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 80 users online :: 4 registered, 0 hidden and 76 guests


Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

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

surface calculation? (don't really know what to write here)

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.
Sponsored 
  • Author
    Message

surface calculation? (don't really know what to write here)

Unread postAuthor: SpudFarm » Thu Jun 19, 2008 5:21 pm

so:
my calculations say that the golf ball barrel on larda is 6.92 times as big as the barrel on my marble gun. so i multiplied 5 with 6.92 and got 34.6. that means that the projectile in larda needs to be 34.6g in larda to get the same surface area:weight.

does that mean if i had the same c:b ratio, barrel length, pressure and valve i would have got the same velocities on both the cannons?

correct my crappy math.
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Unread postAuthor: bigbob12345 » Thu Jun 19, 2008 7:07 pm

No it doesnt
There are many other factors that influence the power the cannon can put out. I would say that the velocity of your GB hybrid would be within 100fps of your marble gun(not considering heated gases)if you had those things you listed.

And isnt your marble gun a pneumatic? If it is than the gasses wouldnt be heated so your hybrid would get a faster muzzel velocity, I wouldnt know how much faster but it should be noticeable.

Hopefully that made sense
  • 0

User avatar
bigbob12345
Major General
Major General
 
Posts: 1516
Joined: Sat Dec 01, 2007 9:13 am
Location: Mercer Island,Washington
Reputation: 0

Unread postAuthor: SpudFarm » Thu Jun 19, 2008 7:14 pm

it did, but i am talking about 150psi pneumatic shot
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Unread postAuthor: POLAND_SPUD » Thu Jun 19, 2008 8:21 pm

mass does not increase linearly as you increase the size of a spherical projectile.... (note that I am assuming that you meant 'diameter' when you said surface)


here is a simple example -> a 6mm ball bearing weights about 0.8g but a 12.7mm one weights about 7.5 g...
  • 0

Children are the future

unless we stop them now
User avatar
POLAND_SPUD
Chief of Staff
Chief of Staff
 
Posts: 5405
Joined: Sat Oct 13, 2007 4:43 pm
Country: Israel (il)
Reputation: 10

Unread postAuthor: rp181 » Thu Jun 19, 2008 9:09 pm

Wouldnt the mass go up exponentially. As you add layer after layer, theres more surface area to cover, increasing more and more weight.
  • 0

User avatar
rp181
Brigadier General
Brigadier General
 
Posts: 1090
Joined: Thu Dec 13, 2007 8:47 pm
Reputation: 0

Unread postAuthor: jimmy101 » Fri Jun 20, 2008 12:44 am

rp181 wrote:Wouldnt the mass go up exponentially. As you add layer after layer, theres more surface area to cover, increasing more and more weight.

Mass goes up as the cube of the radius. The volume of a sphere is 4/3*Pi*r^3. So, if you double the radius you increase the volume by 2^3=8. If the material the two spheres is made of is the same the mass goes up by the same amount.

The cross sectional area goes up as the square of the radius. Area=Pi*r^2

As you increase the ammo and barrel diameter, and if you are using spherical ammo, then the mass of the ammo goes up much faster than the cross sectional area. That means it takes higher pressures to get the larger ammo up to the same speed as the smaller ammo from a smaller barrel. This only applies to spherical ammo, the relationship is different for other shapes.

In ballistics, there is a realtionship called the sectional density; mass/(cross sectional area). Ammos with high sectional density carry farther than ammo with low sectional density. Compare a golfball with a ping-pong ball. About the same areas but the golfball is much more massive and will carry farther for a given muzzle velocity than will the pinp-pong ball. Sectional density is also relevant when you are talking about internal ballistics (the ammo moving through the barrel). As you change the sectional density the acceleration of the ammo changes. Higher sectinal densities means the ammo accelerates slower, all other things being equal. Since sectional density is
mass/area = 4/3*Pi*r^3*density/Pi*r^2 = 4/3*r*density

So the sectional density changes with the radius and that affects how much pressure you need to get to a particular muzzle velocity.

I believe if you maintain the same sectional density as you scale the gun the muzzle velocity should stay the same, but we are neglecting differences in friction, valves, CB ratio etc.
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Unread postAuthor: SpudFarm » Fri Jun 20, 2008 9:54 am

i PM you Jimmy since you talk to advanced for my math :D
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Sponsored

Sponsor
 


Return to Pneumatic Cannon Discussion

Who is online

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

Reputation System ©'