Note: Strong as-in-hitpoints, not damage.
In GritEcon, I used values on the order of:
1 round ground combat.
100% damage factor.
Militia: 1 damage/30hp. 1 militia per million population
Infantry troops: 1-2 damage, 50-70 hp (depending on design and cost) Average build rates are hundreds/turn on planet yards.
Light Tanks: 10-20 damage, 200-300 hp. build rate of a handful per turn (5-10 or so)
Heavy Tanks: 50-100 damage, 300-400 hp. About one a turn probably.
Artillery: 300 damage, 50 hp. A turn or two each.
Two evenly matched infantry squads would give you trench warfare Lasting forever as they wear each other down at the same rate.
Basically, you need bigger tanks or artillery to kill things, and you need lots of infantry reinforcements to act as cheap ablative meat armor for your tanks.
When dropping from ships, the bigger units have the most damage per kt-space, but lack in hp per space and are the most expensive by an exponential factor.
You need to either drop a pile of infantry first to sop up the massive hits from the defenders and establish a beach head for your heavier units, or drop mixed troops from your boats
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As an extra bonus from the 1 round GC, having lots of small, lightly shielded transports deploy your troops is a valid strategy. The idea being that the enemy can't nail all of them, and they're cheap enough to replace often. It works because you know your troops won't all be killed before the single round of GC ends and next transport drops its troopers
PS:
Making orbital bombardment and glassings expensive and time consuming is a key part of the scheme. Otherwise they'll just sweep through, glassing and recolonizing. The troop hitpoints above help that a lot. Adding some heavy-armor "bunker" platforms to boost planetary hitpoints can help too.
Just be sure to remove planet-based weapons. If you leave them in, players will have way too much trouble getting their transports to attack, and they'll be forced to use a glassing strategy. Without planet based weapons, the transports will merrily charge in and drop while the orbital battle rages.
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In one game against SkyAshton, we did some nice fighting over a chokepoint system. I surprised him with an early assault on a key spaceyard planet before he started building infantry. Then with waves of ships duking it out across the system, I had my transports running in with infantry. Ground combat broke out on most of his planets, effectively blockading them from the inside. Meanwhile, in space my fleet was held off, and with a massive maintenance defecit, they fell apart. As I worked at home to rebuild the fleet with more maintenance-efficient hulls, Ashton has the chance to fight back (build cost is proportional to size
cubed while maintenance is constant independent of size).
He started with only the infantry he could Ebuild before my waves of troops hit, and his militia would have been getting thinned out pretty badly by now.
However, now that he held most of the system and could dump reinforcements from his homeworld and its stack of BSYs, time was on his side.
The spaceyard I had captured managed to fight off his attack, though, as it had been busy building some anti-ship fighters and sats(modded 50kt base hull). I came back with carriers full of anti-ship fighter-bombers and a few medium warships. With the help of the spaceyard I pushed him to the warppoint and held there while I flooded the system with troops in an attmept to secure it.
Most of the planets had fallen to my light tanks by the time Ashton had come up with another counter; kamikaze rammers.
Since I was close to his homeworld, he could pump out tons of tiny ships and not be killed by the maintenance costs, since they would all die ramming huge holes in my ships on the same turn.
The game ended before he had a serious chance to attack the chokepoint system again.
Overall this was about 50-70 turns, about a third of which was pre-meeting expansion and buildup.