The Tech 2 Anti-Air Flak Artillery are mid-level fixed-position anti-air defenses. Their Flak cannons are direct fire weapons, but the shells explode in mid-air, potentially damaging several aircraft at once. Flak artillery can be built on water or land. They are great for defending mass extractors from air raids, or really anything from air raids. They should comprise a plurality or majority of your AA defense unless you really love SAM launchers.