Here is a couple of Planetary Defense Ships I designed lately - the Scutum Class System Defens Ship and The Pilum Class Trans Atmospheric Fighter.
None of those vessels is able to go to warp. And the Pilum Class Fighter is not much more than a flying Seat.
I figured that planetary Defense Ships normally have not much need of a Warp Engine in the First Place, so i decided power them with Fusion reactors instead.
enjoy!