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Equipped with gravity well projectors designed to pull ships from hyperspace and/or prevent them making the jump to lightspeed, the Interdictor became a valuable addition to the Imperial Navy.
Interdictor cruisers, like their Vindicator-class cousins, had the appearance of small Star Destroyers. However, their lines were broken by four large globes housing gravity well projectors.
The gravity wells mimicked a large mass in space and thus restricted hyperspace travel. Nearby vessels were automatically prevented from engaging their hyperdrive, and any ships passing through the area via hyperspace were forced into realspace.
As well as gravity wells, the Sienar design was also equipped with 20 quad laser cannons for short range combat against other capital ships and starfighters. The Interdictor's hull was nearly identical to that of the Vindicator-class heavy cruiser and could support several additional weapons depending on mission profile. Such modifications (like those of the Corusca Rainbow, formerly the Black Asp) included the addition of ion cannons used to disable pirates and small craft attempting to flee. Enemy fire was absorbed by a deflector shield created by more than a dozen shield generators.
A variant of the Interdictor design existed, equipped with two large gravity wells rather than the usual four: a ship of this type appeared in Grand Admiral Thrawn's ambush of Luke Skywalker at Jomark in 9 ABY.
The Immobilizer 418 was developed as a counter to the Rebel Alliance's hyperdrive-equipped starfighters in particular, and hyperdrive-equipped vessels in general. This was primarily to counter Alliance hit and run tactics, where a small group of starfighters or light starships would emerge from hyperspace, attack preselected targets, and re-enter hyperspace before sufficient force could be brought to bear on the raiders.
An Immobilizer at the site of attack, or able to access the area with a brief hyperspace jump, would be tasked to power up its gravity well projectors in an attempt to prevent the escape of the raiding party. Starfighter and capital ship support would then be used to eliminate the attackers, whose only options were to try to run for the edge of the interdiction field, or to attack the Interdictor and hope to inflict enough damage to disable the cruiser's influence. Either path would result in significant damage to the Alliance forces. Rebel pilots often mistook it for a miniature Star Destroyer when it was accompanied by Star Destroyers.[source?] It was only when they tried to escape into hyperspace that they realized it was an Interdictor. Interdictors could also be used when blockading planets or moons, such as Yavin 4.
Another duty of this class was to patrol trade routes. Freighters pulled out of hyperspace by an Interdictor would be in a position to have their cargo inspected. Interdictors were a welcome addition to the Imperial Customs assets, because while many smugglers and pirates wouldn't hesitate to outrun—or in some cases even outgun—an Imperial Customs Frigate, few would dare engage with a much more sizable Immobilizer 418, its gravity wells and its two squadrons of TIE starfighters. Interdictors assigned to Customs duties—such as the former Black Asp—would often have some of their quad lasers replaced by ion cannons so as to disable and capture any ship resisting inspection rather than destroy them outright.
Grand Admiral Thrawn was especially famous in connection with Interdictors for developing and refining a tactic that would be named after him—the "Thrawn Pincer" positioned the Interdictor's gravity wells along certain vectors. Other vessels—typically Star Destroyers—would be contacted, and would blindly execute a micro-hyperspace jump straight into the Interdictor's gravity wells, yanking them out of hyperspace near an enemy vessel. Much quicker than subspace movements, the Thrawn Pincer allowed reinforcements to be swiftly deployed on the battlefield and afforded a considerable element of surprise.
Although they were tremendous assets to any fleet, Interdictor cruisers suffered from some drawbacks. First and foremost, it took some time to power up and activate the gravity well projectors after the cruiser positioned itself and, once activated, the tremendous simulated mass they generated made the ship very slow and cumbersome to maneuver. While there was rarely enough time for an entire fleet to make the jump to hyperspace before a newly-arrived Interdictor activated its projectors, fast ships and starfighters could often escape to the edge of an active gravity well, and then make the jump to hyperspace.
The gravity wells were also a large drain on the ship's energy reserves, preventing the optimal use of weaponry and shields. This flaw was exploited on several occasions. When enemy ships were able to inflict significant damage to the Interdictor, the captain was forced to choose between entrapment of the enemy and his ship's own protection. This flaw was compounded by the fact that Interdictors were poorly protected and armed for a capital ship of their size. Enemy forces could send a smaller warship, such as an Nebulon-B frigate or Thranta-class War Cruiser to weaken an Interdictor's defenses and quickly force it to power down its gravity wells to fend for itself. Although the abilities of the Immobilizer 418 were fairly effective against the missile launchers of the Marauder-class corvette often employed by the Rebel Alliance, the ship was also known for its notorious weaknesses against larger capital ships such as the Mon Calamari MC80 Star Cruiser and bombers such as the BTL Y-wing starfighter. Another problem was the fact that active gravity wells attracted space debris. An Interdictor could—and would—suffer external damage from larger pieces of such hazards. While most damage was minor—as the largest debris would be detected and fired upon by the gunnery crews—it was one reason Interdictors required more maintenance than many other fleet vehicles.
Additionally, the engine and hyperdrive-specific areas of an Immobilizer's interior sections were crewed only by maintenance droids, and were totally unprotected. Raiders and saboteurs were thus able to infiltrate and either disassemble or destroy the vital parts of the cruiser's propulsion systems.
Due to their importance and weaknesses, Immobilizer 418 cruisers were usually found on the periphery of the battle they were affecting, escorted by larger and better equipped capital ships and starfighters to protect them from harm and deliver the necessary damage.