SpudFiles

Hello all

This may be a little long but I greatly appreciate any help you can provide.

So my friends and I have played around with our spray spudguns since highschool, it’s been years since mine was out and this past weekend one of my friends brought his out and I had forgotten how much fun they are.
So I want to build us matching adult ones and give him his on his wedding day, I want to make them probably propane so we can get fuel from anywhere and regulate it to get some consistency.

Our old guns were simple 20” section of 3” for combustion and 29” section of 1.5 pipe with a clean out on the end. If I’m doing my math correctly that makes for a 2.75:1 ratio.

My new plan is to build a detachable barrel breach loading eclectic ignition launcher and use a 12” x 3” combustion, with a 24” x 1.5” for a barrel yielding a 2:1 magic ratio, at this new ratio can I expect an increase or decrease in FPS of a similar projectile? Also will I notice an increase or decrease in the noise? Would you recommend a larger chamber and longer barrel while maintaining the ratio and why?

Additionally we made ours originally out of Sch40 ABS since we were under the impression that ABS failures were safer, and the pipe more impact resistant. Is PVC that brittle? Would PVC survive the bumps and bangs of normal travel and use without failing? I ask because I would maybe like to include a clear pipe combustion chamber and that seems to only be available in PVC.

I am also considering adding one fan to the rear of the chamber is what are the pro’s and cons of laying the fan on its side vs. having it laterally in the chamber?

I have also read that there is a specific fuel air ratio to get maximum combustion, and that its somewhere between 3% and 4% can anyone help me figure out how to calculate how to achieve this mix given the pressure out of a standard Coleman type bottle and a given volume of combustion chamber?

Thanks all.

space_weazel wrote:My new plan is to build a detachable barrel breach loading eclectic ignition launcher and use a 12” x 3” combustion, with a 24” x 1.5” for a barrel yielding a 2:1 magic ratio, at this new ratio can I expect an increase or decrease in FPS of a similar projectile?

Have a look at these chamber:barrel ratio studies for a 1.5 inch barrel.



As you can see, best results were achieved with a 0.7:1 chamber:barrel ratio, and having a bigger chamber doesn't necessarily mean more power.

Also will I notice an increase or decrease in the noise?

Loud launchers are generally inefficient ones. If you want less noise, use a smaller chamber, and vice versa.

Would you recommend a larger chamber and longer barrel while maintaining the ratio and why?

It depends on your priorities - what do you want most, power, compactness, noise?

Additionally we made ours originally out of Sch40 ABS since we were under the impression that ABS failures were safer, and the pipe more impact resistant. Is PVC that brittle? Would PVC survive the bumps and bangs of normal travel and use without failing? I ask because I would maybe like to include a clear pipe combustion chamber and that seems to only be available in PVC.

Regular combustions usually generate well below 100 psi, and for a brief instance, so even unrated DWV PVC is usually adequate. With pressure rated PVC you can't go wrong as long as it's well put together.

I am also considering adding one fan to the rear of the chamber is what are the pro’s and cons of laying the fan on its side vs. having it laterally in the chamber?

Having the fan blowing down the barrel obviously helps venting more but in terms of mixing the gasses all you want to do is create turbulence so you should see no visible difference in power.

I have also read that there is a specific fuel air ratio to get maximum combustion, and that its somewhere between 3% and 4% can anyone help me figure out how to calculate how to achieve this mix given the pressure out of a standard Coleman type bottle and a given volume of combustion chamber?

My best advice if you want to avoid the hassle of a regulator and pressure gauge to meter the fuel in:

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My best advice if you want to avoid the hassle of a regulator and pressure gauge to meter the fuel in:

But a proper propane meter/reg can be more easily bolted to the side of the gun, increasing the complexity look and providing additional cool factor

If you're going to be bumping the gun around at all, I'd recommend ABS. You'll never have to worry about it if you do your solvent welds properly.

I think at agreed upon C:B ratios are:
0.7:1 for the most power when the chamber is constant
~1.3:1 for the most power when the barrel is constant.

But if nothing else, you can't go wrong with around 1:1

This will be gas powered, I'm not interested in syringing fuel in, I want it to be a little mechanically complicated (cool factor) and quick and easy.

I don't plan on banging it around but, I am trying to asses risk since to my understanding PVC shatters while ABS ruptures, I know the PVC is rated well above the pressures at had here, my concern is if some one knocks it over while its sitting on, say, pavement then we shoot it and it has developed a weak point, I don't want it going boom in some ones hands.

These are generally going to be friends and family shooting it.

Fnord wrote: I think at agreed upon C:B ratios are:
0.7:1 for the most power when the chamber is constant
~1.3:1 for the most power when the barrel is constant.

But if nothing else, you can't go wrong with around 1:1

Can you explain this further? What do you mean when saying one or the other is "constant"?

Thanks

space_weazel wrote:Can you explain this further? What do you mean when saying one or the other is "constant"?

Yeah Fnord, what on earth are you on about

space_weazel wrote:

Fnord wrote: I think at agreed upon C:B ratios are:
0.7:1 for the most power when the chamber is constant
~1.3:1 for the most power when the barrel is constant.

But if nothing else, you can't go wrong with around 1:1

Can you explain this further? What do you mean when saying one or the other is "constant"?

Thanks

He's saying that if you have a fixed barrel length, eg. it must be 3' long, then a C:B ratio of 1.3:1 will roughly yield maximum power before diminishing returns.

And if the chamber volume is fixed, but the barrel length can be changed, then a C:B ratio of 0.7:1 will give the best results

I don't see that making sense, why should they be different?

I don't see that making sense, why should they be different?

Come on Jack, you of all people should know that sarcasm isn't always obvious in text. You had me confused for a second there