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Need Help with Ideas on Venting the Chamber

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Need Help with Ideas on Venting the Chamber

Unread postAuthor: MaxuS the 2nd » Wed Feb 27, 2008 6:33 pm

Hey Guys.

I didn't particularly want to reveal any of it yet, but:

Image
Image

Right, so this is my first combustion. It's going to be advanced, with onboard propane metering, chamber fan, detachable barrels, high voltage ignition...The Works.

The chamber is 100cm of 140mm PVC pipe.
The barrel will be 1.8m of 110mm, giving me a very rough ratio of about 1:1

Now, due to the fact that PVC fittings over in the Uk have absolutely crippling prices, I need to find an 'advanced' way to vent the chamber. A 140mm - 4" Male BSP and a 4" threaded cap will cost over $120 (£60), which I just don't have the money for. So the plan was to use a 140mm Plain Glue Endcap and whack it on the end of the chamber. This however presents two problems. Firstly, there will be no way for fresh air to enter the chamber and secondly, if a component breaks inside the chamber, that would be another $40 down the drain because I'd then have to saw off the end cap, replace the component and then glue another cap back on. I could stick a schrader through the end cap to blow fresh air behind the fan when it's on to circulate some fresh air, but then this makes the launcher somewhat less portable. I could build a Latke style Quick Vent system (like that used on Gort's Blue Flame III), but the inside of the cap is curved to help with strength, so a flat disc like shape, covered in rubber is out of the question as it wouldn't sit directly against the hole. This also presents another problem. If a component were to break inside the chamber, I'd have no way to reach in and pull it out to replace it, rendering the spudgun completely useless. I do not have 1m long arms where the circumference is less than 4"..

So, would the chamber vent properly if I were to use high powered fans blowing for a minute or two after a shot whilst I go look for the ammo, or does the fan need a source of fresh air behind it?

How likely is a component to break, and how often has it happened to you? It will almost entirely depend on the way I build it inside of the chamber.

Even though it would allow me full access to the chamber, a threaded cap would not make for an 'advanced' way to vent the chamber, unscrew it after every shot, running the fan and screwing the cap back on. In that case, I may aswell just use Hairspray and a BBQ Ignitor.

Does anyone have any ideas that would allow me to:
1. Be able to full access the chamber.
2. To be able to be a source of fresh air behind the fan.
3. To be an 'advanced' venting method.
4. To cost less than $120.
5. To not be horribly unsightly.

Thanks in advance Guys.

Max

Edit: Wow, those paragraphs have a horrible structure. Oh well, it's 11:35pm.
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Unread postAuthor: mopherman » Wed Feb 27, 2008 6:41 pm

for venting i would recommend a mattress blower and a large check valve. for the caps, you could cut some 1" thick circles of something strong, slap some giant o-rings on them, and bolt em in place. With this method you could sacrifice some chamber length and house most of you venting blower inside the back of the pipe.
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Unread postAuthor: BigGrib » Wed Feb 27, 2008 6:42 pm

Well I got a great Idea for you :idea: why don't you just order it from BC Arms or send me some money and i'll send you what you need
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: MaxuS the 2nd » Thu Feb 28, 2008 2:34 am

The shipping is $38 for the simplest of things, and american stuff is not compatable with stuff from the Uk.
I thought about using a Check Valve, but this setup would end up costing more than $120 and I'd have to be in range of a wall socket.
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Unread postAuthor: MrCrowley » Thu Feb 28, 2008 3:09 am

Have you tried American fittings with British fittings? PVC fittings whether from America or NZ/Aus are compatable here. And same with threaded fittings except for the taper which may leak at high pressures (200psi+)

I also got some PVC fittings shipped over by sgort87 for $18USD, i'm sure you could get it cheaper.
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Unread postAuthor: Novacastrian » Thu Feb 28, 2008 3:11 am

You really need a stand for that mother. :P
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Unread postAuthor: MaxuS the 2nd » Thu Feb 28, 2008 11:49 am

:D I've heard that many times from my friends, but I'll see how it fairs first.

I think I'll just bite the bullet and buy the bloody fittings.
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 28, 2008 11:55 am

How about a combination of wood, epoxy and bolts? You can save a fortune by making your own fittings instead of buying standard ones.
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Unread postAuthor: MaxuS the 2nd » Thu Feb 28, 2008 11:58 am

Hmm, it requires a damn good thinking session.
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Unread postAuthor: Jared Haehnel » Thu Feb 28, 2008 12:17 pm

If you make an inter changeble barrel you should be able to screw off the barrel and have some room to fiddle with things...like the spark gap..

You could also thread a copper ball valve to fit any where on the chamber after a shot you open the ball valve to let air into the chamber...perhaps include it in the propane injection design so it didn't look random or ugly

There should be no why a chamber fan would fail on you the only thing you might have problems is with a spark gap but if you make it rugged enough even that won't need adjusting. Place that gap near enough to the barrel you can adjust it easily

The only part the would ever fial on you that would be inside of the chamber would be the chamber fan how ever that should last a long time...

Just some food for thought
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Unread postAuthor: SPG » Thu Feb 28, 2008 12:36 pm

Is there no way to hold the "Plain Glue Endcap" in without glue? Not being inthe UK now I'm not totally certain what the thing looks like, but I'm sure there must be a way. But assuming it looks something like this:

Image

then could you not bolt it on drilling and tapping x-number of holes through the end cap and chamber and then voila, you can just remove it when you want. Doesn't solve the venting, but at least you're ok for getting inside if you need to.
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Unread postAuthor: Jared Haehnel » Thu Feb 28, 2008 1:03 pm

on drilling and tapping x-number of holes through the end cap and chamber and then voila, you can just remove it when you want


Add a rubber gastket or put in some vasoline or grease perhaps even silicon and it will help to make a good seal....

Plus a series of bolts may make it look cooler.... 8)
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Unread postAuthor: MaxuS the 2nd » Thu Feb 28, 2008 3:45 pm

Thanks guys, I hadn't thought of that, that may be the way to go then.
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