WARNING! Below are two longish posts. They explain how the aerial combat model in BTS 3.17 work, why I dislike it, and what I have done to correct the shortcomings.
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The aerial combat system in 3.17 is a very basic model:
1. First, the code determines the best interceptor in a given area. This is the unit with the highest current interception (current means that the health of the unit is taken into consideration. If a unit is at half strength, its interception chance is halved). Current interception is the ONLY factor that determines the selection of best interceptor.
2. Second, there is a test to see if the attacking air unit can evade. A die roll of 100 against the unit's evasion attribute is made, an if the roll is succesful, the attacking unit evades interception. Note that in 3.13, only the Stealth Bomber and missiles has an evasion level greater than zero.
3. Third, the best interceptor makes a test (100 sided die roll) against its current interception level. Again, current means that the units' interception attribute is modified by health. If the roll is succesful, the interceptor engages the attacking unit, and the aerial combat code is run.
4. Aerial combat consists of five rounds of combat. In each round, the odds for the attacking unit and the interceptor are calculated. It's a very basic (current strength for the attacker) / (total current strength of the two units combined) = attacker's odds. Conversely, the interceptors' odds are 100% - attackers' odds. A 100 sided die is then rolled to determine the winner of the round.
5. The unit which wins the round does damage to the other unit (thereby affecting all 'current' values in the subsequent rounds of combat). The amount of damage dealt is the current interception level divided by 2 (this division can be adjusted in XML through a global variable). So BOTH fighters and jet fighters deal out 100% interception / 2 = 50% damage for each round. For units without interception attributes (such as bombers), a minimum damage of 10% pr. round is applied (also a global variable controllable in XML). Incidently, there's also a maximum damage of 50% pr. combat round.
6. The remaining rounds of combat are conducted, until one of the units are dead, or five rounds have been fought.
Now let's take a look at the air units in 3.17. There are airships, fighters, jet fighters, bombers and stealth bombers. Only fighters and jet fighters have interception values greater than zero, both are 100%. Given the damage reduction as described in point 5 above, interceptors will always do 50% of damage pr. round of combat, on the condition that they are fully fit. In the examples below I assume full health for all units involved. I have shown the chance for the interceptor to win the first round of combat:
A. Fighter vs. Airship: 12/(12+4) = 75%
B. Fighter vs. Fighter: 12/(12+12) = 50%
C. Fighter vs. Jet Fighter: 12/(12+24) = 33%
D. Fighter vs. Bomber: 12/(12+16) = 43%
E. Fighter vs. Stealth Bomber: 12/(12+20) = 37,5%, but the stealth bomber has a 50% chance to evade interception.
F. Jet Fighter vs. Airship: 24/(24+4) = 86%
G. Jet Fighter vs. Fighter: 24/(24+12) = 67%
H. Jet Fighter vs. Jet Fighter: 24/(24+24) = 50%
I. Jet Fighter vs. Bomber: 24/(24+16) = 60%
J. Jet Fighter vs. Stealth Bomber: 24/(24+20) = 54,5%, but the stealth bomber has a 50% chance to evade interception.
In the cases where the interceptor (both regular fighter and jet fighter) is up against bombers or airships, it actually stands a significant chance of LOSING the first round - but the amount of damage the interceptor suffers if losing the first round is light, only 10%. Round 2 will of course shift the odds in the favor of the bomber/airship, since we're using the CURRENT STRENGTH for the odds and CURRENT INTERCEPTION for damage. If the interceptor wins the first round, however, the odds are heavily in favor of the interceptor winning in just two rounds.
Moving on to the issue of promotions: Interceptors (i.e. fighters and jet fighters) can, in 3.17, get the Combat line promotions and also the Interception I and II promos, amongst other things. Given the way the model works, choosing promotions can seem counterintuitive. While the Interception I and II do improve the chance the interceptor has to intercept in the first place, a fully fit interceptor does not gain any advantage, because it is already at 100%. Only when the interceptor is damaged does it gain an improvement to its chance to intercept. The interception value is also used for calculating the damage. But again, a fully fit interceptor can not deal out more damage than the 50% maximum damage limit (and 110% interception chance /2 = 55%), so the Interception promotion is again only useful to a damaged interceptor. The Combat line of promotions are actually more beneficial, if for no other reason because both a damaged and un-damaged interceptor can take advantage of it. The effect of it is to improve the odds during the combat rounds. For example, in case B the interceptor with Combat I against an unpromoted fighter would have 13,2/(13,2+12) = 52,4% instead of the normal 50%.
EDIT: It turns out that in vanilla BTS, air units cannot gain the Interception promotion. So the above can be partly disregarded.
So... what is good about this model? Combat results are acceptable. Generally, the combat results make sense, in that usually you'd expect the interceptor will win an engagement, if it's not too shot up. Against an equal opponent, it's a 50/50. Against a superior opponent (there's only one case, the fighter vs. the jet fighter), the fighter has a 33% chance to win the first round.
Unfortunately, there are more downsides to this model:
1. The model is not easily grasped, as it is quite un-intuitive. The interception variable determines both the chance to intercept, and the potential damage dealt. Combat strength only governs the odds of winning a combat round.
2. As a direct consequence of 1., the promotions are not very helpful. The interception promotions basically have no effect for a full health interceptor. You should always go for Combat promotions, as they'll help you win combat rounds.
3. The evasion variable is only used for stealth bombers and missiles. This is unfortunate, because a better model could utilize this extra variable.
4. Modding the air aspect of the game becomes next to impossible. Adding more air units, trying to increase interception levels between generations of air units, lead to very weird outcomes. Biplanes with promotions can all of a sudden take on and beat much more modern fighters. This is not unheard of in the civilization history, but the current model does certainly not lend itself well to modding balanced combat results.
I didn't touch that one.
