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Building a new cannon

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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Building a new cannon

Unread postAuthor: mcole254 » Sun Oct 03, 2010 4:26 pm

I was browsing for some parts for my new spudgun I noticed some 6in pvc. The only problem is that it's SCH 35, would that be sufficient for a combustion chamber (will be using metered propane, no pressure mixes). Thanks
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Unread postAuthor: Gun Freak » Sun Oct 03, 2010 4:50 pm

I wouldn't trust it. Advanced metered propane combustions can generate in excess of 120 psi. If it were me, it is at least sch 40 for that.
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Unread postAuthor: mcole254 » Sun Oct 03, 2010 5:35 pm

thanks, it wouldn't make any difference if i wrapped it in fiberglass right? I think I read it wouldn't.
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Unread postAuthor: motorfixer1 » Sun Oct 03, 2010 6:41 pm

What are the other desireable specs for the launcher? Barrel length etc. Do you really even need a 6" chamber? Maybe a 4" or 3" chamber might do the trick and prevent you from wondering if the 6" is strong enough. Also, the cost of 6" P.R. fittings is a great deal higher that the cost of the smaller stuff.
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Unread postAuthor: Gun Freak » Sun Oct 03, 2010 6:41 pm

I wouldn't, I mean I'd just get the right pipe in the first place.
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Unread postAuthor: Moonbogg » Mon Oct 04, 2010 12:18 am

4" sch40 pipe for the chamber and 2" sch40 pipe for the barrel is a good combination and often used. Chamber like 18" long and barrel 5ft long sounds about right. Cell Core ABS is a good choice for a combustion by the way. Cheap, light weight, strong and if it breaks it doesn't shatter like PVC.
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Unread postAuthor: mcole254 » Mon Oct 04, 2010 2:20 pm

I thought it would be fun to make a 6inch chamber with a 2 or 2.5 inch barrel, I've made a couple of 4-2 cannons already. I was never initially going to do this just because trying to find 6inch pipe is a pain and expensive, however I saw a piece the perfect length only sch 35, thats why I was wondering, I'm not going to chance it though. Thanks for the help
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Unread postAuthor: Fnord » Mon Oct 04, 2010 3:30 pm

Well, does it have a pressure rating on it?
Can you find fittings in that size? I'll bet they're expensive.

thanks, it wouldn't make any difference if i wrapped it in fiberglass right? I think I read it wouldn't.


About 3 or 4 wraps of fiberglass cloth would add a decent amount of strength.

Again, finding fittings in that size will probably be the biggest problem. You'll probably need an extra set of hands to solvent weld them correctly.
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Unread postAuthor: jimmy101 » Mon Oct 04, 2010 4:52 pm

Gun Freak wrote:I wouldn't trust it. Advanced metered propane combustions can generate in excess of 120 psi. If it were me, it is at least sch 40 for that.

"Sch40" has nothing to do with the pressure rating. All it means is that all Sch40 parts will fit together properly.
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Unread postAuthor: BeaverRat » Mon Oct 04, 2010 6:49 pm

jimmy101 wrote:
Gun Freak wrote:I wouldn't trust it. Advanced metered propane combustions can generate in excess of 120 psi. If it were me, it is at least sch 40 for that.

"Sch40" has nothing to do with the pressure rating. All it means is that all Sch40 parts will fit together properly.


Yes, and no... Although Sch40 doesn't directly relate to pressure, schedule 40 will have a higher pressure rating than schedule 20, and schedule 80 will have a higher pressure rating than schedule 40. Higher schedule means thicker walls, which naturally raises the pressure rating.
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Unread postAuthor: Pete Zaria » Mon Oct 04, 2010 7:03 pm

BeaverRat wrote:
jimmy101 wrote:
Gun Freak wrote:I wouldn't trust it. Advanced metered propane combustions can generate in excess of 120 psi. If it were me, it is at least sch 40 for that.

"Sch40" has nothing to do with the pressure rating. All it means is that all Sch40 parts will fit together properly.


Yes, and no... Although Sch40 doesn't directly relate to pressure, schedule 40 will have a higher pressure rating than schedule 20, and schedule 80 will have a higher pressure rating than schedule 40. Higher schedule means thicker walls, which naturally raises the pressure rating.


Bingo - schedule refers to wall thickness but does not necessarily imply pressure rating.

For instance, SCH40 ABS, SCH40 PVC, and SCH40 NSF-PW PVC all have different ratings.

Technically, ABS and non-NSF PVC aren't actually rated.

Here's a chart for the rated, NSF-PW PVC: http://www.engineeringtoolbox.com/pvc-c ... d_796.html

ABS and non-rated PVC are usually fine for combustion guns. I'm not sure I'd use sch35, but if it was pressure rated, I wouldn't hesitate.

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Unread postAuthor: mcole254 » Mon Oct 04, 2010 8:27 pm

The pvc was pressure rated (don't recall what though) would be a good pressure rating to aim for (would it be like Gun Freak said with 120 psi) with using shots of propane for the fuel?
On a side note I thought that the only pvc that wasn't pressure rated was ones that don't have a sch rating or foam cell pvc.
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Unread postAuthor: Lockednloaded » Mon Oct 04, 2010 8:43 pm

SCH IS NOT A RATING, do not trust the SCH number to tell you what it can handle. SCH is a Thickness of pipe.

Your chamber may not have to be rated to 120psi. 120psi was the peak pressure of a perfectly setup advanced combustion, burst disk and all. Also, your chamber only holds that pressure for a split second. It is not recommended, but foam cell has been used on advanced combustions.
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Unread postAuthor: mcole254 » Mon Oct 04, 2010 8:49 pm

I know that sch is not a pressure rating, I was wondering the minimum pressure rating I should use if I decide to make this. I will not be using any burst disks or pistons, just a simple combustion with low propane mixture fuel.
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Unread postAuthor: Pete Zaria » Mon Oct 04, 2010 9:01 pm

mcole254 wrote:I know that sch is not a pressure rating, I was wondering the minimum pressure rating I should use if I decide to make this. I will not be using any burst disks or pistons, just a simple combustion with low propane mixture fuel.


For a non-hybrid combustion, ABS pipe is one of the best choices. Even though it isn't pressure rated, if ABS does fail, it tends to "rip" open and depressurize without any shrapnel. ABS pipe can handle the low (<100psi) pressures generated by a combustion gun. I have a metered propane ABS gun with hundreds of rounds downrange with no failures or visible damage. I've also pressure-tested 4" ABS to 125psi overnight (don't recommend trying that at home, though) with no problems.

The other good option is pressure-rated (pipe needs to say NSF-PW on it) Sch40 PVC.

In the USA, Sch40 PVC that doesn't say NSF-PW is NOT pressure rated and isn't recommended. This is because, if PVC fails, it frequently produces shrapnel, which can cause injury. So if you're going to use PVC, make sure it's rated. ABS doesn't have this problem, however, and is frequently cheaper.

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