When Princess Leia Organa of now-defunct Alderaan shows up on Yavin IV with the plans of the Death Star, Sabine Wren gets to work. These schematics won't decipher themselves, after all…
AN:
I saw Rogue One and spotted the references to Rebels, and wondered about the characters. Obviously Hera and Chopper were on Yavin IV, and Kanan and Ezra couldn't be around or they'd have ended up stealing the show when the time came to get rid of the Death Star shortly afterwards. Of the core crew, that leaves Zeb and Sabine, who might be either with Hera or the boys (I'm assuming nobody dies between Rebels and Rogue One here). I don't know about Zeb, but here is Sabine's story.
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The instant Princess Leia Organa shows up in some dilapidated Corellian freighter that must be at least thrice as old as the Ghost, plans of the Death Star in hand, Sabine Wren, weapons expert extraordinaire (and let's be honest, they don't have a lot of those on base at the moment), gets to work.
They don't have much time, and those plans are extremely complex.
Oh, they find the parts related to the reactor relatively easily, sure, but after that…
None of those on the team tasked with finding the weakness implanted there by the late Galen Erso is a true specialist, and it shows – those who come the closest are two of the engineers who joined the Rebellion with the X-Wing's plans and the Mon Calamari psycho who came up with the B-Wing, and even they are scrambling to make sense of the plans, let alone find out that fekking weakness. Sabine herself might have extensive knowledge of weapons of all sorts, but unfortunately, gigantic weapons of mass destruction aren't quite her forte – for obvious practical reasons, she's always concentrated more on the stuff that the Ghost might carry.
What they really need is the genius scientist himself, but he's dead, so they will just have to do their best.
Frowning, Sabine goes around the projector table to see the reactor's blueprints from another angle. Like most products of Imperial engineering, they are clean and precise, each tiny detail clearly labeled – with unfortunately very obscure abbreviations and acronyms they need to figure out before they can do anything.
An hour into their examination, the only thing the assembled Rebels have managed to figure out is that the Death Star's weapon is not the super-turbolaser they thought it was.
"This is completely new technology", Quarrie the Mon Cal grumbles beside her. "How are we supposed to find the one thing that doesn't belong?"
They're all getting very frustrated.
Heaving a stressed sigh, Sabine moves again, picking up a datapad listing what they have managed to learn about the deadly space station as she goes.
Main engineers…
Engine capacities…
Fuel requirements…
Kyber crystals.
She freezes.
"Guys? Anyone knows what they needed kyber crystals for?" There are at least three separate uses she can think of at the top of her head, all three very obscure, but only one makes sense – barely.
"Didn't the Jedi use them for their lightsabers?" Somebody responds hesitantly. Sabine nods at him.
"So we don't know?" A confirming shake of the head from the X-Wing engineer.
Datapad forgotten, the young Mandalorian comes back to the blueprints once again.
It is obviously not a turbolaser, so she should stop trying to find the components of one.
Instead she thinks of crystals, and of the way she once observed Ezra while he was building his second lightsaber – officially to make sure he didn't blow himself up in Kanan's absence, but officiously because she'd wanted to learn how it worked.
The crystal was used as a focaliser, but it truly was the heart of the construction.
Where is the Death Star's heart?
Ignoring the startled inquiries from her colleagues, Sabine backs up several steps, climbs to stand on another table.
With this new plunging view of the blueprints, she is too far to read the tiny labels or distinguish the details, but that's exactly what she needs: to pick up the overall pattern.
And suddenly, she can recognize it.
There the generator, there a regulator, there the focaliser – or multiple ones, as appears to be the case here. She would bet some of the labels around it read "Ky", "KC" or some other arcane abbreviation for "kyber crystals".
The whole thing is a grossly oversized lightsaber, its energy needs so huge it may only be lit for a fraction of a second at a time.
The irony of that weapon design being used precisely for everything the Jedi once stood against is typical of the Empire, too.
In an instant, Sabine is off that table and explaining her theory to the others. Hope reborn, they pour over the blueprints with renewed vigor, and eventually they do find that crucial flaw, now that they know what they're looking at.
A tiny vent, going down all the way to the reactor. Passing through the chamber housing a truly huge kyber crystal that serves as the final, main focaliser. And the security measures on that vent are faulty, it is found eventually – according to their programming, they won't close off the chamber automatically in case of fire.
And kyber crystals are notoriously explosive under certain conditions, say if a proton torpedo were to find its way down there…
It is their only hope, but it is a fragile hope indeed, considering the impossibility of such a shot – and Sabine knows all too well that Kanan and Ezra, the Rebels' experts on such impossibilities, are currently away on a crucial, secret mission, and could never be called back in time even if communicating with them were possible at the moment.
They will have to hope the Force doesn't mind smiling down on other people than almost-Jedi, for once.