For a Close Shave, You Need a Fo Mi Chin

Moderator: Mike Everman

Postby Bruno Ogorelec » Sat Jul 10, 2004 4:02 pm

Mike Everman wrote: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.


Yes; a pint of the compound would certainly be enough to make 65 Fo-Mi-Chin pulsejets, pushing the coast down to one dollar per engine, but how do you find the other 64 boys who also crave a homemade pulsejet?
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1000 degF Cement

Postby larry cottrill » Sat Jul 10, 2004 5:31 pm

I now have a little experience using Red Devil Fireplace & Stove Silicate Cement, though I haven't seen it "under fire". It is rated at 1000 degF. This might work well for what you're talking about; however, I might question its strength when used sparingly: I have taken a fully cured pencil of it in hand and broken it easily by bending, so the final tensile strength can't be all that great.

I don't think the problem with using something like this for bonded construction will have anything to do with the temperature of the cement per se, nor with the mechanical stresses from chamber pressure [which on average is pretty low in a pulsejet], but rather, the difference in thermal expansion between the cement and the metal it's bonded to. I'm sure the local stresses caused by this could turn out to be pretty high when things start to warm up. So, I think for this project I'll stick with welding, which for me is walking on known steady ground.

The welding is difficult, though, with such thin material -- I'm sure I would never recommend this project for beginners. That's one thing that would change radically if a good bonding agent could be found, just as you've suggested, Bruno -- the project would be more like model building than blacksmithing, sort of like I was getting at with Maggie Muggs. Since then, I've found that the J-B Weld is not really adequate to the task, as Al has put forth above. The first photo below shows the flare I made on Elektra I out of J-B Weld -- beautifully formed, I think [I used a carefully undersized rubber flat washer forced on as an outer mold], but it was unstable when conducted heat got to it, becoming rubbery and detaching from the tube edge [I should have reinforced it somewhat down around the tube wall]. The fuel pipe was added later, by filing a groove and bonding it in with more resin.

The second photo shows forming of the fireplace cement flare. It handles like damp clay or putty, so you can just mold it around the tube as shown, building the fuel pipe assembly right in as you go. In the third photo, after curing for 24 hrs, you just smooth up the inside surface with a half-round file. The finished surface is more like fine sandpaper than a polished surface, as you might expect, but certainly should be good enough to guide lots of air down the chute. I expect this to hold up a lot better than the earlier epoxy attempt.

L Cottrill
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ElI_JBWeld_flare_finished_crop1.jpg
The intake flare molded from J-B Weld epoxy, finished except for adding the fuel pipe assembly. Photo Copyright 2004 Larry Cottrill
ElI_JBWeld_flare_finished_crop1.jpg (32.78 KiB) Viewed 6039 times
ElI_making_cement_flare_crop1.jpg
Hand forming the flare out of silicate fireplace cement. The fuel pipe assembly is simply worked into the process as you go. Photo Copyright 2004 Larry Cottrill
ElI_making_cement_flare_crop1.jpg (61.17 KiB) Viewed 6040 times
ElI_finishing_cement_flare_crop1.jpg
Finishing the inside surface of the fireplace cement flare with a small half-round file. Photo Copyright 2004 Larry Cottrill
ElI_finishing_cement_flare_crop1.jpg (52.46 KiB) Viewed 6040 times
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Postby Bruno Ogorelec » Sat Jul 10, 2004 5:55 pm

What about the flexible stuff? Way back in the early 1950s, neoprene was used to coat pulsejet valve seats. It lasted for at least a few hours. Surely the materials science has progressed since then.
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Postby Mike Everman » Sat Jul 10, 2004 6:03 pm

Seems an awful lot of work to make a flare, Larry. Certainly your torch has graced the rest of the motor for hours, it would make short work of this!
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Sculpt-o'-dyne

Postby Mark » Sat Jul 10, 2004 6:12 pm

It's nice to experiment with new materials. Materials science is such an interesting field. I like your smooth molded intake Larry.
I wish there was some substance you could form like clay, say a thermite of Al and Fe2O3 with binder that could be molded around a seam and light it on fire, welding/bonding the products in a fun easy way.
It would be intriguing to make an entire jam jar out of moldable materials that could take the subdued but fairly high temperatures of putt putting.
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Postby Mark » Sun Jul 11, 2004 2:20 am

I wonder if one could weld with this stuff if it was contained in a little channel/ditch where you wanted the iron to melt and cool, say around the lip of a pipe with your cap or thing to be bonded.
You can buy iron oxide and aluminum powder for cheaper than this of course.
Just something that might find some niche market, perhaps for a very small device one might want to make some day.
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Postby larry cottrill » Sun Jul 11, 2004 4:20 am

Mark wrote:I wonder if one could weld with this stuff if it was contained in a little channel/ditch where you wanted the iron to melt and cool, say around the lip of a pipe with your cap or thing to be bonded.
You can buy iron oxide and aluminum powder for cheaper than this of course.
Just something that might find some niche market, perhaps for a very small device one might want to make some day.
Mark

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It used to be used to weld broken rail iron together, by surrounding it with a fireclay barrier. You kick it off by letting a single drop of glycerine fall on it.

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Postby Mark » Sun Jul 11, 2004 4:46 am

I have made some thermite in the past and have also set it off with glycerine. I use to use glycerine also to set off potassium permanganate and aluminum powder, it would detonate after several tense seconds after I would eye-dropper the gylcerine on it. It would rattle my windows and fire a fast moving, large perfect smoke ring up into the air.
What you need to do though is have some KMNO4 first on the pile, the gylcerine reacts with the purple salt and that classic reaction ignites the thermite. I still have some aluminum powder and some Fe2O3 out in my shed, along with other questionable materials, just in case anything spontaneous should happen, it's away from the house.
Thermite is often very hard to light, you can dip lit matches in it and even gather some on the match without it lighting. I remember the railroad industy using thermite from an old chemistry book.
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Postby Mark » Sun Jul 11, 2004 4:53 am

PS I read where a guy was grinding aluminum powder and potassium permanganate in a large mortar and pestle. It detonated and blew a hole several feet down through a desk with drawers, ripped the skin off his face and arms and he was getting skin grafts for years afterward. Never grind more than you want to blow up in your face! I once shattered a small mortar and pestle while grinding a small amount in the house. Luckily, I was only startled and a fine mist of particles filled the air. Oh, it was very loud too. Sudden wouldn't describe the speed in which it exploded. I don't think it could have been more than a teaspoon, but I did a good job of getting it intimately mixed I think.
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Chamber End Welded On

Postby larry cottrill » Sun Jul 18, 2004 4:27 am

Well, tonight I took about 15 minutes in the garage and got the domed end welded onto the chamber front end rim. Tricky work, so it's a little rough in spots, and I just noticed a couple little pinholes I'll need to touch up ... but it's on, good and solid. The only collateral damage to the chamber wall was a spot where I got careless with my aim and burned a couple of birdshot-sized holes. So, the last step in the process tonight was setting up the tiniest flame I think I've ever run on the little Victor torch and welded up the holes. The patch weld is ten times as thick as the surrounding metal, but it will look all right after I smooth it down a bit with a small flat file. Hey, nobody's perfect ...

I took a couple of shots of it in my hand, but won't be able to post anything until I finish the ten-shot roll of film. Of course, by then, I should have some shots of the bent-back intake and tailpipe, too.

I wonder if there's really any chance that this critter will static run for more than a few seconds before the chamber wall melts through. maybe I should plan on providing some forced air cooling for static running.

If I end up with West Nile disease, it will probably be because of this weld tonight. The mosquitos attacked voraciously all the time I was working on the thing, and there wasn't much I could do for defense. It's amazing the way flies and mosquitos can sense when you have your hands busy with something you can't just drop.

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Postby Mark » Sun Jul 18, 2004 5:14 am

I have the mosquito problem too Larry. I have 2 electric flyswatters that zap the occasion one that gets in the house. They literally explode in a snappy sparky pop. Once I touched the swatter and I must say it hurt quite a bit, more than I would have thought. About the only thing you can do is use a fan, I have even dwelled on buying one of those 4 foot diameter jumbo floor fans for the garage. Everytime I take the dogs out, it seems one sneaks in the house. I have an electric swatter in the garage too, and since I use a spray bottle of methanol from time to time for priming, I mist them with methanol and they fall to the ground and die. I also use a fan in my garage window to blow any fumes outside through the screened window.
Speaking of West Nile virus, I once had a bad case of the flu and went to the hospital. They (our doctors of America) wanted to do a brain scan on me, ( I know most of the people on the forum would have recommended a brain scan, but that's another story), they did test me for West Nile and in the 4 hours I was there they charged my health insurance company almost $1000.00. I told them I just had the flu but I was lucky to get out of there alive! I just wanted something to make me feel better and they were idiots. We've had a few cases of the virus in Pensacola though.
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Postby steve » Mon Jul 19, 2004 2:13 am

you could also try one of those propane fueled co2 (is it CO2? I don't know)generators to repel mosquitos. one of my friends lives near a marsh and the bugs are terrible there but recently his neighbor put one of those things in and now he (my friend) can go outside without being bitten even though the CO2 generator isnt in his yard. Aparently they are somewhat expensive but in someplace like flordia it might be worth it.
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Postby Mark » Mon Jul 19, 2004 6:04 am

I once read an extensive article in the newspaper on mosquitos and the proclivities of several different kinds. Unfortunately, not all kinds of mosquitos in Florida are attracted to the CO2 device. But it would be a good start. Funny the people who made the Dynajet, adapted it to spray insecticide, one of their models uses the same Dynajet redhead and it can also be used to disperse crowds with riot control gas.

http://www.dynafog.com/foggers/pje/index.htm

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Postby larry cottrill » Mon Jul 19, 2004 1:20 pm

One thing that's fascinating [and very different for me] is welding something so light and thin that it hardly holds its heat at all -- after I finished welding the end on this chamber, I was able to handle the assembly after only a couple of minutes of it lying around on the concrete floor of the garage. Very different from welding heavy walled tubing 1/16 inch or more in thickness. I remember when I tried welding my stainless pieces [.062-.070 inch thickness] it seemed as though it took forever before I could pick them up -- that doesn't sound like very heavy work to most people, but stainless really holds the heat.

I wonder how the low thermal mass of the Fo Mi Chin chamber affects cooling effectiveness, in a flight or other non-static situation. I recall how the Dynajet can be flown for a few minutes and all you get for noticeable heat is a little reddening of the nozzle zone. That's stainless, of course, but about as thin as a 3x5 index card, I believe.

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heat

Postby Al Belli » Mon Jul 19, 2004 6:10 pm

Hi Larry,
Stainless steel has a much lower heat conductivity factor than carbon steel. That's why the more expensive stainless steel pots and pans have a laminated structure with stainless on the inside and outside, and an inner core of carbon steel. Cheap stainless steel pots and pans will burn the food at the flame contact area.
Your stainless parts take longer to cool because of the poor thermal conductivity.

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