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Little help needed

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Little help needed

Unread postAuthor: Moonbogg » Fri Aug 28, 2009 2:42 am

I am building a contest entry, so details will be minimal, but the questions will still be valid. I test fired a cannon and it was using PTFE gaskets for the chamber. The gaskets were rated for 3000PSI but they BLEW OUT on the first test shot. Now the gaskets were cut narrow so they ended up looking like narrow circular rings. Maybe they needed to be thicker and extend down beyond the sealing faces? I checked out copper and metal reinforced gaskets but they aren't rated as high as the PTFE! Seems odd to me. What do you think?
Second, I used a PVC spark strip and it seems to have warped from the test shot. I would like to maintain 3 spark gaps like I have, but need something more heat resistant and strong while being non conductive. I would use a spark plug, but again would like to maintain 3 gaps.
The plastic chamber fan got torched on the first shot, LOL! Do you think an aluminum fan would fare better or are chamber fans just not practical for something like a 5x hybrid due to extreme heat?
I have been working really hard and am mentally exhausted and need suggestions.

What would be a good gasket material for a 5x hybrid and how thick should it be cut?

What suggestions would you have for a strong, heat resistant spark strip backing?

Other than that, the build is going well. I test fired it in the garage (unloaded of course) and it was so loud it felt like I got hit in the head with a soccer ball or something! Likely caused by the air pressure increase and shock wave in the closed area.
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Re: Little help needed

Unread postAuthor: jagerbond » Fri Aug 28, 2009 3:01 am

Moonbogg wrote:What would be a good gasket material for a 5x hybrid and how thick should it be cut?


Automotive exhaust gasket material. Your local auto store may even have something close.

Moonbogg wrote:
What suggestions would you have for a strong, heat resistant spark strip backing?


Garolite, G-7

looking forward to seeing this.
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Unread postAuthor: Moonbogg » Fri Aug 28, 2009 3:02 am

Very nice! Thanks man I appreciate it. Those sound like great options.
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Unread postAuthor: inonickname » Fri Aug 28, 2009 3:32 am

As jager mentioned, garolite would be a good material. Near any rating would work. If purchasing from McMaster, then the specifications would be listed.

High-impact polycarbonate, flux capacitor stock, nylon and the likes also come to mind.

How about a gasket compound that is applied in place? I use an aviation grade one. Just as a question, was the gasket used correctly? (eg. tight enough etc)
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Unread postAuthor: Moonbogg » Fri Aug 28, 2009 3:47 am

inonickname wrote:As jager mentioned, garolite would be a good material. Near any rating would work. If purchasing from McMaster, then the specifications would be listed.

High-impact polycarbonate, flux capacitor stock, nylon and the likes also come to mind.

How about a gasket compound that is applied in place? I use an aviation grade one. Just as a question, was the gasket used correctly? (eg. tight enough etc)



When you say gasket compound do you mean like a paste or spray on substance? This would be really great for convenience as I wouldn't have to cut gaskets. I could just gum up the whole damn thing really good.
As far as the tightness goes, I used a torque setting of 20ft-lbs on my hand drill which is about right for 5/16-24 B7 bolts/studs. So it was pretty friggin tight as far as I can tell.
Garolite is expensive for this application I have just learned. Its like $50.00 for the garolite or $9.00 for the PTFE from what I saw based on my quick price check. I am considering a PTFE strip perhaps or something similar.
I might have to go with some high-temp wires as well as the ones I used show some sexy looking charring from just 1 shot. I am entering uncharted territory for myself here with these insane temps and pressures, so I have had to do some trial and error on a few things and it gets annoying and time consuming, but once its all worked out then its all worked out for good. Can't wait to test this thing for real.

EDIT: Oh, what about the chamber fan? Do you think an aluminum one would work out? Anyone done this before?
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Unread postAuthor: inonickname » Fri Aug 28, 2009 4:46 am

Perhaps find a fan in a color other than black, if it is black? An aluminum one would help.

A consideration with the PTFE under massive temperatures, you could end up with flourine compounds (read; HF) which is a bad thing. Just something to consider.

High temp paste/gum gasket makers should work/
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Unread postAuthor: dewey-1 » Fri Aug 28, 2009 9:11 am

Moonbogg;

For high voltage wire you could use solid core copper spark plug wire that has silicone jacket that is commonly called 8 or 8.5 mm spark plug "racing" ignition wire. Available at local auto store that carries Accel, MSD. This type of wire should work OK in that environment.

If you are referring to the low voltage wires, you may get silicone rubber sleeving, PTFE (Teflon) sleeving or fiberglass sleeving.

fiberglass:
http://www.varflex.com/

PTFE sleeving:
http://www.zeusinc.com/extrusionservice ... DQod1W_CJQ

Fiberglass G10 (printed circuit board type) backing on PTFE stock may hold up. It is a variation of Garolite. 400 degress F max.

Like jagerbond said the Garolite G7 would be best, it has 425 degree F operating temperature and 500 max.
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Unread postAuthor: jimmy101 » Fri Aug 28, 2009 12:03 pm

For the wires, you probably don't need any insulation at all. As long as the wires are held in place and don't touch each other, or anything metal, then they don't need insulation.
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Unread postAuthor: Moonbogg » Fri Aug 28, 2009 3:35 pm

jimmy101 wrote:For the wires, you probably don't need any insulation at all. As long as the wires are held in place and don't touch each other, or anything metal, then they don't need insulation.


I was thinking the same thing. They are locked in with epoxy and I figured i'd just let the rubber insulation burn off all it wants lol. If it shorts out one day and stops sparking, i'll know it failed haha.
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Unread postAuthor: theBOOM » Tue Sep 08, 2009 7:08 pm

Not to old of a thread but for spark strip material did you ever think of using jb weld or some other type of high strength epoxy???

Also make sure you build your spark strip as small and compact as possible as this will help you convserve heat and not lose it beacause of all the space in the chamber, the more the unessary space, the more the heat loss.... Well atleast that's what Jimmy told me o.o

Lemmie find the thread where jimmy explains it a little

AHA . found it http://www.spudfiles.com/forums/spark-s ... 16974.html

Towards the end of the thread :P
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Unread postAuthor: USGF » Thu Sep 10, 2009 3:27 am

Moonbogg,

I gotta think you did not constrain the PTFE well enough. The stuff is close to inert and will withstand a lot of pressure but it has very poor mechanical strength. I often see it in square cut or o ring form placed in a tight groove, the lip of the vessel then entraps the seal in the groove. You would then require a lot of force to extrude the stuff. Clearances would be a few thou. You can also get stem packing rope, in graphite, PTFE and other materials Nice since it conforms to any shape but it still needs the deep groove.

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