Does Rogue Trader mention how long psychic powers last, or anything about how to sustain them? The only thing I can find is a bit on how sustaining two powers at a time takes a check.

I'm just worried about abuse of things like Dominate and Compel.

From the Deathwatch core book: Some Psychic Powers can be sustained beyond a single round as noted in their description. A psyker may sustain such powers without the need to make further rolls. However, he must use a Half Action each turn to maintain concentration. Should the psyker be unable to expend a Half Action, fall unconscious, die or otherwise be unable to maintain the power, it will end.

Given that they have more in common with each other than the Dark Heresy interpretation, I'd say that's the intended method for sustaining powers in Rogue Trader.

The Rogue Trader books says it is possible to sustain 3 or more powers at the same time. Given that, how can they each take a half action?

Sustaining any amount of powers is a free action.

Any suicidal action forced on the Dominated/Compelled subject forces 'another' opposed willpower test so the power, as long as it is maintained, will remain in effect until another OWT is forced and failed by the psyker. Compel actually allows a +20 on the test for suicidal actions, where non suicidal actions for Dominate do not get the bonus at all.

Suicidal is pretty broad depending on whats going on so if they do more than hold the guy still while they escape or tell him to go away he'll be forcing opposed will tests every round.

Both are powerful but not overwhelmingly so.

Thanks for the replies.

However, preventing suicidal actions doesn't cut it for me--in regards to dominate feels entirely unreasonable to me to let the PCs mind control npcs indefinitely. Dominate an admiral and suddenly you have a fleet?

I think as a house rule I'm going with a slightly more liberal version of Errant's suggestion. Half action to sustain any amount of powers during combat; "upkeep" test every hour thereafter.

Dominating a member of the navy will get you all sorts of problems. That Admiral is likely to be a lot tougher than you think. Not to mention the fact that his subordinates aren't going to be stupid and obey if he starts doing things against the normal operating procedure. So, no, dominate an admiral and you don't have a fleet. Remember the psyker is going to have to be on the same ship, likely nearby or in the same base. Around thousand sof psyker hating armsmen and naval personnel not t omention some few sanctioned psykers liek other astropaths and navis. So think more about the situation and the likely 'problems' with the use of the power.

PS: Admirals have commisars looking over their shoulders too. Commisars are not to be f&^%$d with.