Why didn't the Imperials block the exhaust port on the Death Star during the Battle of Yavin?

Question

The rebel plan is to fire torpedoes into the Death Star's exhaust port, causing a chain reaction that will destroy the Death Star.

One of the Imperial officers tells Tarkin:

OFFICER: We've analyzed their attack, sir, and there is a danger. Should I have your ship standing by?

_{Star Wars: Episode IV - A New Hope}

Once the Imperials know what the rebels are trying to do, why don't they do the obvious thing - either close the exhaust port, or put something big (like a ship) in front of it to block it?

Answer 1

The relative difficulty of getting to the exhaust port - location and defenses - was the entire reason for the trench run. It was easier to do with a small craft than a more powerful vessel.

Dodonna: Its defenses are designed around a direct large-scale assault. A small one-man fighter should be able to penetrate the outer defense. [...] The approach will not be easy. You are required to maneuver straight down this trench and skim the surface to this point. The target area is only two meters wide. It's a small thermal exhaust port, right below the main port. The shaft leads directly to the reactor system. A precise hit will start a chain reaction which should destroy the station.

If it takes a small craft to get close enough to it to attack it, "put[ting] something big (like a ship) in front of it to block it" isn't going to work. The target is in a trench measured in meters on an object measured in kilometers. As odd as it may sound to say, you're not going to block it with another kilometer-scale object.

Furthermore, blocking or closing an exhaust port for a reactor is a bad idea. The exact science of Star Wars is frequently suspect, but basic engineering principles still hold: they would not have an exhaust port if they did not need an exhaust port. Stuffing a potato in the exhaust pipe of a car stalls the car. You don't want to stall the Death Star, do you?

There were people who believed the required shot was...

Wedge: [...] impossible, even for a computer.

Answer 2

Tarkin's arrogance prevented him from taking the threat seriously.

Let's say you've decided to construct a technological terror of some kind. A large spherical battle station that can blow up a planet. Sure, Vader keeps reminding you that the ability to destroy a planet is insignificant next to the power of the Force. But, you've got big dreams and you won't let his sorcerer's ways get you down. But, you realize that, while very powerful, your station is not invulnerable so you wisely decide to give some thought to its defense.

When you're trying to defend something, you can't defend against every possible threat. Resources (money, ships, people, energy) aren't infinite. Because of that, you have to decide where you're going to spend the limited resources you do have. A good strategy is to look at all the possible threats and prioritize that list based on whatever criteria you deem important. Usually this would mean those threats that you're most likely to face along with some unlikely ones that would be especially dangerous if they happen. Anything that doesn't meet whatever threshold you've set gets ignored, either because it's so unlikely as to be impossible, or not dangerous enough to merit attention.

In the case of the Death Star, the Empire seems to have decided that large scale frontal assaults were the only threat worth considering. They looked at the potential threat of small fighters and decided that the turbo lasers and TIE Fighter squadrons that would be used in the defense against a large scale assault would be sufficient against a small scale assault as well.

So, when Tarkin was presented with information that small fighters might pose more a threat than originally thought, he ignored it. His response shows that he deemed this new threat too unlikely to give special response too.

Evacuate in our moment of triumph? I think you overestimate their chances.

He was too sure of the power of the Death Star to properly gauge the threat the Rebels posed.

Edit: I'll add that, when you think about it, you can't really fault Tarkin for his decision. If you remove Luke's force abilities and Han showing up at the last minute, the attack would have failed. He was correct in saying the threat wasn't great enough to pay attention too.

I can't really blame Tarkin for not considering whether an incredibly powerful Force user might be one of the pilots. Especially when the Jedi had been mostly wiped out for years at that point. And even if he did consider that, Luke's force abilities alone still would have failed. Vader very nearly killed Luke.

For Tarkin to have properly gauged the threat he faced, he would have had to know that Luke was an incredibly power force user and could therefore make an otherwise impossible shot and that Han would have shown up at the last minute to take out the TIE fighters. I don't think anyone would be expected to anticipate both of those incredibly unlikely events.

Answer 3

Regardless of the origin of the exhaust port on the Death Star Mk. 1, as noted in the quote in the question, the Imperials weren't even aware of the hazard until the battle was in progress, with Rebel fighters already in close contact.

Trying to assemble a crew and shield or block the port in the middle of a battle would be futile (and blocking the port would most likely compromise some part of the station's operation) -- the work couldn't be completed in time to matter. The chosen tactic, for their best pilot and his hand-picked wing men to go out in T.I.E. fighters to intercept the attackers, was the best option available to disrupt the attacks long enough for the tower cannons and fighters to destroy the light Rebel fighters. It was only a pull of the trigger from success, if not for the unexpected intervention of Han Solo and the Millennium Falcon.

Answer 4

Exhaust port was a quick fix for big problem, it had to be operational, especially during the firing sequence

As we all know, Galen Erso deliberately sabotaged reactor core, ostensibly to reduce weapon recharge time, but in reality to make it unstable. Here is excerpt from Rouge One novelization

Document #YM3884M (“Reply to Waste Radiation Distribution Solutions”), sent from Galen Erso to Engineering Operations Manager Shaith Vodran.]

Vodran:

Sincere apologies. I fully agree that this is unacceptable. The modifications are intended to reduce primary weapon recharge times to satisfactory levels (I’m sure you saw Tarkin’s directive) but sloppy work is sloppy work.

I assume you alerted Director Krennic to the report as well?

More as soon as I’ve spoken with my team.

He then "offered solutions", but only No 3 (exhaust port) was feasible in allocated time, which he planned all along .

Vodran:

I alerted the director personally, at your suggestion.

I also spoke to my team and we identified the problem. The reactor core modifications are resulting in radiation buildup, which in turn has the potential of interfering with the hypermatter annihilator.

The buildup is caused by the inner shield actively reflecting excess particles and metaphorically “cooking” the reactor core. Had the shielding team’s research not been so heavily compartmentalized this might have been avoided.

Nonetheless:

The reactor core modifications must remain as-is. Therefore, we are left with three possible ways of avoiding radiation buildup.

Option one: construction of a particle funnel and recycler. This is known and tested technology. I am confident it will function. Physical requirements mean the recycler would need to replace existing noncritical mechanisms under the northern command sector, but I estimate the needed disassembly would take under two weeks.

Option two: further refinement of our reactor technology to reduce waste particles. I have several team members keen on this possibility. They are excited about the potential for a technological breakthrough.

Option three: construction of manual venting shafts and thermal exhaust ports. This should reduce particle buildup to within tolerances but not to a degree I find personally acceptable. In addition, adding venting shafts risks additional incompatibilities with noncritical systems.

Please alert me if you have concerns.

Krennic, as expected, selected quick and dirty modification

[Document #YM3884R (“Reply to Particle Buildup”), sent from Advanced Weapons Research Director Orson Krennic to Galen Erso.]

Galen:

New research and technological development is out of the question at this juncture. Work up a full proposal for the exhaust port solution and send the plans to Vodran for SSCR.

Even when fully opened, exhaust ports did not eliminate problem completely

[Document #YM3884S (“Venting Shafts”), sent from Engineering Operations Manager Shaith Vodran to Galen Erso.]

Erso:

What is this trash? The Systems Safety and Compatibility Report quit running after two hundred redlines. I only reviewed the first dozen, but it looks like you’re flooding half the station with radiation?

I thought these venting shafts were supposed to solve the problem.

No changes are approved.

Of course I should have warned you that your droids might register dangers. The venting shafts are designed to expel the majority of the heat and particle buildup, but some radiation leakage is inevitable. We estimate that human crewmembers stationed in any of fifteen sections would—in the event that the battle station fires the primary weapon three times within one hour—be placed at increased risk for a wide variety of long-term health problems. The SSCR, of course, detected this in those “two hundred redlines.”

So, as you can see, Death Star reactors and exhaust ports were deliberately made as a sloppy work and barely held together as it is. Closing exhaust ports (even if possible at such short time) would have killed half of Death Star's crew even without Luke's intervention. Therefore, it could not be done at that time.

Answer 5

"There is a danger" is not incredibly specific. Perhaps they had an idea of what the Rebels were up to, but didn't know the exact details.

That being said, I don't remember the sequence of the battle. If Tarkin is told that after the first pilot (whose name escapes me) takes his shot, then they may have had a better idea of what the Rebels were attempting, but possibly not the potential repercussions.

Answer 6

One other possibility is that, as far as the Imperials were concerned, they thought that they had sufficiently blocked it already. It is stated that the exhaust port is ray shielded:(https://www.imdb.com/title/tt0076759/quotes/qt0440661) so it is not terribly unlikely they thought that would be sufficient to handle anything that happened to hit it, or they might have overlooked it entirely because it was already shielded.

This is mostly speculation, but in the new canon post Rogue One, it seems likely that they wouldn't expect an exhaust port to lead directly to the main reactor from the outside of the station. There are potentially an extremely large number of such exhaust ports though considering the scale of the Death Star, making it not too difficult for the main designer to sneak in/cover up one slightly odd 2 meter port on a small-moon sized battle station.

Answer 7

The Empire became aware during the battle that the Rebels were targetting an exhaust port. To be so focussed on such a specific target would lead the Imperial analysts to presume that the Rebels had reason to believe that it would do something to the Death Star.

However - were the Rebels attempting to cause an explosion inside the port to cascade back to the main core? Or, were they trying to damage the port and block/obstruct it to cause a stall/backfire?

Deciding to arbitrarily block the port (and at short notice - what were they going to do, park a TIE fighter on top?) may have actually accomplished the Rebels' goal rather than hindered it.