For a Close Shave, You Need a Fo Mi Chin

[larry cottrill]

Here's my latest proposed design -- based on what I think I've learned from the Elektra prototype. The main design change here is in moving the intake cutoff rearward, into the front of the tailpipe. Another important difference is the use of a longer, circular cross-section chamber. Lightweight materials are used throughout, in hopes of getting a flyable powerplant [eventually, liquid fueled]. The name is derived from the materials used, and the vague resemblance to the more complex 'Chinese' design [from which this engine is not derived].

The chamber is a modified Gillette Foamy shaving foam can. The domed bottom of the can is sawed off and welded back on, inverted. This comprises a very lightweight chamber, of very good steel. The tailpipe is also enlarged and lightened from the Elektra prototype design, by using 1.25 inch OD antenna mast tubing. The rolled rim of the can is trimmed back at the bottom, and the tailpipe/intake assembly is fully welded around the top opening of the can, with the rim used only as a means of centering it properly.

Because the nozzle area of the can will become very weak under running temperatures, the midpoint of the pipe is supported by a pair of flexible struts anchored to the extended mount rails. These struts provide bracing for all combined pitch & yaw inertial forces, while flexing to allow forward and rearward movement of the tailpipe for thermal expansion/contraction.

The bottom end of the intake pipe is flattened to streamline exhaust flow around it, and for increased intake impedance. Airflow from the gap is deflected forward much more sharply than in the Elektra, and undergoes immediate shear as it passes forward over the nozzle rim, providing significant turbulence and slowing for good mixing. It is probable that the gap at the bottom will be narrower than shown, and also likely that this might turn out to be a critical dimension [and difficult to change experimentally!].

The moving of the intake pipe rearward and the use of the larger section tubing will be tested as a modification to the Elektra II design, for which I have the materials on hand now.

All comments welcome, as always.

L Cottrill

[mk]

Hello Larry!

You are really one of the most iventive I ever got to "know".

The design looks a bit like the chinese design, but I think your intake port could be more succesful at least at small engine sizes by getting a better mixture of fuel and air.
I don't know, but perhaps jou get some trouble with the intake air entering at the lip section of the can, PERHAPS. I would place the intake a few milimeters more to the CC, entering just or partial mostly behind the lip.

Nice work! Looks good!

[larry cottrill]

Hello Larry!

You are really one of the most iventive I ever got to "know".

The design looks a bit like the chinese design, but I think your intake port could be more succesful at least at small engine sizes by getting a better mixture of fuel and air.
I don't know, but perhaps jou get some trouble with the intake air entering at the lip section of the can, PERHAPS. I would place the intake a few milimeters more to the CC, entering just or partial mostly behind the lip.

Nice work! Looks good!

Marten -

Thank you. My gut feel is that the area you're talking about there is fairly critical. Here's why I think this location is an improvement over the original Elektra I design:

First, with this one, we're operating in a region where the flow is almost fully accelerated, and the excess static pressure is almost fully expanded away. That means we should get very little 'leakage' out through the intake. Obviously, it would be advantageous for the next intake charge to be merely at a standstill until the pressure drops, rather than displaced with hot gas flowing outward.

Second, I really see a lot of value in the intake flow being 'sheared' and roughened up by this kind of 'pinched' flow forward into the chamber. That should be really good for mixing and braking of the forward flow, which should give us a good setup for the next explosion.

And, the intake acting as a 'pinch' in the tailpipe should give a little better compression than a fully unhindered pipe. I think this is what Mike E was getting at in one of his suggestions re the Elektra I.

I think the action of the Elektra I, as built, is that of a crudely formed Lockwood [i.e. lots of exhaust out through the intake], which was not what I intended. The new design should come a lot closer to real 'Reynst pattern' breathing, which is what I'm really after.

Anyway, one of these days, we'll see.

L Cottrill

[Bruno Ogorelec]

Larry,

Of course, I have no idea how well your 'close shave' would work. I am looking forward to your practical tests.

However, knowing that you are after as simple and as accessible a design as possible, to encourage young enthusiasts, I humbly propose the following.

The 'pinch' in teh middle of the chamber will offer increased structural rigidity and may even improve mixing.

[larry cottrill]

Larry,
. . .
The 'pinch' in the middle of the chamber will offer increased structural rigidity and may even improve mixing.

... and provide a good deal of practice cutting and welding thin pieces!

Very good, Bruno ... the 'Siamese Chin'.

L Cottrill

[larry cottrill]

There's usually a lot of grief in trying the hold the parts of a pulsejet in alignment with external cold bracing. However, it seems to me that the pressure can is ideal in this regard, because of the inherent flexibility in the way the nozzle zone of the can is made -- sort of rolled down into the chamber just inside the folded rim, then domed outward around the rolled zone. It's a sort of mini bellows, especially when hot.

The idea here is that you lock the front plate and the tailpipe into alignment with the cold frame [mounting rail and tubular trusses], and allow the domed nozzle to flex in and out of the expanding cylindrical shell as it heats and cools. Since the nozzle area will always be the hottest zone, the fatigue is relieved by the heating of the metal, and the nozzle zone is essentially annealed with each run of the engine.

What do you think?

L Cottrill

[larry cottrill]

First shots of building Fo Mi Chin. The first picture shows cutting the domed end off a shaving cream can. I optimistically expected that after turning this over and getting the spark plug mount nut welded on, I could just tack it back onto the can [dome side out] and weld it all around. No such luck -- I ruined the thin edge of the cylindrical can wall trying it.

Then, I grabbed another can with the intent that this time I would hog out the domed bottom while leaving the rolled rim to hold the shape and give me some beef to weld to. The second shot shows me punching my way around the domed bottom, just inside the rim. Smoothness and precision have nothing whatever to do with this operation. Once enough little holes were ganged up side-by-side, it was easy to knock the scrap piece out with a tiny member of my family of stone carving chisels [you just can't have too many past hobbies in the pulsejet game!].

I didn't get any pictures of welding up the plug mount on the front plate, but here's how I did it: I used a good sized flat washer [about 3/4 inch OD] held in place with Vise Grips(TM) while I tacked it onto the inverted can bottom, then rearranged the Vise Grips to support the piece from the opposite side of the rim while I finish welded it all around the rim of the washer. Yes, I did blow a couple of holes, but I'm used to repairing them, so it wasn't much of an issue. Finally, after that had cooled, I used the torch to blow a hole through the dome in the middle of the washer, and finished rounding it out with a round file. Then, I carefully positioned the 3/8-24 nut over the hole in the washer and clamped it with the Vise Grips while tacking it to the washer at a couple of opposite faces. Then, I repositioned the Vise Grips again and mounted them in the vise, and welded the nut to the face of the washer all around. What I ended up with is a thing that looks like a little steel volcano with a threaded crater. I used my 3/8-24 tap once again to clean up the threads. Amazingly, after all that, the rolled rim was not warped out of flatness in the slightest -- amazing!

The third shot shows the finished domed front plate being placed over the end of the chamber's rolled rim. The rough edge left from punching out its scrap dome is simply rolled down in and left there. It should provide a nice little braking/roughening zone for the air/fuel mix that gets to the front of the chamber.

L Cottrill

[Bruno Ogorelec]

Larry, I think you are onto something with those shaving foam cans.

Looking at those can parts, I comes to my mind that you should perhaps concentrate on a way to bond (not weld) metal parts together. A way that will stand up to the heat.

Maybe you should explore sealing compounds like Loctite. Do you have Loctite in the US or is it a British product? The way I remember, you can use it instead of a cylinder head gasket for a while, so it should be up to the task.

I can see the bottom of one can and the body of another, each with the rolled rim, bonded together and the join perhaps helped with a hose clip. there are some hose clips that use twin wire loops in lieu of a serrated metal strip. Each loop could go to one side of the rolled lip join.

The join between the exhaust tube and the domed mouth of the 'combustion chamber' (which also has a rolled lip) might also be bonded in a similar way -- a slip fit helped by a sealing compound and a hose clip.

To me, it looks like a very easy way towards a no-weld pulsejet. Provided that it works, of course.

[Al Belli]

Locktite compounds are anerobic cure sealing and locking compounds which are rated at 300 or 400 degrees F.
As a head gasket on a watercooled engine, the compound stays well below the rated temperature.
In a pulsejet, several puffs of smoke is all that I would expect of this material!

Al Belli

[Mike Everman]

Yah, it's got to be a ceramic bonding material that is under virtually no shear or peel stress whatsoever.

[Bruno Ogorelec]

Yah, it's got to be a ceramic bonding material that is under virtually no shear or peel stress whatsoever.

Yes, that's why I suggested bent wire hose clips to do the mechanical work and some gunk inside to provide the sealing barrier.

Al, thanks for the Loctite info. I had no idea about its thermal rating.

[Bruno Ogorelec]

What about those 'liquid metal' solutions used to stitch broken crankcases together? That's probably also rated for much lower temperatures. Anyone with experience with those?

[Tom]

Bruno,
I have seen these in the farnell catalouges we have downstairs, i remember them being good for somewhere in the 600-800 degrees in one case, I will take another look later tonight.

Tom

[Al Belli]

Metal repair stuff is almost universally made from epoxy resin mixed with metal powder. The epoxy limits the working temp. to 350 - 400 Deg. F.
McMaster - Carr part # 7356A33 is a ceramic and stainless steel putty that is rated for 2,000 Deg. F.
The best stuff for a pulsejet, probably would be P/N 7564A11 , which is a ceramic adhesive that will bond metals, and will take 4,000 Deg. F. with no problem ! It is expensive, ~$65.00 for a Pint can.

Al Belli

[Mike Everman]

Yes, I saw that one too, Al. A pint should go pretty far if you use it sparingly, which is the name of the game in bonding.