Well how does the forcefield hold up against mark v bombers, say even 30 of them? Cause that's what you'll be getting at exogalactic waves.
Also, it'll be pretty late in the game when you can actually use your Knowledge for those mark III forcefields. Unless you want to sacrifice everything else like new ships and command station upgrades, which you shouldn't cause then the exos will shred you to pieces.
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I'd be fine with more expensive but much more durable forcefields. Cause right now what the hell can you do when the first exowaves just decide to beeline straight to your base? You have no time to kill them and you won't have mark III shields either. Which means, you're dead no matter how you approach it.
Well, I don't have AI war available to me at the moment, but I can work off my previous math.
If a Mk. III has 28,800 base damage, then a Mk. I has a third of that, which is 9,600. Thus, a Mk. V has five times of that, which is 48,000. Once again, 6x structural bonus, so 288,000. With 30 Mk. V bombers, that's 8,640,000. Again, Mk. III forcefield has 56,000,000. Thus ~=6.481 salvos, which with 12 seconds per firing, and depending on how well they are "staggering" their shots, is 77.78 to 84 seconds for 30 Mk. V bombers to take out a Mk. III forcefield.
So yea, Mk. V bombers hurt, but not enough that you can severely cut down on their numbers and still expect just as good results.
EDIT: Typos fixed
EDIT2: Oh you are wondering what would happen if you didn't have Mk. III forcefields. Well considering that 30 Mk. V bombers have less firepower than 100 Mk. III bombers, I can safely say that the Mk. II forcefield will last even longer than the previous scenario. So in other words, a Mk. II forcefield would hold up against 30 Mk. V bombers a good deal longer than 20 seconds.