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Portable Soda Bottle Bazooka UPDATE: NEED HELP

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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Portable Soda Bottle Bazooka UPDATE: NEED HELP

Unread postAuthor: GinsuGuy585 » Sat May 19, 2007 9:09 pm

UPDATE:

I am having a problem with my piston. When I pressurize the gun, the piston works great and holds the pressure. However when I dump air from behind the piston, the air goes backwards past the piston instead of pushing the piston back and fireing.

Anyone got a bright idea on how to seal the piston with common household supplies?


Basic design:
Body: 3" through 4" coax gun, 50" chamber 45" barrel (~150ci)
Piston: 3" endcap with neopene seal
Dump valve: 3/4" QEV (from McMaster)

My only concern is that my c:b ratio is 1:2. GGDT says I can get a 20oz bottle (600g) to about 200fps at 100psi, but has anyone had problems with the piston opening with such a small barrel to chamber footprint? (Area between barrel and chamber wall is 11.77sq in)

Pics taken with camera phone, not the best quality but 2Mp nonetheless
Edit: pics are in reverse order, oops.
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Attachments
M01A0092.JPG
Components used (some not pictured) note the neoprene disk I just cut using titanium sissors =D
M01A0093.JPG
More parts
M01A0094.JPG
Hi-Tech drying rig, and roommate.
The primer will be covered with fittings after final assembaly.
M01A0099b.jpg
Barrel and chamber glued together.
Piston assembly complete.
(had to crop because of filesize)
M01A0100b.jpg
Almost done, just need to assemble the back end (3 parts to the lower left with primer applied)

Cudos to the first person to identify the tallest bottle on the table in the background. =D
Last edited by GinsuGuy585 on Wed May 23, 2007 9:14 pm, edited 2 times in total.

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Unread postAuthor: hi » Sat May 19, 2007 9:24 pm

it will work, but as you said, it isnt going to go very far very fast. it not only the C:B ratio, but also the flow of the valve. im going to assum that its a 3'' barrel? if so then a simple ball valvewud work better than a QEV in this situation. i would say (if you are going for the most speed possible with the skills of a beginner) then say the qevand get a ball valve. it will open slower, but will have much more flow.
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Unread postAuthor: GinsuGuy585 » Sat May 19, 2007 9:43 pm

No, the qev will activate the 3" piston valve. (you posted before I got the pics up)

Edit: I've only just joined this forum, but I'm fairly experienced with making spud chuckers. This was an exercise in large caliber portability.
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Unread postAuthor: iknowmy3tables » Sat May 19, 2007 9:57 pm

yes and yes
but I were you I would put a 4" tee some were in there to increase the chammber size, and I think the deaceleration due to drag will prove unimpresive
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Unread postAuthor: homedepotpro » Sat May 19, 2007 11:02 pm

aren't you not supposed to let the primer dry completely. i like the gun though. are those all pressure rated pipes, i was looking at the price of 3'' pressure rated pvc and it was like 3.50 a foot. how much has this project cost
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Unread postAuthor: Velocity » Sat May 19, 2007 11:10 pm

Do not glue those fittings together yet! First, add a fresh layer of wet PVC primer, and then immediately put the glue onto the still-wet primer layer. Then twist them together, hold for thirty seconds, and you are good. PVC primer should not be used when dry. Its purpose is to soften up the PVC pipe, and if it dries, it has no purpose.
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Unread postAuthor: GinsuGuy585 » Sat May 19, 2007 11:11 pm

Oh well I guess I learned something today, dry primer = bad.

It's cost around $100 probably a little more, all the pipe is pressure rated (gimme a little credit huh?) The worst part was buying 10' of 4" pipe cause lowes doesn't let you buy it by the foot. :x Oh, I had the 3" pipe left over from an earlier build.

I just pressure tested it to 100psi, no leaks. I was worried about the threaded coupling, but teflon tape rocks!

Unfortunately I can't get anywhere neer enough flow to fire it, and I'm going to have to try wraping my piston (duct tape probably) to get a better seal for fireing. The neoprine held against the barrel and that's whats important.
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Attachments
M01A0102.JPG
Here's what it looks like assembled, the back is barely screwed on because I just removed the piston for inspection.

Helmet is there for size comparison.
M01A0103.JPG
This is the unit I am going to use for filling and triggering the QEV.

The guage tells me current chamber pressure.

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Unread postAuthor: mega_swordman » Sun May 20, 2007 2:49 am

Very nice. It looks good. I can't wait to see it with the QEV. Are you going to paint it?
Also, I can't help but to find it funny that everyone generalizes a first poster on the forum as a complete beginner in spudgunning.
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Unread postAuthor: GinsuGuy585 » Sun May 20, 2007 10:03 am

I'm going to paint it eventually, idk what color(s) though. I might turn it into a mock-up of a stinger missile platform. I always wanted to try making a model rocket ignite after being launched from a spud gun. :twisted:





My next project is going to be re-creating an m-72 LAW. It will even feature a sliding barrel like the real one! :idea:
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Unread postAuthor: frocksie » Sun May 20, 2007 10:39 am

Paint it a matte drab green, or a desert sand. Nice job, it is obvious that you put some time into it, and researched it thoroughly.
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Unread postAuthor: hi » Sun May 20, 2007 1:11 pm

GinsuGuy585 wrote:No, the qev will activate the 3" piston valve. (you posted before I got the pics up)

Edit: I've only just joined this forum, but I'm fairly experienced with making spud chuckers. This was an exercise in large caliber portability.


im so sorry, i posted that before you posted pictures. i thought you were asking if it would work or not. im an idiot.
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Unread postAuthor: Pete Zaria » Sun May 20, 2007 1:19 pm

GinsuGuy585 wrote:I'm going to have to try wraping my piston (duct tape probably) to get a better seal for fireing. The neoprine held against the barrel and that's whats important.


If you do wrap your piston in tape, make sure to lubricate it. Duct tape doesn't slide on the inside of PVC very well. But it does with some liquid teflon or lithium grease.

Nice design, and pretty darn clean build. I hope it works great with the QEV for you. Let us know how it goes and try to get some damage pics for us :)

Peace,
Pete Zaria.
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Unread postAuthor: GinsuGuy585 » Sun May 20, 2007 1:54 pm

hi: Technically I still don't know if it will work :lol: The small chamber cross section doesn't have a good ratio to the rear (vent) cross section, so I will have to dump the air furiously to make the piston move. (QEV Power!)

Pete: I've been using veggie oil so far (great for the neoprine) it might work with the duct tape too, but will probably get real nasty after a few weeks. I've been debating graphite vs. lithium because graphite is alot easier to get, but may adversely effect the piston's seal with the barrel.


Also since the piston and rear valve assy. is threaded, I can make more barrel/chambers as long as I add male threads. (and have alot of teflon tape on hand)
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Unread postAuthor: iknowmy3tables » Sun May 20, 2007 2:42 pm

instead of duct tape you can use that stretch and seal plumber's tape used for leaky pipe, it's like rubber tape
why would cooking oil make the piston "nasty" over time? (when I question that it just sounds so funny)
and why is dry primer so bad? just ad a new layer right?
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Unread postAuthor: GinsuGuy585 » Sun May 20, 2007 10:12 pm

I'm thinking mold/bacteria growth. As per the dry primer, I was glueing after the primer dried. Won't adversely effect the joints per se but it's not optimal.

I found some aerotech rocket engines that could work with my stinger idea the K250PW engine is only 2.1" in diameter and weighs 1400g, but puts out 100lbs thrust for 3 seconds, which drops over the next 7 sec. Considering a net vehicle weight of 4000g I can fire it at around 75ft/s let it coast a second then get the motor to fire...

Still, just wishful thinking until my qev arrives.

As for more standard ammo, I am going to cut the bottom off 20oz bottles and fill the top with plaster/concrete. This should make it nose heavy and ballistically stable.
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