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Spudchukker III - Vented Piston Hybrid build log

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: jackssmirkingrevenge » Thu May 02, 2013 2:37 pm

[youtube]http://www.youtube.com/watch?v=MFDIIhDa10U[/youtube]

Pretty cool, love the almost imperceptible delay between muzzle blast and target impact in the slow motion :)
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Unread postAuthor: sudpuzzer » Tue May 14, 2013 9:14 pm

I have now switched to manometric metering, for two reasons:
A) I really had no idea where I was at with fueling before. I thought I was around 5X, but not really sure since I couldn't get any other mix numbers to work. I can't be bothered to measure the precise volume of the meter nor chamber, so manometric seems the natural choice. And
B) It takes a bunch of stuff off the actual cannon, so it will be easier to perfect the mounting of my ignitor box, and I think my manometric meter looks pretty cool :wink: pics will be coming.

I taped off the breech end of the barrel with duct tape, and filled the back 10 inches of barrel with peas. After loading the gun to 7X ( 0.644psi*7 = 4.5 psi propane + 6*14.7psi (88.2 psi) air = 92.7 psi total preignition pressure, correct?) and squeezing the trigger, it promptly laid me on my back. Unfortunately, I wasn't on camera at the time.

I am quite liking this cannon!

I know I keep saying this, but I will get more videos up soon. Just gathering footage for a cool video :D
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Unread postAuthor: ramses » Wed May 15, 2013 5:30 pm

I trust there was something thick and transparent in front of the camera?
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Unread postAuthor: sudpuzzer » Mon Jun 03, 2013 12:51 am

Nope, just the standard GoPro housing. Tougher than nails that thing is.

Anyways, haven't posted for a while because I've been having some trouble with my piston. This is mainly due to the fact that I have very little patience to wait for epoxy or JB Weld to dry, and I couldn't get my large diameter washers to seal around the threads at each of the large o-rings and on the sealing face.

To solve this, I went a similar route as Mr. Crowley and designed a piston which I had machined. I didn't go the solid cylinder route as I figured I could reduce the weight somewhat by using threaded rods to span the gap from the piston to the sealing face:

Sitting beside the old piston
Image
(As you can see in this pic, my old bumper idea didn't really pan out. The outsides of the rubber disk sheared off and I suspect it has been working basically like an air bumper. It hadn't failed yet exactly, but I don't particularly like the look of it or know how much longer it would last.)

I made it with 1/2 inch threads in the front of the piston and 5/16 inch threads at the back of the piston. The rods thread directly in and the sealing face goes on the 1/2 inch rod and the vent seal goes on the 5/16 inch rod.

I had it machined out of steel since the shop didn't have any aluminum. It is fairly heavy, but I have a good bumper I am going to use (one of those rubber pipe joint couplers).


Image

While I was at it, I also got a vent seal made which has an o-ring in it as well to seal off the pilot volume:

Image

Plus, I got a 1/4" thick, 2" diameter disk machined with 1/2 inch threads which holds the actual sealing face and supports it. I did this because the threads are easy to seal with pipe tape, and it works much better than the thinner washers I was using before. This overall design reduces the number of joints requiring sealing (just the threads), and is much more resilient than my old piston.

Image

More info coming as soon as I get it firing again. I have tested it as a pneumatic and it holds 140 psi with no air leaks, which is sufficient. I have to install my bumper and I will film the first couple of shots with the new piston for you guys.

EDIT:
Here's a render I was working on before I switched the meter, not current but I figured I'd show it:
Image
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 03, 2013 3:09 am

Your overengineered pistom makes JSR a sad panda :(

Image

Cut some grooves, drill some holes, anything!

http://www.spudfiles.com/forums/what-ma ... 24254.html

Image rendering btw :D
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Unread postAuthor: sudpuzzer » Mon Jun 03, 2013 8:59 pm

Obviously, my next step in design would be weight reduction. However, I'll just leave this here...
[youtube]http://www.youtube.com/watch?v=Ngv1sZJJwyY[/youtube]

(It works pretty good)
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 03, 2013 10:19 pm

BOOOOOM!

Nice!

Some kind of shoulder pad seems to be in order :) and since machining facilities appear to be available, a muzzle brake of sorts would also ease the pressure.
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Unread postAuthor: sudpuzzer » Mon Jun 03, 2013 10:52 pm

You'd be surprised how well the recoil absorbs into the shoulder with no pad. Even with just a T-shirt, it's entirely manageable (at 7X with a small potato I suppose).

The machine shop is in the next town over, and to be honest I've spent all I want to spend on machining. After I got it, I realized I could have just got the O-Ring holders made and threaded like the disk, rather than a full cylinder, but regardless I am happy with the results. I made it adjustable with minimal effort for if I ever need to change the sealing face and the distances are different. I sort of like the recoil and the noise :). My bumper seems to work good too, you can see the piston hitting it in the video, and it doesn't bounce back too bad.

This shot was at about 40 yards, and I put the potato in the center of my target, which I think is pretty good. That was the first one I shot at the 3/4" plywood at 7X, and it put a clean circular hole straight through with massive splintering on the other side.
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 03, 2013 11:27 pm

sudpuzzer wrote:You'd be surprised how well the recoil absorbs into the shoulder with no pad. Even with just a T-shirt, it's entirely manageable (at 7X with a small potato I suppose).


Sooner or later you're going to want to start putting some real projectiles through it, which weigh more and that will increase recoil considerably.

A muzzle brake doesn't need to be complex, a simple tee suitably perforated will do:

Image

regardless I am happy with the results


As you should be :) not many folks get to this level of starch acceleration.

This shot was at about 40 yards, and I put the potato in the center of my target, which I think is pretty good. That was the first one I shot at the 3/4" plywood at 7X, and it put a clean circular hole straight through with massive splintering on the other side.


This is the time you start asking friends and relatives for discarded domestic appliances to be sacrificed as targets :)
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Unread postAuthor: PeteS » Thu Sep 26, 2013 6:23 am

I was surprised to see you used EMT for the barrel. I thought being seamed it might be a problem. Good to know it is an option.

Do you have any chrono readings for this setup?

Anyway, congrats on an awesome project. Very well done.
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Re: Spudchukker III - Vented Piston Hybrid build log

Unread postAuthor: sudpuzzer » Thu Oct 10, 2013 11:41 pm

No, unfortunately I do not have any chrono readings on this gun. The EMT seems to work fine, and it is a great deal lighter than a similar length of Schedule 40 iron pipe. Since my last posts, I've leaned out my 7X mixture a little bit and am actually having a bit of trouble finding potatoes that stay in one piece. This thing just seems to make everything disintegrate. Time to move to more solid ammunition I suppose.
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Last edited by sudpuzzer on Sun Oct 20, 2013 2:27 pm, edited 2 times in total.
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Re: Spudchukker III - Vented Piston Hybrid build log

Unread postAuthor: dart guy » Wed Oct 16, 2013 4:38 pm

I just thought about this what about a sideways muzzle break/silencer?
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Re: Spudchukker III - Vented Piston Hybrid build log

Unread postAuthor: Gun Freak » Wed Oct 16, 2013 6:58 pm

dart guy wrote:I just thought about this what about a sideways muzzle break/silencer?


Did you not read Jack's post 3 lines up?
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Re: Spudchukker III - Vented Piston Hybrid build log

Unread postAuthor: jackssmirkingrevenge » Thu Oct 17, 2013 4:55 am

Gun Freak wrote:Did you not read Jack's post 3 lines up?


The tiny tank muzzle brake image was hard to miss :roll:
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Re: Spudchukker III - Vented Piston Hybrid build log

Unread postAuthor: dart guy » Fri Oct 18, 2013 2:50 pm

I meant to have a silencer that is open on both sides but will still muffle the shot.
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