Hyperdrive

Well, almost invulnerable. Lemelisk had disappointed him in that instance. The greatest challenge in designing the battle station, he had said, was not creating a beam cannon big enough to destroy a planet, nor was it building a moon-sized station that would be driven by a Class Three hyperdrive. The greatest challenge was powering both of them. There must be trade-offs, he had said. In order to mount a weapon of mundicidal means, shielding capabilities would have to be downgraded to a rudimentary level.

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Star Wars: Death Star Part 1 Chapter 2