Ram Tah, at Phoenix Base in the Meene system! If not yet unlocked, the path is Dweller followed by Cheung.
You need:
- Caustic Crystals found drifting around in an active Maelstrom cloud, or on rare occasions around a destroyed Titan.
- Hardened Surface Fragments from the surface of an active Titan; they are quite small and you need to get close to target them, and I may make a video if many Commanders have trouble with it. They can appear on either side, although usually I check the Core side. They need to be shot free with a weapon, although using a Abrasion Blaster gives many more items than using a laser.
- Tactical Core Chips, which can be salvaged occasionally from destroyed Revenants and Banshees. As mentioned recently here alongside a means of listing viable locations, said locations are becoming increasingly rare. The best is an active Spire site, which we need T. Cocijo to reoccupy before that is available, but otherwise you can use Military settlements in Thargoid-controlled systems—the few which remain!
Assuming you reach engineer rank 5 first, each weapon needs
two Fragments, one Crystal and one Core Chip.
Additional: I forgot to mention that salvaging a Tactical Core Chip can be done from the air; despite being too small for sensors beyond ~50 m, Collector limpets are very apt at sensing items on the ground!