A good starting point this this:
However, it doesn't speak of range; whereas this does, as well as appearing to be basically what we seem to be after:
Using the "large" variety, they seem to have range (up to around 6000ft), accuracy (being able to be used against naval targets at that range effectively).
How this would translate into D&D stats would be, without a doubt, an area of effect attack, using the indirect fire rules.
PHB's indirect fire rules; involve BAB + Int - Range Increment & misc factors such as not having line of sight and previous shots. Target AC for a square is 15, and if target is missed a scatter dice is rolled, followed by moving the shot 3 five foot squares per range increment of the attack.
Range increments, we're looking at roughly 600ft per increment; this makes the 2 kilometers just outside its maximum range but at a nasty penalty.
Damage… its rather hard to say. They don't give examples at what the large rockets can do; only that the almost quarter in size medium can blast a 30cm deep crater into a patch of sand. All things considered however, it's likely to be a small amount of piercing damage to represent some shrapnel and primarily fire damage. 1d6 piercing and 4d6 fire doesn't seem too unreasonable. Radius of say 10ft?