Why did the rebels need the Death Star plans?

Question

In the Rogue One film, Galen Erso sent a message to Saw Gerrera and the rebels, in which he has revealed that the Death Star has a serious backdoor vulnerability: shoot a torpedo into the reactor and the whole Death Star will get overloaded and explode. A New Hope proved that Galen was right: Luke shot the torpedo and indeed the Death Star exploded.

But Galen also instructed the rebels that they should get the plans of the Death Star from the imperial citadel on Scarif. Why was this necessary? Wouldn't it have been better if Galen told in the same message which hole on the Death Star it is where the rebels have to shoot the torpedo to the reactor, and just ignore the plans altogether? Getting the plans from an imperial complex was dangerous, and Galen would surely have anticipated that danger.

Answer 1

According to the official novelization by Alexander Freed, he didn't know how to get the explosion to the reactor, only that it needs to be exploded:

Galen snapped back into focus, no longer hesitant or soft. “Saw, the reactor system, that’s the key. That’s the place I’ve laid my trap. It’s unstable, so one blast to any part of it will destroy the entire station.”
...
“You’ll need the plans, the structural plans, to find your way, but they exist. Sabotage from the inside is impossible: Krennic is too paranoid. But I’ve thought about this, Saw, prepared everything for you I could.”

And he clearly didn't have access to engineering plans to the whole station himself, being just in charge of the weapon. He knew where plans existed but didn't have access himself (else, he'd have said Eadu has them too, or as you said, would have sent them with the pilot):

...
“I know there’s at least one complete engineering archive in the data vault at the Citadel Tower on Scarif. Use what I’ve told you, run the analysis, and you’ll be able to plan your attack. Any pressurized explosion to the reactor module will set off a chain reaction that will-”
(Chapter 6)

Answer 2

There were two parts to the Rebels destruction of the Death Star. The first was the knowledge that the reactor was unstable, that any explosion there could cause a chain reaction. That was the easy part.

The second part was that there was a tiny vulnerability, a way to get ordnance down into the core to trigger that explosion. To find that vulnerability, the Rebels needed the plans.

Without the plans, they would have had no way to exploit the weakness Galen revealed. Without the weakness, they'd have had no idea what target to look for in the plans.

Answer 3

He did not have the technical details to give or else he would have. He was working as an administrator of a kyber crystal refining operation. He worked on the reactor design and was able to create the thermal exhaust port weakness, but that does not imply that he had access to the final architectural schematics. In fact, it is pretty clear he didn't as if the plans were available anywhere easier to get to than Scarif he would have said that. So when he submitted his part of the designs the vent was probably just a line drawn on a circle. So he knows the vent exists, but without the final plans he does not know the coordinates of the vent on the Death Star. Even if he had a general idea from, say, seeing the designs in a meeting, that would probably not be good enough for the sort of fast precision strike needed.

Answer 4

A common sense answer here is that Galen didn't have the exact plans on him, of course, so he would have had to describe it from memory. Keep in mind the Death Star is the size of an entire moon. And as we see in ANH, its surface features look pretty much the same everywhere. So, it'd probably go something like this:

Galen: "So...what you need to do is, you see...so there's a trench somewhere on the north-east hemisphere that leads to the weakness. Well, there's about seven thousand trenches there, but you want the one that's...well it's near a tower. One of the identical four billion towers. Also, there's an exhaust port, but THIS ONE goes to straight to the reactor. Oh, but there's also about seven hundred thousand exhaust ports in that section. Hmm...you know what. Just go get the plans, it'll be easier."

The rebels didn't have time to fly around the Death Star looking for the right port, just shooting into every one they saw to see if it stuck. They needed to know exactly where to go, immediately, or risk getting annihilated before the plan would work.

Answer 5

The real-world situation is this:

In the 1977 film, there was never any indication of a deliberate weakening of defenses. The rebels stole the plans in the hope of finding an accidental weakness, and found one.

Once Star Wars became the best film evah, and subject to endless analysis by people who analyze movies instead of getting laid, this sequence of events began to seem rather implausible. The builders of the Death Star were unable to find the flaw, despite decades of intimate familiarity with the plan, but the rebels were able to uncover it in just a few hours?

When the new film came out, the producers wanted to patch the problem: in the revised story line, the weakness (the unshielded "thermal exhaust port") was deliberately introduced to the design and deliberately disclosed to the rebels. None of this was mentioned in the previous movie, but neither was it contradicted.

There is still a problem, albeit a smaller one. The natural behavior of the saboteur, Galen Erso, would be to commit the location of the weakness (and whatever other information necessary to exploit it) to memory, or at worst, to some summary form, and pass that information along directly.

Of course, if the screenwriters allowed their character to do that, A New Hope would be retroactively made pointless, and they wouldn't have much of a story for Rogue One to boot. Instead, Erso carefully creates the weakness and only passes along enough information to tell the rebels what to look for in the plans, which the rebels need to obtain for themselves.

Had I been the screenwriter, I would have made Erso's plan to be the sensible one: to tell the rebels, "If you put an torpedo down the unshielded vent at 38.061927°N, 122.652542°W, the whole thing will blow up."

Then, of course, I'd kill off Erso before he could convey the details and give Jyn a line like "Alas, my father died before he could give us the exact location of the hole in the Imperial defense! Wait, the hole must be in there somewhere. If we analyze the plans carefully, we can find it. All we need now is to steal the plans."

Answer 6

It was necessary for purposes of data security. Galen made the rebels aware there was A vulnerability. If the message had fallen into the wrong hands (a third party who could sell the vulnerability back to the Empire) then all would have been for naught and the empire could dish out credits to the vendor and then patched the hole. Instead, he gives clues that really only the rebels could leverage which wouldn't be valuable to anyone else. The rebels could retrieve the information themselves and know all the details without anyone in between potentially being the wiser.

They call this a handshake in tech circles. I suspect similar patterns of communication are common in tradecraft as well.

Answer 7

After reading all these comments, I had a great epiphany. Erso was in charge of only the superlaser, not the whole Death Star. He embedded the unstable reactor as his trap but did not know how to exploit it. The vent was a coincidence or a result of the reactor needing it, as requested by Erso. However, he did not know how the vent would be built or where it would lead to on the surface. Thus he told the rebels to get the plans and find a weakness for themselves.The imperial records on Scarif contained the entire engineering plans for the Death Star, codename "Stardust". Since the planning and building of the Death Star began long before Erso joined to build the superlaser, "Stardust" as the codename for the Death Star PRECEDED Jyn's nickname. So Jyn was named after the Death Star...