They utilize extremely high power projectors to create a zone of targeted nuclear strong force, a force field, around the ship. Shield effectiveness depends upon a number of factors including number of projectors, amount of fusion power used, and the physical area covered by the shield. More field projectors require more power, but allow more selective reinforcement.
When energy in any form is projected against the shield, power is drawn to counter that force. Shields are more effective against more energetic attacks, meaning they're very effective against plasma, less effective against nuclear weapons and particle beams, and least effective against laser weapons and small ballistic projectiles.
Starship shields require roughly 10x (15x for lasers) the power of the attacking weapon to be effective. A 5 gigawatt explosive impact takes 50 gigwatts of shield power to stop or 75 gigawatts if it is from a laser. Thus it is harder to defend than deal damage. However those values are in true 'delivered' energy. Thus a 5 gigawatt laser may only deliver 1 gigawatt of energy, thus requiring just 15 gigwatts of shield power.
Sufficient fusion power must be available to the shield projectors to allow them to intercept attacks. Shield ratings are expressed in the (amount of power required) / (multiplier required to handle its rated attack). A 500/4 gigawatt shield required 500 gigawatts of fusion power to operate on standby, and 4x500 gigawatt (2 terawatts) of additional power to handle its rated attack, totalling an expected load of 2.5 terawatts.
Shields have capacitors equal to their force rating, as it is not possible to shift such huge amounts of energy during the picoseconds of the attack's impact. The powerplants then continually recharge the shields.
Advanced shields can link and overlap, as well as being directed to reinforce certain areas of the shielded ship.
A shield fails when it takes more damage (energy) than it is capable of resisting, or its emitters are taken off line through overstress or damage. A sudden massive influx of energy equal to several multiples of capacitor rating will usually damageor destroy said capacitors, rendering the shields offline.
Warships often carry replacements, though not always. Shields are designed to 'overload' in these modes, absorbing as much as they can handle then tripping, like a circuit breaker. These overloads are quite energetic in nature, and not always successful.
Shields that successfully overload can be reset in various amounts of time, depending on the design and the severity of the overload. Emitters channel large amounts of power, thus they can be stressed over a battle by repeated impacts close to their threshold. Effectiveness then begins to degrade to the point the shield must be taken offline, or risk catastrophic emitter failure.
In practical use, shields come in three forms.
- Generators to cover an entire ship, usually for small ships.
- Multiple generators generally covering the front and rear arcs on small to medium ships.
- Multiphase overlapping shields using as many as a dozen shield generators that can project in overlapping layers and sometimes reinforce each other.
Due to a diminishing returns, overlapping shield tend to only be used on static defenses. Capital ships use multiphase shields, which can be set to sectors of the ship, moved, and reinforced as necessary. This requires either sophisticated automation or a large crew to manage the task. Multiphase shields most often cover quadrants of larger ships, allowing for ease of moving nearby shields to cover those which have gone down.