The document discusses four potential economic models for upgrading ships in a game:
1) A research points system where enemies drop points that can be used to upgrade weapons, engines, or shields.
2) A modular system where ships have upgrade slots that can be filled with modular upgrades.
3) An acquire parts system where enemies drop random upgrade parts that must be collected to improve systems.
4) A significant upgrade system where defeating squad leaders guarantees a large upgrade to a chosen system.