Why would something so grand contain such a fatal flaw?
Was it really a flaw or was the only flaw the fact the "The Chosen One" existed. Who else in the Galaxy would have been able to fire that shot?
Would the Rebel Alliance had even attempted the mission if Leia didn't think Luke was the one?
Why would something so grand contain such a fatal flaw?"
Other than "how else would ANH work" reason, you mean?
Because whoever designed it didn't realize it was a fatal flaw. If you were a software developer (or designer) you'd know that it's impossible to design bugless software of any complexity. A Space Shuttle was destroyed on the account of a piece of foam. Mars-bound lander was lost due to a metric-imperial unit conversion bug. And so on and so forth.
In-universe, it was retconned to be not an uncommon design flaw, . To quote from SW Wiki:
In another example, Ice Station Beta was destroyed by Kyle Katarn when he planted sequencer charges in thermal exhaust ports throughout the facility.
"Would the Rebel Alliance had even attempted the mission if Leia didn't think Luke was the one?"
Leia did NOT think Luke was "the one". Or anything beyond a capable pilot. He was just another X-Wing pilot, not a designated shooter.
He didn't "have the force" up until he took that shot. Leia had no reason to know he had Jedi abilities.
There are several theories. David Morgan-Mar of Irregular Webcomic! has two.
One of them puts the blame on a social issue. It says that the Death Star was probably built to impress chicks, and withstanding a space battle was a secondary concern:
Another one blames physics. It says that it wasn't a bug, it was a feature built to withstand the tyranny of thermodynamics:
Considering the importance of physics in the Star Wars universe, it's probably the former.
Withstand the tyranny of thermodynamics, sure.
– Gallifreyan
Dec 18 '16 at 07:56
The in universe answer, according to the novel Death Star, is that there was a covering designed, it's just that the architect was sick for a couple days and didn't get the designs to the work crew.
The exhaust port flaw was included by Galen Erso, one of the designers of the Death Star, as a means of deliberately sabotaging the Empire:
“My colleagues,” Galen said, “many of them, have fooled themselves into thinking they are creating something so terrible and powerful it will never be used. But they’re wrong. No weapon has ever been left on the shelf. And the day is coming soon when it will be unleashed.”
His head turned from the recorder as if what he said next, more than anything, he feared to say aloud.
“I’ve placed a flaw deep within the system. A scar so small and powerful, they’ll never find it.”
Rogue One: A Star Wars Story
As for the nature of the flaw he included:
And as he envisioned the cataclysmic energies building within the vast station, he saw it—a detail he had overlooked and forgotten, some trivial adjustment of Galen’s: a single exhaust port leading from a narrow trench down and down, down kilometers of blackness, past conduits and hatches and radiation plating, down and down—
—and into the main reactor.
The primary weapon of the Death Star battle station fired.
Orson Krennic, advanced weapons research director and father of the Death Star, died alone on Scarif, screaming in fury at Galen Erso, at Jyn Erso, at Wilhuff Tarkin, and at all the galaxy.
Rogue One: A Star Wars Story
This of course contradicts the various explanations proposed by the previous Legends canon.
I do not believe that the exhaust port was really a design flaw. The path to the port was heavily guarded by laser towers and the Death Star was able to launch fighters to defend itself.
I think the real flaw was that the Death Star only launched three fighters. It should have launched enough fighters to where anyone trying to run the trench would be immediately under such an intense barrage of laser fire that they'd have to take evasive maneuver out of the trench to survive or die like most of them did in the movie.
In the end it was the over confidence of the commanders of the Death Star that lead to its destruction, not the existence of some small reactor exhaust port.
In the Star Wars novel, the explanation is given that the port would be useless if it were to be particle-shielded; however, it is ray-shielded. Obviously someone knew there may be a problem if that were to be targeted, and gave it the best protection possible while still allowing the port to do its job.
There is still a question; this is an auxiliary exhaust port "right below the main port". Why not target the main port? The main port would be bigger, right? Maybe the main port can be closed, or particle-shielded, to protect it from attack, and the station (or that part of it) could rely on the auxiliary port.
I think it was clearly a design flaw - the 2nd Death Start engineered a way around it (I believe it used multiple, smaller exhaust vents, which lacked the direct path to the reactor core) and it allowed a single fighter to destroy millions of people and uncounted trillions in resources for the cost of a proton torpedo (and a handful of fighters). I'd call that a design flaw.
It likely wouldn't have been such a big problem if the Empire had been smarter - the Death Star should have had screening elements to deal with smaller ships. The only reason they didn't have them is that they rushed to Yavin as soon as they confirmed it was the base, hoping to reach it before the rebels could evacuate or summon fleet elements.
Giant expensive contracts often have fatal flaws in our universe, so it follows that things would be similar in the Galaxy Far, Far Away (especially when it is convenient for the plot). In-universe, according to the Essential Guide to Vehicles and Vessels (1996) Death Star entry:
For weaponry, in addition to the superlaser, the Death Star has 15,000 capital ship turbolasers and over 700 tractor-beam projectors. There projectors could capture virtually any ship and, when working in conjunction with one another, could reel in even an Imperial Star Destroyer. The Death Star also maintained an awesome array of support ships and vehicles, with 7,000 TIE fighters, four strike cruisers, over 20,000 military and transport vessels, and over 11,000 combat vehicles.
Despite all these awesome advantages, simple errors doomed the station. First, the Death Star's defenses were built around the idea of repelling a capital ship attack; starfights were considered "insignificant" by Imperial military strategists. And when the Alliance mounted its assault over Yavin, the Rebels could only send a small force of X-wing and Y-wing fighters. Grand Moff Tarkin considered the attack inconsequential. He refused to launch TIE fighters in defense of the station, and only the TIE fights under Lord Vader's direct command were deployed against the Alliance's fighters.
As a result of those errors of arrogance, the mightiest weapon in the history of the galaxy was destroyed by a simple proton torpedo fired down an unshielded exhaust vent. This Achille's heel allowed the Empire's enemies to detonate the Death Star's power core and win the day.
According to the Death Star II entry:
Not only was the second Death Star larger and more deadly, but several flaws in the original design were corrected. The vulnerable thermal exhaust ports were replaced with millimeter-wide heat dispersion ducts, leaving no weak spots in this battle station's armor.
From a purely technical standpoint, it was clearly a design flaw in that it went STRAIGHT DOWN. A simple P-trap (like the one under your sink) near the exit would have been enough to capture any incoming debris (including photon torpedos) near the surface of the Death Star where they would explode. They'd do damage there of course, but not allow a straight shot to the reactor core.
Or even multiple of these. Or having the vent make many turns or angles on its way up. Any of a number of simple engineering solutions would overcome this issue.
Well, it's only a fail because it allowed direct access to a vital area that potentially could be exploited for destructive means. But how big was the actual chance of anyone being able to exploit this weakness? It was reasonably well protected for such a "flaw".
Take a wall that's protecting you from shooters on the other side. There's a very small peephole in the wall that allows you to see what happens on the other side. Now there is a very, very small chance that one of the bullets will go through the hole and hit you. A better design would have been to install a mirror systen either externally or in a non-straight hole with an inbuilt reflection system to allow you to see what's on the other side or to protect the peephole by bulletproof glass.
According to the Star Wars book Death Star, the exhaust port was actually put there for no real reason. One of the characters, who was an Imperial slave, tried to prevent it from being built as there was already another port that would have rendered the battle-station safe, but alas it was built against her wishes. So, really, the port had no business being there; it was just bad luck for the Imperial Navy.
