Well, the next phase needs new meshes and a new CSM

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However, most of the work on the SIVB would be compatible with Skylab; we'd need an option to create a Skylab mesh with or without a J-2 engine, and to have different docking meshes on the front. If someone wants to build those meshes, then the code for that part shouldn't be hard.
Plus as discussed on the Orbiter Forum, that would also mean we could do the Lunar Skylab.