Actually, shipboard weapons pretty much have to have targeting and guidance programs if they're to have even the slightest chance of hitting something. Well, of hitting what you're aiming at, anyway.

Think about it- you're shooting at a target at most 5km in it's greatest dimension (until BC, GC and BB/DN ships start showing up, anyway), travelling at velocities up to what? 200,000km/h or thereabouts? At ranges of up to ~100,000km (180,000km in the case of the Sunsear's max range)? Do you really expect people to make those kinds of shots, taking into account relative motion etc. in half an hour ?

With that in mind, there almost certainly has to be at least some comp...cogitator support when plotting a firing solution. That being the case, a little basic rocket science to fire torpedoes off-bore isn't that much of a stretch.

As an extra note, torpedoes in 40k fluff aren't actually "dumb", although they are fire-and-forget. They actually have relatively decent seeker heads (see above for the difficulties in targeting), but they travel slower than macrocannon shells, and have limited delta-v for manoeuvring. Of course, they more than make up for that with the size of their warheads, and the fact they can pass straight through shields.

A comment about barrel rolls : the speeed need to keep another ship into one's firing arc is extremely varaiable, but at anything but close range it should be more than enough : a ship at 10 VU moving 10 VU 'up' or 'down' can be kept in sight with a mere 45% roll. You're onlu changing orientation wheras your target would need to change it's whole motion.

You might even be able to spin heels-over-head to shoot at something behind you, but odds are it won't be good for the ship's keel as you would generate quite a lot of torque. Unless it's designed for that sort of acrobatics, which one can thinka cruiser isn't.

Which brings to mind a question : how do an imperial decelerate ? Current space vhecile basically orient themesleves end-first and turn on the main propulsion. The alternative would be to keep going nose first but use some sort of deflector baffle or the like to reorient the main drive's push toward the front.