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Woody's Hybrid Build

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: ramses » Sun Jul 03, 2011 12:00 pm

jackssmirkingrevenge wrote:One must consider however that the stress is mostly on the burst disk. What the cam-lock has to do is hold the disk in place, it's not really being directly affected by the pressure. Once the disk bursts there should be no stress on the locking mechanism at all.


I disagree, depending on how the disk is held. Any force exerted by the barrel "half" of the disk would be held by the cam-lock mechanism itself. This is likely quite appreciable. At the very least you need to compress the disk with u*Fn/(2 sin( burst disk strain cone angle)) to keep it from slipping.
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Unread postAuthor: Woody » Sun Jul 03, 2011 7:14 pm

What's the best way to grind that lip on the one half of the union to make it flush? I'm guessing a belt sander, a few belts and a lot of time... Any other ideas?
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Unread postAuthor: MrCrowley » Sun Jul 03, 2011 7:14 pm

Angle grinder is what I used
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Unread postAuthor: Lockednloaded » Sun Jul 03, 2011 7:17 pm

Woody wrote:What's the best way to grind that lip on the one half of the union to make it flush? I'm guessing a belt sander, a few belts and a lot of time... Any other ideas?


If it's a thick enough lip, I'd try to saw it off with a rotary tool or hack saw, then clean it up from there
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 03, 2011 10:53 pm

ramses wrote:I disagree, depending on how the disk is held. Any force exerted by the barrel "half" of the disk would be held by the cam-lock mechanism itself. This is likely quite appreciable. At the very least you need to compress the disk with u*Fn/(2 sin( burst disk strain cone angle)) to keep it from slipping.


My point is that the worst thing that can happen is the disk slips and lets out your mix, but it's not going to blow off your barrel on firing.
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Unread postAuthor: Woody » Sun Jul 03, 2011 10:59 pm

Ok, well I got that squared away. Used angle grinder and a piece of sandpaper. Everything is together including ignition. Going to have to figure out fueling. Checking out that fueling 101 thread now.

What about burst disks though? Is there a thread out there that estimates psi generated at each x mix and list burst disk materials that break well for that? I'm thinking there's something better than cutting out 150 layers of aluminum foil. Maybe 2 liter soda bottle plastic? I'm thinking about going 5x mix right now.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 03, 2011 11:21 pm

Woody wrote:Is there a thread out there that estimates psi generated at each x mix and list burst disk materials that break well for that?


The problem is that x material bursts at y psi only at a given diameter, so there is no guide as such. As to ignition pressure, the rule of thumb is 100 psi generated per mix number, so calculate 5x will give you roughly 500 psi.

I've had good results with photo paper, you might also want to try a single layer of aluminum from a soda can.
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Unread postAuthor: Woody » Sun Jul 03, 2011 11:31 pm

Well I read through that and I am puzzeled. Maybe I'm just tired and need to think about it in the mourning. I took ap chem last year, but things are so idealized in class. What I took from it was that the ideal air fuel mix for propane is 4.2%, and that's it. I calculated I have approximately 6.69ci chamber for measuring the propane and a 236ci combustion chamber.

So, from my understanding I have to fill the propane up to a certain psi at which would give me the desirable 4.2% once the air is filled for the certain mix I am doing. Once that psi is achieved open the first ball valve out from the chamber, letting the propane into the chamber. Now I open the ball valve for the air and fill it to the set psi for that x mix which is like 14.4, 22.x,... (I have a chart somewhere). Once that's done it's ready to go. Close first ball valve out and fire.

Is my train of thought right? Could someone help me understand how to fuel this?


Edit: Happy 4th of July! Time to blow some crap up.
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 04, 2011 12:16 am

4.2% is for atmospheric pressure, ie what we call a normal combustion or 1x.

If you pressurise to 14.7 psi, we're now at 2x, so you need twice the amount of propane, ie 8.4%.

For 5x, you're pressurising to 4 times (not 5, because you already have one atmosphere in the chamber) atmospheric pressure (approx 60 psi).

You also need 21% propane concentration.
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Unread postAuthor: Woody » Mon Jul 04, 2011 1:38 pm

Many thanks to the spreadsheet that saefroch made up. Got it fired at 3x just now, but I'm having problems. Don't have small enough hose clamps for the propane hose, so I'm leaking out of those. I'm also leaking out the threads on the union and out around the burst disk.

I've been cutting out pop cans (1 layer broke at around 80psi) using two layers. I'm sure they're breaking premature, but i don't care enough to find something better right now.

The bigger problem is the leak which starts even at 20psi which is intolerable. I even cut out gaskets to go on both sides of the burst disk which helped some, but the leak is still there. The work I did making the half of the union flush may not be perfect, but it is pretty damn flush.

I may order a set of 1.5" and 1" cam lock couplings. First off I'll probably pick up a few more rolls of tethlon tape and try to the union as flat as can be. It's the 4th and I've been fooling around with this the past 3 hours. I think it's time to have some fun.

Time to light the golf ball howitzer!
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Unread postAuthor: POLAND_SPUD » Mon Jul 04, 2011 2:35 pm

The bigger problem is the leak which starts even at 20psi which is intolerable. I even cut out gaskets to go on both sides of the burst disk which helped some, but the leak is still there. The work I did making the half of the union flush may not be perfect, but it is pretty damn flush.
get two rubber washers and a large pipe wrench to tighten the union

if that doesn't work put the hybrid underwater and pressurise - maybe ithe union is not the problem but the leak is somewhere else
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Unread postAuthor: saefroch » Mon Jul 04, 2011 3:02 pm

Woody wrote:Many thanks to the spreadsheet that saefroch made up.
:D Glad somebody's making use of it!
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Unread postAuthor: ramses » Wed Jul 06, 2011 9:39 am

Is it leaking where the two halves of the union meet, or where one of the halves is screwed to the pipe?
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