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

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.

McFear · Sat Aug 16, 2008 10:05 pm

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

jimmy101 · Sun Aug 17, 2008 3:57 pm

McFear wrote:

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.

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.

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