This is The Bulwark, aka Rock Cruiser. When The Kestrel came to Rock Homeworlds, they met a Rockmen assault ship that hailed them. It seems that Rockmen want to help but they will help only to those who are worthy. So The Kestrel’s Crew was invited to a challenging fight. After having some of their system-less rooms cleverly hit by the Kestrel, the Rockmen surrendered and considered The Kestrel’s crewmen worthy of their help. And that’s why Federation received blueprints of one of the toughest, bulkiest, most ■■■■■■ ships in the known universe.
The in-game appearance: