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Combustion performance help

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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Combustion performance help

Unread postAuthor: Bokonon » Thu Mar 08, 2012 11:05 pm

So Im from the states and live on a small island in the middle of the Taiwan straight where it is illegal to own or possess a firearm. I made a spud gun with a 3.5 inch combustion chamber that is about 16 inch long and reduces to a small pvc barrel that shoots standard marbles. The problem is that here on the island I could not get a screw cap so I had to glue an end-cap on the stock of the combustion chamber so the air never really cycles. I use a small fan but it takes about ten minutes. Im spraying butane into the chamber by way of the handle. I cannot locate any oxygen on the island so what can I do to remedy the rich fuel ratio. Is there anything else I can use. Do you think a bigger bore and bigger projectiles would achieve deeper penetration into a Styrofoam block? Right now the marble goes in about a foot but I would like to get two feet of penetration.
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Last edited by Bokonon on Thu Mar 08, 2012 11:34 pm, edited 1 time in total.

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Unread postAuthor: jackssmirkingrevenge » Thu Mar 08, 2012 11:11 pm

Welcome to spudfiles :)

Noble intentions but the forum rules are pretty clear on this one:

Discussion of shooting anything at any person and/or animal is prohibited, and may result in a permanent banning of your account.

I suggest you edit your post title in order to avoid this thread being locked.
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Unread postAuthor: Lockednloaded » Thu Mar 08, 2012 11:46 pm

Good edit, now you can get some help solving your problem.

If you insist on using a combustion, which I personally wouldn't recommend for the goal you're tring to achieve, you shoul invest in a syringe for metering your butane, and a ball valve for venting your chamber.
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Unread postAuthor: Bokonon » Thu Mar 08, 2012 11:57 pm

I live on a 3 mile island and to say its space would be an understatement. The closest real hardware store is either a plan ride or 6 hour boat shuttle. I was hoping to find some oxygen but it is proving more difficult than I thought.
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Unread postAuthor: kydavies » Fri Mar 09, 2012 12:07 am

hydrolysis.... only requires electricity (batterys) and water.
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mikemurph wrote:yea, the ammo has been changed to a more aero dynamic projectile. it consists of the back half of a dart, and a hot glue ball
AHahahahah!!
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Unread postAuthor: Zeus » Fri Mar 09, 2012 12:13 am

All I can say on the topic.

Inb4 lock/deletion.
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/sarcasm, /hyperbole
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Unread postAuthor: Lockednloaded » Fri Mar 09, 2012 12:17 am

Also, a larger bore barrel with sabot'd marbles would give much better performance for your chamber
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 09, 2012 12:33 am

Lockednloaded wrote:Good edit, now you can get some help solving your problem.


Still needs to change the title and remove the "styrofoam block" picture.

Some practical advice:

- as LnL suggested, you need to meter your fuel accurately, and a syringe is a great way to do it:

http://www.spudfiles.com/forums/how-to- ... 22301.html

- forget marble barrel, go big bore. If you have access to 3.5" pipe, use that as both your barrel and chamber. The launcher used in the video I linked to above uses that very concept, all you have to do is make it as long as possible. If you have say a 1 metre long pipe, put the projectile base around 35cm from the end of the chamber.

- since accuracy is not going to be terribly good, a shotgun round is your best bet. A soup can wrapped in tape will make a decent light strong sabot which you can fill with projectiles. Marbles are quite poor projectiles due to the relatively low density of glass, if you can get your hands on say 3/8" ball bearings you have a much better chance of penetrating that block. A relatively small number of projectiles (say 9-12) will give you light enough total weight to get good velocity.

edit:

Image wrote:Also, a larger bore barrel with sabot'd marbles would give much better performance for your chamber


:D
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Unread postAuthor: Bokonon » Fri Mar 09, 2012 12:50 am

thanks alot for the idea. I will make one like you suggest. I just figured if I kept a big chamber, reduced the barrel size it would shoot better but it doesn't seem to be the case.
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Unread postAuthor: MrCrowley » Fri Mar 09, 2012 1:35 am

If firearms are illegal in your area, dare I say combustion cannons probably are as well. As for the original topic of this thread (that has now been removed), without a lathe or mill you will probably never make a cannon humane enough for your purposes. Not only are marbles a ridiculous projectile for such a use, but a spray and pray is pretty bad as well.

I suggest buying a bow and arrow.
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 09, 2012 2:16 am

MrCrowley wrote:If firearms are illegal in your area, dare I say combustion cannons probably are as well.


Fair point, and a big bore combustions is not exactly "low profile", especially on a tiny island, so if attracting attention to yourself is not an option, go for the bow and arrow.

As for the original topic of this thread (that has now been removed), without a lathe or mill you will probably never make a cannon humane enough for your purposes.


I would disagree, your larger hybrids are made from standard fittings and with a proper sabot loaded with buckshot at close (below 50 metres) range would bring a styrofoam block down pretty quickly.

Not only are marbles a ridiculous projectile for such a use...


Agreed, though didn't the nutty Russians supposedly make a wooden gun shooting a spherical glass projectile to get through metal detectors...

a spray and pray is pretty bad as well.


With syringe metered fuel it should work a treat at close range.
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Unread postAuthor: Zeus » Fri Mar 09, 2012 2:32 am

Since the thread's been edited, here's some other ideas.

A 1" bore hybrid used at 10X will have more than enough power, it's a matter of the projectile really. An arrow or crossbow bolt saboted will be adequate, broadheads increase the efficacy further.

If the sabots are made well, it will be accurate to 50m or more, you'd have to test its' effective range against milk bottles before use.

Unless you make a hybrid on the scale of SB's or MrCrowley's, I don't recommend hybrids whatsoever.

No links because I CBA on this phone.
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Unread postAuthor: Bokonon » Fri Mar 09, 2012 2:47 am

I wish I would have joined before I made the cannon. I did practice shooting crossbow bolts out of it with good results. I did notice however the shorter the bolt the more accurate it was to my surprise. I was told that I could get pure oxygen here but have yet to find it. Have you ever used it before?
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 09, 2012 2:57 am

I would stay away from oxygen, it can cause some pretty nasty accidents if not handled properly and unless you have strong enough construction (PVC pipe won't cut it...) it's liable to just blow your launcher up in your face.

Stick with a big bore combustion and carefully metered fuel.
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Last edited by jackssmirkingrevenge on Fri Mar 09, 2012 2:58 am, edited 1 time in total.
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Unread postAuthor: MrCrowley » Fri Mar 09, 2012 2:57 am

jackssmirkingrevenge wrote:I would disagree, your larger hybrids are made from standard fittings and with a proper sabot loaded with buckshot at close (below 50 metres) range would bring a styrofoam block down pretty quickly.
indeed but I mean more in terms of reliability, projectile, accuracy and sabot.
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