invading with multiple transports

I don't think it matters whether you do it all at once or one at a time, except that you would get to "re-roll" the "advantage ratio" for each ship instead of just doing one for the whole stack. it's just a matter of throwing enough troops at the planet until you overwhelm them.

"At the moment when I invade, I have no real idea what I am doing and just send hordes of ships until I win."

I'm in the same situation. I have thousands of minions in my army of storm troopers - surely some of them are capable of giving me an "estimate of success" calculation.

There is a definite element of risk involved in planetary assaults as there is no way of knowing whether they have built up a bunch of planetary defenses.

Comparing your respective soldering stats would give you a feel. The attacker gets an additional advantage. My personal preference is to only go in where possible with superior soldering.

It is a good idea to send in one transport on your very first attack on an AI just to see the odds.

Bring as many as you can. Even if you don't take the planet, you cause more damage than you take, so you reduce their econ accordingly.
What I do is look at star system population first. It's often easier to conquer the less populated star in a sector if there are several, then to build culture form there and flip the others. Otherwise, after the first battle, you will realize how many you need, but you have to land to get an idea. Then you can decide to use devious forms of warfare before attacking.
I wich there was a nice cancel button if you realized you don't want to attack.

Here's a tip for taking high-population systems with a limited number of transports.

First, send them in to attack one at a time, knowing that you'll loose the first few battles. This is a good thing, because you can use some of the other attack methods (besides "traditional attack") to give you a big bonus. You'll kill a lot more of the enemy this way before you die than with the "traditional attack." Here's the nice thing - if you use one of the alternate attack methods and loose the battle, none of the negative consequences from that attack method happen to the planet, such as PQ and infrastructure drop.

Repeat this process until you can win using the "traditional attack" and you'll have taken the planet quickly with fewer transports. Just be careful that you don't inadvertently win using a non-traditional attack, or the planet will suffer the consequences.

There is one attack worth trying too, which is the psychological warfare (or whatever the name). It costs 2200bc but doesn't deprive PQ or whatever. It just takes some opponents and brings them on your side. You may want to spend on destabilization the turn before you attack in order to augment the number of converted people. This won't work for high morale planets, but I found it a very good way to win battles with less troops. Furthermore, you get more people on the planet after the win.

I guess this would work but doesn't seem very fair (it's ok for tle human player I think, since he would wim, but for the AI... (:( ).

I usually stack a lot of them, and it can take ages. Then I attack. If I have to level the planet surface in orer to take it, I will. But that goes against my better judgement. I usually go for traditional warfare, except if I'm loosing and taking a vital planet from the AI is imperative in order to survive. then, all means to stay alive are justified. Including rushbuilding a terror star, rushing it to the right sector at hyper-snail-speed and vaporasing 2 or 3 enture star systems in the heart of their empire, thus buying me time to reenforce myself and take the other planets the "old-fashioned-way".

My previous post was a comment of Callipygea post. But LDiCesare posted before I do.

By the way, LDiCesare does this method wrok properly? I read somewhere that it would actually increase the enemy pop due to some calculation eeror with integers or something.

I have run into a bug where as I try to invade a planet, the scary combat music starts and the rest of the game freezes. It is endlessly frustrating! I think it has been a few weeks since I've been able to finish a game because of weird bugs like this.

The method that flips part of the planet's population to fight on your side is 'information warfare', and it's currently bugged, so that is sort of a 'use at your own risk' method until they get it fixed. CariElf knows about it, so I imagine the fix will be in soon.

If you have a culture advantage I agree with LDi, it's a lot more manpower efficient to conquer the easiest world and then flip the rest. Invasions are just so costly that you want to minimize them as much as you can. You can also bring a bunch of constructors in with your invasion, take one planet, then build a party palace. Either way I normally try to take a central planet when invading, one where there are enemy planets in all the surrounding sectors, that way when the flipping starts it gets really brutal for your opponent.

Also note that by taking the soft targets, then culture flipping the rest of the sector, you're boosting your population instead of depleting it.

The method to my madness is to always bring extras. Even though I win in the end with a culture victory, on the way up that tech tree I make sure I got some +soldier mods. Then I assume that I am only going to come out with a 2 to 1 advantage when, in fact, I may have upwards of a 5 to 1 due to bonuses. So, if I got 2 to 1, then I have to bring 50% of the planet's population in invading troops. And, of course, since I am a cheap bum that doesn't want to wreck the place, I use traditional warfare. The only shortfall to this is that sometimes I am so successful that I do not have enough defensive ships left in the stack after leaving one behind to defend the latest conquest to take the next planet with all my extra transports and leave a ship there as well.

Now, that was attacking to capture. When just raiding the enemy, it matters not if I do not have the 50% troops that I prefer. Then I just throw anything I have to spare at the target. If I whittle them down a bit, great. If I take them with a small force, then I will try and figure out how to hold onto the place if my stack does not have enough extras to leave something behind.

But, I tend to go into invasion wars slowly and surely. I know that brushfire wars can start at any time, but I will not be rushed into a ground war prematurely. I would much rather bide my time in an air war holding the enemy at bay in space until my billions and billions of loyal troopers and their escort fleet are ready to slowly trudge their way to whatever the first target on my list is.

Back to the original question, here's what I've observed with attacking with several transports.

If you attack with a stack, all of the marines aboard all the ships will take part in the invasion. (More marines seem to improve your odds, too, though I wouldn't bet on that.) After it's over, if you have a lot of people left over, they will reboard the transports (but you always use up at least one in the invasion).

Some examples:
- You attack with 3x5000 transports and lose. All transports disappear.
- You attack with 3x5000 transports and win with 500 people left; all transports disappear.
- You attack with 3x5000 transports and win with 8000 people left; you end up with 3000 people on the planet and 1x5000 transport.
- You attack with 3x5000 transports and win with 12000 people left; you end up with 2000 people on the planet and 2x5000 transports.

(I'm pretty sure the regular transports work the same way as the combat transports, but I don't think I've ever mixed them.)

Everybody else see similar results?

Yes, that is how it works in my experience, Feargus. If you win, the transports that can be fully filled with surviving troops are returned. The remaining fraction of a billion (assuming one is using normal transports) are the new colonists. Using traditional warfare, there are two advantage gained by using single transports one at a time versus a full assault. One, the exact advantage an enemy has is unknown until actually engaged, so sacrificing a single transport to see what the odds range is could be very helpful. That way the rest of the force can sit back while you bring friends if it doesn't look pretty. The other advantage is, of course, the reroll of the odds each time.