Is it possible there is a correlation between the total combined ranks of airships and the amount of zones unlocked. Looking at the spreadsheet it seems people either went 15 19 20 21 22 or 12 16 17 18. Very few have them both unlocked until their 3rd and 4th ship are getting higher ranked. Again it is probably just rng still though. As soon as my ships got to 50 50 and 15 18 unlocked after making 8 trips to sector 17.