Board the freighter and you will come across a weapons locker next to a watertight door. A control panel to open the next door will also be in that small room.

Before operating the control panel, equip the Dethex Launcher and use the secondary function to put a guided grenade (push the GREEN 'A' button to jump) through the grate maze in the wall and floor of the room.

Use the bomb to blow open the locked watertight door. Inside, you can get a weapon tech pick-up and revert a message for this achievement.