Hi we unlocked our 2nd airship with completing the sector that is level 20 missions. We unlocked our 3rd airship with completing the level 40 mission sector. I imagine the 4th airship will unlock from the 60th level mission sector. Note this is not sector #20, 40, or 60.

We have yet to explore sector 10. We have a ship there right now. People report it drops black clay. We received the athero-chemical pieces for the last prototype from Sector 14 a few days ago.

I believe it is RNG for unlocking new islands. But I feel if you have an island on the map by sector 7 that needs to be unlocked and your searching sector 2 across the map, its less likely to unlock the sector 7 missing island. But this is just my guess. IDK.