OK, I think I made it work:
ALL ships in a fleet must be Troop transports, ALL must have troops on board and weapons. This way they all fly toward planet firing weapons !
The first troop transport that reaches planet will drop troops. Even if there are still weapon platforms on the planet !
However, if ground combat fails, this ship will become "attack ship" - no troops on board ! Other troop transports will try to fly away untill the unlucky ship is destroyed. Then all of them turn back on the planet like sharks!