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Old 10-21-2013, 09:27 AM   #1
vicky_molokh
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Default [Spaceships] Conventional and EM Gun ranges in atmosphere?

Greetings, all!

Spaceships guns are normally treated as having infinite range (but minuscule chance of hitting stuff) because in vacuum there's nothing to stop the shells. However, when used in atmosphere, things change. So, how do I figure the D and Max ranges for the various ballistic Weapon Batteries?

Thanks in advance!
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Old 10-21-2013, 09:41 AM   #2
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

While they're a bit large, you might be able to use Douglas Cole's spreadsheet (if you have it; the Google doc for it appears to be down) for the calculations. Caliber and muzzle velocity (1 mps for conventional, 2 mps for EM) are easy enough. Rounds designed for use in space might not be very aerodynamic, but it's probably best to assume they have the same general shape as typical artillery shells.
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Old 10-21-2013, 11:09 AM   #3
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

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Originally Posted by Varyon View Post
While they're a bit large, you might be able to use Douglas Cole's spreadsheet (if you have it; the Google doc for it appears to be down) for the calculations. Caliber and muzzle velocity (1 mps for conventional, 2 mps for EM) are easy enough. Rounds designed for use in space might not be very aerodynamic, but it's probably best to assume they have the same general shape as typical artillery shells.
Also a gun designed to fire in space may have problems in an atmosphere because the shells may fire too fast. And since shells seem to be mini-missiles they probably have the wrong type of sensors. Though the last can be solved by using a shell type more appropriate for atmosphere.
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Old 10-21-2013, 11:28 AM   #4
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

As a horribly crude approximation, figure that atmospheric half damage range is about 50,000x projectile diameter (so 100m per mm of size). This assumes fairly long streamlined projectiles, and is probably overly generous, but GURPS tends to be rather generous with its 1/2d ranges.
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Old 10-21-2013, 11:51 AM   #5
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

Using DouglasCole's formulas, I get a 2cm shell with a 1 mps muzzle velocity having the following stats:

Damage: 69d+2 pi++
Range: 3,800/26,000 yd

This assumes an aspect ratio of 3:1, and DU slug. Increasing the aspect ratio to the upper end of modern 20mm shells (6:1), you get:

Damage: 99d+2 pi++
Range: 9,700/48,000 yd

Running with a muzzle velocity of 2mps, you get the following:

3:1 Aspect Ratio
Damage: 142d+3 pi++
Range: 3,800/35,000 yd

6:1 Aspect Ratio
Damage: 205d pi++
Range: 9,700/64,000 yd
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Old 10-21-2013, 11:56 AM   #6
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

Quote:
Originally Posted by Anthony View Post
As a horribly crude approximation, figure that atmospheric half damage range is about 50,000x projectile diameter (so 100m per mm of size). This assumes fairly long streamlined projectiles, and is probably overly generous, but GURPS tends to be rather generous with its 1/2d ranges.
I think Max is typically right around 6x 1/2D, so with this you end up with, for a weapon of Caliber n cm, a range of 1000n/6000n.

According to Wikipedia, the M198 Howitzer is going to be right around 16cm, and the RAP (rocket assisted projectile) configuration is probably about right for Spaceships. That has a maximum range of around 33,000 yards, which is about 1/3 what we'd get using the above... so 1000n/2000n might give us a better approximation.
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Old 10-22-2013, 01:51 AM   #7
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

Quote:
Originally Posted by Humabout View Post
Using DouglasCole's formulas, I get a 2cm shell with a 1 mps muzzle velocity having the following stats:

Damage: 69d+2 pi++
Range: 3,800/26,000 yd

This assumes an aspect ratio of 3:1, and DU slug. Increasing the aspect ratio to the upper end of modern 20mm shells (6:1), you get:
Hmm. A 2cm conventional slug from SS does 3d(2) dDamage, i.e. 30d normal damage.

Quote:
Originally Posted by Humabout View Post
Damage: 99d+2 pi++
Range: 9,700/48,000 yd

Running with a muzzle velocity of 2mps, you get the following:

3:1 Aspect Ratio
Damage: 142d+3 pi++
Range: 3,800/35,000 yd

6:1 Aspect Ratio
Damage: 205d pi++
Range: 9,700/64,000 yd
Again 2cm VRF EM Gun does 60d, i.e. your guns seem to be better.

Just saying.
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Old 10-22-2013, 01:55 AM   #8
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

Quote:
Originally Posted by Anthony View Post
As a horribly crude approximation, figure that atmospheric half damage range is about 50,000x projectile diameter (so 100m per mm of size). This assumes fairly long streamlined projectiles, and is probably overly generous, but GURPS tends to be rather generous with its 1/2d ranges.
Thanks. Seems in the right ballpark - a jet-mounted 30mm autocannon in one of the Pyramids has roughly half the D of your calculation. I was just unsure with what it should scale, and with what it shouldn't. E.g. higher-speed bullets from ETC, LP, Gauss and Grav guns seem to have greater ranges than conventional ones.

Edit: that's half, not twice. That is, the Pyramid autocannon has less D.
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Last edited by vicky_molokh; 10-22-2013 at 02:04 AM.
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Old 10-22-2013, 01:58 AM   #9
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

Quote:
Originally Posted by vicky_molokh View Post
Thanks. Seems in the right ballpark - a jet-mounted 30mm autocannon in one of the Pyramids has roughly twice the D of your calculation. I was just unsure with what it should scale, and with what it shouldn't. E.g. higher-speed bullets from ETC, LP, Gauss and Grav guns seem to have greater ranges than conventional ones.
Realistically, they shouldn't unless they have bullets with different ballistic properties or computed range is comparable to maximum vacuum range (for a 1 km/sec bullet, about 100 km at 1G).
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Old 10-22-2013, 02:06 AM   #10
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Default Re: [Spaceships] Conventional and EM Gun ranges in atmosphere?

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Originally Posted by Anthony View Post
Realistically, they shouldn't unless they have bullets with different ballistic properties or computed range is comparable to maximum vacuum range (for a 1 km/sec bullet, about 100 km at 1G).
I'm not sure, that would imply that an object will drop its kinetic energy to zero at the same distance regardless of the kinetic energy it had. How would that happen?
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