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How much would it cost to make a....

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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Unread postAuthor: jimmy101 » Sat Aug 16, 2008 10:54 am

McFear wrote:is there a thread for HGDT cause its kinda confusing when not using it for a hybrid. i know it can be done, cause i have had people do it for me, but its kinda confusing to use for the first time.

You should read through the help pages on D_Halls site; http://www.thehalls-in-bfe.com/HGDT/index.html

To model a standard combustion gun set the mix (NX) to 1 and set the burst disk pressure to zero. A WAG for the friction value for a 2" spud is 10 PSI (30 pounds of static friction).

is there anyway of theoretically finding out how much distance i would get with a spud if i know the distance that a tennis ball went.

tennis ball (67g) went 180m at 5 degrees to the horizontal (video footage on youtube, search for scab1991 and watch) if i were to make a slug thats maybe 150 or 300 grams, is there anyway to hypothesise how far it would go.

It's kind of tricky to model the ballistics of a spud. Nobody knows what the coefficient of drag is and the spud tumbles so the drag and sectional density changes during flight. A tennis ball is much easier to model. I suspect that if you use D_hall's ballistic calculator you'll probably have to use a Cd of about 1.5~2.0. The calculator probably assumes the ammo is either a perfect cylinder or spherical. The distance estimate you get is probably not going to be very accurate. Joanna and ____? were working on a ballistic calculator for spudguns. Not sure what the current status is but I don't hink it has been released.

Someday, someone will finally measure the Cd of spuds from typical barrel sizes.
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Unread postAuthor: McFear » Sat Aug 16, 2008 10:05 pm

Image
Image

the above pics show the perfect shots for my cannons with a tennis ball.
at the moment, the fastest recorded speed i have gotten is 189m/s without a burst disk, this hgt was theorized having a burst disk in it. i am using aerostart and its spray and pray, does this sound right?

ok sorry for hijacking thread
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Unread postAuthor: jimmy101 » Sun Aug 17, 2008 3:57 pm

McFear wrote:Image
Image

the above pics show the perfect shots for my cannons with a tennis ball.
at the moment, the fastest recorded speed i have gotten is 189m/s without a burst disk, this hgt was theorized having a burst disk in it. i am using aerostart and its spray and pray, does this sound right?

ok sorry for hijacking thread

The second HGDT window gives a velocity of 209m/s. You're getting 189m/s. Sounds pretty close to me. Do you actually have three spark gaps? BTW, you told HGDT that there wasn't a burst disk (disk failure=0psi) but there was a constriction at the chamber to barrel junction (disc diam=5cm). Normally, you set the "diac diam" to either the chamber or the barrel ID.
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Unread postAuthor: McFear » Tue Aug 19, 2008 3:40 am

oh, is that what the 3 means, no i only got one spark gap. i thought that if i said that the burst disk failed at 0 then the diameter of it wouldnt matter. o well i will go back and change it again and see what the new data is.
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Unread postAuthor: AgiTator » Wed Sep 17, 2008 10:35 pm

I'm in the process of measuring how far my spuds go.. 2" x 6' barrel powered by a 4x16" chamber... I nearly lose sight of 'em. I do know were they're landing but it's in the water. I have to do a distance measuring between where they land and where I stand while shooting. it seems like a hell of along way away, but it may be only 100 yards. It's so hard to tell.

Good luck with your project. If you must use combustion, make a big chamber and a long barrel. Dual ignition and a fan will help, as others have said. I'll be interested to hear how this thing turns-out.
I'd be very surprised if you can get up to 350 yards. Like others have said... drag is the enemy. No matter how fast it exits the gun, it slows down a lot in the air.

If you can't afford a fan, the pvc supplies should only cost $35 if you salvage the barbecue sparker from the dump (off a grill). $35 may sound low but I bought all the pressure rated stuff and the cement/primer for that price.

Good luck!

~Tator
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