Parts for combustion cannon

[niglch]

After building a pneumatic cannon last summer, I'm considering expanding my horizons and making an "advanced" (metered propane/MAPP gas + chamber fan) combustion cannon. I built a couple of simple spray and prays in the past, but this time I'm being a bit more cautious about safety because of the added power of a more "advanced" design. Mainly, I am having a difficult time finding the right pressure rated PVC fittings for my chamber. In every hardware store I have looked in, they don't have anything NSF-PW for anything above a 2" diameter pipe (and I know from experience with the pneumatic that even these can be tough to find). I even checked McMasterCarr.com and they didn't seem to have anything. Some extensive googling also failed to turn anything up. Among other things, I'm probably going to need a 4" or 3" x 2" bushing (depending on my chamber diameter), and some sort of mechanism to vent the chamber after firing. I mention the venting issue because I'm pretty sure I read somewhere that pressure rated clean-out caps simply do not exist, so I'm wondering what my alternatives are for airing out the chamber. Would anyone be able to point me in the right direction?

Also, I have a quick side question about chamber diameter. I'm considering using a 3" pipe rather than a 4" one since the necessary fittings might be cheaper and/or easier to find. Of course, this means making a relatively long (~2 ft) chamber to reach my desired volume. I heard somewhere that shorter, wider chambers are better since they make it easier for fuel to diffuse evenly throughout the chamber. However, I was wondering if using a chamber fan and 2 spark gaps positioned at opposite ends of the chamber would sufficienty remedy this problem.

Thanks guys

[boom_o_matic_2.0]

well I havent done testing myself but I think someone said that most combustions are pushing it to even reach 50psi. and thats not alot. the pressure of fittings dont really count in making combustions.

[MountainousDew]

well I havent done testing myself but I think someone said that most combustions are pushing it to even reach 50psi. and thats not alot. the pressure of fittings dont really count in making combustions.

When they said, they were most likely referring to "Spray & Prays", or they were wrong.
Advanced combustions can exceed 50 psi easy!

Niglch, check out Triple Thunder, he used 3" PVC, with great success. You might want to build off his design...

[Technician1002]

A quick note on venting, a ball valve works to pressure. Use the fan with one.

[Gaderelguitarist]

Would you be able to use couplings to double up the layer of the chamber?

Someone answer this as it will help both the author and myself...

[vardas]

that is exactly what i did for my last build.but mine is an advanced,so i felt the extra layers were necessary.&i haven't had any problems yet .but ironnickname told me that he has cracked a three layer sandwich of pvc before .just get your couplers & dremel out the rings in the middle ,use lots of glue ,and be sure to get a good fit (no gaps) then glue over the outside as well. you can always sand off the exess after its completely dry

[psycix]

Also, I have a quick side question about chamber diameter. I'm considering using a 3" pipe rather than a 4" one since the necessary fittings might be cheaper and/or easier to find. Of course, this means making a relatively long (~2 ft) chamber to reach my desired volume. I heard somewhere that shorter, wider chambers are better since they make it easier for fuel to diffuse evenly throughout the chamber. However, I was wondering if using a chamber fan and 2 spark gaps positioned at opposite ends of the chamber would sufficienty remedy this problem.

Place the two spark gaps at 1/4th and 3/4th of the chamber for optimal combustion.
A 2 feet 3" chamber is not disastrous or something, especially with multiple spark gaps. 4" would theoretically be better, but in practice, it wouldn't differ "that" much.

[starman]

Both 3 and 4" fittings are equally difficult to find. You may have to go to your local commercial/industrial pipe supplier to find what you need.

It really depends on how large ammo/barrel "caliber" you will build. I consider 1 1/4" and smaller barrel a small caliber cannon and would tend to match it with a 3" chamber. Barrel calibers of 1 1/2" to 3", I would match with a 4" chamber.

You can vent with a 1" ball valve. For that and other ideas, check out my Trip Thunder....a lot of the issues you brought up are addressed there.

[c11man]

i agree to check out the triple thunder.

also to get 4 and 3inch fittings you could try www.flexpvc.com it is a decent price and shipping is fair. anouther benifit is that all the parts from that site are pressure rated unless it says so

like others have said you dont need presure rated parts for a combustion but for an advanced on you might whant th extra safty