Ships can be armored with thicker hull plating and more durable bulkheads and framework. This can only be performed during construction of the ship, and is only available at Class I SCCs. Light armor doubles the cost to HS x 100,000 Cr and provides 6 hull points per hull size, medium armor doubles again to 200,000 Cr per HS and provides 7 hull points per hull size, and heavy armor doubles once more to 400,000 Cr per hull size and provides 8 hull points per hull size.

Source - House Rules Wiki: Star Frontiers US (dead link)