The Heavy Fighter is a more powerful version of the Light Fighter. Although the Heavy Fighter is more powerful than the Light Fighter, it isn't used as much as the light fighter because of the high Crystal to Metal ratio compared to the Light Fighter. Even though it is not as effective as the more powerful Cruiser, the Heavy Fighter can still cause a reasonable amount of damage when launched in significant numbers.
Uses and Advantages
Against cruisers, the heavy fighter is a better Fodder unit than the light fighter due to the fact that cruisers have no Rapid Fire against them. This is especially useful in the early game where cruisers are the dominant combat ship. With more than double the armor rating of the light fighter, they can better withstand hits from smaller defenses that would easily destroy light fighters. While cruisers are more powerful and have a better metal to crystal ratio, heavy fighters are still better as fodder because you can build more of them, cruisers cost Deuterium to build, and the same ships have rapid fire against both ship types. An often overlooked ability is its rapid fire against Small Cargo Ships, a trait shared only by Battlecruisers and Deathstars which cannot be built until much later in the game. Thus, it could be useful early on to support your fleet when facing small cargo fodder.
The cruiser and Battleship are better combat craft and the light fighter is a much more cost-efficient fodder unit. Most of the advantages heavy fighters have over light fighters are rendered moot if you're not facing a fleet of cruisers, and the better armor is of no use against stronger defenses and capital ships which can easily rip through them in a single shot. They will also be quickly dispatched by a fleet of battlecruisers due to rapid fire, while battlecruisers do not have rapid fire against light fighters. A relatively high crystal cost along with higher fuel cost means these ships require a lot of deuterium and are not as practical, especially as players develop their empire and are more pressed for crystal.
In developing the heavy fighter, researchers reached a point at which conventional drives no longer provided sufficient performance. In order to move the ship optimally, the impulse drive was used for the first time. This increased the costs, but also opened new possibilities. By using this drive, there was more energy left for weapons and shields; in addition, high-quality materials were used for this new family of fighters. With these changes, the heavy fighter represents a new era in ship technology and is the basis for cruiser technology.