A shield is an energy-based defensive barrier which completely surrounds the vessel on which it is equipped, and serves to absorb the damage dealt to that vessel from hostile fire. Shields also deflect the force of explosions and shockwaves, making them an invaluable tool for bomber pilots. FreeSpace 2-era fighter and bomber shields are incapable of repelling energy from beam cannons, and are easily pierced by these weapons. Shields cannot be used whilst in subspace.
Shield integrity drops when it absorbs damage, and increases again over time as it draws power from the ship's reactor to recharge itself. The speed of this regeneration can be increased if energy is diverted to shields, away from engines and weapons, with the Energy Transfer System (ETS). Each shield is split into four quadrants (left, right, forward and rear) and specific quadrants can be recharged by diverting power from other quadrants.
Although shields effectively repel energy blasts and shockwaves, they do not protect the host ship from suffering damage via collisions with solid objects (with the exception of missiles).
All GTVA fighters and bombers are equipped with shield generators. The strength of the shield depends on the quality of the shield generator and on the type of the fighter. Interceptors, for example, typically lack strong shields, but compensate through high speed and maneuverability. Bombers, by contrast, are primarily designed to be tough and resilient, making shield strength a higher priority in their design.
Acquiring the Shield Technology
In 2335, the Shivans appeared for the first time, using advanced technology, including the fighter shield system, which gave them a huge advantage over the Galactic Terran Alliance and Parliamentary Vasudan Navy at the time.
To the 14-Year War-era weapons that were in use at the time, Shivan shields were almost completely impenetrable. This advantage allowed the Shivans to win many critical, early battles, such as the destruction of the GTD Amadeus.
It was immediately apparent that the only way to defeat the Shivans would be to develop weapons which could penetrate their shields. A series of daring raids by Terran pilots eventually succeeded in acquiring both scan data and actual Shivan cargo containers containing shield systems. This allowed for a joint Terran-Vasudan venture to reverse-engineer the shielding system and begin mass production, eventually in Ribos. The technology was soon fitted to every Terran and Vasudan fighter in service, and the ready availability of shielded fighters to test them on allowed Terran and Vasudan scientists to develop weapons capable of penetrating them.
The Lucifer's Shields
The SD Lucifer is the only vessel above bomber size ever encountered by Terrans or Vasudans to be equipped with an energy shield. The shielding system it used was known as 'sheath shielding', and was utterly impenetrable to any weapons at the time. The only weakness of the technology was one shared with typical fighter shields: They were inoperable in subspace, and this left the Lucifer vulnerable to attack by GTA and PVN pilots while en route to Earth. So far, the Galactic Terran-Vasudan Alliance has not been able to replicate this technology.
There is no explicit canonical information about whether FreeSpace 2-era beam weapons could penetrate the Lucifer's shields as they do fighter-mounted shields. However, given that the GTVA Colossus was designed to counter an assault by one or more Lucifer superdestroyers, it is likely that they are capable enough against Lucifer-style shielding that using such a system on other ships (like the Sathanas) is impractical.
By default, basic shield management keys are "Q" to equalize shields (evenly distribute all remaining shield energy between all four quadrants), the directional keys to divert shield energy to each respective quadrant, and the "Home" and "End" keys to transfer energy to or from the shield system. In addition, energy from the ship's primary weapons can immediately be transferred to shields, recharging them instantaneously without having to wait for the ship's reactor to slowly regenerate shield integrity. By default, this function is mapped to the "Scroll Lock" key.
Notes for Modders
If you want your fighter/bomber to have shields, you must make a triangulated sphere and name it as 'shield'. Also, you must apply a single color on it.