At the moment of this writing, there are only 6 possible missions in the Colonial Union Combat Office.
I’m not sure how it is supposed to be, since I didn’t do any combat mission for several months.
Anyhow, 6 missions is just very little, especially since they are divided in 1 LL6, 3 LL7, 2 LL8.
There are no LL9, LL10, LL11, LL12.
So, it seems these are not refreshing.