Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 46 users online :: 4 registered, 0 hidden and 42 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Ammo

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
Sponsored 
  • Author
    Message

Ammo

Unread postAuthor: Scotty » Mon Jul 30, 2007 4:49 am

I have been moulding my own rounds using a lead nose and polyurethane.
I would like to hear anyones suggestions as to the best shape for accuracy.
The size of the rounds vary from 12.7mm to 18mm and are usually 15 to 25 mm long respectively. Cheers.

Peace
Scotty
  • 0


Scotty
Staff Sergeant
Staff Sergeant
 
Posts: 114
Joined: Tue Jan 09, 2007 3:00 am
Reputation: 0

Unread postAuthor: VH_man » Mon Jul 30, 2007 5:51 am

make them shaped like darts.
  • 0

User avatar
VH_man
Major General
Major General
 
Posts: 1827
Joined: Sat Dec 09, 2006 6:00 pm
Location: New Hampshire
Reputation: 1

Unread postAuthor: Scotty » Mon Jul 30, 2007 6:05 am

Easier said than done, however it iz a good idea, i shall try it.

Peace
Scotty
  • 0


Scotty
Staff Sergeant
Staff Sergeant
 
Posts: 114
Joined: Tue Jan 09, 2007 3:00 am
Reputation: 0

Unread postAuthor: jackssmirkingrevenge » Mon Jul 30, 2007 8:57 am

since you're not spinning your rounds, you need them to be drag stabilised, either using fins or, easier option, with a hollow tail. What calibre is your launcher? For a 3/4" caliber I found that (aside from spherical projectiles) these were the best option: slugs for a 3/4" barrel - takes a bit of work to make one but they're fairly sturdy and usually re-usable, depends what you're shooting at.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Unread postAuthor: jimmy101 » Mon Jul 30, 2007 6:20 pm

Like JSR said, hollow tail.

A solid but light tail is the same as a hollow tail, as long as the density of the tail is much less than the density of the nose.

A lead nose with a piece of polyurethane foam rod of the same diameter should be stable. Figure the length of the low density rod as about 3x the length of the lead nose. Wood is enough lighter than lead that it would probably work as well. (The basic aerodynamics of a bottle rocket.)

Easy enough to figure out if a simple bullet shape will be aerodynamically stable or not. Put one together and find the front to back balance point. (This is the front to back center of mass of the projectile). Trace the outline on a piece of cardboard and cut it out. Find the front to back balance point of the piece of cardboard. (This is the approximate center of drag/pressure/thrust of the projectile.)

For the round to be stable the balance point of the actual round should be ahead of the balance point of the piece of cardboard by at least the diameter of the round. (Or is it at least 2x the diameter of the round?)
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3128
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Unread postAuthor: Scotty » Tue Jul 31, 2007 4:14 am

Jimmy that what i thought too kinda- however the hollow tail would improve muzzle velocity due to the lighter weight round, thanks for that idea Jack (i actually had the same thought today, about making a top plate with pegs that are aligned with the centre of the round, i make like 15-20 at a time, jus wasn't sure how that idea would "fly")

Off topic banter- I have a S/S Hybrid that's about 600ml ( it takes 22cc Butane for normal combustion, someone can figure out how big it is for me :wink: 3.12% Butane is optimal!) Anyhoo it had a leak, i fixed, Loaded polynut round, injected 44cc Butane, Put 15psi into her ( i know that means she is running a bit rich maybe) Aimed at 5 gallon metal drum approx 4-5 Metres away, Clicked igniter, Witnessed instant holes in both sides of drum... It didn't slow down... Hit tarp 20 metres behind target a near flat trajectory, Hit and went through 1/2 inch thick plywood box that our panel saw is stored in, Thats it, i have to open the box tommorrow and see what damage it did to the saw!....

Peace
Scotty
  • 0


Scotty
Staff Sergeant
Staff Sergeant
 
Posts: 114
Joined: Tue Jan 09, 2007 3:00 am
Reputation: 0

Unread postAuthor: jackssmirkingrevenge » Tue Jul 31, 2007 4:29 am

think shotgun slugs:

Image

The "air rifling" doesn't have any appreciable effect, what keeps the slug stable when fired from a smoothbore barrel is the lightweight plastic or fibre tail attached to the base.

One advantage of using a hollow tail is that the propellant gasses tend to "inflate" it, sealing it against the barrel and preventing any blowby.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Sponsored

Sponsor
 


Unread postAuthor: Scotty » Tue Jul 31, 2007 4:50 am

Thankyou very much, that info will help me lots i think.

Peace
Scotty

P.s. Jack says hi!
  • 0


Scotty
Staff Sergeant
Staff Sergeant
 
Posts: 114
Joined: Tue Jan 09, 2007 3:00 am
Reputation: 0

Unread postAuthor: jackssmirkingrevenge » Tue Jul 31, 2007 4:54 am

woof! :D

lol
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Return to Combustion Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot]

Reputation System ©'