Ten Dollar Jet Engines

Friday, September 23, 2005

Smooth Lady - Getting It Together

Here's how the two main assemblies - the chamber and the intake / tailpipe assembly - are jigged up and welded. I'll also discuss how to do the intake and tailpipe flares.

To jig this up for tack welding, I just set up firebrick on the garage floor (the best flat, non-flammable surface I had available) and pinned the tailpipe in place with two more slabs of firebrick. The front corners of these slabs extended beyond the front edge of the tailpipe just enough to align the rear of the chamber cone with it. Side-to-side alignment of the chamber is essentially automatic, due to the good fit of the intake pipe to the front end of the oval slot in the top of the chamber. You may have to gently bend the intake up or down slightly at the 'spade' to get a near-perfect fit. Near the rear, I had to push the cone edges down in a bit to lessen the gap - note the fairly wide gaps between the chamber cone and the edges of the 'spade'! (I really should have cut the 'spade' area closer to a full inch wide (25mm) to achieve a better fit.) For me, such gaps are no problem, but they can be rigorous (but excellent!) practice for beginners.

Photo Copyright 2005 Larry Cottrill

You need plenty of tack welds to lock things together. Note that the EMT intake pipe is a little thicker than the chamber cone shell, so you need to concentrate your heat more on the intake tube, less on the cone edge. Here's a shot of the tack welded assembly after cooling. The gaps between the 'spade' and the cone edge can clearly be seen here, near the right edge of this photo. Note that there are tack welds around the seam between the cone and tailpipe also (you can just barely see them in the photo) - for these, you have to pick up the assembly out of the makeshift jig and turn it over.

Photo Copyright 2005 Larry Cottrill

Once the tack welding is in place, you're ready to finish weld the cone to the intake and tailpipe. Start with those two gaps - use a small flame and all the patience you can muster, and bridge them in, working gradually forward from the rear. Be extra careful to bridge right up to the point where the previously done welds along the sides of the spade ended (just a few mm forward of the end of the intake tube). Next, turn the assembly over, and again with a small flame, work around the seam between the tailpipe and the rear edge of the chamber cone. Just weld from tack weld to tack weld, blending them in as you go. Try to use as little filler rod as possible, so the underside of the weld will be kept fairly smooth inside the pipe. Once you've completed smooth, unbroken weld around that seam, you can do the final weld all around the top between the intake tube and the cone shell. You can use just a little larger flame and considerably more filler rod for that weld. It can be done a little at a time, from tack to tack, as long as you get continuous weld - no little gaps or holes. Again, the tube is a little thicker than the cone shell, so you need to keep the bulk of your flame on the tube, and just flow the weld puddle into the shell edge and blend it in. When you're all done, it ought to look something like this:

Photo Copyright 2005 Larry Cottrill

The main thing left to do at this point is form the intake and tailpipe flares. The tailpipe flare can be easily hammered cold with a ball peen hammer, by extending the edge of the pipe 3 or 4 mm over the edge of a workbench and hammering downward on the bottom edge. Slowly roll the pipe as you go, gradually extending the slight flare all around the edge.

For the intake flare, set up the engine firmly clamped in a vise so it overhangs the edge of the bench, with scrap wood supporting the dome at the center (i.e. under the spark plug mount):

Photo Copyright 2005 Larry Cottrill

Heat the edge of the intake tube to red heat a small zone at a time and hammer gently with the ball peen. Then heat the adjoining small area and lightly hammer again. Work very gently and gradually, until you have a well-formed flare all around the top and sides of the end of the tube. You won't need to flare the bottom edge, where the intake touches the 'spade' (now part of the finished combustion chamber). If you take this process slowly and gradually, you can form a very nice flare that will work well. As shown in the original drawing, the flare doesn't have to be overly large.

The final step will be to set up the starting air and fuel tubes. At that point, you're ready to test fire your Smooth Lady!

Larry Cottrill
23 September 2005

Friday, September 16, 2005

Smooth Lady - Important Note to Builders

Before returning to construction details, I wanted to let you know of an important change discovered by running the finished engine. Namely, you should add 3.5 inches (89 mm) to the tailpipe length! It was found that the engine would not resonate well enough for easy starting with the original dimensions. I apologize for the inconvenience this may cause if you are already close to welding up your Smooth Lady. The fact is, the correct running length of a pulsejet cannot be determined by design theory alone - you have to get it running to know for sure! Again, my apologies. Fortunately, it is pretty easy (and really good welding practice) to cut a short length of tailpipe tubing and weld it onto an existing too-short tailpipe - you just have to align the new piece carefully while tack welding in four to six places; then, run continuous weld around between your tack welds, blending them in as you go. Here's the original run of the Smooth Lady prototype - note the temporary extension and the C-clamp keeping it in place near the right edge of the photo:

Photo Copyright 2005 Larry Cottrill

Larry Cottrill
15 Sep 2005

Friday, September 02, 2005

Building the Tailpipe/Intake Assembly

Cutting the pieces:

The intake is a 5-inch long piece of 3/4-inch EMT ("rigid metal conduit"). If you don't live where US measures apply, you will need to find a piece of mild steel tubing about 20 mm ID with a wall thickness of about 1 mm, and cut it to a length of 127 mm. One end is cut square, the other is cut off at an approximately 45 degree angle. (The length is measured from the square-cut end to the outermost "point" of the angle cut.) Be sure to file all the edges smooth so the piece can be handled safely. Try to find the point on the angle-cut end that is the 'bottom' of the cut, i.e. where the tube wall is the shortest. Draw a line from that point straight down the full length of the tube with a fine permanent marker; this is the bottom centerline of the tube. Set this piece aside for the moment.

Cut the tailpipe to its full length from a piece of 1.25-inch OD antenna mast steel tubing, using a hack saw with a new blade. The tube is only about .5 mm thick, and will cut easily. Try to get the cut ends as squarely cut as possible. According to the detail plan given earlier (see my 02 Aug 2005 post), choose one end as the 'front' end and mark the 'spade' on the outside of the tube (if you can tell where the welded seam of the tube is, center this weld in the width of the 'spade'). Also mark the cutoff edge under the 'spade' - NOTE that this cut is slightly slanted, i.e. it is 2mm farther rearward at the 'spade' than at the bottom of the tube! This slant is there to accommodate the chamber cone which slants upward as shown in the overall engine plan.

Secure the tube in your bench vise, not too tightly. Use your hack saw to make cuts forming the sides of the 'spade', just back to the point where they meet the cutoff you've marked. Then, turn the tube over and make the slightly slanted cut across the tube from the bottom JUST to the inner ends of the longitudinal cuts you just finished. If necessary, wiggle the waste section back and forth a while until it breaks free. The waste piece can be discarded. Finally, use a small half-round file to smooth up the cut edges all around:

Photo Copyright 2005 Larry Cottrill

Now for the most critical part -
Jigging the intake and tailpipe for welding:

On the outside surface of the tailpipe 'spade', mark the 0.47 inch (12 mm) setback using your fine permanent marker (refer to the Intake / Tailpipe Detail plan) - this is where the square-cut end of the intake will be positioned. Use a C-clamp or (better yet) Vise Grips (TM) pliers to clamp the pieces together. BE PATIENT! This is likely to take a few tries before you get it just right. What you want is to simultaneously achieve THREE correct alignments: (1) the rear end of the intake must be at the 12 mm line just forward of the base of the 'spade'; (2) the bottom centerline marked on the intake tube must be centered on the 'spade'; and (3) the centerline of the intake must be parallel to the tailpipe tube centerline.

To get it, mount the tailpipe in the vise (not too tightly!) so the 'spade' is oriented away from you, as shown in the photo:

Photo Copyright 2005 Larry Cottrill

Now, move the intake into position and set your clamp so it just lightly clamps the intake to the 'spade' (if you use Vise Grips, these should be oriented as shown in the photo above). Patiently work with the setup until you get the bottom centerline of the intake where it belongs, while keeping the rear end of the intake on the mark. Then, rotate the front end of the intake up or down slightly until it appears to be perfectly parallel to the tailpipe centerline. Once you have it exactly where it should be, tighten up the clamp - don't worry about flattening the 'spade' somewhat, as long as the positioning of the intake is correct. Once it's clamped solidly together, carefully move the assembly to your welding area and tack weld before releasing the clamp.


In this case, it helps to hold the assembly in a vise for welding. First, tack weld the intake to the front edge of the 'spade' at its center. Next, turn the assembly over and make a tack weld between the 'spade' and the intake side wall about 1/4 inch (about 6 mm) in front of the rear end of the intake. Then do the same on the opposite side. Once these tack welds are in place, you can remove your clamp and set it aside.

At the front edge of the 'spade', start at the center tack weld and run a fillet weld from there out to the corner of the 'spade'. Do the same from the center to the opposite corner. What you want is unbroken weld between the 'spade' and the intake tube, clear across the front 'spade' edge. Then, choose one side and make a good fillet weld from the tack weld near the rear right up onto the front-end weld you just completed. Repeat this on the other side. What you should end up with is a continuous, unbroken weld between the 'spade' and the intake like this (again, flattening of the 'spade' doesn't matter, and should be expected):

Photo Copyright 2005 Larry Cottrill

Photo Copyright 2005 Larry Cottrill

Photo Copyright 2005 Larry Cottrill

The last photo above shows the front-end 'spout' of the intake. To form this, you grip the rear portion of the intake tube (not the tailpipe) in a vise, then heat the top front area of the intake red hot and gently hammer it down. As shown here, it is not necessary to deform it as much as shown in the Intake / Tailpipe Detail plan - you only need enough of a bend to divert the intake air stream downward slightly into the front of the chamber.

This description makes the procedure sound like a lot more work than it is. If you're an experienced metalworker, all this can be done in a couple of hours or less. Once this has been completed, we're ready for the exciting part - bringing our two main subassemblies together to build the engine as a whole!

Larry Cottrill
02 Sep 2005

Thursday, September 01, 2005

Finishing Up the Combustion Chamber

The chamber dome just completed should closely fit the large end of the combustion chamber cone. If the front edge of the cone is a little "out of round", adjust it slightly by hand until the edge of the dome meets the cone edge with reasonable accuracy all around. You should be able to get it to fit with gaps of less than 1 mm anywhere around the edge. The cone and dome can be "jigged" for welding with a few pieces of firebrick, as shown here. Lay down three pieces butted together; set the cone on top of these and stabilize it with a firebrick piece along each side; and finally, carefully position the finished dome snug against the front edge of the cone while positioning another piece of brick up against the plug mount to hold the dome in place:

Photo Copyright 2005 Larry Cottrill

Next, apply just a couple of small tack welds about 60 degrees apart along the top edge of the dome to lock it onto the chamber cone:

Photo Copyright 2005 Larry Cottrill

Now, you can remove the brick that was holding the dome in place and use pliers at the small end of the cone to rotate the assembly so you can make a couple more tack welds. Keep your tack welds small and spaced at approximately 60 degree intervals; i.e. the assembly will be tack welded at six points around the front rim.

Once the last tack weld is done, you can start at that point and begin finish welding all around, blending in your tack welds as you run into them. Again, you'll have to use your pliers to rotate the assembly as you work your way around the edge. This MUST be continuous weld, without gaps or holes! Try to make this a neat weld without using excessive filler rod - however, getting a strong weld without holes is more important than appearance. My finished chamber looked like this, right after letting it cool and scouring it with the wire brush:

Photo Copyright 2005 Larry Cottrill

The next step will be building the intake / tailpipe assembly. That will complete the major subassemblies of our engine. Then, the REAL welding begins - everything up to that point will be "just practice" compared to the final assembly! (But, this shouldn't really be difficult welding - there'll just be a lot of it.)

Larry Cottrill
01 Sep 2005

Add an element of visual excitement to your home, shop or office with one of our full-color 11 x 17 inch 'Year of the Valveless Pulsejet' mini posters! See them here:

Thursday, August 25, 2005

Smooth Lady - Forming and Welding the Dome

After cutting out the dome sheet from the 24 gauge black sheet steel material and filing the edges smooth, I was ready to roll this small piece into a shallow cone shape. Here's how I set up a 3/8-inch diam. twist drill in the vise - the smooth shank of the drill was left protruding about 1.5 inches (about 36mm) so I could use it as a crude "mandrel" for the bending. The work is done very gradually, going slowly around the whole circumference of the piece while pushing downward gently as you go:

Photo Copyright 2005 Larry Cottrill

The two straight edges needed light hammering to get the adjoining areas properly curved instead of ending up as "flat zones":

Photo Copyright 2005 Larry Cottrill

Once the conical shape looks good and the edges come close to meeting properly, the edges are welded, again using the very small no. 00 welding tip and 1/16-inch mild steel filler rod. The next photo clearly shows the welded seam. The idea of the bolt and nuts is that the lower nut will be welded onto the dome to form the spark plug mount. However, I decided that a normal size hex nut like these would be too massive to go on such a lightweight dome. The thread is metric, M10x1, to fit the CM-6 type spark plugs that I like to use:

Photo Copyright 2005 Larry Cottrill

A better choice for the plug mount was an M10x1 bicycle wheel hub nut. This is a hex nut made from fairly thick sheet steel. As before, the bolt is pushed through from the bottom side and the bicycle nut threaded down finger tight. Then, the thicker hex nuts are threaded on. The large nuts protect the threads from being damaged by the welding operation, and also help hold heat close to the thin nut as it gets joined to the dome by welding all around. You have to concentrate the torch flame on the side faces of the nut while heating the thin dome very gently, to keep from burning through. Also, with such a thin plug mount, you have to be very careful not to run any weld up onto the thicker nut above! So, here's how the setup looked just before I started welding:

Photo Copyright 2005 Larry Cottrill

Here's the finished dome with the plug mount welded in place. Note that, though it doesn't show in the photo, the thread in the plug mount remains smooth and clean. And, the threads of the bolt and extra nuts are still in good shape after welding, too - these can be re-used the same way indefinitely for future projects:

Photo Copyright 2005 Larry Cottrill

I think the next thing I'll do is weld the dome onto the front of the chamber. Then, I'll be able to work on the tailpipe and intake assembly to get it ready to join onto the rear of the chamber. Finally joining these two main assemblies will be the majority of welding on this engine. Once that's done, I'll just need to add the starting air pipe / fuel pipe assembly to finish up.

And then - our first test run!

Larry Cottrill
25 Aug 2005

Our beautiful, full-color 11 x 17 inch mini posters make great gifts! See the entire selection at http://www.jetzilla.com/PJ_posters/YOVP_posters.html

Friday, August 19, 2005

Smooth Lady - Welding the Chamber Cone

The chamber cone rolled to shape. Photo Copyright 2005 Larry Cottrill

The photo above shows the cone after rolling it with my crude 'hydrant water pipe' tool described earlier. The edges have been filed smooth for safe handling. There is only a little run of weld needed to fasten the cone together - just the couple of inches where the straight edges meet.

I had little trouble welding the rather thin (24 gauge, or about .5mm) material, except that great care was required at the ends of the weld, where it's awfully easy to melt back the edges. The best way to proceed is to pull your cone together with a large hose clamp, so the butted edges are held in good alignment; then make a couple of tack welds at the butted joint, not too close the the edges. Then, loosen and remove the clamp and weld from the middle of the butt joint out to one edge, going right over the tack weld. Then start again in the middle and weld out toward the other end, again blending in your tack weld as you go. I used my No. 00 welding tip and 1/16-inch mild steel copper-coated filler rod, very sparingly applied. The flame was set very small, even for a tip this size. Welding took only a few minutes to complete.

And, here's is the final result:

The initial weld of the chamber cone. Photo Copyright 2005 Larry Cottrill

Once I have the front dome built, the chamber can be completed and I can get started on the intake and tailpipe assembly. So far, this engine is proving to be about as easy to build as I had imagined, and I hope the final steps will be just as easy.

L Cottrill
19 Aug 2005

Friday, August 05, 2005

Smooth Lady - Making the Chamber Cone

Scalable pattern for the chamber cone

For the cone material, I bought a piece of 24 gauge black stovepipe. The smallest piece I could get was a 6-inch diameter x 24-inch pipe, so the sheet was a little over 19 x 24 inches, and had quite a bit of curvature to it, right off the shelf. A flat sheet of steel would have been easier to work with, I'm sure. This piece cost about $5.00 US, but then I can make quite a few cone sheets from it, if I need more in the future. I made a pattern by printing out the jpeg above and blowing it up to full size with a copier at work, then tracing it onto an old manila folder and cutting it out. I then pressed the pattern onto the steel surface by hand and scribed around the pattern with a small nail, scratching easily through the black coating onto the steel. By carefully holding the pattern down tight, I got a virtually perfect outline to cut on.

I cut the piece night before last, in less than an hour total work time. One of the things I did right was buy a PAIR of shears from Menards - one for right curves and one for left. Unless you're using some of the new transparent sheet metal ( ;-) that's almost a must, in my opinion. I don't have very strong hands, so at times I thought it was pretty tough going; and, there was some edge distortion, though that was taken out after cutting by very light hammering. But to my amazement, by carefully choosing the correct shear (right or left), I had no trouble at all making the curved cuts, including the inward curve that forms each side of the 'oval slot', even at the front end where the curve is really tight! I almost couldn't believe how well it worked (of course, it helps to have brand-new cutting edges on your tools!). After cutting out the part, I gently hammered down the edges and hit all the edges with small flat and half round files, taking out little flaws and dressing down all the sharp edge on both sides for safe handling. The finished piece looked really good, and weighed in at exactly 5.0 ounces on a postal scale! The thickness of this steel is only about 2/3 mm - that should be adequate for a small prototype chamber. If I can put this together successfully, it will be the first engine I've done where I will have made my own cones from scratch. (The cones for my original FWE prototype were provided by my young friend from Connecticut, Steve Bukowsky. )

I rolled the cone to shape last night. The only suitable "tool" I could find that was solidly mounted was the 1-inch galvanized pipe that sticks up out of the floor of our horse barn (it has a freezeless hydrant at the top). Using leather work gloves, I gently pulled the piece around the pipe, working out from the center gradually in both directions. You have to pull hardest at the "small end" (i.e. the end that will be the rear of the finished chamber) because that's the tightest part of the cone. This technique worked well for all but the side edges, where my fingers couldn't provide the needed leverage, so along those edges I had to resort to careful light hammering. The result seems wonderful - much better than I would ever have expected for a first effort.

One disadvantage of my "vertical roll" tool is that it has the clunky hydrant at the top, so I had to "spring" the piece off the pipe from the side. But, it was easily brought back to shape by hand so that the rather short straight edges come together almost perfectly. And, the front and rear edges seem almost perfectly circular, too. The 24 gauge steel is really a nice material to work with - even a very young person could do it, equipped with patience and a good pair of work gloves.

Now I have to make the front dome. That's a lot smaller, but it will be harder to cut AND harder to roll, because of the very blunt conical shape and almost no clearance at the "small end" (where the plug mount goes). I'll probably start by drilling and filing the center hole, then shear cut the rest. I have the manila pattern cut out and ready to use. I'll have to come up with a different tool for rolling this piece.

Larry Cottrill
05 Aug 2005

Add some warmth and color to your home, shop or office with a beautiful valveless pulsejet mini-poster: