Death of Fo Mi Chin

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Death of Fo Mi Chin

Postby larry cottrill » Thu Sep 08, 2005 3:40 pm

The Fo Mi Chin II engine is now nothing more than an artifact of Third Millenium pulsejet technology.

After a couple of successful runs yesterday afternoon, I decided I needed to get one more photo of the engine running hot in semi-darkness, to try to show the intake flame, chamber heating pattern, etc. So I got the shot metered, got the camera into position and fired her up. Started nicely and I got it running just right and was just about to circle around to the camera to snap the shutter, when suddenly the pulsation pitch went up about two full steps! I knew something was very wrong. After two or three seconds running like this, the engine quit running, with yellow flame billowing from the exhaust port and some point on the chamber opposite my position (naturally!) so I quickly shut down the propane.

The hole opened up low on the right side of the chamber. It is a roughly crescent-shaped, rough-edged hole almost an inch (25mm) end-to-end. The hole is centered perhaps 4 cm aft of the front welded seam (where the dome is welded on). The metal remaining at the edge is basically tissue paper thin, and the edges appear quite sharp, not melted back. Here are some interesting things to think about regarding this engine and its ultimate failure:

- This is the lightest and most compact valveless engine I have ever gotten to run well: 2.6 inches max diameter (67 mm), 22.5 inches length (ignoring spark plug) (572 mm) and 15 ounces total weight (including plug)
- Due to the thin material, this was one of the most difficult welding projects ever - but, it was ultimately doable
- Intake flame visible during the final run was straight out, parallel to the tailpipe surface
- This kind of ultimate failure was expected sooner or later, because of the very thin steel can used for the chamber
- The engine continued to run after the hole opened up, but at a higher frequency, presumably due to the creation of an additional pressure node
- Since the hole was on the opposite side where I couldn't see it, I can only speculate that it started small, then enlarged, killing pulsation
- The steel material was NOT blown out - it was sucked INTO the chamber (I shook it out after the engine cooled); the material consisted of a variety of tissue-paper-thin flakes, rough and grey in appearance
- The hole is the result of brittle fracturing, not melting

There is absolutely no point I can see in trying to repair this engine. All the metal around the breach is probably thinned out to the point that it will be virtually impossible to weld to it. This wasn't a very 'serious' project anyway, just something to try to see how well it would work. As is, it makes a nice artifact of the modern valveless pulsejet era. It would be possible (even fairly easy) to re-use the intake/tailpipe assembly to make a prototype Reynstodyne (TM) FMC II engine, with a custom-built chamber of reasonable weight material; I don't know if I want to tear it up for that, though. Maybe it would just be better to mount it on a nice piece of oiled walnut and use it as a wall hanging.

This is where I'll post the final photos of Fo Mi Chin as a memorial, when I get them.

L Cottrill
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re: Death of Fo Mi Chin

Postby ed knesl » Fri Sep 09, 2005 4:17 am

Larry,

You need to start cutting stainless steel, CR built engines will never last
more than few runs. It is a vaste of valuable work time.

Ed
...Nobody is right, nobody is wrong...
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re: Death of Fo Mi Chin

Postby Bruno Ogorelec » Fri Sep 09, 2005 10:20 am

R.I.P.
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re: Death of Fo Mi Chin

Postby Mark » Fri Sep 09, 2005 2:10 pm

Aerosol cans might be more suited for jam jars if you could come up with a clever design, perhaps pushing a little car or boat around.
Or for a simple short run "rocket/pulsejet", (disposable), maybe make your V-2 shaped rocket out of paper, but lined with the shells from light weight aerosol cans to protect the walls from burnout. Then let her rev up and lift off straight up into the wild blue yonder for a few seconds of burn time.
It wouldn't cost hardly anything, all the materials to make a short running pulsejet could be made out of garbage. A pulsating combustion "rocket" would be a lot more fun than those store-bought bottle rockets and they could be very light in weight.
Mark
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Re: re: Death of Fo Mi Chin

Postby larry cottrill » Fri Sep 09, 2005 2:53 pm

Mark wrote:Or for a simple short run "rocket/pulsejet", (disposable), maybe make your V-2 shaped rocket out of paper, but lined with the shells from light weight aerosol cans to protect the walls from burnout. Then let her rev up and lift off straight up into the wild blue yonder for a few seconds of burn time.
It would cost hardly anything, all the materials to make a short running pulsejet could be made out of garbage. A pulsating combustion "rocket" would be a lot more fun than those store-bought bottle rockets and they could be very light in weight.

That's almost exactly what I wanted to do with the Dynajet years ago, but of course I never did because I had no clue how to make a reliable recovery system that would give me back my engine unharmed. With a junk jet that would run well on a lightweight butane refill cylinder, you wouldn't care, as long as it didn't fall on anything of value. As you say, the vehicle itself would need to be little more than a paper shell with aluminum foil lining and some stabilizing fins.

Tailpipes can be the "curtain rod" from a piece of Sauder u-build-it furniture: 1-inch OD, tightly seamed, paper-thin chromed steel tube, perfectly round. I had no trouble welding such a tube into the top of a (too large) pressure can to make Long Tall Sally. The end of the tube fits so tightly, you probably wouldn't even need to weld it for a one-time shot, just have enough 1/16-inch wire bracing to keep it lined up in the rocket body. With one of the miniature cans (like hobby enamel comes in), your engine could be put together simply and cheaply. There would be virtually no slosh in the butane cylinder, because of constant acceleration (ha!) during the burn.

An ideal demonstration of the "jets from junk" philosophy.

Of course, actually firing such a vehicle would be utterly irresponsible ;-)

L Cottrill
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Memorial Photos

Postby larry cottrill » Fri Oct 14, 2005 7:08 pm

As promised, photos of the last moments of Fo Mi Chin II.

L Cottrill
Attachments
FoMiChinII_running_day_crop1_small.jpg
Fo Mi Chin II running full grease while there's still some daylight left. Interesting heat pattern; the barely visible hint of trouble brewing at the bottom toward the front went unnoticed. Photo Copyright 2005 Larry Cottrill
FoMiChinII_running_day_crop1_small.jpg (148.25 KiB) Viewed 7186 times
FoMiChinII_running_night_crop1_small.jpg
Fo Mi Chin II running after dark. I still didn't see the hot spot developing at the bottom, but the camera caught it perfectly here. Photo Copyright 2005 Larry Cottrill
FoMiChinII_running_night_crop1_small.jpg (139.09 KiB) Viewed 7188 times
FoMiChinII_burnthrough_crop1_small.jpg
Nature always sides with the hidden flaw. The scale is in centimetres. Photo Copyright 2005 Larry Cottrill
FoMiChinII_burnthrough_crop1_small.jpg (134.94 KiB) Viewed 7185 times
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re: Death of Fo Mi Chin

Postby steve » Sun Oct 16, 2005 4:00 am

that was such a great looking little engine too! why did you have to break it?

oh well, at least you got some nice shots of it before it died.
Image
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