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Need help with SMAW styled launcher

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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Unread postAuthor: Berkut » Fri Jul 08, 2011 11:02 pm

To inlet the brass tubes, you drilled out a hole and pushed the tubes in, right? and how big are those tubes? also, I plan to use a 2" union, so do you think running the wire straight across would allow a reliable, uniform rupture?

I would like to thank you guys for all of your help. I am going to go get parts next weekend, and do as much work as I can before I go on vacation.
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 09, 2011 12:36 am

Berkut wrote:To inlet the brass tubes, you drilled out a hole and pushed the tubes in, right? and how big are those tubes?


I actually cast the epoxy around them. In your case it shouldn't be necessary as you'll be using cartridges. Make sure the nichrome wire isn't touching the PVC though, and your finger on the trigger switch is very light.

also, I plan to use a 2" union, so do you think running the wire straight across would allow a reliable, uniform rupture?


Yes, you should get enough flow for your purposes.
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Unread postAuthor: Berkut » Sun Jul 10, 2011 12:32 am

Do you mean it wouldn't be necessary to cast the epoxy, or it wouldn't be necessary to inlet the brass tubes?
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 5:44 am

You're going to have to figure out a way to have the wire not touch any of the PVC parts, otherwise it will melt through them and compromise sealing for the next shot.
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Unread postAuthor: JDP12 » Sun Jul 10, 2011 4:10 pm

JSR- what sort of battery can you use to heat that wire- what sort of voltage is required? This may have to lead to another prototype
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Unread postAuthor: saefroch » Sun Jul 10, 2011 9:25 pm

Right here. Mylar melts at about 254C
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 10:10 pm

saefroch wrote:Right here. Mylar melts at about 254C


Good find!

I was using a single 9V battery so not much.
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Unread postAuthor: JDP12 » Sun Jul 10, 2011 10:19 pm

god I love the internet..

Thanks for the link
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Unread postAuthor: saefroch » Mon Jul 11, 2011 12:18 pm

Credit for the link goes to jimmy101, from back on this thread.
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Unread postAuthor: Berkut » Wed Jul 13, 2011 12:15 am

Here is my idea for the triggered burst disk. The Nichrome wire will be held to the mylar disk with duct tape, and the wire will be connected to the rest of the circuit using quick disconnects. The disconnects give a more rigid structure for easier connection.
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Attachments
Untitled.jpg
yellow boxes are the disconnects
red is the nichrome
light grey is mylar
dark grey is duct tape
blue is insulated wire
black is pvc

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Unread postAuthor: 3VIL G3NIUS » Wed Jul 13, 2011 4:16 am

JDP12 wrote:JSR- what sort of battery can you use to heat that wire- what sort of voltage is required? This may have to lead to another prototype

I'd be more worried about the current it draws. The nichrome wire I have is 13Ω/m so for a 10cm length at 9v it would be attempting to draw ~6.9A. Don't expect the battery to last long. And as jack said, be very light on the trigger, you're pretty much shorting the battery.
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Unread postAuthor: saefroch » Wed Jul 13, 2011 8:11 am

3VIL G3NIUS wrote:I'd be more worried about the current it draws. The nichrome wire I have is 13Ω/m so for a 10cm length at 9v it would be attempting to draw ~6.9A. Don't expect the battery to last long. And as jack said, be very light on the trigger, you're pretty much shorting the battery.
That resistance is at what temperature? The temperature coefficient becomes very significant, because nichrome heats up quickly, and not much charge actually needs to be passed through it until it heats up enough to trigger the valve.

About that picture: Leads inside the barrel? I'd be concerned they'd be torn off during a shot...
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Unread postAuthor: Berkut » Wed Jul 13, 2011 8:34 am

I will primarily firing those finned nerf howler footballs, as per my area's AT weapon regulations. The leads will only go an inch or two before passing through the barrel walls, and the rocket fins are about five inches long. The leads will also be secured to the inside of the barrel using some type of adhesive, maybe some of that liquid nails stuff.
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Unread postAuthor: noname » Wed Jul 13, 2011 1:16 pm

Cool project. Are there any velocity/range restrictions? I would think that anywhere above 200-250 fps would be pretty dangerous in the case of shooting an unintended target (someone) at unintended range (close). Those nerf footballs are capable of breaking ribs, I'm sure.

Either way, if you can build it to meet the restrictions it shouldn't be too difficult to get the performance you want.

I think something along the lines of 2-3 seperate pressurized chambers would work sufficiently. A 3" PVC chamber of whatever length, a foot or two probably, with a coupler and bushing down to a 2" union would provide enough oomph, while being easy to slide into the gun's casing and simply screw in, putting the burst disc in contact with the nichrome wire in the barrel, if you get what I'm saying. If not I'll attempt to draw something up with Paint.
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Unread postAuthor: Berkut » Wed Jul 13, 2011 2:07 pm

I sent a pm to one of the area's admins, and have yet to receive a response, but changing the muzzle velocity to suit regulations shouldn't be hard, because I could increase or decrease the pressure when I refill the casing.

I think I see what you are getting at, but I want to put some of that expanding foam insulation between the inner and outer tubes.
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