Well, 100 cm is the minimum needed for an object to be immune to fighter weapons. Those weapons capable of penetrating phase shields of that strength do so by bypassing the shields, not by tearing them down.
Regarding the shield/weapon upgrade cycles, what seems to happen is that, as more powerful weapons become available, shield power and frequency is increased. Shield frequency is the rate at which the ship's onboard computers can modulate its phase shields--this is important because unless the shields and weapons can be synchronized, all weapons fired by the ship would impact against the inside of its own shields.
Anyway, advances in computers make it possible to modulate a capship's phase shields faster than enemy torpedoes can handle, which results in said shields becoming impenetrable until the enemies can successfully make computers fast enough to match the modulation that are also small enough to be carried aboard torpedoes. Likewise, fighter computers must catch up with the modulation frequency of capships in order for fighter guns to damage capships. Then the cycle starts over again, with the development of computers to modulate the capship's phase shields even FASTER.
We see this cycle repeated at least twice in Wing Commander. At the time of Action Stations, capship shields are impenetrable, but then the Kilrathi create shield-penetrating torpedoes. By the time of WC1, all fighter weapons can penetrate capship shields, but in WC2, only torpedoes and capship-based guns can penetrate again. In WC3/4, the balance has swung the other way again, and all fighter guns and missiles can punch through capship shields, although the Vesuvius shows that the balance is swinging back since Blair must fly inside the hangar in order to destroy it. Finally, in WCP/SOP. only torpedoes and plasma guns are capable of destroying capships, although BuWeaps is probably working on getting around that...
Regarding the shield/weapon upgrade cycles, what seems to happen is that, as more powerful weapons become available, shield power and frequency is increased. Shield frequency is the rate at which the ship's onboard computers can modulate its phase shields--this is important because unless the shields and weapons can be synchronized, all weapons fired by the ship would impact against the inside of its own shields.
Anyway, advances in computers make it possible to modulate a capship's phase shields faster than enemy torpedoes can handle, which results in said shields becoming impenetrable until the enemies can successfully make computers fast enough to match the modulation that are also small enough to be carried aboard torpedoes. Likewise, fighter computers must catch up with the modulation frequency of capships in order for fighter guns to damage capships. Then the cycle starts over again, with the development of computers to modulate the capship's phase shields even FASTER.
We see this cycle repeated at least twice in Wing Commander. At the time of Action Stations, capship shields are impenetrable, but then the Kilrathi create shield-penetrating torpedoes. By the time of WC1, all fighter weapons can penetrate capship shields, but in WC2, only torpedoes and capship-based guns can penetrate again. In WC3/4, the balance has swung the other way again, and all fighter guns and missiles can punch through capship shields, although the Vesuvius shows that the balance is swinging back since Blair must fly inside the hangar in order to destroy it. Finally, in WCP/SOP. only torpedoes and plasma guns are capable of destroying capships, although BuWeaps is probably working on getting around that...