The Combustion Chamber, part 2

Fuel injection sticks and combustion chamber rear


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At the rear of the combustion chamber, held by a diameter of the NGV, are a set of 12 curved SS sticks which receive the fuel from the fuel pump via a set of hypodermic needles. The sticks, as well as the combustion chamber (cc) rear, come as part of the cc pack of parts, available from Wren and J.D. Enterprises. These would be difficult to make, and the pack of parts is highly recommended.

The plans call for the sticks to be secured to the cc rear via spot welding, and the front of the sticks form a 54 mm circle towards the front of the chamber.

The sticks as supplied from Wren are close but not exactly to print. Some of the sticks have more acute bends than others, which makes even positioning of the tips on the 54 mm circle difficult. I tried to bend them a bit more evenly, but once they are in this cut state, they are almost impossible to bend.

Trimming to length is easy, though... I applied a bit of blue, layed them over the plans, and marked them with a scribe. A fine-toothed hack saw, followed up with a bit of sanding, took care of them.

It is desireable to have as even a distribution of fuel at the front of the cc as possible. Rather than guessing, I created a thin aluminum jig, with 12 holes drilled on the desired 54 mm circle. A bit of reaming and fidgeting allowed me to assemble the sticks into the correct formation on the cc rear.

Wren suggests using four spots per stick flange to secure the sticks into position. I was not able to get more than one per stick, and even then I blew 2 very small holes in the thin sheet steel. I elected then to seal and secure the flanges to the cc rear with silver braze. I selected Harris 45 Safety-Silv as the alloy of choice, as it fillets nicely and flows sluggishly at 1370 degrees f. This will provide a more secure joint than a thinner, more fluid braze.

The joints are cleaned and wire brushed with a stainless brush, soap, and water. I used Harris flux for the brazing, and applied this material liberally... a piece the size of a lima bean smeared around each joint. Apply the flux to all 12 joints, and do the brazing in one sitting.

The wire diameter I used was 1/32", and this worked perfectly. A very small propane torch is all that is necessary, as stainless steel does not conduct heat well and the local area heats quickly. Don't use oxy-acetylene, you would surely melt the steel, and it is simply not necessary.

The silver braze flowed beautifully around the sticks, and produced a perfect fillet on the underside of the joint.

With the jig removed, the sticks maintain their perfect and even spacing.

On to the swirl jets!

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