My ships are named after eagles since I am quite fond of birds.
t1 - USS Osprey
t2 - USS Harpy (best bird ever since it eats monkeys and I hate monkeys)
t3 - USS Martial (2nd most awesome eagle)
t4 - probably USS Aquila as I think the Exploration Cruisers deserve such a cool name. Aquila is latin for a genus of eagles including the Golden eagle.
t5 - not sure what it should be. I don't really have any love for the sovereign so I might buy a expl. cruiser retrofit. Maybe call it USS Harpy II or something. Or do you have better ideas?