Full Arsenal is a Ship Achievement for the Kestrel Cruiser.
The Kestrel Cruiser is missing five systems initially. These systems and their respective Scrap costs are as follows: Teleporter (90), Drone Control (75/85, depending on which Drone it comes with), Cloaking (150), Hacking (80), and Mind Control (75). However, as there are a possible maximum of eight systems altogether, only three out of the five need to be purchased to earn this achievement. Advanced Edition systems can still count towards the total. Stores commonly offer each of these systems in various sectors throughout the game, so there is little need to rush and buy them early.
The newly-introduced Backup Battery (35) Subsystem counts as a system for the purpose of this achievement.
(Note: It seems that at least in the advanced edition the only requirement is "11 systems". Namely, "primary systems" and "subsystems" are not distinguished from each other.)