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Combustion or Air? +Intro

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Combustion or Air? +Intro

Unread postAuthor: loganc58 » Tue Mar 22, 2011 10:37 pm

Hello,
I have always been a long time forum lurking among several boards. This past weekend I was browsing around and found this pretty cool site and community.
I then took a stab at building my very own combustion (hairspray) spud launcher. After finishing it, I and a few other friends have been firing it almost everyday since then. It is something different to pass the time than the normal sports or video games that have become rather dull over the years.
So this cannon that i built was the basic hairspray gun with 3" x 2' chamber and a 2" x 4' barrel. The farthest it has shoot is around 175-200 yards.

My friends and I have now the longing for something that will shoot a potato farther.

I am trying to decide if i should just add a propane meter to the cannon I have now, or make a pneumatic cannon. The reason why this is posted in the pneumatic section is because i am leaning greatly towards the pneumatic side.
My thoughts and ideas on the matter is to build a compressed air cannon, with a modified 1" or 3/4" sprinkler valve.

I was just wondering which one would be a good 'next step' in spud gun making, and which route i should take.

Also, we have been looking into getting the SDR-21 for a golf ball cannon. This would need the pneumatic fuel source I would think. How far would one be able to launch a golf ball with around a 4" x 3'(or 4') air chamber.

Thank you for any comments,
again I basically am a forum and a spud gun newbie. But not a complete idiot, so I am capable of using a brain and problem solving so keep that in mind when it comes to suggestions. :wink:
Thanks, Logan.
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Unread postAuthor: Lockednloaded » Tue Mar 22, 2011 10:47 pm

Always good to see new members with their heads on straight, you've certainly come to the right place.

With combustion cannons with similar chamber dimensions as your, people tend to use 1.5" chambers to make a 1:1 Chamber to Barrel ratio. The best improvement to any spray and pray is a CPU fan in the chamber, they mix fuel and add loads of power to your cannon. Only after the adition of a fan would I consider a propane meter, but it can be done cheaply and simply

I'm more of a pneumatics guy, but when you're going for large bore, low price, and simplicity... there's not much to do short of making a large piston valve and getting an air compressor (Bike pumps work if you're willing to put the pain in for the gain :wink: )

Welcome to the forums!
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Unread postAuthor: ilovefire » Tue Mar 22, 2011 10:49 pm

make a piston valve piloted be the sprinkler valve, you will get much better performance, something like this http://www.spudfiles.com/forums/2-pneum ... 20224.html
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why make it if it dosent shoot?
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 22, 2011 11:04 pm

Welcome to spudfiles :D

For big bores, unless you have a substantial budget and an automated portable filling source, I would steer away from pneumatics.

As Lockednloaded suggested, a combustion cannon can be made more cheaply and simply with better performance than a similarly sized pneumatic using shop pressures. If you browse through the finished cannon section, there are plently of good examples of the features that will give you the performance you're after:

- proper fuel metering
- good chamber:barrel ratio
- mixing fan
- decent ignition

Have a look at the Image for some general pointers.

Another thing, if you want to shoot further, consider that potatoes are just about the worst possible projectile shape and density imaginable. You can get a lot more range optimising your projectile than your launcher, something which most people neglect.
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Unread postAuthor: loganc58 » Tue Mar 22, 2011 11:08 pm

Thanks for the replies, guys.

Honestly, I am kinda having a difficult time deciding what all to build, with the resources/money that is available. I often wonder if any body else had a certain kind of 'rush' to want and try to build all the different kinds of cannons.

Cost is certainly an issue when deciding since being seventeen has a limit on play around money.

But thanks for the suggestion for the fan and also the links. I will look into getting an old fan and rigging it in there.
How far does an average 'spray and pray' usually shoot a potato?
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 22, 2011 11:13 pm

loganc58 wrote:How far does an average 'spray and pray' usually shoot a potato?


175-200 yards is about as good as you can hope for.

Fuel metering really does make a tremendous difference.

Given your chamber volume you'll be fine with roughly 100mL of propane or butane, if you can get your hands on a 50mL syringe that's 2 easy squirts.

Advice on syringe filling and metering here ;)
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Unread postAuthor: Technician1002 » Wed Mar 23, 2011 1:13 am

I had the opportunity to shoot apples next to a spray and pray fueled with Propane. The combustion was very close to the size of your launcher. I was using my ABS cannon. We were very closely matched in performance when I was using 30~35 PSI. Even though the combustion sounded powerful compared to the air cannon, the distance told the truth. When I cranked up to ~65 PSI it was game over.

I have not had the opportunity to shoot next to an advanced combustion of the same size with chamber fan, meter, etc, but I suspect I would need to go higher in pressure than 65 PSI to keep up with one based on chrony shots shared here. I would need to run 85-110 PSI to match an advanced combustion (not hybrid) of about the same size.

See my SIG for info on the 2inch barrel air cannon.

Don't try feeding a 2 inch barrel with a 3/4 inch sprinkler valve.. Not enough flow for serious range. A 2 inch ball valve is much better.

Just for fun, I have a 2.5 inch barrel for one of my small 1 inch valve launchers. A big chamber and valve works much better.
Image
OK for lobbing tennis balls and t shirts. Range ~200 feet.
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Unread postAuthor: saefroch » Wed Mar 23, 2011 2:14 pm

I don't think there should be any doubt to anyone that because of the scalability of a compressed air launcher, a pneumatic design can comfortably beat an atmospheric combustion. However, I think it may be appropriate to go for an atmospheric combustion system for very large bores if an easy compressed air source is not available.

But I do think you should listen to Tech, because he has demonstrated the effectiveness of large-bore pneumatics with exceedingly fast valves at low pressure, which is actually practical to fill from a bike pump. He pretty much demonstrated he can beat an atmospheric combustion with no more pressure than you'd put into your bike tires.
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