I would have offered my own services, but it seems you're selling the book commercially, so that counts me out. Sorry.
EDIT: Thinking about it a bit more, and because I realised the above statement isn't so helpful, I think I'd go with a strong logo as a central image. The Earthfleet of your story and center that on all the book covers. Then you could denote each part simply by a change in background colour and obviously title. Doing it this way would mean you'd not have to focus on generating a brand new cover for every installment.
Let me work on a mockup to show you what I mean.