I expect my players will sooner or later want to acquire the services of mercenaries. I couldn't find any rules in any of the books, so does anybody have any house rules of how to deal with acquisition rolls to acquire mercenaries? What kind of modifier should they receive? The key difference of course being that a mercenary unit is only available for one or a limited number of actions, while house troops are permanently available.

I'm particularly interested too in mercenary ships. What kind of acquisition roll would it take to hire ships for a limited amount of time. Say, for instance, patrolling a site while salvage operations are in progress.

If you are looking for official rules they are in Battlefleet Koronus but you can pull together some simple ones from the Aquisition guidelines in the core book: Quality of the troops and their gear, size of the unit and availability depending on the location that players are trying to recruit from. But I'd recommend the BK book for a more in depth breakdown (and a discussion of merc organisations in the Expanse).

I can find some mentioning of mercenaries on page 121 in BK, but I can't really find any acquisition modifier anywhere. Unless all the modifiers further on (table 4-2 to 4-6) are for mercenaries only? I always assumed they were for permanently acquired forces.

Permanently acquired forces tend to get paid wages as well, so it's really quite similar.

I'd make the players make Upkeep rolls if they want to retain a force permanently but I use the same adjustmenst to the "recruitment" Aquisition roll.