Sorry for that I still experience the same problem.
May be I am not clever enough the system.
Here i put a video link on how I try to save a fighter.
Would you mind please take a look and see what I am doing wrong?
I think the game requires every fighter to have a fuel tank, and you can’t save the ship design unless that module is present.
It seems to be the case with every hull type that certain conditions must be met before you can save the design.
Aside from fighters needing fuel tanks, every ship must have sufficient crew and power.
Yes this is true, you need a fuel tank, but this is clearly not obvious. I’ll include a tutorial pop up for when people try this…
I would suggest a hover notice on why a ship cannot be saved 
As it is quite frustrated that no hints ever tell me I need a fuel tank for the fighter, it would probably lead to some player leave and think the game is bugged.
I have been fans of the GSB since it first release, still I cannot find why, I think it will be a serious problem for new player to figure out why they cannot save.
The latest version actually pops up a tutorial window to explain when you try it.