Bit of a powerful bug.
Fighters can get upgraded on the fly. Changing the design of the fighter in the ship design menu instantly changes all existing fighters, it also doesn't seem to care if the new design is not a fighter anymore, I can send a swarm of fighters and turn them from having 1-5 armour 475 tons into 150,000 ton behemoths with 40-253 armour and 20 meson cannons and 1000 engines.
My guess is the game needs to lock built and building fighter designs like it does for other ships and then be able to obsolete old fighter designs so they don't clog up the ship designs list.
You're supposed to lock all designs prior to building(fighter/ship/pdc). Steve added to lock control to the design screen for this purpose. If I recall correctly, the reason Steve has not added a check to constuction is that in SM mode the lock can be turned off.
All ship designs are auto-locked once a shipyard is retooling for it and locked while it is tooled for a design. Fighters though aren't built in shipyards so they don't get locked. I can understand the SM mode to unlock existing designs on shipyards for when people forget to add that one component to their design. The optional lock design option is so you don't make accidental changes if you are just browsing your design. This bug exists for fighters and PDC's and any ships which don't have design on shipyard.
What needs to be changed though is locking designs if there are ships existing with that design, the same bug occurs if you retool a shipyard from one design to another but still have ships on the old design, any changes to that design will affect any ships in existence.
Current
If shipyard has design as assigned class or shipyard is retooling to design then lock design
Proposal
If shipyard has design as assigned class or shipyard is retooling to design or population industry building design then lock design
If ship has design then lock design
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My understanding is that SM mode is for RP purposes and bypassing any minor bugs that might occur (like PDC going on Task Group training and ending up in the middle of the Sun), while playing without SM mode the game should run as a normal challenge and shouldn't really have exploits, im guessing this is true because ship designs in shipyards are automatically locked by the game and cannot be unlocked unless using SM mode or no longer having a shipyard tooled for the design.
It just looks like PDC's and Fighters are overlooked as well as Ships that are no longer tooled for in the Shipyard, so my idea of solving the problem would be checking if the ship exists (so is on ships list, doesn't matter if dead) and if ship exists then the design is automatically locked by the game, it will also lock the design if fighter or PDC/PDC component is being built. Wrecks aren't affected by design change (I think, not 100% confirmed as don't have presence in sector where I have a wrecked ship of my design) so that shouldn't be a problem. I don't think Aliens can board your ships yet and I'm not sure what happens if you gift/surrender a ship to an alien and then change the design, I suppose that could be solved when ship changes ownership the faction that gets the ship automatically creates a design of the ship which is locked so the original faction of the ship can change the design if that was the last ship of theirs and it shouldn't affect the new owners.
Right now it's a bit silly to be able to put a bunch of layers of armour on a ship just before missile impact, although would be interesting to see how the game reacts when you take away that armour and then when you add it again, I wonder if it would refresh the armour or if it would remember the damage allocation.