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Another New Member Introduction and Question

A place to ask general spud cannon related questions.
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Another New Member Introduction and Question

Unread postAuthor: yoshi » Sat Sep 24, 2016 1:50 pm

New to spuds and caught the velocity bug. Anyways, a few questions:

Are differences between air and hybrid significant? Could I launch a >1 lb spud at supersonic (1116fps) with air given a $300 budget? Or do I need to hybrid for anything heavy supersonic?

Still browsing the guides section; any recommendations on supersonic spud cannon design and specific brands/methods for tanks/ignition greatly appreciated.

I have all the manual machining and probably scrap available, though I'd prefer not to work steel too much...
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Re: Another New Member Introduction and Question

Unread postAuthor: jakethebeast » Mon Sep 26, 2016 6:39 am

The problem with supersonic spuds is the strenght of the projectile. You'd need so large acceleration forces that a simple spud without a decent wadding would never stay in one piece. With a given 300$ budged you'd easily build a hybrid launcher capable of launching a given projectile supersonic.

The basic difference between pneumatic and hybrid is quite small. Though you achieve the pressure quite differently. Basically a hybrid and a burst disk pneumatic launcher work similarly if the maximum pressure of the burst disk is similar. To put it simply, if you have a 1 liter chamber and a 1" diameter barrel, divided by a burst disk that bursts at 1000psi, it doesn't really matter how you get the 1000psi behind it
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Re: Another New Member Introduction and Question

Unread postAuthor: abukhansa » Wed Sep 28, 2016 10:16 am

Hi I'm new to potatoe launchers

Have made one and need assistance

Chamber = 3.28 ' long x 3" diameter

What would be ideal barrel length to use 10oz projectile at over 300 yards ???
Trying to post a pic but not allowing me to
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Re: Another New Member Introduction and Question

Unread postAuthor: yoshi » Tue Nov 01, 2016 5:53 pm

Well, I found a simple online calculator to get barrel length, though there's definitely better programs out there:
http://www.ken-zoe.net/Potato/
3' combustion chamber is nuts! Are you making a howitzer???

Just finished drilling and tapping holes into a 12" x 2" chamber, 48" x 1.5" barrel spud gun, very similar to Pete's "Easy Peasy Hybrid". Hoping to finish it this weekend!
Image

Where the hoopla do you test this stuff, especially on the East Coast? Are beaches cool with this? Living near Boston. Wouldn't mind getting bunch of cinder blocks for a hard stop.
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Re: Another New Member Introduction and Question

Unread postAuthor: D_Hall » Mon May 29, 2017 6:01 pm

jakethebeast wrote:The basic difference between pneumatic and hybrid is quite small. Though you achieve the pressure quite differently. Basically a hybrid and a burst disk pneumatic launcher work similarly if the maximum pressure of the burst disk is similar. To put it simply, if you have a 1 liter chamber and a 1" diameter barrel, divided by a burst disk that bursts at 1000psi, it doesn't really matter how you get the 1000psi behind it


Only true if your goal is a safely subsonic projectile. Once you start going supersonic the "ease" at which a gas flows becomes very important. Assuming similar chemistry, hot gases (such as those found in a hybrid) flow much more quickly than cool gasses (such as those found in a simple pneumatic). Or, more to the point.... The gases flowing through a choked orifice are going to be moving at Mach 1 conditions *within that gas*. The speed of sound (aka, Mach 1) for a given gas is roughly = sqrt(G * R * T) where G = ratio of specific heats ("gamma"), R = gas constant (lower molecular weights are better!), and T = temperature. For a better explanation, see https://en.wikipedia.org/wiki/Speed_of_ ... es_and_air
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Simulation geek (GGDT / HGDT) and designer of Vera.
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Re: Another New Member Introduction and Question

Unread postAuthor: Zeus » Mon May 29, 2017 6:22 pm

Good to see new members building hybrids!

Another factor is simply the amount of air you need. Do you want to cart a beefy generator, air compressor or spend all day pumping, or build a hybrid with significantly less demanding air requirements and the potential to exceed the speed of sound very easily.

For determining barrel length, check out D_Hall's sig
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