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Piston vs Burst Disk

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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Piston vs Burst Disk

Unread postAuthor: PeteS » Tue Dec 17, 2013 5:53 am

I am contemplating my next hybrid build (or rebuild of one of my current ones) and am leaning toward another burst disk cannon, but wonder about piston hybrids. I have built a bunch of pneumatic piston guns, but not a piston hybrid.

Given that burst disk hybrids are simple and very effective I have to wonder about the attraction folks have to piston hybrids and whether I am missing something.

My naive take on piston hybrids:
+ You don't need to mess with making burst disks
+ They are pretty cool looking
+ You get to fiddle with more parts
- You have to fiddle with more parts
- You don't have the easy swapping of different burst disk materials and thicknesses to tune the results
- There are more parts to wear out or break
- They seem more likely to be a bit fiddly

Feel free to correct any of my assumptions or to add additional comments.
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Re: Piston vs Burst Disk

Unread postAuthor: MrCrowley » Tue Dec 17, 2013 6:17 am

PeteS wrote:- You don't have the easy swapping of different burst disk materials and thicknesses to tune the results

This should actually be a + as piston hybrids are arguably more easy to fine-tune than burst disk hybrids as you can precisely adjust the pilot pressure, which is responsible for timing the opening of the valve. Burst disk adjustment is discrete, you can either add an extra disk or remove one (though, admittedly, of different materials); piston valve adjustment is done with pressure, which is on a continuous scale. However, we still don't know enough about piston hybrids in terms of how adjustments in pilot pressure can affect performance or how close it mirrors the theoretical calculations used to find the pressure required. For example, we may calculate that 350PSI in the pilot area is just the right amount to hold back the piston until 900PSI is reached after ignition but for all we know the valve could be opening before then due to some weird physics or valve design. Perhaps another 25PSI won't make much difference and you'll really need to add an extra 100PSI even though the calculations suggest otherwise. There's just not enough data at this stage to say with confidence, I think. On top of that, peak combustion pressure is never known, only estimated, so you have to factor in a fairly large error anyway (something like +/- 10% expected peak pressure).

Apart from that, you pretty much got it down. Between not having to make burst disks, not having to load burst disks for each shot, and having more precise control over the valve opening, I decided to go with a piston over a burst disk for my largest hybrid. A piston makes more sense the larger the valve diameter too, as 2" burst disks would be a real pain to 'manufacture'.

They're also quite fun to build and a bit more of a challenge. Making a burst disk cannon isn't really the same as you're more or less just assembling parts. On the other hand, a piston hybrid is more closely related to that feeling of building your own device from scratch.
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Re: Piston vs Burst Disk

Unread postAuthor: PeteS » Tue Dec 17, 2013 1:44 pm

Interesting. Thanks.
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Re: Piston vs Burst Disk

Unread postAuthor: AdaLena147 » Thu Dec 26, 2013 4:42 am

Would it be a bad idea to use a pop off valve to evacuate the area behind the piston rather than have a little piston connected to the main piston?
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Re: Piston vs Burst Disk

Unread postAuthor: jackssmirkingrevenge » Thu Dec 26, 2013 6:32 am

AdaLena147 wrote:Would it be a bad idea to use a pop off valve to evacuate the area behind the piston rather than have a little piston connected to the main piston?


Hmmm... copy-pasted from here.

PeteS wrote:Would it be a bad idea to use a pop off valve to evacuate the area behind the piston rather than have a little piston connected to the main piston?


Are bots getting more sophisticated or is this an underpaid thirdworldster?
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Re: Piston vs Burst Disk

Unread postAuthor: jrrdw » Thu Dec 26, 2013 6:35 am

Are bots getting more sophisticated or is this an underpaid thirdworldster?


Just a nembie who is asking before searching... :roll:
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When life gives you lemons...throw them back they suck!
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Re: Piston vs Burst Disk

Unread postAuthor: jackssmirkingrevenge » Thu Dec 26, 2013 6:49 am

Naturally ;)

Same here: miniature-cannons/topic23218.html

AdaLena147 wrote:Or would this be weaker than just blowing?


From here: miniature-cannons/topic24519.html

PaperNinja wrote:I had this idea to "optimize" simple pen-blowguns which would be to simply add a piston valve so your breath is at it's full pressure before it blows the pellet. This is opposed to the gradual build up in pressure you get by just blowing, which may lead to the pellet leaving the barrel before you are blowing at maximum strength. Is this idea feasible at all? Or would this be weaker than just blowing?

I don't intend for this to have any kind of high power, just something that would be easy to make, slightly more powerful than just blowing into a barrel, and what won't leave me light-headed.
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Re: Piston vs Burst Disk

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Re: Piston vs Burst Disk

Unread postAuthor: jrrdw » Fri Dec 27, 2013 5:14 am

"leave me lightheaded" That's funny.
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