the turbine section. The turbines extract some of the pressure and convert it into rotational energy like a windmill. This energy is transferred via a shaft to the compressor to allow it to compress more inlet air, thus the process is self sustaining. The remainder of thrust is diverted out of the back of the engine and can be used for propulsion or other needs.

To build a home made GT, you will need a Compressor and Turbine connected via a shaft. Fortunately these exact specifications exist in turbochargers for internal combustion engines. Almost any automotive turbocharger will work, as well as very large turbochargers from big trucks or earth moving equipment. Obviously, the larger the turbo, the more thrust the completed engine can produce. For this project, I have chosen a large Cummins ST-50 turbocharger from a big truck. This should produce enough thrust to propel the bike at a pretty decent rate of speed, and acceleration. Remember, speed is how fast the vehicle can travel, acceleration is how fast it can achieve that speed. Since this bike will be run on a dragstrip eventually, it will need to accelerate very quickly. Making the bike street legal is more for show than anything as it will not be very practical as far as gas mileage goes. We will discuss that part later however.

The next part of the equation is a Combustor or Combustion Chamber. The combustion chamber must be able to sustain combustion, a flame, without the flame being extinguished by the fast moving gasses. To achieve this, air is fed from the compressor into the combustor (usually made from a length of pipe) and inside the combustor is the flame tube. The flame tube (also a piece of pipe) runs from one end of the combustor to the other. One end of the combustor is sealed off, and the other is open only in the middle to allow the hot gasses to escape from the flame tube. The flame tube is comprised of pipe with strategically placed holes to allow the air to enter the flame tube. Fuel is fed in and combustion occurs only within the flame tube, while fresh air is constantly fed in around the perimeter of the combustion chamber. This allows even flow to the flame tube, and also serves to cool the combustion chambers outside walls. This design will be obvious when viewing the pictures on these pages.

I chose to make my combustion chamber smaller that what is normally recommended for this size of turbo. I am doing this as a test to see if it is possible, because I need the extra space for everything to fit into this tiny bike. If this design does not work out, I will make a larger one. With this being the case, do not be surprised if the construction lasts longer and I have to take a step backwards and build a new one. Now on to the pictures!

Cummins ST-50 Turbo

This is the Cummins ST-50 turbocharger. There is also a model VT-50 that is almost identical except for the compressor housing. The gloves give some idea as to the size of this beast.

The inducer vanes of the compressor

The inducer, visible here, is the visible part of the compressor's inlet vanes (the things here that look like fan blades). Usually the flame tube would be double the diameter of the inducer. The inducer on this turbo measures 2.9 inches in diameter. Our flame tube will be 2.5 inches in diameter however, as I am trying something new to save space.

Note the size in proportion to my large gloves

Just a picture from the other side of the turbo.

Combustion chamber pieces

These are the basic parts of the flame tube. There are three of the rings in total, one of which is drilled and threaded for the six screws that hold the end cap on. That ring is attached to the back of the end cap at the bottom of the picture with 6 screws.

Combustion chamber pieces close up

Here you can see the inner flame tube at the upper left (the holes for the air to pass through have not yet been drilled). The outer tube of the combustor is on the right. The hole in the chamber is drilled to the side to create swirl as the air enters. The tube in the middle will attach to the combustor at the hole on the side, hence the funny cut on it.

Air inlet to the combustion chamber

Here is a better view of the offset hole in the combustor housing. It was drilled in the drill press with a hole saw and some very careful clamping to keep it from moving while the cut was made.

Combustor parts layout

Here the parts are laid out to show how they will be welded together.

Combustor as it will be viewed from the front of the bike

You can see the need for the scooped cut on the pipe feeding the combustor. If the pipe was left straight on the end, it would protrude into the combustion chamber, which would be undesirable.

Combustor welded together

Here the combustor is welded together. Weld penetration in welding this part is critical as you do not want any air leaks. It is fairly easy to achieve good penetration because the steel is only 1/8 inch thick in the combustor. The steel used could have a thicker wall, say 1/4 inch, but would be unnecessary and would just add more weight to the project. 1/8 inch steel is perfectly suited as far as weight and strength goes.

Combustor showing end cap attached

Here you can see the six screws that hold the end cap of the combustor on. This allows for easy inspection of the flame tube inside, and also aids in changing fuel jets and ignitors.

Combustor opened to show flame tube holding rings

As you can see, the rings visible in the earlier pictures are located at both ends of the combustor. There are 2 rings at the opposite end. One is welded flush with the outlet pipe, the other being closer to the inside. The third ring is visible here at the front of the combustor. It is threaded to allow the 6 cap retaining screws to bolt right to it, eliminating the need for separate nuts. The flame tube is made of 1/16 inch inch wall thickness 304 stainless steel and is 2.5 inches in diameter. I chose to use the stainless for the flame tube because of its excellent ability to withstand high heat, and used mild steel for the rest of the combustor for its ease of machining.

Flame tube being inserted into the combustor

The flame tube slides right into the rings at both ends and is held in the dead center. The end cap is then secured by 6 allen head cap screws with washers. The allen head screws are better than phillips, as they are less likely to strip out, and are also easier to get at in tight spaces. The cap will be right behind the front tire of the bike, so ease of accessibility is a must.

Flame tube inserted into combustor

You can see here how the flame tube fits. The ring, or flange, around the tube also provides a good seal when the cap is bolted on. If you look closely at the end cap, you can see a hole in the center. This hole will be used to mount the fuel injector. Another hole will be provided for the ignitor. Also noted for the astute viewer is the fact that the flame tube here is not the stainless tube, but a piece of 1/8 inch pipe. The stainless tube has half the wall thickness. I stuck in the wrong piece of pipe before this photo.

Before I can go any further I need to build a test stand for the GT so that I can work out any bugs before installing it in the bike. I will try to document that for the next installment in case anyone needs some basic ideas for their own, not that I am advocating building one however. If you do happen to build one of these though, would you be kind enough to offer me a race on the dragstrip as I think it could be fun and would really wow the crowd!

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