measure of shields, hull armor, and the structural durability of
starships, armored vehicles, etc., is difficult in most cases. A shield
(or armor plate) resists different types of weapons differently,
depending on what kind of armor, shield, and the type of weapon.
It may be highly vulnerable to a particular kind of attack - and sometimes, there are holes or weak spots in shields or armor, making it particularly difficult to quantify how much of a beating shields can take.
Will a small fast projectile with the same energy hurt more than a large slow projectile? Would you be better off trying to melt the hull, or smash it?
We have many methods of examining shield and hull toughness:
All the problems about the consistency of weapon power are multiplied by the uncertainty in the angle of impact, number of attacks employed, and open questions about what type of attack the shields or armor are best suited to block.
The last item on the list, though, happens to be a matter of considerable study. The study of ballistic impacts on armor has been a matter of pressing concern since the invention of tanks and warships with enough armor to resist modern guns.
Although the penetration of shaped charges on armor is fairly complex, simple ballistic impacts by well-designed shells has been exhaustively formulated. The depth of armor that a projectile can penetrate depends on its mass, radius, and speed.
All things being equal, a narrower and denser projectile penetrates better. In most cases, penetration is directly proportional to kinetic energy, but in others, penetration may be proportional to the square root.
The original penetration formulae generalize over all shells of a certain shape by measuring penetration in calibers. On these pages, we somewhat abuse the empirical armor penetration formulae, pretending that ships, asteroids, logs, etc., are as well shaped for hull penetration - and as solidly constructed - as bullets. This may tend to produce overestimates.
However, the formulae available give us very good estimations for the equivalent thickness of steel armor needed to match the resistance to physical impact displayed in Star Wars and Star Trek.
An article on empirical armor penetration formulae by noted expert Nathan Okun.