The ST-50 engine project

I have a new jet engine project in mind, an X-Project. This new project will require an engine that puts out more thrust that any of my previous designs. The X-Project is under wraps for now and will be disclosed at a later date, but the engine is in production and you will get to see the build first hand as it unravels.

The new engine will produce more thrust the the HR-1 or HR-1A engines. To achieve this goal, I am going to try to slow down the air in the combustion chamber as much as possible using techniques that I have learned during the construction of the HR series of engines. By slowing the air down, I can increase the dwell time the flame has to reach full combustion inside the combustor and dramatically reduce overheating of the turbine.

There will also be a diffuser inlet similar to the one designed for the HR-1A engine which should help to further slow the air. Slowing the air down creates extra pressure with the same amount of energy being input to the compressor, so the end result is free energy that would otherwise be wasted by inefficient design. If this design proves to increase P2 pressure, it will be a leap forward in home built gas turbine technology.

ST-50 Turbocharger

The basis of the new ST-50 engine is the ST-50 turbocharger pictured above. The ST-50 has an inducer opening of 3 inches which should generate approximately 60 to 65 pounds of thrust as calculated by the Jet Spec program. Other builders, such as Russ from Bad Brothers Racing have had success with this turbo, and it seems to be a good match for the new project. If my new designs do indeed work, there will be more thrust output than what early calculations show.

Combustor and flame tube material

The material for the combustor and flame tube are steel tubing with diameters of 9 inches and 6 inches respectively. The larger combustor housing should help to slow the air further and produce more compression inside the chamber, while the large diameter flame tube should give the area more lateral room to burn thereby making the burn process complete sooner in the primary region of the flame tube.

Turbine flange

I purchased a new Mini-Mill from Harbor Freight Tools before beginning construction on the ST-50 engine. With the help of a few experienced friends I have become quite good at machining parts and will use the mill during the fabrication of the engine.

The piece being machined above is the turbine inlet plate. This plate will bolt to the turbine where the hot gasses from the automotive exhaust would normally enter. I laid out the pattern and took careful measurements and then proceeded to mill the part and drill the holes for mounting.

Roughing end mill

I milled the hole on the turbine flange to a rectangular opening of 2 x 3 inches to match the turbine inlet exactly. A roughing end mill was used to machine the piece, and I can tell you that it does a great job of removing metal quickly. The end mill was purchased through McMaster Carr at www.McMaster.com and it is a hog for chewing out the metal in a hurry.

Stitch welding the flange

Plate steel was cut and tack welded to the turbine flange to create the box section inlet to the turbine housing. After tack welding, I stitch welded the pieces together making sure to keep everything in alignment. By making numerous stitch welds, the metal is less likely to deform and warp out of shape.

Finished flange

With all of the welds fully closed the entire flange was cleaned up and checked again for proper alignment. The extra length of metal to the right of the plate can be used later as an extra support when mounting the engine. If it does not prove to be necessary, it can be cut off quickly and finished off with the mill.

End plates

To cut out the end plates for the combustor housing, I turned again to my Mini-Mill. I purchased a rotary table that is indexed and calibrated in degrees. If you look closely at the picture above you can see that the plates are mounted to the table via the bolt in the center, and the handle that is used to rotate the table is barely visible to the upper right of the plates.

By placing metal blanks on the table and slowly turning them, the end mill can cut away the metal around the edges to leave nice perfect circles. The drilling process can then be used to drill the bolt holes for the caps by rotating the table the required number of degrees between each hole and then plunging the drill through the material.

Although I still use Auto Cad to layout my parts, it is much easier now to fabricate them directly with the machine tools than it is to use a pattern and trying to match it perfectly. I don't know how I ever got along with the mill!

End caps and nuts

Bolts will be used to secure the end caps to the combustor body. To make the assembly in the easiest way possible, I turned two caps on the mill, and then turned two rings as well. All of the holes were laid out to make them universal in the way they bolt to one another. This is important to allow maximum flexibility in the installation and maintenance of the completed engine.

Once the caps and rings were milled to shape, bolts and nuts were used to temporarily attach the cap and ring assemblies together.

Welded nuts

Since it would be impossible to hold the back of the nuts from turning while bolting on the caps, the nuts were tack welded to the inside of the rings before the rings get final welded to the combustor body. The also serves a nice feature over just threading the holes into the rings with a thread tap. If the threads ever become damaged for any reason, it will be a simple matter to remove the old nut, grind the area smooth, and weld a new nut in place. Modular construction is definitely the way to go!

Welding the end rings

The end rings were placed into position on the outside of the combustor housing and aligned carefully. Again, I tack welded the rings into position and checked alignment, then used careful stitch welds to button things up. Alignment was checked often to make sure that no more issues crept up on me as happened with the HR-1A during the rebuild of that engine.

Fully welded

Once the stitch welds were completed, the whole assembly was inspected for good weld penetration. Having a leak here is not something you want to happen. Even though a hole might be almost invisible, hot gasses can find their way out. The heat of the air leaving even a small hole is enough to case surrounding objects to spontaneously burst into flame. Since the surrounding objet could be me, I took extra care in examining the welds.

After good weld penetration was insured, the welds were ground and cleaned so that the combustor would look good from the outside and the caps would sit flush. Welds were then re-inspected to make sure no holes were opened during the grinding process.

Test fit of caps

After grinding and inspection of the main combustor body welds, I test fit the end caps. They are a perfect fit! The whole body of the combustor was then given a polish with an angle grinder outfitted with a flap disc. I used a 140 grit flap disc, and it put a nice sheen on the metal without removing anything but the surface rust and mill scale. Mill scale is the oxide like coating that is on the steel from the production process. The steel sure does shine up nicely.

Well, that's it for this time around. There is lots more to go before the engine is completed so check in often to see the progress. Remember to support the site!

Gary Richards

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