When's the megafactory?

Right now I'm on phase three of the space elevator, and it looked very easy. I've seen huge megafactories all over the internet and I want to build one. I had a factory for phase one of the space elevator, I built it, it produced everything I needed, and then I dismantled it. The same thing happened with phase two. The question is, will it just be an endless cycle of building and dismantling? Or will there be no point in doing this later in the game? When will the "megafactory" become a thing? Will it? I'm so lost.