[Moejoe]

So here's a quick mockup of how I'd approach it. First I'd design some kind of military insignia. In my example I quickly mocked up an eye overlooking three bars, but if I had time I'd include wings of some kind and a symbol for the earth. Center this insignia on every cover, then all you need change is the colour of the background and the text and you're done.

The added bonus of this approach is that your 'Earthfleet' insignia then becomes a powerful t-shirt design and brand, which you can then use across the board. Think of the Star Trek upended 'V'.