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Hello everyone, first let me say a few things. I have been building cannons for almost 10 years. I consider myself to be pretty knowledgable of pnuematics and combustion cannons. Recently I decided to explore the world of hybrids, and having never built one I did tons of research. I wanted to use steel pipe but its hard to come by and expensive where I live. I saw quite a few people using pvc for theres and at much higher mixes than I planned to use so I decided to use pvc and ease into it. I proceeded to put together a cannon, not pretty but just to test some things out. After it was built I did about a dozen shots at 1x 2x and 3x with no projectile and hiding behind a blast sheild. All was good so I then tried a few shots with potatoes at 1x and 2x and about 5 shots at 3x. Everything was going fine and I was getting confident that the cannon would not rupture so I did away with the blast sheild. I went to fire one last shot at 3x before I put it away for the day, and also decided to record it. For some reason the cannon exploded violently this time. It seemed several times as powerful as my other 3x shots. I was about 15 ft away and some how everything besides me was hit with shrapnel. I even found shrapnel in my neighbors yards several hundred feet away. The blast felt like a punch to the head/chest, and left me disorientated and without hearing for a while. I do not plan on building another hybrid any time soon. I am putting this up as a cautionary tale. Im not sure what exactly went wrong but I could have been easily killed, and I do not want this to happen to any fellow enthusiasts of this hobby. Any ideas of what could have gone wrong? It seemed 10 times as powerful as any other 3x shot I had done, which makes me wonder if the propane could have gone threw a deflagration to detonation transition. Scary stuff, be careful guy!

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What schedule pipe did you use to build this?

Welcome to Spudfiles.

Damn! That must have been LOUD.

Glad you're ok!

Some questions, what size and type of PVC were you using?

What were you using as a burst disk?

Damn, those are some nasty shards of plastic... if I didn't understand probability I would advise you to go and buy a lottery ticket.

Haha thanks guys. The pvc was sch 40 pipe from my local hardware store, not the thin HWV stuff. Probably should have used sch 80 As for a burst disk I did some tests with aluminum foil and found that a 5 layer disk ruptured at around 40-45 psi and that is what I used for this shot. The disk did rupture but did not let the pressure out fast enough.

According to this, you should have been good for more than a couple of hundred psi: http://www.engineeringtoolbox.com/pvc-c ... d_796.html

On the other hand, PVC by nature is brittle and does not take well to the sudden pressure shock which hybrids tend to produce. There could also have been an unseen manufacturing defect or hairline crack from abuse.

So, are we all in agreement by now that PVC hybrids = bad things? Especially SCH-40 ones?

Glad to hear you're OK. Perhaps try some smaller bore hybrids in steel/copper if you can't find steel large enough affordably?

PVC hybrid for me are suicide machines.
However, how did you know the shot was much stronger? I guess it's because it sound much louder to you right?
Then probably nothing special happened, just the PVC give up after a couple o abuse. Sound louder doesn't mean it's stronger. The gas has lot more area/space to expand when the chamber exploded than just exiting from the barrel so the expansion it's much faster also creating a more intense pressure shock that reaches your ear. Same reason why the same cannon fired without a barrel sound louder than with a barrel.

Thanks for posting the video. Vicarious.

Lesson learned on the PVC.

I wonder if your metering system was very consistent? I know when I dialed in my hybrid it made a big difference in power.

Let this be a warning to everybody to never build a hybrid. Ever... (noname)

Your pipe may be PVC but your balls are solid STEEL! :violent1:

But seriously, nobody should ever even think about building a hybrid with PVC... not even for 2x. What made you think that was a good idea? Must have been the total lack of any PVC hybrids in the showcase... Wyz hit the nail on the head when he said PVC hybrids are suicide machines!

I'm very glad to hear you are OK. I too have experienced violent explosion of PVC in my face and I know how scary it is. One small shard in your eye and you're done... You're very lucky!!

Is DDT at all a possibility here? I mean it did survive a few shots before... Just something that came to me, I'm not at all knowledgeable about it.

Gun Freak wrote:I'm very glad to hear you are OK. I too have experienced violent explosion of PVC in my face and I know how scary it is. One small shard in your eye and you're done... You're very lucky!!

Another reason to wear "proper" ear and eye protection, every. Single. Time, that you pick up a launcher. I didn't when I was younger, but now I wear completely enclosed goggles (rated for impact, not just chemical exposure) and foam earplugs when shooting my homemade contraptions.

On another note, I take the time to work safely and wear my PPE at work, too, much to the chagrin of other people. Having had two surgeries, both for repetitive stress injuries, I know how much your health, and more importantly, money is at stake. Don't let idiots kill you.

Gun Freak wrote:I too have experienced violent explosion of PVC in my face

That almost sounds kinky

On another note, I take the time to work safely and wear my PPE at work, too, much to the chagrin of other people. Having had two surgeries, both for repetitive stress injuries, I know how much your health, and more importantly, money is at stake. Don't let idiots kill you.

Wise words!

I had this up in the lab:

Sh*t, that's a nasty explosion

I used a PVC connector for my hybrid, failed violently after a good amount of shots hehe
I love explosions, but they need to happen when I want them to

I had this up in the lab:

Nice one, that's something I might actually put that one on a wall in the school workshop

There are some amusing variations: http://knowyourmeme.com/memes/carol%E2% ... ty-goggles



Something similar to this is on every Sherline motor:

Been there done that:


Hooray for PVC grenades!

Me and my friend have done it intentionally in a semi-controlled manner with some questionable propellants before.

We didn't have any shards to take a picture of, though.

On a serious note, I'm very thankful I've never had this happen to me, even with my 2006-era plastic hybrid. A lot of us built them back then...