when used skillfully, corvettes are excellent weapons
they have decent attack power which allows them to take out most starbases, attack trade routes, take out colony ships, chase down constructors and freighters, etc. and since they are so cheap to make and maintain, it doesn't hurt to have a lot of them. If I have the tech, I never really stop making them.
If I am frigate rushing an AI and I have the corvette technology, I'll have corvettes near to destroy the colony ships parked in orbit.
In fact my military numbers looks like the following:
at INTERSTELLAR TACTICS - frigates
NORMAL WARFARE
33% frigates
67% corvettes/star fighters
corvettes/star fighters used for protecting transports, harrassment, and random attacks. Here the corvette is far better than the star fighter. Corvettes actually have a chance at taking out an enemy transport.
ALPHA STRIKE
67% frigates
33% corvettes
a surprise attack, transports do not need protection, and I have a few corvettes on hand to do mop up, I have a lot less ships than under normal circumstances. Frigates are really expensive.
after BATTLESHIP TECH - capital ships
40% capital ships
20% frigates
40% corvettes/star fighters/battlecruisers
ATTACKING UNKNOWN TERRITORY
capital ships are the center the attack attack group, followed (sometimes at a distance) by combat transporters
frigates leads the capital ships with corvettes at point
corvettes are placed with transports, at right and left
flank of attack, sometimes frigates are placed at the flank, too
when enemy ships come into sight
capital ships engage enemy captial ships, combat transporters, and frigates
frigates pick off non-capital ships and clear systems off transports, colony ships, freighters, etc. sometimes I uses corvettes for this job too.
corvettes attack trade routes / protect combat transports if I have a stack greater than 5 / luring enemy ships into attack range / protect flanks of a attack group (advance warning anyways - I try to take sensor drones, but sometimes they are just too slow)
corvettes are just so cheap I can make an endless supply even on undeveloped colonies with low military spending rates and the starship bonus that you would like for your capital ships isn't really necessary.
On higher spending rates, I build battlecruisers instead of corvettes and less frigates, since battlecruisers can do most of the things a frigate can do and are far cheaper to maintain
As for leaving the UP
I haven't found it necessary to leave, but it would be quite useful in several cases:
20% warfaring tax
terror stars
economy tax
you can still trade with minors right??
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