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Smaller bore combustion launchers

A place to ask general spud cannon related questions.
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Smaller bore combustion launchers

Unread postAuthor: niglch » Mon Aug 10, 2009 1:05 am

Back when I was a complete newcomer to spudding, one of the first differences I noticed between typical combustion launchers compared to pneumatic launchers was size. Combustions are usually pretty big and cannon-like while pneumatics tend to be smaller, less bulky, and more gun/rifle-like. Of course, there are advantages to not being bulky. For example, my pneumatic is lighter and much easier to aim than any of the larger combustion cannons I have built. This makes it much better when I want to try shooting targets from more than a few yards away.

The disadvantage is that I am stuck with a bike pump to pressurize the thing. I live in a very residential area, so using my back yard as a firing range isn't a good idea. I usually go down the road to an abandoned dump for all my spudding, but there is obviously no place to plug in a compressor which means I have to pump. It's a great workout, but it really gets to you after 10-15 shots...

Anyway, I started wondering how a similarly constructed combustion would compare to my pneumatic as far as performance. So, out of curiosity, I did a quick estimated comparison using HGDT and GGDT. I figured the pneumatic would cream the combustion @ 120 psi (barrel sealing piston valve), but I was wrong. The muzzle velocities turned out to be about the same. I used 120 psi as a reference since my gun is made from PVC and that probably about the max pressure I consider safe.

I have only seen a few smaller size combustions in the showcase. However, elitesniper's advanced extreme small combustion seems to perform very well and similar to what I had in mind. Of course, simulations don't always turn out to match the real thing. Has anyone else looked at this?
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Re: Smaller bore combustion launchers

Unread postAuthor: jackssmirkingrevenge » Mon Aug 10, 2009 2:01 am

niglch wrote:I have only seen a few smaller size combustions in the showcase. However, elitesniper's advanced extreme small combustion seems to perform very well and similar to what I had in mind. Of course, simulations don't always turn out to match the real thing. Has anyone else looked at this?


Pressure is the big issue. When you go down in calibre, the surface area of the projectile (which the air or gasses act upon to push the projectile) goes down exponentially, ie if you go from 1" to 0.5", the area has gone down to 25% of the original. This means that to provide the same force, pressure has to increase by 4 times.

With combustions, you're stuck at pretty much the same pressure so by definition, the smaller you go, the less powerful it will be. Pneumatics on the other hand give you the option of upping the pressure, for example my pengun type launchers would be absolute manure as a combustion, but a small chamber at 400 psi gives them enough power to be worthwhile.

Another thing, the pneumatic you modelled can afford some optimisation, for example the pilot volume should be below 2 cubic inches, the pilot valve is very restricted and in a piston valve you would expect dead volume to be zero.
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Unread postAuthor: jimmy101 » Mon Aug 10, 2009 11:33 am

A well made combustion, especially if it is the optimal combustion size range, will usually outperform a poorly made pneumatic of similar dimensions. For example a pneumatic with a ball valve usualy isn't all that much better than a combustion, and often isn't even as good. If the pneumatic is pressurized with a typical shop compressor then that ~120 PSI limits the performance to about the same as a combustion.

The big advantage of pneum'os is the ability to go to much higher pressures than the ~120 PSIG limit of 1X combustions.
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Unread postAuthor: Moonbogg » Mon Aug 10, 2009 12:31 pm

I feel your pain. I don't know if this is an option for you, but consider using aluminum and making a low mix hybrid. Around 3x range will give you plenty of power even from a smaller cannon and will outperform a full sized advanced combustion. If you hate pumping use oxygen from a disposable welding tank to get your levels up.
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Unread postAuthor: theBOOM » Mon Aug 10, 2009 12:54 pm

I agree with moonbogg.. or if you build your chamber out of metal you could use hydrogen as fuel... which would generate more pressure than mapp or propane thus giving you an increase in power...I'd just build a 2x or 3x hybrid if you want more power out of combustion type launchers.
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Unread postAuthor: Moonbogg » Mon Aug 10, 2009 1:30 pm

Exactly like Boom said. Its like a combustion on steroids. Not really a hybrid like most are (small with high mixes).
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Unread postAuthor: Technician1002 » Mon Aug 10, 2009 2:44 pm

Using oxygen to enrich the mix is not recommended. Ignition temperatures drop, Non flammables become combustible and flammables become explosive.

Safety first.

As far as the side by side comparison shown, well done. Chambers are most often not insulated. Many combustions are not optimum. In a side by side with a combustion (spray and pray) of the same size as my ABS pneumatic, we were pretty evenly matched where I was running about 35-40 PSI. At higher pressure pneumatics most often beat spray and pray combustions.
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Unread postAuthor: niglch » Thu Aug 13, 2009 12:31 am

Thanks for the helpful replies. Just as a general question, would it be safe to construct a relatively small ultra-low mix hybrid (no more than maybe 1.5x air) out of sch80 PVC? Perhaps something like a 2" diameter chamber. According to McMaster, this is rated at 400psi and the theoretical max pressure (w/ closed chamber) is approximately 160psi.
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Unread postAuthor: Moonbogg » Thu Aug 13, 2009 12:55 am

People have done it without issue, however, others have done it WITH issue. I believe it has become a common rule to simply not use plastics for hybrid use, especially if hand held, regardless of the mix. It seems the higher shock loads will eventually weaken the plastic and cause it to break, sometimes quickly, sometimes slowly. PVC is also prone to cracking when dropped or banged and you may not notice the crack until the chamber blows apart from a 1.5-2x shot. We'll see what others have to say.
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Unread postAuthor: skyjive » Thu Aug 13, 2009 1:06 am

Why not just get a CO2 setup? This will allow you to fire a pneumatic effortlessly at high pressures, and it's portable.
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Unread postAuthor: niglch » Thu Aug 13, 2009 3:13 pm

Thanks Moonbogg. I should have caught that detail in the wiki. No sense in making exploding guns. I would love to be able to build an aluminum cannon like you did with the cobra venom, but I'm afraid I don't have the money or metalworking tools and skills (wish I did).

CO2 does sound like a good idea though. It's all stuff I can check out next year if/when I build another cannon.
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