If you want to deal with STOs by orbital bombardment with the minimum amount of civilian casualties, the "secret" is to use lower-power weapons rather than the biggest guns you have. This may seem counterintuitive so some explanation is needed:
Per the wiki page on Ground Combat,The damage in ground combat for an energy weapon is equal to 20x the square root of its point blank damage in ship-to-ship combat. Armour penetration is equal to half the damage. Fractions are retained.
Weapon effect against ground units is calculated by ATK= 20 * sqrt(damage) and AP = 10 * sqrt(damage) or half the ATK, where 'damage' is the point-blank damage of the weapon. This has two implications: (1) we can calculate the effect of the weapon against the expected ground forces, and (2) damage done to ground units is not affected by damage falloff at longer ranges. Furthermore,
Orbital bombardment ships have the same chance to hit as ground units,
we also know that the to-hit chance against ground targets is not affected by range. Thus we prefer a longer-ranged weapon to minimize the damage taken from STO fire (from both loss of accuracy and damage falloff at long range).
Let's consider a "typical" NPR STO unit, which uses the Static base class and has the minimal armor level. Perhaps the most common early-game opponents will have a racial armor tech level of 12, so the STO in question will have 36 HP and 12 armor. Let us consider firing at this STO unit with railguns of varying calibers, and that the STO has a fortification level of 3 with no terrain modifiers (chance to hit is thus 1/15 or 6.67% per shot):
- 10cm railguns (1 damage per shot): ATK = 20, AP = 10 -- kill probability on a hit is (20/36)^2 * (10/12)^2 = 21.4% -- expect
70 0.56 collateral damage per kill (see formula below). - 12cm railguns (2 damage per shot): ATK = 28.3, AP = 14.1 -- kill probability on a hit is (28.3/36)^2 * 1.0 (overmatch) = 61.7% -- expect
49 0.55 damage per kill. - 15cm railguns (3 damage per shot): ATK = 34.6, AP = 17.3 -- kill probability on a hit is (34.6/36)^2 * 1.0 (overmatch) = 92.6% -- expect
49 0.67 damage per kill. - 20cm railguns (4 damage per shot): ATK = 40, AP = 20 -- complete overmatch, 100% kill probability on a hit -- expect
60 0.96 damage per kill. - 25cm railguns (5 damage per shot): complete overmatch -- expect
75 1.34 damage per kill. - 30cm railguns (6 damage per shot): complete overmatch -- expect
90 1.76 damage per kill.
And so on. Once you have complete overmatch of the enemy STO (or at least what you think the enemy STO stats look like), using a higher-damage weapon only increases the collateral damage done. Once you overmatch one stat (usually armor as here, but would be HP if the STOs were armored), there's no benefit to increasing weapon damage*, and once you overmatch both stats then all you're doing is pumping up the collateral damage needlessly.
*Technical aside: The reason why is this: as long as both ATK and AP remain below the HP and armor of the target, respectively, increasing damage causes the kill probability to increase with the square of damage. Once you overmatch one stat, kill probability only increases linearly with damage. This means, in the example above, that a 2-damage or 3-damage weapon both have equal kill probabilities, however a 3-damage weapon is usually larger and more expensive, so the 2-damage weapon would be preferable.EDIT: Furthermore,
Collateral damage is not linear with ground combat damage. The effect on the population increases exponentially for ground combat weapons with higher damage. The damage value of each weapon that fires is cubed, then the total damage is divided by one million.
therefore, collateral damage increases with the 3/2 power of the weapon damage. I've edited the calculations above to reflect this.
Given this, what weapon types are preferable for combating STOs? I've used railguns in this example for their damage scaling of +1 per caliber, but in practice low-damage railguns tend to be rather short-range and thus ships with railguns are vulnerable to taking a lot of damage from the STOs (they're fine for taking out Gauss turrets though). This also tends to be the case for low-damage lasers. Perhaps unexpectedly, the best weapon choice for taking out STOs is thus low-damage particle beams, since particle beam range and damage are completely independent and you can have a 2/3-damage particle beam with 200,000 or 240,000 km range at a fairly low tech level. Of course, since low-damage particle beams are not exactly a typical choice of main weapon, this implies that you want to build highly specialized anti-STO ships to do the job. Low-damage missiles are another option, but probably significantly more expensive and vulnerable to being taken out by STO Gauss turrets unless you fire sufficiently large salvos - I also think missile collateral damage is rather worse than beam weapon collateral damage, so I would recommend avoiding missile bombardment in the case where you want to recover the planet more or less intact.
EDIT: On the basis of the above edit, another weapon type which is very well-suited for the job is the much-maligned
Meson Cannon, which always does 1 damage but can have a very long range once you develop the tech line sufficiently. Meson cannons are particularly attractive here since they are also, in principle, viable as a main beam weapon - mesons bypass shields,
might not be as bad against unshielded opponents as we used to think, and apparently improve ground unit attack (I think this is a recent change in 2.0 or later).
Side note: one consequence of the mechanics here is that armoring STOs can be effective in a niche application: increasing the amount of collateral damage an opponent must deal to defeat the STOs. If we use heavy STO armor, which multiplies the racial armor by a factor of 3 instead of 1, the collateral damage incurred will increase by at least a factor of 3 against weapons which do not achieve total overmatch against minimal armor, but increases by even more if the opponent does not have overmatch, potentially increasing the total collateral damage by a factor of up to 9 if the opponent is using very low-damage weapons which do not even overmatch a minimal-armor STO. This is indeed a very niche application, but if you expect an opponent to take a planet and you don't expect to get it back, this can be a way to make the planet even less useful for your enemy.EDIT 2: Corrected the numbers in the examples again, I think they should be correct against the collateral damage mechanics in the wiki now. I recall there was at some point an 80% reduction to collateral damage as well, which I have not factored in since I don't know if it affects naval weapons - in any case, the ratios remain the same.
