As was seen in the previous section, the HR-1A engine suffered some major setbacks when the combustor warped during fabrication. There is always good that comes with the bad though, and while I was having a new combustor housing rolled I thought of many things that would help the engine to be even better. In this installment, we will rebuild the engine with new life, and even better designs.

Mild steel combustor

I did have another combustor housing rolled for me out of mild steel. I salvaged the end rings and combustor caps from the stainless combustor and welded it all back together. Working with stainless is definitely an art, and until I master it I will stick to mild steel. The final product looks good, and I think it will work out fine. The exit funnel from the combustor can be seen here bolted back to the bottom of the combustor.

Combustor end cap in place

Now that the new combustor housing is all welded together, the end caps were reattached and I was ready to fabricated a new inlet diffuser. I am also taking the lessons learned previously and making another design change to the diffuser as well.

Inlet diffuser # 2

This is the start of the new diffuser. The general construction is the same, with the tubing being split down the middle and the triangular pieces welded in. It has the same 3 degree taper along the length. I am happy with the outcome, and it seems that the experience from the previous mishap has paid off as the new diffuser looks even better.

The diffuser bend

The big change in the new diffuser is the bend that will allow the gasses to enter the combustor housing more easily. To make the bend I cut slits along the side of the pipe so that it would flex and could be shaped into position. With the diffuser held to keep the bend at the proper angle, every slit had to be carefully welded closed.

The finished diffuser

The diffuser was finished with the flap disc on the angle grinder, and it really made it shine. This was a quite complex piece to build, and since there are lots of welds it will need to be tested to see if it holds pressure. I will cap the ends and run up the inside pressure with my shop compressor, then I can use a soapy water solution to test for leaks.

New flame tube

Since there were already changes being made to the engine, I decided to make another to the flame tube. The new flame tube is still a 5 inch diameter tube, but it will now have a taper in it to help the gasses to flow even better. My thinking is that by tapering the end of the flame tube to match the 3 x 2 inch inlet of the turbine housing, the gasses will be sped up by the gradual restriction. The milder taper in the flame tube should also produce better flow characteristics and the dilution holes will be able to cool the narrow air column more efficiently before it enter the turbine. With the combination of increased pressure from the new diffuser and the better flow from the flame tube, the thrust should be increased considerably.

Exit funnel modification

As the new flame tube now has a much narrower exhaust, the exit funnel was modified to fit the flame tube. This now means that the exhaust is funneled into the turbine gradually over a length of about 8 inches, as opposed to the previous setup where it has only about 2 inches to make the transition.

Flame tube hole pattern

The hole pattern on the flame tube consists of several different hole sizes and different placements. The smallest holes at the left are called the primary holes, and these supply the incoming air to the flame tube in a position closest to the fuel injector. When all works properly, most of the fuel is burnt in the primary section, and the smaller holes also help to keep the flame front near the injector side of the flame tube.

The second row of larger holes in the middle are called the secondary air inlets. The air supplied by these holes provides all of the needed air to complete the combustion process in the flame tube, and all combustion should be finished by the time the fuel reaches these holes.

The last set of holes to the right, are called the dilution or tertiary holes. These are the largest holes in the flame tube, and their job is to cool down the column of hot gasses before they reach the turbine. Without these holes the turbine would melt away from the intense heat generated inside of the flame tube.

Once again, a great piece of software for calculating the size, pattern and placement of flame tube holes is Jet Spec designer. I highly recommend downloading this software from my site for free if you plan to build one of these engines for yourself. It will help you to calculate the flame tube, as well as being able to run analysis on turbos to determine the thrust they are able to produce when configured as a jet engine.

Flame tube exit

The very small holes that are visible along with the gradual taper are additional cooling holes in the flame tube. The thin sheet of air that these holes provide will further enhance the cooling properties of the dilution holes and will make certain that the ends of the flame tube do not overheat and melt away.

Flame tube injector end

This end of the flame tube will hold the nozzles or fuel injectors. I will have to fabricate a manifold to hold the nozzles in place and keep them aligned. The end of the flame tube is capped off, and unlike other designs where the flame tube runs from one end of the combustor housing to the other, mine will stop short of the end cap on the injector side. This design approach allows cooling air to flow over the top of the flame tube so that heat will be kept away from the end cap of the combustor. Using this method, the end cap should be less subject to warping which could ruin yet another combustor housing.

Combustor and flame tube

The combustor and flame tube are almost finished now. I must still cut the inlet air hole into the side of the combustor carefully weld the inlet diffuser into place. With any luck the new engine will be running soon, and testing can resume to achieve maximum thrust. Fortunately, I have acquired some new toys in the shop that should make the rest of the build process easier, and I look forward to using them to complete the engine. You can read more about my shop in the Garage section here on the site.

Thanks for following along during the construction so far. Be sure to check in regularly for updates to the site and to watch the engine progress. See you soon!

Gary Richards

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