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Fixing my first spud gun

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Fixing my first spud gun

Unread postAuthor: Wayfast73 » Sat Jun 26, 2010 5:16 pm

Hey guys I'm new here. I made my first spud gun yesturday. It has a 3"x24" chamber and a 2"x36" barrel. Both have 226 cubic inches of volume if my math is correct. I shot it using a russet potatoe and 6 seconds of aquanet, but it only went about 20 yards. It fired first time I hit the grill igniter so I don't believe it's rich. Should I go for more spray or cut the barrel shorter?
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Unread postAuthor: SpudFarm » Sat Jun 26, 2010 5:19 pm

Why don't you just try different mixes?

There is no need to cut the barrel of that cannon.
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Unread postAuthor: jrrdw » Sat Jun 26, 2010 5:42 pm

Try putting a spud into the muzzel, then spray your fuel and cap off the chamber, then push the spud the rest of the way down the barrel and see if it improves any. Doing it that way mixes the fuel and air better, be sure to blow the spent fuel out of the chamber after each shot.

In fact one of our moderators (sgort87) even put the cap off of the fuel can in the chamber and shake the cannon to mix the fuel and air better.

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Unread postAuthor: Wayfast73 » Sat Jun 26, 2010 6:50 pm

I'll give that a try tonight. I know this should have the potential to go further than 20 yards. I do eventually want to try setting it up with metered mapp.
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Unread postAuthor: clemsonguy1125 » Sat Jun 26, 2010 6:53 pm

Could you post a picture?
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Unread postAuthor: Ranger » Sat Jun 26, 2010 8:25 pm

With a peumatic you would not have that problem :wink:

Seriously, just try different fuel ratios. Six seconds sounds like too much, if it fired the first time that means it is rich.

With a spray and pray, you "spray" and you "pray" :D
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Last edited by Ranger on Sat Jun 26, 2010 9:22 pm, edited 1 time in total.

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Unread postAuthor: kydavies » Sat Jun 26, 2010 8:43 pm

if your hung up on the idea of a combustion spud gun do metered propane (ive heard its really easy to do)
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Unread postAuthor: mark.f » Sat Jun 26, 2010 9:00 pm

Six seconds is way too much. Start off with about a half a second and go up from there until you get good, consistent performance.
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Unread postAuthor: Wayfast73 » Sun Jun 27, 2010 2:53 am

I was shooting it tonight. anymore than 7 seconds of spray makes it too rich, but it still won't go further than 90'. I'll try a ecu fan but this is discouraging it makes plenty of noise but no go.
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Unread postAuthor: SpudBlaster15 » Sun Jun 27, 2010 4:27 am

You're adding way too much fuel. For a chamber of that size, you shouldn't need more than 3 seconds worth of spray.

Also, think about switching to a better fuel. Hairspray is considered the standard spray 'n' pray fuel, but it's one of the worst propellants out there. It contains large amounts of non-flammable solids that will gunk up your chamber, cleanout cap threads, and ignition electrodes. Look into deodorants with flammable components, fast evaporating liquids (Gasoline, methyl alcohol, etc.; they can be aerosolized using a spray bottle), or starting fluid. All will work much better than hairspray.
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Unread postAuthor: jrrdw » Sun Jun 27, 2010 4:30 am

The spud has to fit as nice as possible, the more gaps the worse performance you will have. Try some wadding to fill in any gaps. Also try different ammo, Nerf footballs, squishy balls, anything that fits snugly.

Also experiment with different fuels, "aqua-net" may have went by way of "Right Guard" thus not much good for spudding. I've heard that "Static Guard" still works well. "Propane", 2 seconds worth dumped (propane sinks it's heavier then air) into the chamber should give you great power.

Less is sometimes more, like the others said above. At one point in time your fuel mixture will be at it's strongest. Start at one second then two etc...

Don't give up! You can do this. :wink:
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Unread postAuthor: SpudBlaster15 » Sun Jun 27, 2010 4:51 am

jrrdw wrote:Also experiment with different fuels, "aqua-net" may have went by way of "Right Guard" thus not much good for spudding. I've heard that "Static Guard" still works well.


I have some relatively new (Date of manufacture: 2006) cans of Right Guard which have the same composition (Denatured alcohol, propane, butane, isobutane) as the product once commonly used by spudders. They do have a new formula which contains only non-flammable components, but the original brown can is still available.

Static Guard works well; tylerthetatertosser and I used it in his golf ball cannon, and we were able to fire golf balls more than 300 yards using only a 3ft barrel.

Technically, there isn't much variation (Heat per mole oxygen) between any of the common spudgun fuels, but most people find that some work better than others. I suspect it's mostly an issue of aerosol can nozzles and air/fuel mixing; a finer mist will mix more quickly and effectively with air than will a coarse spray, giving the launcher more powerful and consistent shots. Static Guard and some deodorant cans produce a very fine spray, making them ideal for spray 'n' pray launchers.
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Unread postAuthor: jimmy101 » Sun Jun 27, 2010 10:03 am

1. You have too much fuel. A rich mixture burns very slowly and trashes the gun's performance. (It also lowers the combustion energy but that is less of a problem than is the slowly burn speed.)

2. The fuel is poorly mixed. This also trashes performance. A chamber fan will fix this problem. To test if a fan will help without actually installing a fan, try fueling then waiting a minute or two before firing, slowly invert the gun a few times to help mix the fuel. If the performance goes up then mixing is a problem. (Gases mix surprisingly slowly.)

3. The spud should make a tight, basically an air tight fit, with the barrel. This is easily done if you sharpen the muzzle of the gun. You then shove an over sized spud into the barrel and the sharpened muzzle will slice the spud to the exact size of the barrel and give an airtight fit. A small amount of bevel on the inside of the muzzle will give a slightly over sized spud with a lot of friction. Surprisingly, the extra friction actually helps performance. (I usually suggest a first time spudder make a gun with a 1.5" barrel since spuds big enough for a 2" barrel are hard to find and kind of expensive. With the smaller diameter barrel all kinds of veggies can be used, apple, cucumbers...)

Your gun should be able to launch a half-spud at least 300 feet.

I use half spuds cause you get twice as many shots per spud. Slice the spud in half perpendicular to its long axis to give a "bullet" like shape with a roundish end and a flat end. To load, shove the nice flat end into the barrel first and your muzzle spud cutter will slice it perfectly to size. With a perfectly sharpened muzzle knife it'll take a lot of force to ram the spud home, perhaps as much as 50 pounds of force. Make sure the breech is open when ramming to (1) prevent accidental firing (never ram a fueled gun) and (2) to allow the air you are displacing from the barrel to escape through the breech. Once rammed home you can add fuel.
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Unread postAuthor: Wayfast73 » Sun Jun 27, 2010 5:08 pm

Im thinking about trying some propane. Should it be about the same amount 3 seconds or so? Heres a pic of it if it loaded right.
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Unread postAuthor: SpudBlaster15 » Sun Jun 27, 2010 5:26 pm

Wayfast73 wrote:Im thinking about trying some propane. Should it be about the same amount 3 seconds or so? Heres a pic of it if it loaded right.


Propane is difficult to use without a meter and injection system. If you don't plan to build such a setup, I'd just stick with aerosol fuels. Try automotive starting fluid; a 1 second burst should be good for that chamber.
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