SS Combat Rules Systems Precis:
CT Bk2: Miniatures ruleset for ship combat. Newtonian. Sinple, ast, but at listed scale, takes considerable set-up space.
CT Bk5: High Guard: table based combat ruleset which abstracts combat extensively.essentially 3 ranges: Close, Long, and disengaged.
Mayday: a streamlined boardgame version of Bk 2 combat, using past/present/future markers. Later printings included advice on using Bk 5 combat tables.
MT: a High-Guard variant, using non-vector, non-newtonian movement, but using movement (squares per turn = Agility).
TNE: Used a simplified version of Brilliant Lances, and narrative style of play.
Brillian Lances: Full set of tactical starship rules, showing combined materials from T2K and Mayday.
Battle Rider: Streamlined, large scale tactical fleet combat wargame. Derived from (but not integratable with) BL.
T4: Not certain. I have it, I've read it, and never made much sense to me. Derived from hybrid of HG and BL, from what I understand.
GT: GURPS Space 3 based rules. Don't play GURPS anymore, so no comment.
T20: HG and D20 hybrid. Works as well as the rest of T20. (So, based upon your opinion of T20....) Actually not directly mechanically related to HG for comabt mechanics, but has very similar results in many cases.
T5: FNORD!