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Some of the better combat systems I have seen have two weapon factors: Penetration and Damage. In world war two, there were two typical weapon types: Armor Piercing and High Explosive. Armor Piercing actually had little explosive power (usually none) but would penetrate the armor of a tank sending shell and hull fragments around the interior killing the crew, igniting fuel and ammunition. High Explosive would detonate on the outside and need a powerful enough of a bLast to get through the armor.

Each weapon should have a Penetration Factor that is compared agains the targets armor thickness. If Penetration is achieved, the full weapon damage is applied, otherwise, it is reduced by the armors natural emmissive property.

Also, to give unform armor coverage, larger hulls should require more KT of armor, based on surface area. An object that is 1 meter cubed, has 6 square meters of surface area. a 2 meter cube, has 24 squate meters of surface area. It contains 8 times the volume, but has 4 times the surface area. It needs 4 times the armor to achieve the same thinkness. Assuming all ships are cubes (kind of boring), each ship's armor thinkness would be (KT of Armor) / (6 x (KT Hull)^(2/3)). You can improve that ratio a bit if you use the perfect shape - a sphere.

In reality, tanks tend to be have facings with most of the armor facing forward. This allows armor thinkness to be increased in the most likely direction of attack, thus increasing armor thickness without increase cost as much. This could be acheived by reducing the multiplier (6 x).

You can go ever further by using sloping armor to increase the chances of a ricochette. Also, a shot that is not perpendicular to the armor must traverse greater thinkness, but slope gets real complicated.

Elongated ships or ships with lots of appendages require an even higher ratio of armor because they have so much surface area. But hey, they look neat don't they. The U.S.S Enterprise is a bad design based on surface area. Thats why they need spherical shields. Borg Ships on the other hand make a lot of sense surface area to volume wise. However, the Enterprise has better slope on impacts than Borg ships.