Design before manufacture

[Thrilla]

I've been lurking around the forums for about 2 hours straight now lol, couldn't resist but to design my 1st spud gun. (there was a first, but it was pretty dumb lol, built it after seeing it on youtube )

This time, I want to make a powerful looking yet simple one, so I'm going with combustion since this is pretty much going to be a show piece.


The pictures should be self explanatory. Mounting frame not shown in any of the pics. (Took me another 2 hours to draw this thing hahaha)

• Fuel and O2 adapters are from Bernz-O-Matic torches, built in valves.
• All tubing are HVAC/R grade copper, rated over 600psi, joints brazed shut.
• Fuel chamber has its own constant presure gauges (no valves in between)
• Bicycle valve is used to feed presurized air or NOS
• The yellow gauge is constantly showing explosion chamber in psi, rated up to 350psi, and 700psi in retard.
• The scope is just a short tubing with acrylic lens, no magnification.
• A piece of metal O ring is glued to the fixed barrel, preventing potato from falling. Added rough surface where a tight fitting ammo would make contact, creating a weak burst disk.
• The silencer is open cell foam sanwiched between metal pipings. It's slightly bigger than barrel ID, no contact with ammo.
• A seperate potato cutter will be used, easier to work with.

Destruction Sequence

• Tight fitting ammo is fed into the barrel
• Propane is opened, fed into the mixing chamber until a preset value, then released into combustion chamber.
• A small amount of oxidizer is metered (same as propane) and fed into combustion chamber.
• Brushless fan turns on, mixes fuel, turn off.
• FIRE IN THE HOLE, note how much propane and oxidizer used, and run some calculations for both safety and performance, and stochiometry.
• Then either open rear cap to vent, or feed compressed air through bicycle valve.
• Use previous firing calculations to feed propane and oxidizer, so that explosion psi stays within 85% of rated bursting psi.

Using tight fitting ammo and a section of rough surface as a simple and safe burst disk, it kind of fits under hybrids lol, but there will be less than 5psi of pressure inside the combustion chamber anyway, before the ammo starts to move.

What do you guys think of the design? Any constructive comments are welcome

I'm not sure if I should use a fixed or threaded/removable barrel yet, comments on that would be great as well -

and it's going to be shoulder mount, full PVC (acrylic barrel if i can find them for cheap)

[spudbud101]

Wow. Very nice design! Looks nice, too. It'll have some power, too.

[noname]

I would strongly advise against oxygen and propane; PVC can't take it and you'll be digging fragments out of your shoulder for quite some time. Other than that, it looks like you know what you're doing. Use a threaded barrel with the female threads on the barrel and the male threads on the chamber.
Acrylic tubing for barrels can be found <a href="http://www.tapplastics.com/shop/product ... ">here.</a>

[PVC Arsenal 17]

What program did you use for those diagrams? They look freehand but awesome nonetheless!

[Gepard]

Google Sketchup.


What's the point of trying to get the exact mix required for combustion. As soon as you vent it into the combustion chamber you have increased the oxygen levels.....

Michael

[jimmy101]

If you go to the effort to build a gun that complex you definitely want a removable barrel. Use either cam-locks or threaded fittings.

The meter chamber looks way to big, probably more like 1/2" pipe?

Add a couple more sets of screws for spark gaps. Perhaps one at the middle of the chamber and one towards the barrel end of the chamber. Personally, I would put on in the center, one a chamber radius in front of the cleanout plug and a third a chamber radius before the chamber-barrel junction.

To use the "scope" it'll need a butt, otherwise it'll be very awkward to hold. You really need a butt to shoulder fire a weapon to keep the scope or gun from smacking you in the head. (A gun this size will have a pretty good kick)

Perhaps move the tanks from the top of the gun to the bottom? It'll have much better balance if it isn't so top-heavy.

I would omit the "silencer" for two reasons;
1. It'll make loading more difficult (for a muzzle loader, no affect if it is loaded from the chamber end of the barrel).
2. In the US, a silencer is a big no-no. You cross over into a whole new set of laws with a silencer. An anal cop and court could ream you big time for just having it in your possession.

Overall a good design. Perhaps a bit ambitious but what the heck.

Welcome to the forums.

[Thrilla]

noname
thanks for the link, after some more lurking, I'll probably go with lexan polycarb. If further research shows that it's stronger than sch 80 PVC, I'll go full lexan then.


Michael
I didn't take that into account lol. Guess I don't have to vent after all, I can measure O2 and propane and just put it all in the combustion chamber. Instead of having nitrogen, i'll have CO2 and some water vapor.
5O2 + C3H8 --> 4H2O + 3CO2
So fuel will be 5:1


jimmy101
Yea the mixing chamber looks a bit tooooo big lol, I was thinking of compressed air from a bicycle pump to vent the combustion chamber, but now I figured it's not needed.

I'm going to use a spud cutter and a knife to make it aerodynamic, then insert the ammo by removing the barrel.


Thanks for the input, I'll make a rev 2 and post it up.

[Jumpin Jehosaphat]

Manufacture eh? Just how many of these things are you going to build.

I agree with noname, enrich and die! (in PVC)

[Thrilla]

Oh and I forgot, in Canada, I think a silencer is allowed. I've seen one being used at a paintball field, and it works really really well. If the cops ask, I'll just say it's a orange-tipped (recreational firearm lol) preasure equalizer and blast shield, in case the ammo explodes upon exit. And call it a counter weight lol.

Enrich from 16% to 20% isn't too much lol oxygen will allow firing without venting.

[Jumpin Jehosaphat]

I think the bottle of propane might catch their eye before the silencer does. As far as what I've heard spudguns are legal in Canada as long as the muzzle velocity stays under a certain limit.

Edit: Even if you enrich, you will have byproducts of combustion in your chamber. I would recommend you vent, you are correct that adding the oxygen manually would allow for combustions without venting. If you're going to go spend money on O2 tanks, you may as well avoid choking the combustion with CO2 from previous shots (you will get more power from less fuel)

[Thrilla]

hahaha I'll get away by calling it a metalwork project or science fair project. I have lots of restricted items in my high school locker, but the principal says it's okay for me as long as I keep it low profile so other kids don't bring them lol.

Yea spud guns are legal in Canada, my friend has a basic combustion that runs on hairspray, over time the chamber is very sticky and gooy hahaha.

[SpudBlaster15]

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[Thrilla]

I got a bit carried away and made some designs lol

Internal



rocket luncher style backfire to counter the kick



Hydraulics tray, bottom part not drawn






And behold, the damn-you-better-get-the-hell-outta-here cannon





With a supressor, same volumn as the combustion chamber.



size comparison