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something..... abit different

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something..... abit different

Unread postAuthor: jojofelix » Wed Mar 04, 2015 5:27 pm

i have been invited to an event that has a contest to see who can launch a 2 liter bottle the farthest. i am curious if anyone has any thought s on a design, and method of acheiving this with compressed air, or even cumbustable propulsion. i work in a metal shop and have access to tools, machines and materials to work with steel. this seems like a possible "upgrade" to typical pvc designs for the extra pressure needed for the mass of a full 2 liter. ANY constructive feedback is very appreciated. i want to blow some minds!
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Re: something..... abit different

Unread postAuthor: jrrdw » Wed Mar 04, 2015 6:18 pm

Ever watch Pumkin Chunkin?
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When life gives you lemons...throw them back they suck!
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Re: something..... abit different

Unread postAuthor: DYI » Wed Mar 04, 2015 6:48 pm

What are the restrictions on attaching things to the bottle to make it more stable in flight? Or is that against the spirit of the contest? :lol:

Regardless, the best way to go about this is probably going to be a low mix hybrid, oxy/fuel combustion, or burst disk pneumatic, depending on how much base pressure you find the bottle to be able to resist. Slightly over 200psi initial base pressure is possible with oxygen/fuel mixes starting at atmospheric pressure. I doubt that the bottle will survive a high enough base pressure to make a metal barrel worthwhile - I've used 3" SCH40 solid wall ABS barrels successfully up to about 400psi on pneumatic burst disk cannons.

Some quick modelling in GGDT tells me that, with a 12' long barrel, a very large (8" x 72" long) chamber, and a reasonable maximum base pressure value of 150psi, you won't be getting much over 450ft/s with a 2kg, 4" diameter projectile. Depending on the drag coefficient (an unstabilized bottle will be tumbling), this will result in a range between 1700 and 2500 feet.

You may be able to do a bit better with a combustion by using the reaction kinetics to maintain a higher ratio of average base pressure to maximum barrel pressure, but that becomes a complicated design problem.

Obviously if you're willing to go to longer barrels and larger chambers higher performance is achievable. Use GGDT (freely available, easy to use) to predict performance and range to decide on the launcher's parameters, if you decide to go with a pneumatic. Otherwise use HGDT, and GasEq for gas phase combustion calculations if necessary.
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Re: something..... abit different

Unread postAuthor: Brian the brain » Thu Mar 05, 2015 1:03 pm

What are the rules?

Does the bottle have to be a plastic one?
Or could it be made out of metal?

If not I suggest looking up how to build a pulse-jet...

Should go until it runs out of fuel...

A far simpler method would be to use a helium balloon to get it up into the clouds...
Not what they expect but I'm sure you can't loose..
All depends on what is allowed and what is not...
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Re: something..... abit different

Unread postAuthor: jojofelix » Tue Mar 17, 2015 5:26 am

well.... those sound very good... in theory, but i'm not independantly wealthy. as its an outdoor contest, in the middle of a music festival, i'm most limited by how much i can lug in. i have a smaller air comp that is capable of getting to 120 psi. was researching piston valve mechanism cannons. can only have up to 8 ft sections of the final build(my vehicle only has a 6 ft long roof)thought maybe having two 6inch dia reseviors at 6 ftlengths and a 8 ft barrel is as big as i can go. i have no idea what range i can expect from such a rig, or where to look to find out. i'm expecting the vast majority of my compition will bring elastic/springloaded slingshot/catapaults, and i think that 500 ft is probably as far as the average hippy attending the show is gonna produce(if the challenge were waterbong crafting i'd be well beaten :wink: ), if that helps to decide if this rig would be worthwhile. also, the bottle, with an aluminum sleeve(work at a metal fab shop) for the top/tapered end with 4 fins would add appr 1.5 lbs and create a resonable amount of stability to the bottle. it is howeevr 4 1/4 inches dia plus the 1/8th inch dia minimum for the sleave, meaning that 4" sched40 wouldnt be big enough and 6 is WAY too big. is there a place online to get 4 1/2" sch40" the alt would be to fashion a barrel of steel(14 gauge?) with a flange in the exchange end to keep it secured to the pvc body. again ANY help is appreciated as i'm a complete beginner to this.
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Re: something..... abit different

Unread postAuthor: DYI » Tue Mar 17, 2015 5:22 pm

i have no idea what range i can expect from such a rig, or where to look to find out.


DYI wrote:Use GGDT (freely available, easy to use) to predict performance and range to decide on the launcher's parameters, if you decide to go with a pneumatic.


As I mentioned soon after you asked the question initially, GGDT will provide good performance estimates for a low pressure pneumatic launcher like this one.

Your plan for the launcher (8' long barrel, two 6" diameter 6' long chambers) sounds fine, but building a piston valve can be finicky. Unless you're wanting highly repeatable performance of fast reloading, a burst disk valve would be much easier, seeing as how it requires only a flange, some sheet metal or aluminum foil (probably around 32 layers for a 4" union at 120psi) discs, and suitable gaskets. Also, performance will be higher if the system is inline when assembled. Doesn't mean it needs to be inline for transport.

Projectile stabilization is most easily achieved here by adding a rigid tail to the bottle. The tail can be PVC pipe with 4 slots cut in end, two PVC sheets that fit together to produce 4 fins when inserted into the slots, then a cap on the end of the pipe to hold the fins on. Securely attaching it to the bottle is left as an exercise to the student.

In all honesty, with no stabilization at all, you'll still win. It'll also give your competitors less to gripe about.
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