Following up on my previous post, our next two voyages:

Visit sector 05 (only), Rank 19->20, unlock 07.
Visit sector 07 (only), Rank 20->21, unlock 08.

Enterprise Forecastle, Invincible Rigging, Bronco Hull and Aftcastle.
Previously made at least 2 voyages to 05 (not alone) without unlocking anything, plus one which unlocked 06. Those were all made with Enterprise Fore, but don't know which Aft (Bronco/Invincible).

The idea that the unlocks are tied to rank is, I'm pretty sure, incorrect. We unlocked sector 04 before we were high enough rank to go there, while sector 05 wasn't unlocked until after we exceeded the min rank by one or two. Meanwhile, a friend's FC unlocked sector 05 at a lower rank. I'm fairly certain you need to explore a certain sector or sectors (associated by proximity on the map) in order to unlock new sectors. If Exploration (or any other stat) is a factor, I'd guess that it contributes to an RNG roll, rather than straight-up determining the unlock.