I've always thought the generational lineage of Freespace ships to look like this:
1st Gen. - Early exploration era, subspace discovery, so colonisation vessels and soforth.
2nd Gen. - First military warships, Orion et al. Years prior to and including 14 Year War & GW.
3rd Gen. - Reconstruction, advent of primitive Beam Weapons. 3rd Gen weapons on 2nd Gen Hulls; early retrofits.
4th Gen. - NTF Reb/GWII; Modern Beam Weapons, Flak. 4th Gen Weapons on 2nd Gen hulls; modern retrofits. New 4th Gen hulls; Hecate et al. Start of phase-out for 2nd Gen. hulls.
That really covers the major generational changes in Terran ships, well, capital ships at least. Of course, fighters would be a wide spectrum, ranging from the 2nd Gen. Medusas and Ulyse.. Ullyse.. Uyy.. The Bat, to the 3rd Gen. Myrmidon, to the 4th Gen. Erinyes.