Hey just had an idea- sorry if someones had it before but...
If you want to make a planetary mission (or at least a mission on a small moon considering the possible size limitations of the Freespace engine), the obvious objective is to have a ground strike. How about simply making the target a subsystem of the planet, and also any "Surface to Air" sites would just be turrets. You'd have to be careful about the damage rating of the moon and stuff though. Or, given the fact that two models can be placed over one another in Fred, you could have one which is the moon and one which is the installation... I know what I'm going on about

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"Supernova, ya supernova, supernova goes pop
supernova, ya think it's over but the supernova don't stop" PM5K
Death:"Arnold Judas Rimmer... your life is over. You will travel with me to the-"
#knees death in the groin#
Rimmer:"Remember matey, it's only the good that die yoing"