Elektra I Jet Engine Builders Web Log

This Web log is to encourage building and experimenting with the world's cheapest working pulsejet engine: the Elektra I(TM) - total cost in brand-new materials: Under $10.00 US, not counting the spark plug.

Want to build your own Elektra I pulsejet, with help from designer Larry Cottrill? Then this page is for you!

Friday, July 02, 2004

Construction Techniques for Elektra I

The Combustion Chamber Box

On the Valveless Forum yesterday, Steve asked about the availability of the 'octagon box' for the combustion chamber. My answer was that any good hardware store should have them. However, it turns out that there are two styles you can get -- one is designed for conduit, and the other for a mix of conduit and Romex style wiring. I think the Romex style [like the one shown here] is easier to work with, because there are fewer knockouts that have to be welded tight. On the other hand, it does have a few little slots to fill in [you can see one in the photo, while I weld the one beside it]. The other type has five knockouts on the flat face of the box alone! Also, be sure you get an 'octagon box' with the large rounded corners, not a sharp-cornered 'square box' - it should be just like what you see here:
Welding up the smaller 'Romex' knockouts in the octagon box (c) 2004 Larry Cottrill

When you buy the box, be sure to get the matching cover plate as well. You'll find a single knockout to seal up, right in the center. This plate is shown lying behind the box in the photo, above.

Welding On the Tailpipe Tube

Note on the dimensioned drawing posted earlier that [a] the hole in the box for the tailpipe is made intentionally oversize for the pipe; and [b] the pipe is set back from the outer face of the box, leaving a small gap [1/10 inch or a couple of mm]. The reason for this is so that, in bridging the gap with fillet weld all around the joint, you will also be creating a small, reasonably smooth 'nozzle zone' from the chamber into the pipe. Here's how you set this up:

First, position the box loosely in a small vise so that the tailpipe hole is looking out to one side. Now, set the tailpipe so its lower lip is just resting on the bottom edge of the hole, coming up into the hole at a slight angle. Now, rotate the box up or down slightly in the vise until the top lip of the pipe is about .1 inch or about 2mm away from the top of the hole, with the pipe drooping away at about a 10 or 15 degree angle. What you should have at the top of the hole is the lip of the pipe slightly below the edge of the hole, and slightly 'outside' it as well [i.e. away from the box wall]. Now tighten the vise a little to hold that orientation, and make about a 1/8-inch wide tack weld right there at the top. Allow this to cool, loosen the vise and remove the piece.

Now, gently rotate the pipe out straight, bending around the tack weld, until you can see proper alignment of the box and pipe in the vertical plane. Don't worry about side-to side alignment yet. When you've got it lined up, turn it over and make a second tack weld at the bottom point of the hole. Now check the alignment and, if necessary, heat one of the tack welds and bend a bit to get it just right. You should now have the pipe held by two opposite tack welds, with a slight gap between the pipe and the hole, roughly the same all around.

Now, rotate the chamber in relation to the pipe, bending around these two tack welds, until the side-to-side alignment is as good as you can get it. Then, make a little tack weld at one side. re-check the side-to-side alignment and re-heat that weld as you adjust it to get it just right. When satisfied, make a little tack weld at the opposite side. You now should have an assembly with the pipe centered on the hole but a little distance from it, looking more or less like this when you look in from the open side of the box:
View of the tack welds joining the exhaust pipe to the chamber, before finishing the welded joint (c) 2004 Larry Cottrill

Now, you can finish the fillet weld between the pipe and the box, by working around between the tack welds. Do one section, then flip the assembly over and do the opposite section of weld, then do the remaining two. You should end up with a good smooth fillet welded joint, and practically perfect alignment between the box and the pipe.

Welding in the Intake Tube

With your intake tube cut to length and the angular cutoff filed smooth, mark the 2-inch [51 mm] point from the sharp tip of the cutoff end. With the box lying flat on its closed side, run the tube in through the hole until the mark lines up with the flat top of the box. Support the outer part of the tube with vise grips or a small C clamp. Get it into position at the 60-degree angle shown, and get the cutoff positioned as shown in the drawing. Make a single 1/8-inch tack weld between the outside of the box and the upper side of the tube, and allow this to cool. Now, adjust the alignment of the tube until it is properly in line, parallel to the broad face of the box. Tack weld on the opposite side. Re-check alignment, heating and bending as necessary, until it looks right, then finish weld all around the tube to finish it up.

The idea in all this is: Tack weld, adjust, tack weld, adjust, tack weld, adjust ... and finally, finish up with a full-strength fillet weld from the hole edge to the tube, all around, preserving the alignment you've established.


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