This fleet is weak, puny, and terrible, and my fleet will tear it to shreds!!
...and now, I shall stop RPing as
Precursors and give actual commentary.
First, quick check: I can reproduce your engine as Ion Drive, 120% boost, Fuel Consumption 0.6, and size 13.5 HS which is 675 tons. Assuming I have this right, this means you have 1,350 tons of engine on the corvettes and twice that on the cruiser, which if we use the derived optimum engine:fuel ratio of 3:1 means you want to have around 450 tons of fuel or 450,000 litres (and twice that for the cruiser). You definitely have room to work with in this case, and should still see a reasonable range of about 20b km so I'd say your engine design is reasonable. On the other hand if you want the 40b km range you have on the Do You Feel Lucky? class then the same engine with 90% boost will be optimal giving ~3,040 km/s speed. I'd say stick with the engine you have and reduce fuel.
On the Chevalier B, I recommend using two reactors of half the size/power, this is not as tonnage-efficient but gives you redundancy in case one is taken out by battle damage. For the same reason I would double up on the active sensors (thermal and EM can be singular as you don't need them to fight). With the space you gain from cutting fuel you should be able to mount another laser as well - though I thought being able to mount multiples of a spinal weapon was a bug? Ah well, use what you got.
For the Do You Feel Lucky? in the same vein I say double up the FC and actives, however you only need the anti-missile sensor - the RES10 sensor gives you no benefits as the RES1 sensor will still detect anything larger than 50 tons at its maximum distance. You can't fire any farther than 20,000 km anyways. As before reduce the fuel load, I don't like how it is so lightly-armored so I would probably see about beefing that up. Remember, every additional layer N of armor prevents you from taking internal damage from a missile hit from an N^2 strength warhead (on undamaged armor). I would ideally like to see N=3 layers of armor (proof against strength-9 missiles), although N=2 is fine against
Precursor missiles in my experience. The idea here is not to have an invulnerable ship, just to be able to take a few stray missile hits before internal damage cripples the ship.
Command cruiser looks fine for what it does, again cut the fuel load and mount either extra armor or (my preference) some PD guns. You can make space by removing the redundant passive sensors which again are not critical in combat situations, but since you may want to maintain passive sensor contact in some circumstances this isn't a requirement.
General fleet design: For what it is - a bunch of ships quickly churned out for emergency defense - it's fine aside from individual ship issues above. However for a more permanent fleet I would prefer to see the main combat ships be considerably larger - not battleship-sized monstrosities but 10,000-15,000 ton mainline cruisers which are a lot more capable and
durable than small corvettes. The big benefit here is that you can design your main line cruisers to be truly capable of independent operations by putting both anti-ship and PD weapon batteries on the same platform, which means your fleet overall becomes much less vulnerable to losing all of one kind of ship. In one recent battle against a missile-wielding enemy fleet I had to retreat-in-order after the initial missile spam took out all of my plasma-armed ships while my PD capabilities remained mostly intact (this...wasn't my best fleet design). I still had the majority of my fleet but lost the capability to actually win the battle even after closing in.
I would suggest having corvette classes for PD and sensor platforms and main line cruisers carrying the lasers and some PD, along those lines of discussion. All that said, fleet doctrine is ultimately a matter of personal taste and philosophy, and the fleet you have here is certainly capable of succeeding as long as you're not horribly overmatched in tech or numbers.
