At what range do ships enter "combat"?

Once combat begins, fleets mostly leave direct player control (except for the "retreat" option). What is the distance (in in-game units, i.e. the units that weapon range are expressed in) between two fleets when they enter this "combat" stage?

Solution 1:

According to the game's "defines" file (Stellaris/common/defines/00_defines.txt), the relevant constants are

COMBAT_DETECT_RANGE_MULT                = 1.05
# Range to engage in combat with other ships is <fleets max attack range> * COMBAT_DETECT_RANGE_MULT
COMBAT_DETECT_RANGE_MIN                 = 10

So for instance a fleet equipped entirely with Large Gamma Lasers (range 80) will become locked in combat with an enemy fleet at range 84. (Unless the enemy fleet has longer-ranged weapons - entering combat is reciprocal). For a Tachyon Lance (range 150), the combat detect range is 157.5.

However, most fleets will not actually fire at their theoretical maximum range because of weapon windup. Returning to the example of the LGL, its rated cooldown is 4.6 days, but this is actually split between a cooldown (after firing) of 3.0 and a warmup (before firing) of between 0.2 and 2.0 days. Likewise the Tachyon Lance's warmup is between 0.1 and 1.9 days with a 7-day cooldown. (These values can be found in common/component_templates/weapon_components.csv)

Unless your fleet and your opponent's are composed entirely of long-range warships with Artillery or Carrier combat computers, they will continue to approach each other during the initial windup time so the gap between combat detect range and the range where the first shots are fired will appear larger than 5%.