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| SpudFarm |
 Posted: 06/19/2008 17:21 PM Post subject: surface calculation? (don't really know what to write here) |
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Hybrid Guy
Joined: 04 Nov 2006
Posts: 2036
1056.38 Spud Bux
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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. |
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| bigbob12345 |
| Posted: 06/19/2008 19:07 PM Post subject: |
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Experienced n00b
Joined: 01 Dec 2007
Posts: 1516
370.57 Spud Bux
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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 |
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| SpudFarm |
| Posted: 06/19/2008 19:14 PM Post subject: |
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Hybrid Guy
Joined: 04 Nov 2006
Posts: 2036
1056.38 Spud Bux
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| it did, but i am talking about 150psi pneumatic shot |
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| POLAND_SPUD |
| Posted: 06/19/2008 20:21 PM Post subject: |
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Brigadier General
Joined: 13 Oct 2007
Posts: 876
2174.38 Spud Bux
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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... |
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| rp181 |
| Posted: 06/19/2008 21:09 PM Post subject: |
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Major
Joined: 13 Dec 2007
Posts: 356
446.12 Spud Bux
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| Wouldnt the mass go up exponentially. As you add layer after layer, theres more surface area to cover, increasing more and more weight. |
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| jimmy101 |
| Posted: 06/20/2008 0:44 AM Post subject: |
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Major General
Joined: 28 Mar 2007
Posts: 1594
6760.52 Spud Bux
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| 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. |
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| SpudFarm |
| Posted: 06/20/2008 9:54 AM Post subject: |
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Hybrid Guy
Joined: 04 Nov 2006
Posts: 2036
1056.38 Spud Bux
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i PM you Jimmy since you talk to advanced for my math  |
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