Welcome back to the ST-50 engine build. This time out I will be fabricating the exhaust flanges to the turbine and making the inlet diffuser. I'll start by getting ready on the end mill to turn away some more metal for the engine destined for the X-Project.

Exhaust cap

One end cap would be the main cover of the combustor housing where the fuel injectors are located, and the other would be for the exhaust side. The exhaust side needed an opening to match the turbine flange, so the Mini-Mill was brought into service yet again. The end cap was lined up and clamped to the mill table with step clamps and the roughing end mill was used to open up the exhaust port.

Turbine side end cap assembly

With the exhaust opening milled out, the next step was to weld the exhaust end cap to the turbine flange I made previously. As I was preparing to weld I had a sudden idea. There would be a thermocouple temperature probe installed in the rectangular opening of the exhaust flange. If I had to remove the combustor for maintenance, it would also mean removing the thermocouple and all of its wiring. This led to a design change for the end cap. I was happy that I finally made a design change before a mistake instead of a fix-up after one. I think I am getting the hang of this jet engine business!

Separable end cap pieces

To make the end cap separable from the turbine flange I used the Mini-Mill again to turn a smaller disc of metal. Holes were indexed and drilled to mate the turbine flange cap to the combustor end cap.

Welding the nuts

I used the same technique as before to weld nuts to the inside of the combustor exhaust cap as I did with the housing rings. This will allow the turbine flange to bolt directly on the the end cap. By now it should be obvious that my construction techniques have vastly improved with the purchase of my new machine tools. The ease with which I can make these new parts surprises even me, and I believe that I will be able to achieve my goal of making the engine a true beast.

New turbine flange

The intermediate plate was welded to the new turbine flange assembly using the tack and stitch method. The outcome is a compact assembly that can be modified later without major changes to the entire combustor. It will also allow the thermocouple to remain attached to the turbine assembly when the combustor is removed. The thermocouple will be attached to the side of the rectangular tunnel section of the flange with a threaded compression fitting.

Test fit

With all of the main combustor body pieces finished, the parts were assembled for a test fit. Visible here is a close-up of the turbine flange assembly bolted to the main combustor body. It becomes obvious why this is a good choice for the unit, and the pieces truly become modular assemblies.

Finished assembly

The turbine flange has a straight through opening, but one of the benefits of this type of construction is that if the need arises, a new flange can be made to help funnel the gasses from the combustor. For now, the flame tube will have the taper built into it. Another design that would also be easy to implement would be an exhaust flange with a 90 degree bend similar to the original HR-1 engine exhaust. All things said, I am very pleased with the outcome so far.

Engine layout

Next up on the agenda is to build the inlet diffuser to get the air from the compressor to the combustor. As in the HR-1A redesign, I will be making the tube have a gradual taper to help slow the gasses and further increase the pressure.

Tapered diffuser

The diffuser starts out as a piece of 3.5 inch diameter pipe and needs to be enlarged to have a final diameter of 4.5 inches. By slowing the air down, it will compress more easily, and more compression in the combustion chamber means more power. I started by making a series of 2.5 degree cuts in the tube and bending it into the desired shape to make the bend into the combustor housing. The pipe was then split down the middle and a triangular filler strip was tacked into place.

Diffuser

The filler strip has a taper in it of 3 degrees down the length of the tube. I have found that expanding the tube more than 3 degrees will cause the air to separate from the pipe wall and cavitate which could cause a major loss of pressure. While this design is still very much experimental, I believe that I have hit upon just the right recipe for success.

The filler strip and wedge cuts

Although it is hard to see from the outside, the filler strip and cuts needed to make the radius bend are visible from the inside. The welds on the inside are smooth, but have the look of being raised because of the discoloration of the metal during the welding process. The diffuser was surface finished on the outside after welding, and then pressure tested with the shop air compressor and soapy water solution. A few holes had to be patched up with the welder, and the finish was restored to a shine.

The diffuser in place

With the diffuser finished up, it was test fit against the combustor while mounted to the turbo. I believe that this engine will be capable of producing more thrust than the ratings returned by Jet Spec Designer. I still have to cut the opening in the combustor and weld in the diffuser before I can go any further. After the problems I had with the HR-1A rebuild, I will take a bit of time to let my nerves settle and plan this out well. I do not want to see another major mistake at this point in the game.

Be sure to check back often, as more updates are on the way. Thanks for stopping by, and be sure to support us so that we can continue to bring you projects like the ST-50 engine.

Gary Richards

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