First of all - do not discount original force fields so quickly.
- While they may look cumbersome when taken as is, they actually are pretty lightweight (just 1 simple percentile roll per hit) in comparison to other options. For example, the first force fields, described in Dark Heresy supplement - Inquisitor’s Handbook worked like this:
This amulet functions as a force field. Any time the wearer is attacked, he may roll 3d10 and reduce the Damage dealt by the sum of the dice. Should the wearer roll double ones, the attack bypasses the field completely (do not reduce the Damage). If the wearer rolls double tens, the field overloads and shuts down for 1d10 Rounds. In the case of an overload, the wearer must roll an extra 1d10. On a result of an 8, the amulet is burned out and cannot be used again.
Complete hit negation it’s up to the GM. If the attack power is too high - he can just say that field couldn’t protect wearer from death (time to burn Fate Points!). Or that force field negate most of the attack’s power (leaving wearer critically damaged) but gets destroyed in process.
And don’t forget about overloading. A Dark Eldar Archon with Shadowfield can ignore a number of shots, but then he rolls a low number, and suddenly his force field just turns off, leaving poor bastard exposed to angry PC-s (to add some fun, overload rating of such device could be even higher than on crude Imperial force fields. After all, Dark Eldar don’t mind doing a little bit of sabotage even to their own kin). Also you can raise an overload rating for hits delivered by some heavy weaponry.
But, if you are strongly inclined to use more “fluff”-y force fields, you probably need the rechargeable shield concept.
In such a concept, your force field has a number of “hit points” but it’s not fixed. Instead there is a maximum value. You can restore field’s HP by a number of ways, for example:
Regeneration - a force field could slowly restore itself. There could be various types of force fields - some have a really good HP pool but a poor or no regeneration rate (so it will be pretty useless in a continuous battle), while other can fully recharge themselves in a few rounds, but for a cost of maximum HP value (so a concentrated assault could quickly overrun such a field).
Recharge by some energy source, like generator, or powerful battery (power armor energy source, for example). Requires a successful Tech-Use test with at least -20 difficulty, so it would be wise to leave such a task to a techpriest.
Luminen Charge - requires a techpriest (obviously)
Full recharge and repair - requires some heavy machinery, powerful energy source and qualified tech assistance. A Rogue Trader ship should have all of this.
Optional rules - As force field is a first line of defence, Armor should not affect damage dealt to field HP. This leaves high-Penetration weapons slightly ineffective. To fix this, you can either:
- Give a field its own armor-like damage reduction negated by weapon Penetration. This can further increase force fields variety - those with high damage reduction will be much more effective against high numbers of lesser enemies
- Simply add Penetration to weapon Damage when hit a force field.