Tuesday, 5 April 2011

Welding the Combustion Chamber

Today I finished welding together the combustion chamber for the rocket engine. The time spent on trialling the welding set-up proved its worth; the whole process went without a hitch.

Before I assembled the chamber, I gave the outer surface of the inner tube a coat of black high temperature paint. The idea being to improve its performance as a heat radiator. Most of the thermal transfer in a rocket engine takes place by convection, but every little helps. I welded the outer tube in position first.

The assembled chamber was clamped together through its central hole using clamp bars and a threaded rod.

 I then tacked the outer tube to the flanges in approximately eight places each, as per the fabrication trials. The tacks were diametrically opposed to minimise distortion. These were joined up with short runs of bead, again as per the trials. I detected no discernible distortion of the chamber once the outer tube was fully welded.

Here is the chamber with the outer tube finished:-

The Swagelok nuts on the coolant fittings protected the threads whilst the chamber was moved around.

With the outer tube completed, it was time to start on the inner tube. This had to be welded to the flanges at each end. The image of the pre-assembled chamber in the Fabrication Experiments post reveals the welds have to be done close to the inner tube edges.

The pulse TIG really proved its worth here. The welds were completed without incident. I kept the pulse settings the same as for the outer tube, with an average current of 70A.

Here is one of the inner tube to flange welds:-

Again, no discernible distortion was detected post welding. The chamber will be returned to the lathe next, for a tidy up. This will involve polishing the unit to remove any heat discolouration and small nicks caused by handling. The flange faces will be restored to  flat, mating surfaces by skimming the inner tube to flange beads.

The chamber is now able to act as a foundation element for the injector and nozzle. It has a hot gas enclosure which is surrounded by a cooling jacket. Fittings are provided for coolant inlet and outlet. The cooling jacket and hot gas side tubes also form structural members to withstand the chamber pressure forces.

Today has been a productive one, and I am very happy with the way the welding went. The chamber looks pleasingly like the original sketches I did for it. These are in one of my notebooks, and were drawn in the summer of 2009. This is the joy of designing, calculating and constructing. A problem exists. An idea forms in the mind. A rough sketch is made. The idea takes shape. Trials begin, problems are solved, the concept evolves. And then one day a thing of paper becomes a thing of metal. A tangible object that serves a real purpose, its form, one with its function, the confluence of equations, experience and expedience.


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