Sienar Fleet Systems or SFS (owned by the massive company Santhe/Sienar Technologies) was a major starship manufacturer. Through their Sienar Design Systems subsidiary, they were also responsible for many drive and power systems.
Sienar Fleet Systems, along with Kuat Drive Yards and the Corellian Engineering Corporation, was one of the major suppliers of military vessels for the Galactic Empire: whereas KDY was famous for designing massive Star Destroyers, SFS was best-known as the supplier of the small, yet deadly TIE fighters. Yet SFS also created the massive I-a2b solar ionization reactor that powered the Imperial-class Star Destroyer and dictated the size and design of its mile-long hull: in effect, KDY built the Imperial-class around a huge Sienar drive system.
SFS, originally Republic Sienar Systems, was founded over 15 millennia before the Battle of Yavin by the wealthy Sienar family, apparently on Corulag. Over the millennia, they produced designs such as the Sienar Battleship and the Interdictor-class cruiser. A century before the Battle of Yavin, the Sienars united with the Santhe family of Lianna, who owned Santhe Security, due to an arranged marriage. Although this gave the Santhe family something of the status of a ruling dynasty within the merged Santhe/Sienar, scions of the Sienar lineage retained operational control of the ship-design section of the conglomerate: father and son Narro Sienar and Raith Sienar served as successive CEOs for many years in the last decades of the Old Republic and under the Galactic Empire.
Under the covert patronage of Senator Palpatine, Raith Sienar established Sienar Design Systems and its Advanced Projects Laboratory, which were apparently responsible for designing Darth Maul's Sith Infiltrator: although Raith Sienar fiercely denied any connection to the craft, it does seem that it was connected to Advanced Projects designs, including the starfighter prototypes that would develop into the T.I.E. starfighter and early TIE series designs. By 29 BBY, Raith Sienar had also produced the original Expeditionary Battle Planetoid design that later became the Death Star. Even after the Clone Wars ended with the Declaration of a New Order in 19 BBY, the Sienar Fleet System, alongside Kuat Drive Yards, continued to produce warships, ground vehicles and other war machines at a quick pace, because of Palpatine's desire to extend Imperial rule throughout the civilized galaxy. This in turn led to the Reconquest of the Rim taking place from 19 BBY to 17 BBY.
Under the Empire's rule, SFS produced an immense variety of military spacecraft. Most were relatively small—starfighters, patrol ships and planetary defense pickets—but as well as the power systems for the Imperial-class design, Sienar also built the 600 meter Vindicator-class heavy cruiser to a design produced by Wilhuff Tarkin, and its Immobilizer 418 interdictor variant.
Raith Sienar seems to have remained in control of SFS until after the Battle of Endor, but when he was killed in an assassination, Lady Valles Santhe of the Santhes dynasty took direct control of the company, and continued to control it after the collapse of the Empire. She then created Sienar Army Systems when the Emperor Palpatine was reborn in the body of a clone. After the defeat of Grand Admiral Thrawn, the company began supplying vessels to all buyers, including the New Republic.
SFS was loyal to the Galactic Alliance by 40 ABY, producing the Freebooter-class transport and the Aleph-class starfighter. By the time of the Sith–Imperial War, however, they were producing the Predator-class fighter and other spacecraft for the revived Empire.