A normal ballistic missile submarine has one pressure hull, with a large section of ballistic missiles taking up the middle of it. This submarine has two pressure hulls, on either side containing no missiles, but sandwiching the missiles between them. In theory this means that you can torpedo the sub from a side, and the missiles are still okay. But it also means that the sub has ludicrous amounts of space available. No missiles taking up pressure hull space, and two, not one pressure hulls. So everything on this sub got to be more spacious and there was room for extras.