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Pneumatic springs in a piston hybrid

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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Pneumatic springs in a piston hybrid

Unread postAuthor: ramses » Sat Oct 04, 2008 5:42 pm

I plan to make a piston hybrid with the piston valve in a 2" tee, 1.5" porting. I intend to seal the normal "pilot" area off from the rest of the guy (Orings with no equalization hole). I intend to pressurize the pilot area to a pressure determined by the desired equivalent burst pressure. I do not intend to use a pop off to assist it's opening. I was wondering whether I should give the piston a long travel, so the piston slowly stops as the pressure behind it in the pilot area rises, or use a conventional short travel piston, to minimize the acceleration during combustion.

Thanks
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Unread postAuthor: Fnord » Sat Oct 04, 2008 6:31 pm

Are you sure the pipe you've picked will allow for 1.5" porting? 1.5" pipe fits fairly tightly in 2" sch 40, and perfectly in sch 80.
I'd settle for a moderate amount of travel; maybe a little over an inch.

Also, if you are keeping the barrel 1.5 inches or under, you can get away with using 1" porting without flow constrictions affecting performance. I'd try to get a hold of some 1.25" pipe if it were me, though.
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Unread postAuthor: ramses » Sun Oct 05, 2008 10:29 am

Aside from the porting size, would you have the piston travel far or short. Thanks for the advice with the 1.25" pipe. odds are i will go that route.

The barrel was going to be 3", but I can't afford a 4" metal threaded tee.
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Unread postAuthor: spot » Sun Oct 05, 2008 10:37 am

Long is better I think.
With a short piston travel, when your piston moves back it will compress the pilot much more than with a long piston travel.
If you understand what I'm saying :P.

SpoT^
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Unread postAuthor: psycix » Mon Oct 06, 2008 10:42 am

spot wrote:Long is better I think.
With a short piston travel, when your piston moves back it will compress the pilot much more than with a long piston travel.

The shorter the travel, the faster the piston will compress it since there will be less volume to be compressed. When its longer, it may generate higher pressures due to the speed it has.


I would not make the travel too long: theoretically you only need 1/4th of the porting diameter, so more then the whole porting diameter would be unneeded.

You should calculate the force that you expect to be a the front of the piston.
Then compare it with the force you will have with a certain backpressure. The piston travel will stop when the forces are equal, where every time the pilot volume halves, the pressure(and thus force) double.
That way you can calculate the travel it will have.
Then go for a travel of about 1/2 or 3/4th of the porting diameter.
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Unread postAuthor: ramses » Mon Oct 06, 2008 2:23 pm

that does leave out inertia, however. close enough. :D

on an unrelated note, any advice on the massive recoil expected of the whole gun, concentrated on the back of the pilot area(directly behind the barrel), and it's non destructive dissipation?
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