Why were there no Interdictor-class Star Destroyers included in the taskforce hunting the rebels at the Battle of Hoth?

Question

In the Battle of Hoth, the Imperial forces attempted to blockade any escaping rebel ships.

Clearly the idea of capturing at least some of the rebel ships wasn't completely off the table. Yet notably absent from the taskforce was any representation of the Interdictor-class of Star Destroyers.

Interdictors are designed specifically for capturing enemy vessels, employing gravity well projectors to prevent enemy ships from escaping into hyperspace.

They seemed to be a fairly common tool in the Imperial arsenal, at least by the time of Thrawn's rise, and the Wookieepedia article indicates that Darth Vader used an Interdictor-class vessel as his command ship shortly after the Battle of Yavin.

If one had been present, the Ion cannon defense would not have helped the Rebels escape, and while the Millenium Falcon would likely still have managed to escape, an Interdictor-class ship would have seemed a very useful tool for Vader to have around.

Is there any canon explanation given as to why there was no Interdictor-class Star Destroyer present at Hoth?

Answer 1

Most appearances of Interdictors in the Star Wars universe are in Legends and are therefore non-canon. However, they do make an appearance in the canon novel Tarkin. At the time (five years after the creation of the Empire, which would be about 17 years before the Battle of Hoth) the ships were experimental and did not function well. They were used in an attempt to recover Tarkin's stolen ship, the Carrion Spike. Here is the relevant quote from the book (with some emphasis added), which includes their introduction and use:

“By utilizing Interdictor cruisers, my lord -- precisely placed to yank the Carrion Spike from hyperspace short of its destination system and reversion point. Governor Tarkin assures us that any jump from the dissidents’ current location will require at least two reversions to reach potential Imperial targets. Thus, Interdictors can be positioned in advance of the Carrion Spike’s arrival.”

The Emperor looked down at Kren Blista-Vanee.

“The requested Interdictors are being developed as part of the Deep Core Security Zone, my lord.” Fond of wearing flamboyant hats and frequenting the opera, Blista-Vanee was a relative newcomer to the Ruling Council, but had already proven an asset in blazing hyperspace routes into the Deep Core star systems. “I hasten to add, however, that the ships’ gravity well projectors have not been tested in scenarios of this sort.

...

Tarkin angled the microphone of his headset closer to his mouth and opened the battle net to the task force liaison officer, who was aboard the CC-7700 frigate.

“The projectors are powering up to high gain, Governor Tarkin,” the commander said. “The field will be initiated, then disabled, in an effort to keep from dragging vessels other than the quarry from hyperspace. I should caution, however, that that may be unavoidable, given the heavy traffic in this system.”

“I understand, Commander,” Tarkin said. “Order your technicians to be judicious, nonetheless.”

“I will, sir. But the power setting of the gravity wells is dictated to some extent by the relative speed of the targeted ship, and, well, sir, to be blunt about it, there aren’t many as fast as the Carrion Spike.” Tarkin pinched his lower lip in thought. Ideally, local systems would had been notified that Obroa-skai had been designated a no-entry zone, but naval command had opted against issuing the designation for fear of alerting the dissidents. He had other reasons for concern: chiefly the question of why the dissidents would jump to Obroa-skai, which lacked anything in the way of an Imperial target, and was known mostly for its medcenters and libraries.

“T minus thirty and counting,” the specialist in the data pit announced.

Moving to the forward end of the walkway, Tarkin fixed his gaze on the trio of Interdictors. Arms folded across his chest, he counted down in silence even while the voice of the specialist was doing the same in his right ear bead.

The countdown had just reached T minus five when Tarkin was yanked forward, nearly completely off his feet. Fearing another lurch he spread his hands wide and so was kept from being slammed headfirst into the closest viewport panel. Klaxons began to howl throughout the suddenly trembling command bridge as the giant ship groaned and lurched yet again in the direction of the distant Interdictors. Struggling to remain upright, Tarkin caught a glimpse of the middle-distance frigates and pickets being pulled forward, almost as if accelerating.

“Commander,” he shouted into the headset mouthpiece, “the field is too powerful!”

“Working on it, sir,” the commander said with equal volume. “It’s the Immobilizer. The overcurrent resistors failed to prevent the gravitic systems from redlining—”

The comlink connection broke.

Close to the Interdictors, ships began to appear where there had only been star-filled space. Tarkin turned from the forward bay and stumbled back to the data pit to study the magnified view on one of the screens. First to drop out of hyperspace was an outmoded, saucer-shaped YT-1000 freighter, followed by two angular transports and a lustrous space yacht. Then another freighter winked into visibility, followed by two passenger vessels.

Abruptly, Tarkin felt as if he’d been shoved toward the rear of the bridge. With the interdiction field neutralized, the ships that had been caught in the invisible web began to whirl out of control. Two of the ships collided and drifted out of view. The magnification screen showed the sublight engines of other ships flashing, but the ships barely had a chance to flee or correct their spins when the field re-initiated, capturing them once again. Tarkin spread his legs wide in an effort to balance himself; then his eyes went wide as well as he turned to face the viewports. Listing on its port side, an enormous ship that more resembled something grown than built decanted, broadsiding the Detainer CC-2200 before careening into a spin that left its dorsal surface impaled on the Interdictor’s sloping bow.

“Mon Cal star cruiser!” a voice in his ear said, loud enough to be heard over the head-splitting racket of the klaxons. “The luxury liner Stellar Vista out of Corsin. Approximately ten thousand aboard!”

A brief but nova-bright explosion flared in the distance, ferocious enough to leave Tarkin blinking and seeing stars that weren’t there. When he was able to focus through the viewport’s blast-tinting, he saw that the stern of the organically sculpted passenger ship and disappeared and that the Interdictor had been knocked ninety degrees from its former position. In moments podlike lifeboats and flocks of spherical escape pods were streaming from the stricken liner.

"The Stellar Vista reports that it is in imminent distress," the specialist said. "The ship's captain is requesting all the help we can provide."

_{pp. 183, 191-192}

The Interdictors worked, but they nearly created more problems than they solved.

An Interdictor also appeared in Star Wars Rebels episode "Stealth Strike". By this time the Empire seems to have improved on the technology, so the previous disaster depicted in Tarkin does not seem to have halted development. However, the Interdictor ended up destroying itself and other Imperial ships when it was sabotaged by the rebels to uncontrollably pull in all nearby vessels.

We can only speculate why the Empire did not include Interdictors at the Battle of Hoth, but these incidents provide some reasons why the Empire would avoid using them (and perhaps abandon their use entirely by the Battle of Hoth):

• The gravity well generators were difficult to control or could be sabotaged.
• The interdiction field posed a danger to Imperial vessels (including the Interdictors themselves) and personnel if not properly controlled.
• The interdiction field would drag civilian vessels out of hyperspace and into a battle, potentially creating a public relations problem for the Empire.

(Out of universe, Interdictors weren't present because they hadn't been invented when Episode V was released.)

Answer 2

You're not going to find a canon reference to the absence of interdictors, since it doesn't exist... although that may change in the future. AFAIK, the topic has never been addressed in the exiting "Legends" EU. However, in-universe, we do have a little more information regarding the fleet that attacked Hoth.

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The "Death Squadron" fleet commanded by Darth Vader consisted of the Super-class Star Destroyer 'Executor' and five Imperial-class Destroyers. The fleet was put together for a single purpose - find and destroy the headquarters of the Rebel Alliance. For this reason, the fleet was designed to attack a ground-based target. Imperial Star Destroyers are often used as artillery to pound a target from orbit - it's one of their primary functions. By that same token, the fleet had a large contingent of Imperial Army on-board, including multiple AT-AT walkers and their smaller AT-ST cousins. The fleet operated by positioning itself at hyperspace traffic lanes and waiting - ready to reach a destination quickly if word of the Rebel base reached them.

Now, that said, it's highly possible that Vader would have called in interdictors after securing the planet. His plan had been to catch the rebels unaware and then pin them down. Since Admiral Ozzel gave the rebels' advance warning, they were then able to prepare and launch their ships. Presumably, without that warning, the Destroyers may have been able to spread out more or prevent the launches from above. If that were the case, they would only have to hold the rebels there long enough for the interdictors to arrive and the whole thing would have been over.

Typically, Interdictor-class vessels are used for customs duty on the outer fringes of the Empire, and there are only a few hundred in service at the time of The Battle of Hoth. Contrast that with the well over 25,000 destroyers in service, and the interdictors become a rare commodity. It's most likely that Vader or his commanders simply didn't forsee a need to have such a valuable resource in what was essentially a "search-and-destroy" fleet compliment.

Answer 3

Interdictors are most effective in the spacelanes away from natural gravity wells. While it can be argued that the presence of Interdictors would have made escape trickier, their influence would have been marginal. Hoth itself was a natural gravity well that extended in all directions.

Interdictors generate artificial gravity wells in the shape of a cone, so all a ship has to do is manuver out of the cone, and suddently, they can go into hyperspace. The projectors are massive, embedded in the ship's superstructure, so "aiming" sometimes requires the ship to manuver. Which means that they are best used in ambushes, rather than assaults. Although Grand Admiral Thrawn showed that any sort of ship can be used offensively, given the right circumstances.

Also, Interdictors are usually tied up in anti-piracy patrols. There may simply have been no available cruisers that could get to Hoth in time.

Answer 4

The Death Fleet should have had at least a couple of Interdictors regardless of the situation. Thrawn had two at his disposal during his siege/attack at Lothal and that was when Interdictor technology was still in development. Darth Vader's personal fleet for sure would have had at least a couple. The point about projection cones is a good point however. I would say there probably were a couple of Interdictor cruisers at Hoth but they looked like Destroyers because of the size difference in comparison to Executor, they were so small you couldn't make out the projection domes. This isn't a great answer. The other answer I thought of was that they were there but the Rebels targeted them first with the Ion cannon and they were off-screen during the panning shots of the fleet.