I love Star Wars (as do most people I know). At least the original trilogy and most of the original Legends canon. That's part of why it hurt so much to see Disney throw away all the good stuff like Thrawn and the Imperial Civil War and the new Jedi Order and replace it with JJ Abrams redoing A New Hope with less charm and more plot holes. One of the many, many problems with The Force Awakens is the low-hanging fruit that is Starkiller Base. So it is "built out of a planet" (even though based on the comparative size of it and the original Death Star, it's only about 600 km in diameter, so more like a large asteroid or Kuiper belt object). So it uses a "hyperspace laser." So it "consumes the power of a sun." Does not matter. It's still a large, spherical object with a glowing weak spot that takes time to charge up, blows up one target, then is clearly targeting the Rebel/Resistance base. In addition, they spend less time setting it up, describing the weapon and its weaknesses, or showing us why its targets were important, as opposed to the Death Star (mentioned in the title crawl and shown twice before blowing up a planet), the superlaser (the entire movie is about stealing the plans to destroy the weapon), and Alderaan (they keep mentioning Alderaan as the Princess' home and a peaceful, neutral planet with no weapons, and the Empire destroys it anyways).
But the Death Star itself is not without flaws. After all, why not just hyperspace into position to fire on Yavin IV at once instead of giving the Rebels 30 minutes to attack? And what about that stupid thermal exhaust port? What moron would build a massive weak spot in their armored superweapon?
Actually, let's talk about that last one.
The original Death Star gets a lot of flak for having a single direct line to the reactor which, should it take a hit, causes the entire space station to explode. The most recent Star Wars movie, Rogue One, went so far as to having one of the designers install the thermal exhaust port as purposeful weak spot. Even in the Legends canon, some authors tried to write various technicians who identified the thermal exhaust port as a weak spot and tried to fix it, but failed due to budget, time, and bureaucratic constraints.
But is the design such a bad one? Was such a feature necessary, was it truly as vulnerable as it seems, and finally, was destruction via exhaust port inevitable?
Point of order, I am only going to be considering the Legends Canon here. JJ Abrams and the bastardizations he has spawned can go off to the alternate universe where shitty film-making is somehow rewarded instead of punished. But I will get to JJ later. He shall not be spared my wrath. In addition, data from A New Hope, specifically from the original theatrical release of A New Hope, will be rated higher than later authorial works.
In the first place, there are many examples throughout history of invincible warships undone by a single weak point. Multiple Japanese cruisers were sunk when Allied fighters fired machine guns down the smokestacks (a.k.a. "primitive exhaust ports") and into the boilers. The HMS Hood was sunk when a single shell penetrated its lightly-armored deck and detonated an ammunition magazine. HMS Invincible did not partition turrets from ammunition and detonated at Jutland when a German shell blew up a turret, causing a flash fire to ignite cordite in the magazine. Most famously, a single torpedo fired from an antiquated biplane flying below the level of the defender's anti-aircraft guns managed to hit the one vulnerable spot of the battleship Bismarck, the rudder, condemning her to circle and be gunned down by the Royal Navy. So the idea of a massive weapons system having an overlooked weakness is well-established in history.
Moving into Star Wars, let us begin with the necessity of a thermal exhaust port. What does it even do? The movies and other sources are maddeningly vague about the operations of a thermal exhaust port, but they do say that it "dissipates the excess heat produced by large energy reactors."
Logically, this makes sense. Heat dissipation is a major problem of modern power generation. All nuclear reactors have some sort of coolant loop, and in current spacecraft designs, heat removal is one of the most difficult problems to rectify, requiring large radiator panels to ensure the spacecraft does not overheat. Therefore, the existence of some form of thermal release system checks out.
But an exhaust port? You are saying that advanced space reactors produce exhaust?
Maybe. Officially, the Death Star uses a "Hypermatter Annihilation Reactor" to power its weapon, hyperdrives, and other systems. Hypermatter is, of course, fictional. It is supposedly matter composed partially of tachyons that exists in hyperspace, and when confined to real space, the hypermatter self-annihilates as the tachyons "accelerate to infinite velocity." Now, not being versed in fictional reactor design, I cannot tell how such a device would be built, but I could imagine that there is a loop of coolant designed to prevent the reactor from heating up to the point that it melts down and destroys itself. It also seems as though there would be some matter left over from the disassociating tachyons. Therefore, you have a setup for producing hot exhaust as coolant circulates about the reactor and the waste material needs to be scrubbed from the core. Venting this exhaust into the void of space would be an efficient way of removing the waste heat, so the basic design, once again, makes sense.
Excellent, so the thermal exhaust port as an idea works, but is the Death Star's weak point truly as weak as it seems?
No. No it is not.
In the first place, the exhaust port is tiny. This is a space station over 120 kilometers in diameter. That is a surface area of 45,238.93 square kilometers, or 45,238,930,000 square meters. Of this, the exhaust port is "only 2 meters wide," or as I use in my daily measuring system, 1 womp rat wide. Since we see that the exhaust port is circular, this means that it is 3.14 square meters in area (0.785 womp rats squared). The fact that this critical system was made so small to begin with implies that quite a lot of thought went into protecting this system from the beginning. It was not an obvious or easy weak point to find, either. The Rebels had to expend considerable resources finding, stealing, and analyzing the plans for this massive battle station before they could determine that the exhaust port might be a weak spot.
Aside from its small size, the thermal exhaust port was then placed inside a deep and narrow trench. It does not face the sky, but instead projects laterally into the trench. This oblique angling and defensive positioning renders it secure from almost any orbital bombardment, as a capital ship would need to position itself precisely and expose itself to the withering fire of 15,000 turbolaser batteries (compare to 60 heavy turbolasers on an Imperial-1 Star Destroyer). Thus, attacking the thermal exhaust report is restricted to fighters.
"Aha!" you cry, seeking a flaw in my plan. "There you have it! The Rebel Alliance used starfighters! Game, set, match."
Not so fast. Even against starfighters, the thermal exhaust port was fairly well guarded. The port is ray shielded, so lasers, a.k.a. the primary weapon on a starfighter, are useless. Those ray shields render many starfighters worthless, as they cannot attack the exhaust port at all unless they have some form of missile system, but since both X-wings and Y-wings are equipped with proton torpedo launchers, we shall ignore that.
Second, there are literally dozens of anti-starfighter turbolasers placed around the thermal exhaust port. Their positions are varied, some on the surface, some on anti-starfighter towers. While not the most effective defense (the Rebels evade them by flying so fast and so low that the turbolasers could not target the fighters due to auto-safe programs), they still managed to down several of the attacking fighters.
But the final defense is one that is hardly referenced and barely used: the 7,000 TIE Fighters stationed on the Death Star.
Yes, 7,000.
See, one cannot view the Death Star as a single piece of equipment or one weapon platform. It is a full-up multi-layered combat system, including docking ports for Star Destroyers, enough ground troops to occupy a planet, and 7,000 TIE Fighters.
"But we saw the TIE Fighters deploy. Darth Vader led them. They could not stop the Rebel fighters."
You are partially correct. Darth Vader deployed in his personal TIE X-1 Advanced, and then ordered Black Squadron to follow him.
Yes, 1 Squadron. 12 TIE Fighters and 1 TIE X-1 against a frankly uncertain number of Rebel fighters. We know that two squadrons, Red and Gold, are mentioned in the movie. That's 24 fighters. Other sources sometimes add a Blue and Green squadron, which is supported by the fact that there are more than 24 fighters in the scene where all the rebels are flying to attack the Death Star. So that is up to 48 fighters. Officially, in Legends, there were 32 rebel fighters, and in the movie, an Imperial officer says to Vader, “We count 30 Rebel ships.” Either way, that’s at least 30 vs 13. And those 13 Imperial fighters, along with the turbolasers and other defenses, destroyed all but 3 Rebel fighters and nearly stopped them from destroying the Death Star. Had Grand Moff Tarkin agreed to release the other 6,987 TIE Fighters, the little Rebel snub fighters, the only vehicles which even had a chance of hitting the thermal exhaust report, would have been quickly eliminated, long before they got close enough to fire their torpedoes.
There are additional arguments that could be made, but since the technical details surrounding Hypermatter Reactors and Thermal Exhaust Ports are scanty, I can only offer conjecture. One of the main questions is, "why not use a screen?" Or a shield, a door, or some other thing. I am not certain, but if I had to guess, it is that such a screen would impede the flow of the exhaust, which would indicate that the exhaust is itself made of matter. That would also explain why the port is ray shielded, since it needs to let the exhaust through. "Why would hitting the exhaust port cause the whole reactor to explode?" Perhaps the exhaust is so hot, it must be magnetically contained to prevent it melting the port walls, and destroying the powerful super-magnets shuts the entire system down, leading to an exhaust build-up near the reactor and leading to a core breach. Again, I do not know, since the technical schematics are not available to me. The point is, I could come up with a reasonable response to the more conjectural arguments. It does not abolish the fact that had Tarkin deployed his TIE Fighters like he should have, then the Death Star would have never been destroyed. It was Tarkin’s arrogance, not design ineptitude or a secretly-inserted weak spot, which destroyed the Death Star.
The Death Star's Thermal Exhaust Port was a fine idea, possibly necessary and well-protected behind layered defenses. It was not perfect, but I believe that it was actually good enough. It failed not because of any inherent flaw in its design, but because those operating the Death Star failed to use their full range of safety features, arrogantly assured that the incomplete use of their control measures would be enough. The last time a similar installation purposefully ran without critical safety features, Chernobyl occurred. The fact that said safety features were 7,000 TIE fighters simply means that the Death Star designers were expecting a different type of threat than the Soviets when they built a power plant. You do not need a "built-in weakness." You do not need a silly flaw. The story of a decent, sufficient, yet imperfect system compromised by the arrogance and ignorance of an administrator who refused to listen to his subordinates is a much more powerful, nuanced, and interesting message. Even in space, human error is inevitable, and must be considered.
After all, it is that human error which allowed the Rebels to triumph.