Spudchukker III - Vented Piston Hybrid build log

[jackssmirkingrevenge]

[youtube][/youtube]

Pretty cool, love the almost imperceptible delay between muzzle blast and target impact in the slow motion

[sudpuzzer]

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 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

[ramses]

I trust there was something thick and transparent in front of the camera?

[sudpuzzer]

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

(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).




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:



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.



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:

[jackssmirkingrevenge]

Your overengineered pistom makes JSR a sad panda



Cut some grooves, drill some holes, anything!

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

rendering btw

[sudpuzzer]

Obviously, my next step in design would be weight reduction. However, I'll just leave this here...
[youtube][/youtube]

(It works pretty good)

[jackssmirkingrevenge]

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.

[sudpuzzer]

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.

[jackssmirkingrevenge]

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:

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

[PeteS]

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.

[sudpuzzer]

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.

[dart guy]

I just thought about this what about a sideways muzzle break/silencer?

[Gun Freak]

I just thought about this what about a sideways muzzle break/silencer?

Did you not read Jack's post 3 lines up?

[jackssmirkingrevenge]

Did you not read Jack's post 3 lines up?

The tiny tank muzzle brake image was hard to miss

[dart guy]

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