Most warfare is settled through space combat. Though ground warfare is necessary to achieve ultimate victory, landing armies is safer once the fleet has eliminated the threat of enemy vessels intercepting and destroying vulnerable troop transports. Winning battles across the stars is about adapting to the enemy's loadout and using effective counters. Each offensive or defensive component has a counter-measure to it, meaning that component choices are likely to evolve and change as empires encounter new enemies and try to build ships that counter those of the opponent.
Combat in Stellaris takes place in real time, out of the player's control save for the emergency FTL order. Though the player can direct a fleet toward or away from enemies, there is no control over individual ships (unless they make up a fleet on their own) or which ships they target.
Each fleet is shown on the screen with a strength number. This number represents the power of this fleet, measured by the combined offensive strength of all of the ships in the fleet. This is a rough guide to a fleet's prowess, but cannot be understood as an absolute determinant in how combat will play out. A fleet that has a lot of offensive weapons that are unsuited for the enemy it is facing (Alien creatures with regenerating hulls or skins, enemies with sophisticated and appropriate countermeasures) may still lose to this enemy even though its strength number is considerably higher.
When a fleet engages an enemy ship, enemy station or alien creature in combat, the player gets an alert and the fleet listing in the Outliner has a red battle icon beside it. Select this fleet and you will see a real-time report of the ongoing combat.
At the top of the results sub-screen is a bar measuring the approximate strength of the opposing sides in combat. This bar changes as combat progresses and the odds shift in favor of one or the other side. As the battle progresses, the report sub-screen shows breakdowns of the effectiveness of the weapons systems in use and how much damage the shields, armor and hulls have taken. Analyzing this data helps determine whether current ship builds are effective against this opponent and may indicate ways to improve.
Damage of space combat is distributed onto three components: Hull Points, Armor and Shields. Different weapons do different amount of damage to each.
- Hull Points is the most important attribute for any ship. When Hull Points reach 0 the ship will be destroyed and ships will have their speed and combat abilities decreased by a percentage equal to half of the percentage of Hull Points lost. Hull Points are determined by shipsize and a small amount of minor techs.
- Armor is an additional layer of health protecting Hull Points and added by armor components.
- Shields provide additional protection for ships. It is created by shield modules on board, and always regenerate unlike the Hull Points and Armor. While shield components are superior to armor, they also require energy from the ship's reactor. Guided weapons completely ignore shields.
When being attacked, the shield will absorb damage first. After the shield is depleted, the armor will be damaged, and after armor is depleted Hull Points will be lost. However, some weapons ignore shields and/or armor partially or completely.
When the hull and armor are damaged the most common way to repair them is by sending the fleet back to a Starbase. It is possible to repair ships other than by docking, like with the component 'regenerative hull tissue', a commanding admiral with the trait 'engineer', and the titan module 'aura nanobot cloud'. However, self repairing is slower than in-dock repairing (1% per day or 30% per month). Defense Platforms have a base self-repairing rate of 1% per day. It takes 30 days before a ship can be repaired after damage and 10 days for a station to repair itself after combat.
Ship protection modifiers are sparse, only two for each defense layer existing:
|Unyielding admiral trait||+10%|
|Devouring Swarm civic||+25%|
|Volatile Reactive Armor edict||+25%|
|Matter Compression repeatable technology||+5%|
|Exotic Gases for Shield Boost edict||+25%|
|Shield Harmonics repeatable technology||+5%|
Accuracy, Tracking, Evasion
Three parameters determine the chance of a weapon to hit its target: the accuracy and tracking of the attacker, and the evasion of the defender.
Accuracy comes primarily from the weapon stats. Accuracy is the percentage chance to hit a target with no evasion. Weapons with high accuracy tend to have comparatively low damage or other weaknesses. Accuracy is capped at 100.
Tracking comes primarily from the weapon stats as well. Tracking will cancel out some or all of the defender's evasion, and the actual chance to hit is the attacker's accuracy minus the defender's remaining evasion, if any. Smaller weapon sizes usually have significantly higher tracking, with some exceptions.
Evasion is primarily based on hull type and is a defining feature of the Corvette, Destroyer, Transport and similar small ships. It can be increased by thrusters, by the combat computer for some hull sizes, and by Auxiliary modules like the Enigmatic Encoder and by having surplus power in the ship design. The evasion rate is capped at 90%.
The formula to calculate the chance of a specific attack to hit the target is (remember that evasion is capped at 90 and accuracy is capped at 100):
Generally the goal is to pick a weapon that has just enough tracking to maintain the weapon's baseline accuracy, i.e. tracking equal to the enemy's evasion. Thrusters and Sensors mostly cancel each other out when on the same level, so without a serious tech disadvantage, the hull size and weapon sizes matter the most. Large weapons sport much higher damage than smaller weapons, allowing them to destroy their intended targets in just a few shots. However, since said large weapons also tend to have low tracking, ships with high evasion may become nigh-invulnerable to them as they can simply dodge the shots. Weapons with higher tracking, such as small weapons or missiles, are needed when engaging ships with high evasion, such as Corvettes. Conversely, small weapons have high tracking, ensuring that they are able to hit their target without missing much at all. However, they also have low damage and therefore struggle to deal with larger ships such like Cruisers and Battleships due to them having very high shield/armor/hull values, allowing them to absorb shots from small weapons without much difficulty. Large weapons with high damage are needed for dealing with ships that have such high durability.
When in combat, every military ship will advance to within a certain range of its intended target and fire its weapons; combat computers determine what range the ship will engage its target from when in combat. Bold ships will use the combat computer of that behavior by default when the design is auto-generated. Ships that have similar combat computers tend to cluster together, stay in formation with each other and attack in groups.
|Swarm||Corvette||The ship will charge straight at enemies and try to deal as much damage as possible from point-blank range.|
|Picket||Corvette, Destroyer, Cruiser||The ship will move to close (30) range and attempt to intercept incoming enemy ships.|
|Line||Destroyer, Cruiser, Battleship||The ship will advance to medium (50) range, assume formation and engage all enemy ships within range.|
|Artillery||Destroyer, Cruiser, Battleship, Titan||The ship will stay at long (80) range and fire its long range weapons on the target. (This only applies against stationary targets, everything else is engaged as usual; i.e the ship will attack once the target is within range)|
|Ignore||Colossus||The ship will ignore any attackers and carry on with its current task.|
In combat, any ship that takes hull damage while already below 50% health has a chance to disengage from battle, depending primarily on the amount of damage inflicted and secondarily on the ship class (Destroyers and Cruisers have the highest survivability, followed by Titans and Battleships, with Corvettes coming last). A ship that disengages will instantly leave the battle and rejoin the fleet at the end, until then appearing in the combat interface with an icon indicating it has disengaged. If a fleet contains only disengaged ships it will make an emergency retreat.
Ships will never disengage if the owner uses the No Retreat War Doctrine.
Force Disparity is a combat bonus applied when a smaller force is engaged with a larger one in battle and gives a bonus to the Fire Rate of all ships belonging to the smaller force. Force Disparity scales linearly with the fleet size difference, so for example a ship force that is engaging a hostile ship force twice as big will gain a 50% bonus to its firing speed, representing the fact that the smaller force has an easier time maneuvering and targeting the larger enemy force.
Force Disparity will never make a smaller force win unless it has significant technological advantage. It will however ensure that all but the most overwhelming forces will take casualties and bear a cost for their victory.
All ships of a fleet in combat have the opportunity to trigger an emergency retreat in the middle of a battle. This is a sudden escape using each individual ship's equipped F.T.L. engine weather it be a Hyperdrive or Jump Drive that allows the fleet to escape destruction. The order is given by pressing the Retreat button in the combat sub-screen and the retreat can be triggered even from deep within the system. While on Evasive stance, a fleet will automatically retreat once the option is available.
There are some tradeoffs, however:
- Warships require 30 days before they can retreat. Civilian ships require only 7.5 days.
- The shock of the emergency retreat will cause 20% hull damage. Ships with less than 20% HP have a 50% chance of being destroyed, otherwise they will arrive in friendly space with 1 HP.
- The fleet will go "missing in action" upon a retreat for a number of months, depending on how far the ship was from friendly space. As such it can be neither controlled nor attacked while moving back to friendly space.
- There is a 1% chance for each ship to be destroyed when retreating.
AI fleets in neutral or hostile territory will always retreat if half of the fleet is destroyed.